Bioremediation of Smog: Current Trends and Future Perspectives

Abstract

Air pollution has become one of the biggest problems throughout the world. Smog has a severe effect on the pulmonary and circulatory systems, which causes a significant number of deaths globally. Therefore, the remediation of air pollutants to maintain ecosystem processes and functions and to improve human health is a crucial problem confronting mankind today. This review aims to discuss the health effects of smog on humans. This review will also focus on the bioremediation of air pollution (smog) using bacteria, fungi, phytoremediation, nanotechnology, and phylloremediation (using plants and microbes). Phylloremediation is the most effective technology for removing air pollution naturally. The future perspective presents a great need to produce an ecosystem where microbes, plants, and nanoparticles synergistically control smog. In addition, further advancements would be needed to modify the genetic makeup of microbes and plants. Biotechnological approaches like CRISPR-Cas9 can be applied to the editing and cutting of specific genes responsible for the bioremediation of VOCs, NOx, SOx, and harmful hydrocarbons. The extracted genes can then be expressed in biologically modified microorganisms and plants for the enhanced bioremediation of smog.

1. Introduction

Environmental pollution has become a prominent issue, affecting the ecosystem’s biotic and abiotic factors. Sewage systems have transformed water bodies into an ecosystem full of organic and inorganic toxicants. This ultimately affects aquatic life and mankind. Additionally, landfilling is a source of soil pollution, contaminating the land with toxicants and making it unfit for sustaining healthy life. Ultimately, toxicants can also be found in the air. The gaseous toxic molecules react with atmospheric gasses and moisture in the air, becoming more toxic and causing air pollution [1]. The ultimate form of air pollution can be seen as smog. In winter, the air toxicants encounter fog, causing natural non-affecting air-to-air pollution called smog [2].

Historically, there have been two types of smog based on their origin, namely London and Los Angeles. London smog contains a high quantity of sulfur dioxide; hence, it is known as sulfurous smog. In 1952, London smog lasted for a week, affecting 4000–10,000 lives. The current UK guideline criteria for sulfur dioxide is 0.04 ppm and PM₁₀ of 50 μg/m³ (daily average), which was drastic in 1952. The highest recorded concentration of smoke was 4.46 mg/m³, while the concentration of sulfur dioxide reached 1.34 ppm. Los Angeles smog is formed due to the reaction between nitrogen oxides and reactive hydrocarbon organics. High oxidant levels are seen in this type, as it depends on intense solar radiation and, ultimately, secondary pollutants like ozone and trace gas species. Smog directly affects the greenhouse effect, as it disturbs the topography of the land. When surface-level ozone concentrations are above 0.070 ppm for eight hours or more, it is deemed harmful; these circumstances are frequently encountered in urban regions vulnerable to photochemical smog [3]. According to Moses Maimonides, 12th century air pollution was due to rapid urbanization. In the 14th century, there was a significant increase in the utilization of coal, which eventually led to its prohibition by the English Parliament. It was the first law passed to conserve the natural composition of air and prevent the detrimental effects of air pollution. In 1948, a heartbreaking event occurred in England, termed “killer smog”. In this event, a poisonous smog cloud was formed from the emissions of zinc smelter industries, and thirty people died due to excessive breathing difficulties [4].

The air quality index (AQI) is a tool for gauging the health impacts of smog. The public’s understanding of the dangers posed by air pollution to human health can be improved by categorizing air quality into several states. These categorizations could be problematic, though, according to conflicting epidemiological findings. First, there is no universally accepted definition of a “safe level” for the AQI, which is usually defined through agreement among experts [5]. The linear model is the most reported method for forecasting the mortality rate of particulate matter (PM), one of the primary air contaminants. A strong correlation exists between prolonged exposure to PM_2.5 ambient air pollution, even at low concentrations (e.g., 10 μg/m³), and an increased risk of lung cancer and cardiopulmonary death. There are three categories of particulate matter, namely fine PM_2.5, which contains particles with a median aerodynamic diameter of less than 2.5 μm, coarse PM₁₀, which contains particles with a median aerodynamic diameter of more than 10 μm, and ultra-fine PM of a diameter of 0.1 μm [6]. The IQ Air Institute has ranked Jakarta, Indonesia, as one of the world’s most polluted cities. In contrast, according to the World Air Index, Yangon, Myanmar, is among the safest countries, with zero PM_2.5.

Most of the pollution, at least in industrialized countries, is caused by fossil fuel combustion. This includes internal combustion in industrial uses and power plants and exhaust from motor vehicles such as airplanes, cars, trucks, and ships. Emissions encompass a wide range of substances, including gasses like nitric oxide (NO), nitrogen dioxide (NO₂), carbon monoxide (CO), sulfur dioxide (SO₂), particulate matter (PM) in solid and liquid forms like carbon black and organic carbon, transition metals and aromatic hydrocarbons, benzene, toluene, and xylene. Nevertheless, with the potential health risks, several gaseous pollutants have been implicated, including sulfur dioxide (SO₂), nitrogen oxides (NOx), carbon monoxide (CO), and ozone (O₃) [7].

Every day, people and animals are exposed to various environmental contaminants. These contaminants can affect different organ systems differently, leading to negative outcomes. As a result of its adverse impacts on ecosystem health and food security, environmental pollution has emerged as a critical global concern. In a single year, air pollution kills millions of people worldwide. The World Health Organization reports that the leading causes of premature death due to air pollution include lung cancer, COPD, asthma, heart failure, stroke, and respiratory infections. The fact that 99% of the world’s population breathes air with toxins at unsafe levels is notable [8].

There are different bioremediation techniques to tackle smog. The goal to lessen smog is environmental conservation, achieved by protecting ecosystems from harmful chemicals released from industries, vehicles, etc. Bioremediation uses living organisms to clear up harmful chemicals and aerosols. Different methods can be used, including phytoremediation, bio-stimulation, nanotechnology, bio-sparging, biosorption, bioaugmentation, and bioenergy, to deal with traffic-related pollutants to achieve “clean fuel” and to grasp up the waste generated by industries. Along with bioremediation technologies, the effective enforcement of regulatory policies is necessary.

The bioremediation of smog uses a microbial process, and plants have proven effective and reliable due to their eco-friendly features. The bioremediation of air pollutants can either be carried out ex situ or in situ, depending on several factors, including but not limited to cost, site characteristics, type, and the concentration of pollutants [9]. In addition, the bioremediation of smog can be conducted via different living organisms and their synergism. This review aims to discuss the effects of smog on human health and bioremediation techniques to reduce this type of pollution. In addition, different ex situ and in situ bioremediation techniques will be discussed, and the role of bacteria, fungi, nanoparticles, and plants in reducing smog will be examined. The review will also explain the recent discoveries about the synergistic effect of plants, nanoparticles, and microbes in the removal of air pollutants effectively. It summarizes the methods that lead to sustainable and eco-friendly solutions for improving air quality and protecting human health. So, overall, the objective of this review is to discuss how smog is affecting human health globally and how it can be reduced by using different economical and sustainable bioremediation approaches.

2. Methodology

This review synthesizes the current knowledge on air pollution, smog formation, and bioremediation techniques, with a specific focus on identifying research gaps and future priorities. The methodology employed for this review involved a systematic approach to selecting the relevant literature, aiming to ensure a comprehensive and balanced overview of the topic.

2.1. Literature Selection

A structured search was conducted across several academic databases, including Google Scholar, PubMed, Scopus, and Web of Science, to gather peer-reviewed articles, review papers, and case studies related to air pollution, smog, and bioremediation techniques. Keywords used in the search included “air pollution”, “smog”, “bioremediation”, “phytoremediation”, “microbes”, “nanotechnology in pollution control”, and “smog formation”. The search focused on studies published from 2000 to 2024 to capture recent advancements while also including foundational research that shaped the field.

Only studies published in English were considered. Selection criteria prioritized publications that addressed (1) the causes and effects of smog, (2) the role of particulate matter (PM_2.5 and PM₁₀), (3) the application of bioremediation techniques such as phytoremediation, bio-stimulation, bioaugmentation, and nanotechnology, and (4) future research priorities in air pollution mitigation. Studies were excluded if they lacked empirical data, were region-specific without a broader application, or failed to discuss sustainable and scalable solutions.

2.2. Data Extraction and Analysis

Relevant data, findings, and conclusions from the selected literature were carefully reviewed and extracted. The analysis focused on identifying recurring themes, current bioremediation practices, and research gaps. Special attention was given to the potential for integrating plants, microbes, and nanotechnology in future bioremediation efforts and the synergistic effects of combining these approaches.

3. Geographical Aspect of Smog

According to the World Health Organization (2021), the air quality index relies on the annual measurement of the mean concentration of NO₂, PM₁₀, and PM_2.5. Under the Clean Air Act, the Environmental Protection Agency (EPA) regulates five primary air contaminants to safeguard public health, and these measurements are included in the index. Carbon monoxide is a colorless, odorless gas emitted when fuel, such as stove gas, burns. Industrial facilities and wildfires are other sources. Nitrogen dioxide is a highly reactive, reddish-brown gas mainly produced when fossil fuels are burned, particularly in cars and power plants. Ozone is a primary component of the acid mixture known as smog. This gas is created when two pollutants—nitrogen oxides and volatile organic compounds—react in sunshine. According to the WHO, particulate matter concentration is of more significant concern, and smog is a measure of particulate matter [10].

According to the WHO report of 2023, Abbottabad was declared the cleanest city in Pakistan, and Lahore was the most polluted city. According to the AQI world ranking, Lahore is in third place, having an unhealthy AQI. A drastic increase in vehicular traffic has burdened the air with toxic pollutants, directly or indirectly contributing to smog production. The five countries declared to have high mortality rates due to a poor AQI were China, India, Pakistan, Indonesia, and Bangladesh according to State of Global Air rankings (2024) [11]. According to the WHO, the AQI is also a measure of a state’s population. Alleviating levels of PM and NOx were found in America, whereas southeast and eastern Mediterranean regions have more than average levels of particulate matter [12].

The country with the greatest amount of smog in 2023 was Bangladesh, with a PM value of 160, which is six times larger than the WHO recommendations, followed by Pakistan (73.8) and India (54.4) [13]. This profile is due to a significant drawback; among the power plants Bangladesh runs, 80% of them are based on gas, enhancing the generation of airborne particles—ultimately creating more smog. It is even deduced that 30% of particulate matter is released from India into Bangladesh, worsening its AQI [14]. However, the USA has far lower concentrations of particulate matter and NOx due to stringent rules and regulations, the use of green transportation, the regular monitoring of air quality, and the sustainable use of renewable resources. All these factors contribute to the USA having a good AQI and less smog generation, mitigating the perilous effects of smog [15].

According to the World Air Quality Report (2023), the countries that indicated less smog due to negligible particulate matter were New Zealand (2.4 μg/m³) and Canada (2.7 μg/m³), as they have fewer industrial sectors and efficient waste management, as well as the least aerosol emissions [16]. The AQI is around 35, implicating it to be good, and the contributing factor is elite industries using clean energy, whereas the smog extent is much greater in Lahore, Pakistan, because of its geographical region, abrupt climate change, population explosion, and vehicular emissions [17].

Table 1. Air pollution in different geographical regions and remediation techniques.

Geographical Region	Concentration of PM (μg/m3)	NO2 (μg/m3)	Air Quality Index	Smog Status	Eradication Measures	References
Saudi Arabia (Jeddah)	102	15.35	110	Moderate	Biological air purification system to purify air in large buildings and industries	[18]
India (Delhi)	60	15.2	136 (Unhealthy)	Intense	Phytoremediation using transgenic plants	[19]
China	38.15	33	144 (Unhealthy)	Intense	Sulfate-reducing bacteria-assisted remediation of SOx; microbial electrochemical technology	[20,21]
Afghanistan	60	17	157 (Unhealthy)	Intense	Algal bioremediation	[22]
Iran	30	-	92 (Moderate)	Intense	Phytoremediation	[23,24]
South Africa	21	-	89 (Moderate)	Moderate	Microbial remediation	[25]
United States of America	9.6	11.3	10 (Good)	Moderate	Installment of effluent treatment plants to prevent the escape of industrial effluents into air; public transportation to reduce vehicular emissions; bioremediation of e-waste sites; recycling; CRISPR-Cas9-based bioremediation	[26]
Japan	10.84	10	89 (Moderate)	Moderate	Phytoremediation; green buildings; transgenic microbial remediation; microalgae-based treatment; carbon filtering using phylloremediation	[27]
Australia	3.1	8	40 (Good)	Moderate	Microbial remediation; genome editing-based bioremediation	[26]

Table 1. Air pollution in different geographical regions and remediation techniques.

Geographical Region	Concentration of PM (μg/m3)	NO2 (μg/m3)	Air Quality Index	Smog Status	Eradication Measures	References
Saudi Arabia (Jeddah)	102	15.35	110	Moderate	Biological air purification system to purify air in large buildings and industries	[18]
India (Delhi)	60	15.2	136 (Unhealthy)	Intense	Phytoremediation using transgenic plants	[19]
China	38.15	33	144 (Unhealthy)	Intense	Sulfate-reducing bacteria-assisted remediation of SOx; microbial electrochemical technology	[20,21]
Afghanistan	60	17	157 (Unhealthy)	Intense	Algal bioremediation	[22]
Iran	30	-	92 (Moderate)	Intense	Phytoremediation	[23,24]
South Africa	21	-	89 (Moderate)	Moderate	Microbial remediation	[25]
United States of America	9.6	11.3	10 (Good)	Moderate	Installment of effluent treatment plants to prevent the escape of industrial effluents into air; public transportation to reduce vehicular emissions; bioremediation of e-waste sites; recycling; CRISPR-Cas9-based bioremediation	[26]
Japan	10.84	10	89 (Moderate)	Moderate	Phytoremediation; green buildings; transgenic microbial remediation; microalgae-based treatment; carbon filtering using phylloremediation	[27]
Australia	3.1	8	40 (Good)	Moderate	Microbial remediation; genome editing-based bioremediation	[26]

All these bioremediation techniques are being operated in different continents and geographical regions. However, the extent and efficiency of the continents differ depending on technological advancement, bacterial growth systems at a vast scale when dealing with genetically engineered microbes to inhibit gene transfer, proper ventilation system availability, and green energy utilization. According to the USA’s EPA, there has been a 37% decline in PM_2.5 at the national level from 2010 to 2023 due to better prognosis and technological advancement (Table 1). For example, a recent bioremediation technique has been developed in the USA, i.e., the bio-electrochemical system for air pollution control, which showed promising results in the efficient degradation of PM_2.5 and volatile organic compounds. In this system, microbes such as Shewanella oneidensis are cultivated in a reactor on electrodes, and the transfer of an electric current helps bacteria degrade air pollutants aided by an electrochemical reaction [28]. The AQI is dependent on many other important factors, involving managing traffic density, dealing with industrial effluents efficiently, and using fertilizers and pesticides that have the least impact on the environment and humans, but when it comes to technology, America and Australia are treating smog far better because of proper laws, as well as their implementation [29].

4. Health Effects of Smog

Taking in air embedded with toxic ingredients from smog harms human health. The health effects of smog are profound and perturbing, given its instinctive potential to penetrate deeper into the lungs. In the modern era, with the advancement of technology, there are rising levels of smog in different areas of the world, and most of it is attributed to industrial effluents. Particulate matter in smog can transfer bacteria with strong antibiotic-resistant genes into the human lungs [30]. There is a need to develop certain policies to tackle smog and to combat serious illnesses caused by it (Figure 1). Among such environmental crises, alleviating air pollution is a major goal [31].

The World Health Organization has set up research on air quality, energy, and health. According to the WHO, smog leads to many health problems, including stroke, asthma, pulmonary problems, neurological impairment, and pregnancy-related complications [10]. According to the study performed by Usman et al (2020) and Thangavel et al. (2022), most of the pulmonary effects due to smog are because of the reaction of toluene present in the air with different toxic compounds to generate organic aerosols, leading to inflammation in the lungs (Figure 2). In contrast, particulate matter (PM_2.5) generates potential carcinogens that impact the lungs and has a risk factor of 60% for smoking individuals [32,33]. According to the study performed by Gavett & Koren (2001), the effect of PM on asthma, which develops in youngsters who continuously encounter bad air quality [34].

There are certain implications of smog in the male reproductive system as well. According to the study performed by Omolaoye et al. (2024), PM_2.5, SO₂, NO₂, CO, and O₃ can have a serious impact on erectile dysfunction due to the deficiency of an enzyme [35], i.e., nitric oxide synthase in penile tissue, whereas in females, pregnancy is accompanied by extreme stress and trauma due to the heavy burden of ozone and sulfur dioxide, leading to oxidative stress [36]. During COVID-19, an abrupt alteration in air composition significantly impacted the air quality index. During this time, there was an increased incidence of post-partum depression in women due to immense air pollution. There was an association between gestational diabetes mellitus and particulate matter in pregnant women [37].

Smog has also worsened the repercussions of COVID-19. According to the Environmental Protection Agency, 700 chemicals are released from industries daily that cause cancer and other respiratory issues. In COVID-19 complications, pulmonary obstruction was the major symptom. Most patients suffered due to poor air quality due to ozone and PM_2.5, and such patients required mechanical ventilation on compulsion [38]. According to the study by Qayyum et al. (2024), there was no ozone reduction during the COVID-19 era, but there was a decline in particulate matter concentration from 2019 to 2022 [39].

According to the study done by Naveed & Khayyam (2023), most of the pulmonary effects due to the smog are because of the reaction of toluene present in the air with different toxic compounds to generate organic aerosols leading to inflammation in lungs [40]. In contrast, particulate matter (PM2.5) generates potential carcinogens that impact the lungs and has a risk factor of 60% for smoking individuals [41] The air quality index and smog content are reported for different geographical regions of the world in Table 1. Pseudomonas putida has the potential to interact with Fe₂O₃ nanoparticles and break down particulate matter into less toxic compounds like certain organic compounds and ions [42]. Trapping such air pollutants at a vast scale helps minimize the inhalation of toxicants and ultimately reduces air pollutants [43].

The bioremediation of particulate matter is crucial to tackling brain disorders like Alzheimer’s disease. Phytoremediation is beneficial in absorbing or accumulating high levels of PM_2.5 by buffalo grass, boxwood shrubs, and certain urban trees like silver maples through their trichomes, waxy coatings, and roots, and they also take in certain metals that are chelated on particulate matter, a technique known as phyto-sequesteration, which can ultimately lessen indoor air contaminants and reduce the prevalence of autism, Alzheimer’s, and Parkison’s disease. It was found that with a half reduction in PM_2.5, cognitive development in such patients improved by 10% (See Table 2) [44].

Table 2. Chemicals in the air (smog) contribute to different health implications.

Pollution	Chemical	Effects on Body System	Mechanism of Action	Citation
Smog	Ozone	Menstrual and pregnancy-related disturbances in females	Decline in progesterone and increment in estrogen level to alter luteinizing hormones	[45]
Smog	Particulate matter2.5	Reduction in sperm count and disturbance in its motility	Increment in thiobarbituric acid in testes and decline of superoxide dismutase to increase SO2 levels	[46]
Smog	Persistent organic compounds	Obesity	Disruption in metabolic pathways, increment in oxidative stress leading to hormonal instability	[47]
Smog	Nitrogen and sulfur oxides	Acute coronary problems	Increment in blood pressure and obstruction in blood flow, leading to blood clotting	[48]
Smog	Genotoxic carcinogens (benzene)	Brain tumors	Interference with the growth and differentiation of cells at DNA level	[49]
Smog	Non-genotoxic carcinogen (dichlorobenzene)	Brain tumors	Increment of inflammation in the brain along with assisting the accumulation of arsenic to enhance oxidative stress in the brain	[49]
Smog	Ozone, NO2	Bronchitis	Inhibition of β-catenin levels and production of IL-6 and interferon, causing inflammation in lungs	[50]
Smog	Ozone, PM2.5	Intellectual and psychological problems	Impairment in cognitive abilities and social withdrawal due to anxiety and stress posed by air pollutants	[51]
Smog	Volatile organic compounds, SOx, ozone	Retinal problems and rhinorrhea	Release of histamine and neuropeptides to enhance mucous production in nose and dryness in eyes	[52]
Smog	PM2.5, SO2	Alzheimer’s disease	Neuroinflammation due to the passage of cytokines from blood into brain	[53]

Table 2. Chemicals in the air (smog) contribute to different health implications.

Pollution	Chemical	Effects on Body System	Mechanism of Action	Citation
Smog	Ozone	Menstrual and pregnancy-related disturbances in females	Decline in progesterone and increment in estrogen level to alter luteinizing hormones	[45]
Smog	Particulate matter2.5	Reduction in sperm count and disturbance in its motility	Increment in thiobarbituric acid in testes and decline of superoxide dismutase to increase SO2 levels	[46]
Smog	Persistent organic compounds	Obesity	Disruption in metabolic pathways, increment in oxidative stress leading to hormonal instability	[47]
Smog	Nitrogen and sulfur oxides	Acute coronary problems	Increment in blood pressure and obstruction in blood flow, leading to blood clotting	[48]
Smog	Genotoxic carcinogens (benzene)	Brain tumors	Interference with the growth and differentiation of cells at DNA level	[49]
Smog	Non-genotoxic carcinogen (dichlorobenzene)	Brain tumors	Increment of inflammation in the brain along with assisting the accumulation of arsenic to enhance oxidative stress in the brain	[49]
Smog	Ozone, NO2	Bronchitis	Inhibition of β-catenin levels and production of IL-6 and interferon, causing inflammation in lungs	[50]
Smog	Ozone, PM2.5	Intellectual and psychological problems	Impairment in cognitive abilities and social withdrawal due to anxiety and stress posed by air pollutants	[51]
Smog	Volatile organic compounds, SOx, ozone	Retinal problems and rhinorrhea	Release of histamine and neuropeptides to enhance mucous production in nose and dryness in eyes	[52]
Smog	PM2.5, SO2	Alzheimer’s disease	Neuroinflammation due to the passage of cytokines from blood into brain	[53]

There is a crucial need for bioremediate endocrine-disrupting chemicals in the air to prevent reproductive disorders in males and females. Environmentally friendly bioremediation techniques are being used with a lower production of toxic byproducts. There has been tremendous research on generating genetically modified microbes and using in silico prediction of new metabolic pathways to target certain enzymes and protein engineering to bioremediate air pollutants efficiently [54]. Benzene is one of the most problematic chemicals, causing erectile dysfunction accompanied by oxidative stress. Recently, artificially generated P. putida filters have been developed to trap benzene in the air, and they were genetically engineered to produce nitric oxide, which lessens the oxidative stress caused by benzene. However, there is a need for more research to understand the implications of different diseases by different bioremediation strategies [55].

The whole-body system is affected by smog and its ingredient pollutants. Researchers are focusing on combating these air pollutants to lessen deleterious health effects. Ozone can be biodegraded by bacterial biodegradation using Pseudomonas and Achromobacter species [56]. For this, the total hydrocarbon content was measured at 6-week, 8-week, and 12-week intervals, and it was shown that the ozone level had been reduced to 6.4 ppm, 2.8 ppm, and 1.4 ppm, respectively. Such implications could help reduce preterm birth and neonatal mortality in China, where public awareness regarding exposure to ozone and other air pollutants, especially during pregnancy, is minimal [57].

A genetically engineered P. putida biofiltration system was developed to remediate benzene, which is a cause of brain tumors and leukemia. This experiment was carried out in New Jersey, USA, in which a biofiltration system was installed in a proper ventilation system, and it was seen that before experimentation, the benzene levels were 200 µg/m³. Leukemia incidence was 10 cases per 100,000 people. The remarkable results of P. putida showed a decline in benzene to 75 µg/m³, and the leukemia rate was seven cases per 100,000 people. Hence, it was deduced that benzene concentration declined by almost 70% in 6 months [58].

5. Bioremediation of Smog via Bacteria

Bacteria-based bioremediation is a novel and green technology that deploys specific bacterial strains to decompose and detoxify pollutants. They detoxify harmful chemicals, which makes them survive in any place, and they can be used for all types of environmental cleanup. Bacteria like Pseudomonas putida and Bacillus subtilis can break down hydrocarbons and nitrogen oxides, two of the most important components in smog. They break down pollutants into inert byproducts that will not pollute the air [59]. Table 3 presents the bacteria and relevant efficacy to degrade certain smog constituents.

Table 3. Bioremediation of smog components using bacteria.

Name of Bacteria	Effects	References
Corynebacterium sp.	55% reduction in VOCs	[60]
Pseudomonas aeruginosa	60% reduction in hydrocarbons	[61]
Flavobacterium sp.	50% reduction in NOx	[62]
Azotobacter sp.	Sulfur compounds decreased by a whopping 70%	[63]
Nocardia sp.	60% reduction in VOCs	[64]
Burkholderia sp.	55% reduction in hydrocarbons	[65]
Nitrosomonas sp.	65% decrease in nitrogen oxides	[66]
Sphingomonas sp.	60% reduction in VOCs	[67]
Streptomyces sp.	70% drop in carbon monoxide	[68]
Rhodococcus sp.	50% reduction in NOx	[69]
Alcaligenes sp.	45% reduction in VOCs	[70]
Micrococcus luteus	Sulfur compounds reduced by 40%	[71]
Acinetobacter sp.	Up to 55% less sulfur compounds	[72]
Bacillus subtilis	Nitric oxide levels also dropped by 60%	[73]
Pseudomonas putida	50% reduction in hydrocarbons	[74]

Table 3. Bioremediation of smog components using bacteria.

Name of Bacteria	Effects	References
Corynebacterium sp.	55% reduction in VOCs	[60]
Pseudomonas aeruginosa	60% reduction in hydrocarbons	[61]
Flavobacterium sp.	50% reduction in NOx	[62]
Azotobacter sp.	Sulfur compounds decreased by a whopping 70%	[63]
Nocardia sp.	60% reduction in VOCs	[64]
Burkholderia sp.	55% reduction in hydrocarbons	[65]
Nitrosomonas sp.	65% decrease in nitrogen oxides	[66]
Sphingomonas sp.	60% reduction in VOCs	[67]
Streptomyces sp.	70% drop in carbon monoxide	[68]
Rhodococcus sp.	50% reduction in NOx	[69]
Alcaligenes sp.	45% reduction in VOCs	[70]
Micrococcus luteus	Sulfur compounds reduced by 40%	[71]
Acinetobacter sp.	Up to 55% less sulfur compounds	[72]
Bacillus subtilis	Nitric oxide levels also dropped by 60%	[73]
Pseudomonas putida	50% reduction in hydrocarbons	[74]

Qamar et al. (2022) accomplished this with air samples polluted with smog and showed how Pseudomonas putida reduced levels of hydrocarbons by up to 50% [74]. Other research reported similar success with bacterial bioremediation for ameliorating smog. Ganguly et al. (2024) found that using Acinetobacter sp. could also reduce sulfur compounds by 55%, and another study showed a reduction in nitrogen oxides by up to 60% when using Bacillus subtilis [72,73].

When fuel is consumed in industrial activity, waste gasses are released into the atmosphere as flue gas emissions. The amount of wastewater released into the environment is substantial due to these identical activities. There exists research aimed to find out if bacterial consortiums in a bubble column bioreactor could simultaneously treat wastewater and decrease the emissions of flue gasses. The research examined different growth media that were created from wastewater. The highest biomass was achieved with maximal removal efficiencies of 80.77% for CO₂, 77.30% for SO₂, and 3.66 g/L for NO [68].

Similarly, another study investigated the possibility of a bacterial consortium for CO₂, SO₂, and a gaseous mixture to fix both gasses simultaneously. The researchers used a 3 L glass bioreactor and ran extended semi-continuous tests. Bacillus tropicus and Bacillus cereus are a part of a bacterial consortium that uses thiosulphate for energy and DWW with extra minerals for nutrition in this study. For gaseous mixtures of CS and S, the greatest CO₂ mitigation efficiency was found to be 93.8%, while for gaseous mixtures of S, it was 91.4% [75].

Research conducted by Lu et al. (1999) showed that immobilized Pseudomonas putida and P. fluorescens in a fiber-bed bioreactor degraded BTEX (benzene, toluene, ethylbenzene, and o-xylene) under hypoxic conditions. Compared with the free cells, these immobilized cells exhibited a higher reduction in BTEX compounds [76].

In accordance, a study by Yang et al. (2020) investigated the potential of Bacillus subtilis JD-014 for nitrogen removal, focusing on the aerobic denitrification process. A nitrate reduction from 50 to 300 mg/L was reported, identifying the nisir gene as a key regulator in nitrogen reduction. Bacillus spp. was studied for nitrogen oxide reduction in a simulated sewage bioreactor, showing a significant reduction in nitrogen concentrations, especially of nitrite-N and ammonium [77].

Regarding volatile organic compounds (VOCs), Kleinheinz et al. (1999) inspected the reduction in α-pinene utilizing a biofiltration column inoculated with Pseudomonas fluorescens and Alcaligenes xylosoxidans. The microorganisms contributed to the debasement of α-pinene inside after 36 h, outlining a compelling ejection at concentrations averaging 295 ppm. Another approach underscores the potential of biofiltration systems for VOC treatment, contributing to cleaner air [78]. Moehlman (2018) conducted a bench-scale micro-investigation to re-enact field conditions to reach petroleum hydrocarbon-contaminated objectives. It was found that higher phosphorus concentrations made strides in benzene debasement, emphasizing the noteworthiness of site-specific factors and supplement availability in bioremediation triumph [79].

Moreover, the bioremediation potential of Pseudomonas aeruginosa was reviewed by Hu et al. (2023), which showed its excellence in degrading various organic pollutants through its diverse metabolic pathways and biosurfactant production [80]. Sun et al. (2019) investigated the ability of a biofilter for the synchronous removal of nitric oxide (NO) and sulfur dioxide (SO₂) underneath thermophilic and micro-oxygen conditions. The biofilter fulfilled clearing efficiencies, outperforming 90% for both harmful compounds, with consistent execution over specific operational stages for reducing brown fog (smog) components [81].

Another study by Karimi et al. (2015), utilizing Pseudomonas aeruginosa, showed a prominent removal of naphthalene by 90%, an organic air pollutant, using specialized continuous flow systems [82]. The integration of these continuous flow systems into the present industrial settings was adopted for more encouraging results. Lastly, Khan et al. (2024) used a membrane bioreactor (MBR) system to remove phenolic contaminants from industrial effluents. The mixed microbial consortia comprising Pseudomonas sp. and Bacillus sp. were inoculated in the MBR system and exhibited a high removal efficiency over time. Compared to conventional physiochemical setups, this MBR system has more advantages, such as its cost-effectiveness [30].

5.1. Mechanism of Air Pollutant Degradation by Bacteria

The enzymatic mechanisms involved will differ greatly and depend largely on the type of pollutant for bioremediation by specific bacteria. Oxygenase enzymes convert unstable hydrocarbons into stable alcohols or degradation intermediates, which serve as substrates for other metabolic pathways (including the tricarboxylic acid cycle) to yield CO₂ and water byproducts. This is especially useful for smog, where hydrocarbon contaminants are frequently discovered as bacteria work to oxidize all types of organic pollutants. Nitrogen oxides (NOx) are also where different bacterial species like pseudomonas fluorescens use nitrate reductase enzymes that help convert nitrates into nitrogen gas through denitrification, reducing NOx levels a lot in nature. Acinetobacter sp. is a major contributor of sulfur compounds and catabolizes toxic sulfur compounds with the help of sulfur-oxidizing enzymes to obtain sulfates, rendering them less harmful (Table 4) [73].

The mechanisms of bacteria are different in this case and depend on the pollutant concerned. In the oxidative degradation of organic compounds, enzymes can insert oxygen atoms into a given hydrocarbon molecule or a substrate. Then, dehydrogenation or breaking some carbon–carbon bonds, followed by either being eliminated, produces an alkene group and smaller acyclic carbonyl functional group. By comparison, nitrogen oxides must be further “reduced” by transforming the NOx into plain old nonreactive or inert unstable triple-bonded compounds. Sulfur-oxidizing bacteria, such as Acinetobacter sp., convert sulfur compounds by oxidization into non-toxic sulfates [83]. These mechanisms depend on the pollutant chemistry and bacterial metabolism to degrade environmental pollutants through the extraordinary adaptability of bacteria.

Table 4. Bacterial bioremediation mechanisms vs. contaminant types.

Bacterial Species	Pollutant Type	Mechanism of Degradation	Effectiveness	References
Pseudomonas putida	Hydrocarbons	Oxygenase enzymes convert hydrocarbons to alcohols, acids, and CO2	50% reduction in hydrocarbons	[74]
Pseudomonas putida	Nitrogen oxides (NOx)	Nitrate reductase converts NOx to nitrogen gas (N2) via denitrification	60% reduction in NOx	[73]
Acinetobacter sp.	Sulfur compounds	Sulfur-oxidizing enzymes oxidize sulfur compounds into sulfates	55% reduction in sulfur compounds	[72]
Acinetobacter sp.	Ammonia	Ammonia monooxygenase converts ammonia (NH3) into nitrite (NO2−)	65% reduction in ammonia	[76]
Pseudomonas fluorescens	BTEX (benzene, toluene, etc.)	Toluene dioxygenase degrades BTEX compounds in oxygen-limited conditions	Significant reduction in BTEX	[76]
Flavobacterium sp.	Nitrogen oxides (NOx)	Utilizes nitrate reductase to convert NOx into nitrogen gas	50% reduction in NOx	[72]
Burkholderia sp.	Hydrocarbons	Oxygenase enzymes break down hydrocarbons	55% reduction in hydrocarbons	[76]
Sphingomonas sp.	Volatile organic compounds (VOCs)	Degradation of VOCs was complemented by biofilms and biosurfactants	VOCs reduced to 60%	[82]
Streptomyces sp.	Carbon monoxide	Carbon monoxide dehydrogenase oxidizes CO to CO2	70% reduction in CO	[72]
Corynebacterium sp.	Volatile organic compounds (VOCs)	Hydrocarbon-degrading enzymes degrade VOCs into non-toxic byproducts	55% reduction in VOCs	[76]
Rhodococcus sp.	Nitrogen oxides (NOx)	Nitrate reduction enzymes convert NOx into nitrogen gas	50% reduction in NOx	[80]
Micrococcus luteus	Sulfur compounds	Sulfur oxidase enzymes degrade sulfur compounds into less harmful sulfates	40% reduction in sulfur compounds	[72]
Sphingomonas sp.	Polycyclic aromatic hydrocarbons (PAHs)	Enhanced by organic matter in manure compost; utilizes oxygenase enzymes	40% reduction in PAHs	[84]
Thiobacillus ferrooxidans	Inorganic sulfur compounds	Oxidation of sulfur compounds to sulfates using sulfur-oxidizing enzymes	Rapid oxidation of sulfur compounds	[83]
Pseudomonas fluorescens	BTEX (benzene, toluene, etc.)	Toluene dioxygenase (oxidation); breakdown under hypoxic conditions	Significant reduction in BTEX	[76]
Burkholderia sp.	Hydrocarbons	Oxygenase enzymes (oxidation)	55% reduction in hydrocarbons	[82]
Methylosinus trichosporium OB3b	Nitrous oxide (N2O)	Methanobactin inhibits reduction in N2O (inhibition of denitrification)	Reduced N2O production by denitrifying bacteria	[85]
Sphingomonas sp.	PAHs	Oxygenation enhanced by water-extractable organic matter	Significant degradation of PAHs	[84]
Pseudomonas aeruginosa	Toluene	Unique metabolic route for toluene degradation (oxidation)	70% degradation of toluene	[86]
Methanobactin OB3b	Nitrous oxide	Inhibits N2O reduction in denitrifiers	Inhibition of denitrification process	[87]

Table 4. Bacterial bioremediation mechanisms vs. contaminant types.

Bacterial Species	Pollutant Type	Mechanism of Degradation	Effectiveness	References
Pseudomonas putida	Hydrocarbons	Oxygenase enzymes convert hydrocarbons to alcohols, acids, and CO2	50% reduction in hydrocarbons	[74]
Pseudomonas putida	Nitrogen oxides (NOx)	Nitrate reductase converts NOx to nitrogen gas (N2) via denitrification	60% reduction in NOx	[73]
Acinetobacter sp.	Sulfur compounds	Sulfur-oxidizing enzymes oxidize sulfur compounds into sulfates	55% reduction in sulfur compounds	[72]
Acinetobacter sp.	Ammonia	Ammonia monooxygenase converts ammonia (NH3) into nitrite (NO2−)	65% reduction in ammonia	[76]
Pseudomonas fluorescens	BTEX (benzene, toluene, etc.)	Toluene dioxygenase degrades BTEX compounds in oxygen-limited conditions	Significant reduction in BTEX	[76]
Flavobacterium sp.	Nitrogen oxides (NOx)	Utilizes nitrate reductase to convert NOx into nitrogen gas	50% reduction in NOx	[72]
Burkholderia sp.	Hydrocarbons	Oxygenase enzymes break down hydrocarbons	55% reduction in hydrocarbons	[76]
Sphingomonas sp.	Volatile organic compounds (VOCs)	Degradation of VOCs was complemented by biofilms and biosurfactants	VOCs reduced to 60%	[82]
Streptomyces sp.	Carbon monoxide	Carbon monoxide dehydrogenase oxidizes CO to CO2	70% reduction in CO	[72]
Corynebacterium sp.	Volatile organic compounds (VOCs)	Hydrocarbon-degrading enzymes degrade VOCs into non-toxic byproducts	55% reduction in VOCs	[76]
Rhodococcus sp.	Nitrogen oxides (NOx)	Nitrate reduction enzymes convert NOx into nitrogen gas	50% reduction in NOx	[80]
Micrococcus luteus	Sulfur compounds	Sulfur oxidase enzymes degrade sulfur compounds into less harmful sulfates	40% reduction in sulfur compounds	[72]
Sphingomonas sp.	Polycyclic aromatic hydrocarbons (PAHs)	Enhanced by organic matter in manure compost; utilizes oxygenase enzymes	40% reduction in PAHs	[84]
Thiobacillus ferrooxidans	Inorganic sulfur compounds	Oxidation of sulfur compounds to sulfates using sulfur-oxidizing enzymes	Rapid oxidation of sulfur compounds	[83]
Pseudomonas fluorescens	BTEX (benzene, toluene, etc.)	Toluene dioxygenase (oxidation); breakdown under hypoxic conditions	Significant reduction in BTEX	[76]
Burkholderia sp.	Hydrocarbons	Oxygenase enzymes (oxidation)	55% reduction in hydrocarbons	[82]
Methylosinus trichosporium OB3b	Nitrous oxide (N2O)	Methanobactin inhibits reduction in N2O (inhibition of denitrification)	Reduced N2O production by denitrifying bacteria	[85]
Sphingomonas sp.	PAHs	Oxygenation enhanced by water-extractable organic matter	Significant degradation of PAHs	[84]
Pseudomonas aeruginosa	Toluene	Unique metabolic route for toluene degradation (oxidation)	70% degradation of toluene	[86]
Methanobactin OB3b	Nitrous oxide	Inhibits N2O reduction in denitrifiers	Inhibition of denitrification process	[87]

5.2. Limiting Factors in Bacterial Bioremediation

Different factors limit the utility of bacterial bioremediation. One of them is pollutant concentration. If the potent is too high, it can kill bacteria and enzyme activity, decreasing their degradation rate [82]. It also depends on how well bacteria can access pollutants, particularly hydrophobic contaminants that are poorly soluble in water and have low bioavailability. For example, BTEX compounds (benzene, toluene, ethyl benzene, and xylenes) have limited solubility, which can obstruct bioremediation (See Table 5). Bacterial enzymes can be denatured by environmental conditions, such as extremely high or low pH levels; things like this are also of major concern. Additionally, the continuous exposure of bacteria to conditions with a lack of oxygen may decrease their enzymatic activity, hence affecting their pollutant-degrading ability [81]. Meanwhile, mixed pollutants could disrupt bacterial activity, as certain strains cannot co-degrade multiple types of pollutants simultaneously [85].

Table 5. Limiting factors for bacterial bioremediation.

Limiting Factor	Impact on Bioremediation Efficiency	Bacteria	References
Pollutant concentration	High concentrations can inhibit bacterial growth and enzymatic activity	Pseudomonas aeruginosa	[82]
			[86]
Bioavailability of pollutants	Limited solubility of hydrophobic pollutants reduces bacterial access	Pseudomonas fluorescens study on BTEX	[76]
Environmental pH	Extreme pH levels can denature bacterial enzymes, reducing effectiveness	Bacillus subtilis nitrogen removal study	[77]
Temperature fluctuations	Decreases bacterial metabolism and enzyme activity	Biofilter study for VOC removal	[81]
	Cold temperatures slow down bacterial metabolic rates	Degradation of hydrocarbons in cold conditions	[88]
Oxygen levels	Anaerobic conditions limit oxygen-dependent bacterial degradation	Study on BTEX degradation by oxygenase enzymes	[80]
Nutrient availability	Lack of nitrogen or phosphorus limits bacterial growth and activity	Study on phosphorus-enhanced hydrocarbon degradation	[79]
Presence of multiple pollutants	Bacteria may not effectively degrade mixed contaminants simultaneously	Inhibition of nitrous oxide reduction by Methylosinus	[86]
		Bioremediation of CO and sulfur compounds	[72]
Toxicity of pollutants	Highly toxic compounds inhibit bacterial enzyme production and growth	Naphthalene degradation by Pseudomonas spp.	[20]
		Inhibition of nitrous oxide reduction by Methylosinus	[85]
Moisture content	Low moisture reduces bacterial metabolic rates	Study on BTEX in soil bioreactors	[89]
		Study on PAH degradation by Sphingomonas spp.	[84]
Aeration	Poor aeration limits aerobic degradation processes	Bacillus cereus and CO2 removal in bioreactor	[76]

Table 5. Limiting factors for bacterial bioremediation.

Limiting Factor	Impact on Bioremediation Efficiency	Bacteria	References
Pollutant concentration	High concentrations can inhibit bacterial growth and enzymatic activity	Pseudomonas aeruginosa	[82]
			[86]
Bioavailability of pollutants	Limited solubility of hydrophobic pollutants reduces bacterial access	Pseudomonas fluorescens study on BTEX	[76]
Environmental pH	Extreme pH levels can denature bacterial enzymes, reducing effectiveness	Bacillus subtilis nitrogen removal study	[77]
Temperature fluctuations	Decreases bacterial metabolism and enzyme activity	Biofilter study for VOC removal	[81]
	Cold temperatures slow down bacterial metabolic rates	Degradation of hydrocarbons in cold conditions	[88]
Oxygen levels	Anaerobic conditions limit oxygen-dependent bacterial degradation	Study on BTEX degradation by oxygenase enzymes	[80]
Nutrient availability	Lack of nitrogen or phosphorus limits bacterial growth and activity	Study on phosphorus-enhanced hydrocarbon degradation	[79]
Presence of multiple pollutants	Bacteria may not effectively degrade mixed contaminants simultaneously	Inhibition of nitrous oxide reduction by Methylosinus	[86]
		Bioremediation of CO and sulfur compounds	[72]
Toxicity of pollutants	Highly toxic compounds inhibit bacterial enzyme production and growth	Naphthalene degradation by Pseudomonas spp.	[20]
		Inhibition of nitrous oxide reduction by Methylosinus	[85]
Moisture content	Low moisture reduces bacterial metabolic rates	Study on BTEX in soil bioreactors	[89]
		Study on PAH degradation by Sphingomonas spp.	[84]
Aeration	Poor aeration limits aerobic degradation processes	Bacillus cereus and CO2 removal in bioreactor	[76]

6. Mycoremediation of Smog

Fungus-mediated bioremediation should be considered for those classes of pollutants that are unproductively degraded by bacteria, including pollutants such as dioxins and 2,4,6-trinitrotoluene, or human drugs or other chemicals found in environmental matrices (water, aquatic sediments, and soil) [90]. The potency of fungi to convert various hazardous chemicals has aroused interest in bioremediation. Laccases are fungal enzymes that oxidize chlorinated phenolics, nitrogen oxides, and VOCs. According to the study by Soares et al. (2001), a laccase of the fungus Flavodon flavus was shown to decolorize the effluent from a Kraft paper mill bleach plant, releasing harmful chemicals into the atmosphere. The laccase from Coriolopsis gallica has been shown to decolorize alkaline effluents such as the effluents from the pulp and paper industry, i.e., sulfates, nitrates, PCBs, etc. Mycoremediation strategies include in situ and ex situ methods [91].

White-rot fungi are propitious bioremediation agents because of their capability to transform aromatic pollutants that can reach soil and water, thus lessening their toxicity. These pollutants include the BTEX group and petroleum products. Pointing (2001) found that white-rot fungi such as Phanerochaete chrysosporium and Stropharia sp. can degrade immensely toxic environmental pollutants, such as polycyclic aromatic hydrocarbons [92]. Certain pesticides like organochlorines and carbamates can cause immense environmental persistence, detrimental effects on organisms, and bioaccumulation [93].

A group of polymers known as polyhydroxyalkanoates (PHAs) originates from microbial metabolism. Polyhydroxy butyrate (PHB) degradation is carried out by Penicillium spp. and aided by extracellular PHB depolymerase. Aspergillus ustus was shown to degrade PHB under pressure, as in deep sea conditions. There has been excessive use of the insecticide DDT since the 1940s, resulting in an unfavorable ecological imbalance. Ganodema sp. favorably degrades DDT in appropriate conditions [94]. Table 6 shows the bioremediation technique that fungi use to degrade air pollutants.

Table 6. Indicates the mycoremediation of air pollutants.

Type of Bioremediation	Type of Microorganism	Effects	References
Vapor-phase bioreactors for VOC removal	Exophiala lecanii-corni, Cladosporium sphaerospermum, Cladosporium resinae, Mucor rouxii, Phanerochaete chrysosporium	Degradation of VOC	[95]
Biotrickling and biofilters for BTEX removal	Candida subhashii, Fusarium solani	BTEX removal 37.7 ± 3.3 g/m3 h	[96]
Soil bioremediation of TNT	Phanerochaete velutina	70% TNT degradation in 49 days	[97]
Degradation of HMW-PAHs	Fusarium sp. strain ZH-H2	Achieved 85.9% reduction in HMW-PAHs	[98]
Chlorobenzene removal by white-rot fungus	Phanerochaete chrysosporium	Achieved 95% chlorobenzene removal at 550 mg/m3	[99]
Perchloroethylene degradation by white-rot fungus	Trametes versicolor	PCE degradation rates were 0.20 and 0.28 nmol/h mg	[100]
Hydrocarbon degradation	Purpureocillium lilacinum	Up to 15.3% weight loss	[101]
Hydrocarbon degradation	Penicillium chrysogenum	7.6% degradation of hydrocarbons	[101]
VOC removal in biofilters	Arizona cypress, Pseudomonas fluorescens	Co-inoculation showed enhanced bioremediation; effective in reducing fuel pollution	[102]

Table 6. Indicates the mycoremediation of air pollutants.

Type of Bioremediation	Type of Microorganism	Effects	References
Vapor-phase bioreactors for VOC removal	Exophiala lecanii-corni, Cladosporium sphaerospermum, Cladosporium resinae, Mucor rouxii, Phanerochaete chrysosporium	Degradation of VOC	[95]
Biotrickling and biofilters for BTEX removal	Candida subhashii, Fusarium solani	BTEX removal 37.7 ± 3.3 g/m3 h	[96]
Soil bioremediation of TNT	Phanerochaete velutina	70% TNT degradation in 49 days	[97]
Degradation of HMW-PAHs	Fusarium sp. strain ZH-H2	Achieved 85.9% reduction in HMW-PAHs	[98]
Chlorobenzene removal by white-rot fungus	Phanerochaete chrysosporium	Achieved 95% chlorobenzene removal at 550 mg/m3	[99]
Perchloroethylene degradation by white-rot fungus	Trametes versicolor	PCE degradation rates were 0.20 and 0.28 nmol/h mg	[100]
Hydrocarbon degradation	Purpureocillium lilacinum	Up to 15.3% weight loss	[101]
Hydrocarbon degradation	Penicillium chrysogenum	7.6% degradation of hydrocarbons	[101]
VOC removal in biofilters	Arizona cypress, Pseudomonas fluorescens	Co-inoculation showed enhanced bioremediation; effective in reducing fuel pollution	[102]

According to the study performed by Wang et al. (2014), the coordinated action of fungi and minerals has substantiated a more fecund solution for pesticide remediation as compared to the results from the utilization of fungi without minerals. For example, the combined application of P. chrysosporium supplemented with borosilicate glass minerals was utilized to remedy pesticides and aromatic hydrocarbons (PAHs). Multiplicative effects led to a 40% removal of hydrocarbons from agricultural soils, whereas separate treatment with either P. chrysosporium or the mineral showed lower efficiencies of 40% and 30%, respectively [103].

Fungi are considered the ultimate degraders of complex organic matter, known to degrade lignin cellulose and other plant-derived materials, which are considered waste products in agriculture. Furthermore, Aspergillus sydowii, Penicillium miczynskii, and Trichoderma spp. have been prospected for the remediation of the insecticide Dieldrin. P. miczynskii was found to degrade 80% of Dieldrin in 2 weeks. This marine-derived fungal strain manifested phosphodiesterase and was suitable for deleting chlorpyrifos and profenofos. These studies affirm the potency of fungi for the bioremediation of pesticides [104].

6.1. In Situ Strategies and Ex Situ Strategies

The in situ strategies include bioventing and bio-sparging, promoting fungal microbes’ aerobic activity. However, bio-stimulation utilizes nutrients to facilitate enhanced bioremediation, and bioaugmentation entails adding to the pollution site.

In the ex situ strategy, a bioreactor is a method used to remediate pollutants in an aqueous solution. Composting involves a remedial action for a polluted matrix in a small enclosure. Landfarming is based on soil tilling collected on a designated bed. Bio-piling is a system that comprises irrigation, aeration systems, and collection of leachates, and in biopiles, the moisture, oxygen, pH, and nutrients are controlled [105].

From an environmental condition control perspective, the main difference between in situ and ex situ mycoremediation technologies is determined by location. Ex situ methods aim to remediate the fungus at the contaminated site, while in situ aims to eliminate it by using the bioventing and bio-sparging techniques of pumping air or nutrients into soil/groundwater that encourage fungal activity. So far, Phanerochaete chrysosporium has been successfully employed in a bioremediation process through its bioventing in situ technique to notably decrease polycyclic aromatic hydrocarbons (PAHs), leading up to an 85.9% reduction. In situ mycoremediation methods are non-destructive, more affordable, and provide less influence on managing variables like temperature and humidity [106].

On the other hand, ex situ methods can be carried out by lifting high-level radioactive waste from one place to another for treatment in a controlled environment (e.g., bioreactors or composting systems). Certain kinds of fungi, such as Pleurotus pulmonarius, have been studied in controlled settings in ex situ bioreactors for the degradation of recalcitrant pollutants, including dioxins, which enable high environmental control during the entire process and a rapid pollutant removal rate when compared to natural environment methods. The ex situ methods are less effective in the processing of complex and high-concentration pollutants, and they are more expensive and logistically cumbersome (Figure 3) [107].

Table 7. In situ vs. ex situ mycoremediation approaches.

Strategy	Mechanism	Pollutants Targeted	Control over Environmental Factors	Efficiency	Research Findings	References
In situ: bioventing	Oxygen introduced into the subsurface to stimulate aerobic fungal degradation.	VOCs, hydrocarbons, and organic pollutants in shallow soil.	Limited control over temperature, moisture, and airflow.	Moderate	Phanerochaete chrysosporium for PAH remediation (85.9% in 50 days).	[106]
In situ: bio-sparging	Air and nutrients injected into groundwater or soil to stimulate fungal biodegradation.	Hydrocarbons, VOCs, PAHs, and volatile pollutants.	Limited control; dependent on nutrient diffusion and oxygen levels.	Moderate	Pleurotus pulmonarius for dioxin degradation (96% degradation).	[108]
In situ: bio-stimulation	Nutrient addition (e.g., phosphorus) to stimulate fungal activity for pollutant degradation.	Hydrocarbons, nitrogenous compounds, and organic pollutants.	Minimal control; depends on soil nutrient distribution.	Moderate	Phosphorus-enhanced hydrocarbon degradation by Trichoderma in petroleum-contaminated soil.	[79]
Ex situ: bioreactors	Contaminated materials placed in bioreactors with controlled conditions for fungal degradation.	Persistent organic pollutants (POPs), PAHs, dioxins, and VOCs.	High control over temperature, pH, and oxygen levels.	High	White-rot fungi for PAH degradation in bioreactors, showing efficient removal of PAHs.	[109]
Ex situ: composting	Organic matter mixed with fungi in a controlled compost environment to enhance degradation.	Organic pollutants, hydrocarbons, heavy metals, and solid waste.	High control over temperature, moisture, and aeration.	High	Fungal treatment of compost using Trichoderma species shows enhanced pollutant breakdown.	[110]
Ex situ: landfarming	Contaminated soil spread over a designated area and tilled regularly for fungal degradation.	Hydrocarbons, PAHs, and semi-volatile organic compounds.	Moderate control over moisture and aeration.	Moderate	Mixed fungal remediation of hydrocarbon-contaminated soils; significant hydrocarbon degradation achieved.	[111]
Ex situ: biopiling	Contaminated soil piled and treated with controlled aeration and irrigation systems to enhance fungal activity.	Hydrocarbons, PAHs, and organic pollutants in solid waste.	High control over oxygen and moisture.	High	Mixed white-rot fungi used in biopiles for lignite degradation, showing increased floatation efficiency.	[112]
Ex situ: biofilters	Air or gas containing pollutants passed through a biofilter inoculated with fungi for degradation.	VOCs, hydrocarbons, BTEX, and α-pinene.	High control over air flow, moisture, and pollutant concentration.	High	Pseudomonas fluorescens and Alcaligenes xylosoxidans showed complete remediation.	[79]
Ex situ: vapor-phase bioreactors	Airborne pollutants treated in a vapor-phase bioreactor inoculated with fungal strains.	VOCs, hydrocarbons, and BTEX.	High control over all environmental conditions.	High	Fusarium solani for BTEX removal in a vapor-phase bioreactor (37.7 ± 3.3 g/m³ h).	[113]

Table 7. In situ vs. ex situ mycoremediation approaches.

Strategy	Mechanism	Pollutants Targeted	Control over Environmental Factors	Efficiency	Research Findings	References
In situ: bioventing	Oxygen introduced into the subsurface to stimulate aerobic fungal degradation.	VOCs, hydrocarbons, and organic pollutants in shallow soil.	Limited control over temperature, moisture, and airflow.	Moderate	Phanerochaete chrysosporium for PAH remediation (85.9% in 50 days).	[106]
In situ: bio-sparging	Air and nutrients injected into groundwater or soil to stimulate fungal biodegradation.	Hydrocarbons, VOCs, PAHs, and volatile pollutants.	Limited control; dependent on nutrient diffusion and oxygen levels.	Moderate	Pleurotus pulmonarius for dioxin degradation (96% degradation).	[108]
In situ: bio-stimulation	Nutrient addition (e.g., phosphorus) to stimulate fungal activity for pollutant degradation.	Hydrocarbons, nitrogenous compounds, and organic pollutants.	Minimal control; depends on soil nutrient distribution.	Moderate	Phosphorus-enhanced hydrocarbon degradation by Trichoderma in petroleum-contaminated soil.	[79]
Ex situ: bioreactors	Contaminated materials placed in bioreactors with controlled conditions for fungal degradation.	Persistent organic pollutants (POPs), PAHs, dioxins, and VOCs.	High control over temperature, pH, and oxygen levels.	High	White-rot fungi for PAH degradation in bioreactors, showing efficient removal of PAHs.	[109]
Ex situ: composting	Organic matter mixed with fungi in a controlled compost environment to enhance degradation.	Organic pollutants, hydrocarbons, heavy metals, and solid waste.	High control over temperature, moisture, and aeration.	High	Fungal treatment of compost using Trichoderma species shows enhanced pollutant breakdown.	[110]
Ex situ: landfarming	Contaminated soil spread over a designated area and tilled regularly for fungal degradation.	Hydrocarbons, PAHs, and semi-volatile organic compounds.	Moderate control over moisture and aeration.	Moderate	Mixed fungal remediation of hydrocarbon-contaminated soils; significant hydrocarbon degradation achieved.	[111]
Ex situ: biopiling	Contaminated soil piled and treated with controlled aeration and irrigation systems to enhance fungal activity.	Hydrocarbons, PAHs, and organic pollutants in solid waste.	High control over oxygen and moisture.	High	Mixed white-rot fungi used in biopiles for lignite degradation, showing increased floatation efficiency.	[112]
Ex situ: biofilters	Air or gas containing pollutants passed through a biofilter inoculated with fungi for degradation.	VOCs, hydrocarbons, BTEX, and α-pinene.	High control over air flow, moisture, and pollutant concentration.	High	Pseudomonas fluorescens and Alcaligenes xylosoxidans showed complete remediation.	[79]
Ex situ: vapor-phase bioreactors	Airborne pollutants treated in a vapor-phase bioreactor inoculated with fungal strains.	VOCs, hydrocarbons, and BTEX.	High control over all environmental conditions.	High	Fusarium solani for BTEX removal in a vapor-phase bioreactor (37.7 ± 3.3 g/m³ h).	[113]

In situ mycoremediation is less expensive and easier on the environment, but it also takes time because of organic matter decomposition, meaning nature has control over various environmental factors that can affect site remediation as opposed to ex situ-type methods. On the other hand, although in situ methods are more cost-effective and cause less disturbances in an area, ex situ methods are a quicker way of servicing because they provide faster rates and better control over all conditions [107].

Hydrocarbon degradation by fungal strains Purpureocillium lilacinum and Penicillium chrysogenum was studied by Yang et al. (2023), in which 15.3% removal was shown by Purpureocillium lilacinum and 7.6% by Penicillium chrysogenum. It exhibited varying capacities of these fungal strains for hydrocarbon degradation [101]. Hydrocarbon degradation is important, as it directly affects plant growth and photosynthesis, while hydrocarbons can cause severe respiratory disorders in animals [114].

Priyanka and Lens (2022) used semiconductive zinc sulfide nanoparticles and Aspergillus niger cells to investigate the breakdown of volatile aromatic chemicals such as benzene, toluene, and xylene (BTX). The elimination method was clarified; employing nanohybrids in UV-A light, total degradation was accomplished in 75 and 60 min. Based on molecular weight, the removal efficiency was determined. A. niger-ZnS nanohybrids powered by light exhibited a greater removal efficiency [115]. Baron et al. (2021) reported the variety of melanized fungi that can grow and endure toluene as a carbon and energy source in settings rich in hydrocarbons. According to the study, these data may help develop bioindication instruments for hydrocarbon exposure in anthropogenic and natural settings. The study discovered that Exophiala spp. were separated from every sample and that Chaetothyriales species favored hydrocarbonaceous settings. Black fungi are perfect for bioremediation applications because of their high tolerance. However, this tolerance may also make them more virulent [116]. The bioremediation potential of Bacillus altitudinis MT422188 for nickel removal was reported by Babar et al. (2021). This strain reached over 70 to 85 mg/L of nickel, proving this strain could bioremediate heavy metal-contaminated industrial wastewater [117].

Kaewlaoyoung et al. (2020) investigated the reduction in dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) by a white-rot fungus Pleurotus pulmonarius. It showed a 96% degradation of PCDD/Fs, which is a highly toxic compound without any pretreatment [109]. Poli et al. investigated a nano-biocomposite A. flavus Fe₃O₄ bio-based paint. The results of the first application were a 96.1% removal of RB5 and the effective removal of its toxicity, which indicates this biocomposite’s potential for water applications [118]. He et al. studied the removal of cadmium and antimony from Penicillium spp. XK10 results were 32.2% for cadmium and 15.5% for antimony. In addition, this strain is also highly tolerant to both metals, which allows for its industrial use for soil treatment [119]. Prenafeta-Boldú et al. (2004) studied the biodegradation of benzene, toluene, ethylbenzene, xylene, and methyl-ter-butyl ether in soil microcosms. The toluene-metabolizing fungus Cladophialophora sp. strain T1 was more effective in promoting biodegradation in native soil microorganisms than in axenic soil cultures. The presence of the fungus also increased the biodegradation rates of toluene and ethylbenzene under acidic conditions [90]. Kumar and Dwivedi (2021) studied Trichoderma lixii CR700 for its ability to absorb heavy metals, including copper. This strain accomplished an 84.6% Cu²⁺ evacuation, which appeared to have high resilience and protein movement to overwhelming metal contamination [120].

Ryan et al. (2005) assessed Trametes pubescens to treat phenolic wastewater and obtained a high phenol expulsion rate of 0.033 g phenol/g biomass per day. The investigation, moreover, found an increment in laccase action, showing the organism’s viability in corrupting phenolic compounds [121]. Selenium removal by Se-degrading fungus Ascomycota was studied by Sabuda et al. (2020). It showed the excellent potential of Ascomycota for Se (IV) as compared to Se (VI), as well as the significant effects of carbon sources, exhibiting good potential of this strain for Se-contaminated wastewater treatment [122].

Rodrigues et al. (2023) examined Aspergillus niveus 43, Aspergillus terreus 31, and Cladosporium cladosporioides for the biotransformation of 3,4-dichloroaniline. A. terreus 31 showed high potency and sound reduction in phytotoxicity, cytotoxicity, and genotoxicity [123]. Sofia et al. (2016) investigated the potential of Schizophyllum commune for dye elimination. A 95.45% reduction was reported in optimal conditions and increased manganese peroxidase activity. These results showed the higher capacity of this strain for reactive dye reduction [124]. Mixed white-rot fungi were used by Shi et al. (2021), and the fungi used included Schizophyllum spp. for lignite degradation. The study’s results for enzymatic activity in the Czapek–Dox medium showed that the marked increase in the activity of the kick was much higher, and the floatation efficiency and mixed cultures were notably enhanced (See Table 7) [112].

6.2. Advantages and Limitations of Fungal Methods

Fungi perform similar tasks and have a varying complexity of enzymatic competence, and the bioremediation properties range from breaking down simple organic pollutants like dioxins, PAHs, or pesticides to adaptation models with extensive functionality based, among others, on bacterial bioremediation. Enzymes for xenobiotic biodegradation, such as laccases and peroxidases, two fungal enzymes, have been found to be effective against recalcitrant aromatic pollutants, chemicals which standard bacterial metabolism either cannot break down or is achieved only at a very slow rate [108]. Yet, compared with bacterial approaches for bioremediation, fungal degradation is slower; this could be associated with the fungi having lower growth rates and being harder to affect by environmental conditions such as pH and temperature [125].

Fungi have high degradative capabilities and have been seen to be used for the efficient degradation of complex pollutants. Fungi can also be less dependent on site-specific environmental conditions than plants used in phytoremediation (plants generally require shallow roots and fertile soil structures) [1].

Fungal methods require time for slime to degrade and are influenced by the environmental conditions of pH and temperature. They are also less effective for non-specific or recalcitrant pollutants like mixed contaminants and/or high-toxicity-class contaminants and conventional approaches will not be suitable in such instances; however, bacterial bioremediation/nano-remediation might have a chance [115]. The summarized advantages and limitations of fungal as compare to other bioremediation techniques are presented in Table 8.

Table 8. Advantages and limitations of fungal bioremediation vs. other approaches.

Remediation Approach	Advantages	Limitations	Organism	References
Fungal bioremediation	High efficiency in degrading complex organic pollutants like dioxins, PAHs, and pesticides; enzymatic versatility	Slower growth rate; sensitive to environmental factors (pH, temperature); limited for mixed pollutants	Phanerochaete chrysosporium for PAHs	[118]
Bacterial bioremediation	Faster degradation rates; capable of handling mixed pollutants; efficient under various conditions	Limited to simpler organic pollutants; requires specific conditions like oxygen and nutrients	Pseudomonas putida for hydrocarbons	[74]
Phytoremediation	Cost-effective; improves soil structure; long-term solution	Slow process; limited to shallow-rooted plants; not effective for volatile pollutants	Study on heavy metal removal by plants	[120]
Nano-remediation	Highly effective in removing small concentrations of pollutants; rapid degradation	Potential environmental toxicity; high cost; limited large-scale applications	Study on ZnS nanoparticles with A. niger	[115]
Chemical remediation	Immediate pollutant breakdown; effective for a wide range of pollutants	High cost; secondary pollution; not environmentally friendly	General chemical treatment for VOCs	[118]
Phytoremediation (enhanced)	Bio-stimulation can enhance phytoremediation, making it more effective for metal removal	Slow process; dependent on environmental conditions; limited scope	Phyto-enhanced bioremediation	[103]
Nanoparticle-enhanced remediation	Increases the efficiency of microbial degradation by improving bioavailability and pollutant breakdown	Environmental risks due to nanoparticles; potential toxicity	Study with ZnS nanoparticles	[115]

Table 8. Advantages and limitations of fungal bioremediation vs. other approaches.

Remediation Approach	Advantages	Limitations	Organism	References
Fungal bioremediation	High efficiency in degrading complex organic pollutants like dioxins, PAHs, and pesticides; enzymatic versatility	Slower growth rate; sensitive to environmental factors (pH, temperature); limited for mixed pollutants	Phanerochaete chrysosporium for PAHs	[118]
Bacterial bioremediation	Faster degradation rates; capable of handling mixed pollutants; efficient under various conditions	Limited to simpler organic pollutants; requires specific conditions like oxygen and nutrients	Pseudomonas putida for hydrocarbons	[74]
Phytoremediation	Cost-effective; improves soil structure; long-term solution	Slow process; limited to shallow-rooted plants; not effective for volatile pollutants	Study on heavy metal removal by plants	[120]
Nano-remediation	Highly effective in removing small concentrations of pollutants; rapid degradation	Potential environmental toxicity; high cost; limited large-scale applications	Study on ZnS nanoparticles with A. niger	[115]
Chemical remediation	Immediate pollutant breakdown; effective for a wide range of pollutants	High cost; secondary pollution; not environmentally friendly	General chemical treatment for VOCs	[118]
Phytoremediation (enhanced)	Bio-stimulation can enhance phytoremediation, making it more effective for metal removal	Slow process; dependent on environmental conditions; limited scope	Phyto-enhanced bioremediation	[103]
Nanoparticle-enhanced remediation	Increases the efficiency of microbial degradation by improving bioavailability and pollutant breakdown	Environmental risks due to nanoparticles; potential toxicity	Study with ZnS nanoparticles	[115]

7. Nano-Remediation of Smog

One way nanotechnology is used for the bioremediation of harmful pollutants, especially heavy metals, involves the inherent properties of the nanoparticles that are applied to trap or neutralize unwanted uptakes. This method improves bioremediation productivity and can target virtually any contaminant. Silver and titanium dioxide nanoparticles have catalyzed pollutant breakdown in the case of smog. Those particles can address hazardous components, depollute the smog, and help people breathe [126].

Nanotechnology has the potential to significantly address issues such as poor air quality by using nanoscale adsorbents, which can adsorb contaminants such as microbes, volatile organic compounds (VOCs), and metal oxides. Nano-adsorbents have been developed and categorized to detect and adsorb contaminants such as microbes, volatile organic compounds (VOCs), and metal oxides [127].

Various treatment techniques and conditions are used to eliminate and monitor the discharge of toxic gasses and air contaminants by various nano-adsorbent materials. One method for controlling air contamination is using semiconductor materials for photocatalytic remediation, where the material exposure to light with energy equals its band gap, leading to the development of electron–hole pairs. Nanotechnology could also increase the size of the nanoparticle and its molecular distribution/structure for developing advanced nano-catalysts with high surface areas [128].

Another method for controlling air contamination is using nanostructured membranes with small pores to remove various contaminants from the exhaust. Nanofiber-coated filter media have been utilized in industrial plants for air filtration, particularly for volatile organic carbon vapors. For example, electrospun polyacrylonitrile-based carbon nanofiber membranes possess superior microporosity and profuse nitrogen-containing functional groups as efficient adsorption sites [129].

In interior air contaminants, silver-based nanoparticle and copper-based nanoparticle filters have been extensively used as antimicrobial materials in air purification technology to remove bioaerosols through air-conditioning processes [130].

Nanomaterial-enabled sensors are a sensational technology that offers exquisite detection on the nanomolar level to the sub-picomolar level of ecological pollutants. These nanosensors consist of three major components, namely (1) a nanomaterial, (2) a detection element contributing to specificity, and (3) a signal transduction technique offering a means of relaying the analytes’ existence. Recent studies have focused on appropriate gas-sensing materials for detecting hydrogen sulfide, sulfur oxide, nitrogen oxides, sulfur oxide gasses, and ozone [131]. The visualization of pollutants can effectively aid in using a mobile application that is connected to the sensors. The chemicals in the sensors react with the pollutant and produce specific wavelengths of visible light, e.g., polydimethylsiloxane (PDMS) and o-dianisidine, which are the components of a colorimetric optical particle spectrometer (OPS) that is used for the visual detection of average O₃ exposure. In the presence of oxygen, the colorless molecule known as o-diaminisidine undergoes a transformation into a yellow color [132].

Table 9. Bioremediation of air pollutants using nano-remediation strategies.

Type of Nanomaterial	Efficiency	References
Silver nanoparticles	Cutting particulates by 75%	[133]
Titanium dioxide-loaded platinum nanoparticles	2.4-fold increase in CO oxidation to CO2	[134]
Zinc oxide nanoparticles	45–50% decrease in sulfur compounds	[135]
Gold nanoparticles	55% drop in carbon monoxide	[136]
Copper nanoparticles	65% reduction in hydrocarbons	[137]
Silica nanoparticles	60% reduction in VOCs	[138]
Aluminum oxide nanoparticles	Lowered sulfur compounds by 55%	[139]
Platinum nanoparticles	Nitrogen oxides down by 70%	[140]
Nickel nanoparticles	50% reduction in VOCs	[141]
Cobalt nanoparticles	55% reduction in NOx	[142]
Graphene oxide nanoparticles	65% reduction in hydrocarbons	[143]
Cerium oxide nanoparticles	Lowered CO by 60%	[144]
Manganese oxide nanoparticles	55% less sulfur compounds	[145]
Palladium nanoparticles	60% reduction in VOCs	[146]

Table 9. Bioremediation of air pollutants using nano-remediation strategies.

Type of Nanomaterial	Efficiency	References
Silver nanoparticles	Cutting particulates by 75%	[133]
Titanium dioxide-loaded platinum nanoparticles	2.4-fold increase in CO oxidation to CO2	[134]
Zinc oxide nanoparticles	45–50% decrease in sulfur compounds	[135]
Gold nanoparticles	55% drop in carbon monoxide	[136]
Copper nanoparticles	65% reduction in hydrocarbons	[137]
Silica nanoparticles	60% reduction in VOCs	[138]
Aluminum oxide nanoparticles	Lowered sulfur compounds by 55%	[139]
Platinum nanoparticles	Nitrogen oxides down by 70%	[140]
Nickel nanoparticles	50% reduction in VOCs	[141]
Cobalt nanoparticles	55% reduction in NOx	[142]
Graphene oxide nanoparticles	65% reduction in hydrocarbons	[143]
Cerium oxide nanoparticles	Lowered CO by 60%	[144]
Manganese oxide nanoparticles	55% less sulfur compounds	[145]
Palladium nanoparticles	60% reduction in VOCs	[146]

Chakrabarti et al. (2021), using silver nanoparticles in smog-contaminated air, also reported a 75% reduction in PM and other pollutants. The studies conducted on nanotechnology-based bioremediation have yielded appreciative results [133]. Weon et al. (2021) examined a site-specific Pt loading on facet-engineered TiO₂ to accomplish room temperature carbon monoxide (CO) oxidation. Spatially loaded Pt on the {101} facets of TiO₂ effectively drew in photoinduced electrons. Consequently, oxygen dissociation promoted on the platinum surface evidenced a 2.4-fold increase in carbon monoxide oxidation in CO₂ relative to the platinum standard supported on titanium dioxide [134]. In air pollution control and remediation, many studies have investigated the effectiveness of nanoparticles in the treatment of various pollutants. For example, Ahmadian et al. (2020) demonstrated the good performance of CuO-modified SBA-15 and MOR-SBA-15 composite adsorbents in removing sulfur dioxide (SO₂) from exhaust gasses. Their results showed that the 8.7% CuO/MOR-SBA-15-imp adsorbent was effective, showing high desulfurization efficiency and good regeneration in multiple cycles. This study also highlighted the pseudo-first-order kinetics of the adsorption process, and the activation energy was 20.4 kJ/mol and 18.5 kJ/mol for CuO/SBA-15-imp and CuO/MOR-SBA-15-imp, respectively [147].

Similarly, Saucedo-Lucero and Arriaga (2013) investigated the photolysis of hexane vapor using ZnO nanoparticles. Their study showed that ZnO was more degraded than TiO₂ when determined by BET surface area, but TiO₂ had better mineralization activity than hexane. This indicates that ZnO may be more effective in some applications where a large surface area and good photocatalytic activity are important [148]. Discoveries showed a noteworthy decrease in press oxide concentration and a critical contrast in press weight between the return and release gas using plant biofilters. It shows the capacity of organic channel frameworks to control quality in confined zones, but more inquiry is required to progress in general quality and assess the viability of these frameworks under distinctive conditions [149].

The removal of benzene using ZnO porous adsorbent nanoparticles was also studied by Changsuphan et al. (2012). The research showed that the benzene removal efficiency of the coated zeolite was 97.9%, while virgin zeolite achieved a removal rate of only 94.2%. The presence of UV, O₃, and their combination only slightly affected the pollutant removal efficiency with both nets (light absorption/agricultural) [150]. Hussain and coworkers (2011) studied TiO₂ nanoparticles for VOC degradation in the realm of photocatalysis. After the latest round of optimizations, both ethylene and propylene dropped even further, as did toluene [151].

Zhang et al. (2017) considered TiO₂/diatomaceous soil composites for formaldehyde deterioration. It appeared that unadulterated TiO₂ has superior photocatalytic movement and reusability due to better nanoparticle scattering and a higher formaldehyde adsorption capacity. This illustrates the potential of composite materials to move forward with photocatalytic execution for indoor purification [152]. Similarly, Mohamed and Aazam (2013) created Pt–ZnO–hydroxyapatite (HAP) nanoparticles for photocatalytic reduction in benzene. The findings showed that the Pt-ZnO-HAP crossover had excellent results and demonstrated steady photocatalytic movement beneath unmistakable light, making it a great choice for photocatalytic applications under light conditions [153].

For environmental improvement, Khiadani et al. (2014) used iron oxide nanoparticles and magnetic fields to improve urban drainage quality. Their research found that it effectively removes sludge and heavy metals such as lead, zinc, and cadmium, but not acids. This indicates the potential of magnetic nanoparticles to solve urban flow problems and presents an opportunity for further optimization [154]. Pei et al. (2013) investigated Ag-coated activated carbons for indoor air quality control. The addition of silver nanoparticles significantly improved the activated carbon’s antibacterial activity but slightly reduced toluene’s absorption capacity. The ability to kill airborne bacteria within 100 min while maintaining effective toluene absorption makes these nanocomposites useful for air-cleaning applications [155].

Eltouny and Ariya (2012) highlighted the adequacy of Fe₃O₄ nanoparticles in expelling BTEX compounds from the discussion. It highlights evacuation efficiencies for different BTEX components, with Fe₃O₄ nanoparticles appearing proficiently when utilized with carboxymethyl cellulose. This emphasizes the potential of attractive nanoparticles for the adsorption and evacuation of the destructive pollutants under study. Overall, these studies outline nanomaterials’ different applications and benefits in air contamination control, each with interesting preferences for diverse contamination and conditions [156].

NPs can be added in several forms, such as raw suspensions and in a dried format, improving biogas production and microalgae growth. Adding dried NPs did not affect the improvement of biomethane quality or CO₂ absorption. In domestic wastewater treatment, it is observed that the growth of microalgae and their photosynthetic activities were augmented by elevating the concentration of dried nanoparticles [157]. One section of the research investigated potential methods for removing petroleum from river water using iron oxide nanoparticles (IONPs), biochar, immobilized hydrocarbon-degrading bacteria (HCB), and monoammonium phosphate. The efficacy of biochar beads alone in eliminating pollutants was lower than that of beads combined with IONPs. Researchers found a substantial negative correlation between bacterial abundance and concentrations of TPHs and PAHs in the treatment microcosms, and they also saw an increase in the number of hydrocarbon-utilizing and cultivable heterotrophic bacteria [158].

Using Chlorella sorokiniana as a model, another study looked at how carbon-coated zero-valent nanoparticles affected microalgae growth, biogas upgrading, and the efficiency of CO₂ removal. When added, the raw suspension and dried form of NPs enhanced microalgae growth and biogas upgrading. Dried NPs were added, but neither CO₂ absorption nor biomethane quality was improved. The same holds true for home wastewater treatment; increasing the concentration of dried NPs leads to more microalgae growth and photosynthetic activities [159].

Bioremediation and modified Fenton (MF) procedures involved using calcium peroxide (CaO₂) nanoparticles in continuous-flow sand-packed columns to remove benzene. According to the findings, MF boosted benzene remediation, created OH radicals, and ultimately led to completely eliminating benzene. Groundwater microbial biodiversity was also investigated concerning CaO₂ injection [160].

Irrespective of the potential of nanoparticles in the remediation of air pollutants, concerns regarding their toxicity and impacts on human health are still a point of discussion. Carbon nanotubes cause asthma in kids. These carbon nanotubes adsorb HOCs, which are carcinogenic in nature. Additionally, metal-based nanomaterials appear to affect the cardiovascular system and nervous system. Therefore, nano-remediation for removing smog and their effects on human health should also be studied (See Table 9) [131].

8. Phytoremediation of Smog

Phytoremediation, using plants to remove pollutants, presents a cost-effective method for improving indoor air quality [161]. Indoor environments often have higher pollution than outdoor areas due to poor ventilation and indoor activities [162,163]. Pipal et al. (2012) addressed indoor air quality as vital for health [164]. Pollutants can cause respiratory problems, dizziness, and severe conditions like cardiovascular disease and cancer [165,166]. These pollutants also affect the immune and nervous systems, impacting overall well-being and productivity [167].

Effective solutions are essential, as indoor air pollution contributes to nearly 4 million premature deaths annually [168]. Strategies such as reducing pollution sources, improving ventilation, and using advanced HVAC systems are crucial [169]. Phytoremediation augments these approaches by employing plants to absorb volatile organic compounds (VOCs), thereby significantly enhancing air quality (See Table 10) [170,171]. Research indicates that integrating plants into indoor spaces can reduce pollutants and improve health [172].

Table 10. Different plant types and their role in remediating smog.

Type of Plants	Effects	References
Dracaena fragrans (Golden Coast)	Removal of up to 3 ppb NO2 per m2 of leaf area over a 1 h test period.	[173]
Caesalpinia gilliesii and Robinia pseudoacacia	The air pollution tolerance index (APTI) was species-specific; ascorbic acid was crucial for Robinia pseudoacacia (88.1%) and Caesalpinia gilliesii (78.9%).	[174]
Eucalyptus camaldulensis	pH of leaf extract was dominant in Eucalyptus camaldulensis (45.7%).	[174]
Clerics siliquastrum	Total chlorophyll content was most significant in Clerics siliquastrum (56.1%).	[174]
Portable active green wall with unspecified plant species	The active green wall achieved single-pass removal efficiencies of 56.42 ± 21.02% for PM2.5 and 20.73 ± 0.87% for O3.	[175]
Nephrolepis exaltata and Spathiphyllum wallisii	Nephrolepis exaltata and Spathiphyllum wallisii removed CO2 by 45.4–51% and VOCs by 36.2–42.7%.	[176]
Dypsis lutescens and Latania Livistona	Dypsis lutescens and Latania Livistona achieved CO2 removal of 40.9–41.8% and VOC removal of 46–47.8%.	[176]
Epipremnum aureum	Epipremnum aureum removed CO2 by 35.6–38.6% and VOCs by 32–34.3%.	[176]
Vigna radiata	Formaldehyde removal rates increased with microbial addition. Vigna radiata showed the highest enhancement, with 97.6 ± 0.9 μg/h/g and an 88.7% increase over the 25.1 ± 4.2 μg/h/g without microbes.	[177]
Tradescantia zebrina	Tradescantia zebrina had a removal rate of 86.4 ± 0.7 μg/h/g with microbes compared to 59.3 ± 0.2 μg/h/g without, showing a 45.6% increase.	[177]
Aloe vera	Aloe vera achieved 23.1 ± 0.1 μg/h/g with microbes versus 18.5 ± 0.21 μg/h/g without, a 24.9% improvement.	[177]
A vegetation biofilter	The vegetation biofilter achieved an average single-pass removal efficiency of 20% for isobutylene at 5000 ppm.	[178]
Agave americana	18.40 for the air pollution tolerance index (APTI).	[179]
Cassia roxburghii	Tolerance index (APTI) for selected plants is Cassia roxburghii at 17.63.	[179]
Anacardium occidentale	Tolerance index (APTI) is 11.97.	[179]
Cassia fistula	Tolerance index (APTI) for selected plants is Cassia fistula at 11.60.	[179]
Mangifera indica	Tolerance index (APTI) is 11.59.	[179]
Saraca asoca	Tolerance index (APTI) is 10.88.	[179]
Spathiphyllum wallisii	70% reduction in benzene.	[180]
Sansevieria trifasciata	60% reduction in toluene level.	[180]
Gerbera jamesonii	Decreased xylene concentrations by approximately 50–60%.	[180]
No specific particular types	Plant clean air delivery rates (CADRs) were low, with a median value of 0.023 m3/h.	[181]
Madhuca longifolia	Madhuca longifolia had the highest APTI values based on pH, ascorbic acid content, relative water content, and total chlorophyll content	[182]
Cyperus and Brachiaria spp.	Cyperus and Brachiaria spp. showed significant potential in phytoremediation processes.	[183]
Nephrolepis sp.	Nephrolepis also yielded favorable results for organic contaminants.	[183]
Acacia sp.	Oil emulsion: 48% oil, suspension: 23%, settled emulsion: 42%, and sludge emulsion: 36%.	[184]
Lactuca sativa	Dieldrin removal rates: 50–78%.	[185]
Raphanus sativus	50–78%	[185]

Table 10. Different plant types and their role in remediating smog.

Type of Plants	Effects	References
Dracaena fragrans (Golden Coast)	Removal of up to 3 ppb NO2 per m2 of leaf area over a 1 h test period.	[173]
Caesalpinia gilliesii and Robinia pseudoacacia	The air pollution tolerance index (APTI) was species-specific; ascorbic acid was crucial for Robinia pseudoacacia (88.1%) and Caesalpinia gilliesii (78.9%).	[174]
Eucalyptus camaldulensis	pH of leaf extract was dominant in Eucalyptus camaldulensis (45.7%).	[174]
Clerics siliquastrum	Total chlorophyll content was most significant in Clerics siliquastrum (56.1%).	[174]
Portable active green wall with unspecified plant species	The active green wall achieved single-pass removal efficiencies of 56.42 ± 21.02% for PM2.5 and 20.73 ± 0.87% for O3.	[175]
Nephrolepis exaltata and Spathiphyllum wallisii	Nephrolepis exaltata and Spathiphyllum wallisii removed CO2 by 45.4–51% and VOCs by 36.2–42.7%.	[176]
Dypsis lutescens and Latania Livistona	Dypsis lutescens and Latania Livistona achieved CO2 removal of 40.9–41.8% and VOC removal of 46–47.8%.	[176]
Epipremnum aureum	Epipremnum aureum removed CO2 by 35.6–38.6% and VOCs by 32–34.3%.	[176]
Vigna radiata	Formaldehyde removal rates increased with microbial addition. Vigna radiata showed the highest enhancement, with 97.6 ± 0.9 μg/h/g and an 88.7% increase over the 25.1 ± 4.2 μg/h/g without microbes.	[177]
Tradescantia zebrina	Tradescantia zebrina had a removal rate of 86.4 ± 0.7 μg/h/g with microbes compared to 59.3 ± 0.2 μg/h/g without, showing a 45.6% increase.	[177]
Aloe vera	Aloe vera achieved 23.1 ± 0.1 μg/h/g with microbes versus 18.5 ± 0.21 μg/h/g without, a 24.9% improvement.	[177]
A vegetation biofilter	The vegetation biofilter achieved an average single-pass removal efficiency of 20% for isobutylene at 5000 ppm.	[178]
Agave americana	18.40 for the air pollution tolerance index (APTI).	[179]
Cassia roxburghii	Tolerance index (APTI) for selected plants is Cassia roxburghii at 17.63.	[179]
Anacardium occidentale	Tolerance index (APTI) is 11.97.	[179]
Cassia fistula	Tolerance index (APTI) for selected plants is Cassia fistula at 11.60.	[179]
Mangifera indica	Tolerance index (APTI) is 11.59.	[179]
Saraca asoca	Tolerance index (APTI) is 10.88.	[179]
Spathiphyllum wallisii	70% reduction in benzene.	[180]
Sansevieria trifasciata	60% reduction in toluene level.	[180]
Gerbera jamesonii	Decreased xylene concentrations by approximately 50–60%.	[180]
No specific particular types	Plant clean air delivery rates (CADRs) were low, with a median value of 0.023 m3/h.	[181]
Madhuca longifolia	Madhuca longifolia had the highest APTI values based on pH, ascorbic acid content, relative water content, and total chlorophyll content	[182]
Cyperus and Brachiaria spp.	Cyperus and Brachiaria spp. showed significant potential in phytoremediation processes.	[183]
Nephrolepis sp.	Nephrolepis also yielded favorable results for organic contaminants.	[183]
Acacia sp.	Oil emulsion: 48% oil, suspension: 23%, settled emulsion: 42%, and sludge emulsion: 36%.	[184]
Lactuca sativa	Dieldrin removal rates: 50–78%.	[185]
Raphanus sativus	50–78%	[185]

8.1. Phytoremediation Mechanism

Mechanisms of phytoremediation, phytoextraction, phytovolatilization, phytodegradation, phytostabilization, rhizodegradation, and rhizofilteration are discussed below.

8.1.1. Phytoextraction

Phytoextraction is a process that uses the plant’s phyllosphere and rhizosphere to draw pollutants up from the soil and into the plant’s aerial portions [43]. The term “phytoaccumulation” can describe this process just as well as “phytoabsorption” [186]. This technique involves drawing contaminants from contaminated soils and storing them in the phyllosphere using hyperaccumulator plants, which can concentrate 100 times more metals than non-accumulating plants. Plants suitable for this strategy have a high biomass production rate, a high translocation factor of pollutants into their surface biomass, a high detoxification rate, a high tolerance for contaminants, and are easy to harvest. After these plants are collected from polluted areas, incineration is the last step in cleaning them up. Phytoextraction has two main advantages over more traditional methods of pollutant remediation, which are its low upfront cost and the fact that it removes toxins from locations permanently [187].

8.1.2. Phytovolatilization

Phytovolatilization converts pollutants into volatile components after diffusing them into plants’ phyllospheres. Consequently, stomata release this degraded volatile product into the air. Transpired substances may remain in the air as a contaminant or be further broken down by hydroxyl radicals due to phytovolatilization. The approach has the potential benefit of reducing pollutants’ toxicity before discharge into the atmosphere [188].

8.1.3. Phytodegradation

When pollutants are broken down by plants and absorbed into their tissues in the phyllosphere, this process is known as phytodegradation or phytotransformation. Specifically, plant enzymes known as laccases hydrolyze anilines, dehalogenases degrade pesticides and chlorinated solvents, and nitroreductases hydrolyze nitroaromatic compounds, allowing them to be metabolized by plants. This process does not rely on the microorganisms in the rhizosphere and instead makes use of plant enzymes [189].

8.1.4. Phytostabilization

Phytostabilization, phytoimmobilization, or in-place inactivation reduces the movement of pollutants through immobilization in the rhizosphere. Plant lignin or humus binds pollutants, turns them into insoluble molecules, and stores them in the rhizosphere. With this technique, there is no need to worry about disposing dangerous materials. When protecting both the surface and groundwater, this technology is a great tool to have on hand. This strategy can help decrease soil erosion and increase water availability [190].

8.1.5. Rhizodegradation

Phytostimulation occurs in the rhizosphere and is stimulated by microorganisms, a process known as rhizodegradation [191]. Plant roots increase a plant’s surface area, which is useful for oxygen transmission and microbial development. Microbes in the rhizosphere use plant metabolites and exudate as food for growing and decomposing pollutants. They also release biodegrading enzymes [192].

8.1.6. Rhizo-Filtration

The process of rhizo-filtration involves using plants to remove contaminants from water utilizing their rhizosphere, which they then absorb and precipitate. Hydroponic systems allow plants to absorb pollutants via their roots and other rhizosphere organs. The plants are gathered like in phytoextraction once they reached the saturation limit of the pollutants. Common applications of this approach include treating wastewater, surface water, and groundwater. This method is most suited for plants with a high adsorption surface area, a large root biomass, a high accumulation capacity, and the ability to tolerate contaminants. The benefits of rhizo-filtration include that it applies to aquatic and terrestrial plants and eliminates the need to translocate pollutants to the shoots [193].

8.2. Phytoremediation of Particular Matter

Airborne particulate matter (PM) ranks high among the most dangerous air contaminants. For the benefit of human health, many people plant some species in green belts because of their great potential to filter polluted air. On the other hand, plants experience physiological changes and stress when exposed to excessive amounts of PM. Nookongbut et al.’s (2024) study examined the effects of particulate matter (PM) produced by tobacco smoke on eleven native perennial plant species in Thailand. Plants such as Tectona grandis L.f., Wrightia religiosa (Teijsm. & Binn.) Benth. ex-Kurz, and Bauhinia purpurea DC. ex Walp. were able to significantly lower PM levels (usually between 43.95% and 52.97%) [194]. In another study, to find out which native Australian species could accumulate leaf surface (SPM) and in-wax PM (WPM), 12 plants were surveyed across three locations. The plants included two deciduous trees, three evergreen shrubs, and seven evergreen trees. The most effective PM accumulator among the evaluated species was Lagunaria patersonia with 139.22 μg/cm², followed by Ficus obliqua with 131.02 μg/cm². A native tree of Australia, L. patersonia has a thick crown that effectively traps PM, owing to air turbulence between its branches and leaves, and its broad rough-textured leaves further improve its PM-trapping abilities [195].

The buildup of PM is affected not only by the shape of the leaf but also by its micromorphology and structural features [196]. For example, Pittosporum undulatum, which has a much thinner wax covering, gathered more PM because of its wavy and bent leaf morphologies. According to the research, planting efficient PM accumulator species is crucial to protecting susceptible areas from pollution and reducing human exposure to pollutants. Planning urban treescapes with these species in mind can help reduce air pollution and enhance air quality by utilizing their sink capacity [195].

Air pollution is a major environmental issue, particularly in highly populated places such as Delhi, India. However, some plant species can pull pollutants out of the air when they are hung in it. In light of this, Tripathi and Nema (2024) calculated tolerance indices such as the air pollution tolerance index (APTI) and the anticipated performance index (API) utilizing biochemical data, plant morphology, and socioeconomic factors. During the pre-monsoon season, the APTI value was 7.99 for Polyalthia longifolia (Sonn.) Thwaites and 11.94 for Ficus religiosa L. at 1305.46 µg/cm²; Ficus benghalensis L. has the highest SPM adhesion on its leaves, whereas F. religiosa had the lowest, at 56.62 µg/cm². Also, according to the statistical study, R² > 0.6 with the APTI suggests a positive link between ascorbic acid and chlorophyll concentration. The results highlighted the need to choose suitable plant species and consider seasonal fluctuations to establish urban greening strategies that effectively battle air pollution [197].

Bui et al.’s (2024) study set out to determine how well different plants could tolerate air pollution and how much particulate matter they could absorb. Because of this, it will be easier to choose appropriate species for planting along roadways. Consequently, six distinct plant species had their particulate matter buildup assessed in this study. The results showed that the best plants to plant along roadsides to reduce air pollution were Chamaecyparis obtusa, Ligustrum obtusifolium, and Hibiscus syriacus [198]. Researchers in central China measured the air pollution tolerance index (APTI) by analyzing eight common roadside plant species, namely Lilithrum lucidum, Prunus cerasifera, Photinia fraseri, Photinia serratifolia, Nandina domestica, Paeonia suffruticosa Andr., Nerium oleander, and Eriobotrya japonica. Young leaves of E. japonica remained sensitive (with an APTI value of about 5.72 to 8.29) throughout the whole spring in contrast to the more tolerant species of N. oleander and L. lucidum, which displayed higher APTI values (8.90–9.45 and 8.73 to 9.17, respectively) in the three months of testing. The results showed that young leaves of the test plants were more resistant to particle pollution in somewhat dry conditions in the spring when the relative water content was greater than ascorbic acid, which contradicts earlier research (r = 0.996 at p < 0.01 level) [199].

The capacity of phytoremediation to remove VOCs and PM depends on the plants’ state. Recent work has used priming as a straightforward method to investigate how plants can increase their resistance to abiotic stress by accumulating certain metabolites; however, neither the mechanism nor the duration of this “memory” has been thoroughly investigated. Increases in proline and antioxidant enzymes brought about by exogenous ornithine help plants stay efficient and shield them from stress. Plant “memory” processes under PM and VOC stress were never demonstrated until [200].

To enhance the air quality in cities, it is possible to use green biofilters to remove pollutants. During Japan’s spring and summer seasons, a study examined the phytoremediation potential of Prunus × Yedoensis as a biofilter of particulate matter (PM). The coarse, fine, and ultra-fine PM fractions were extracted from two samples. In contrast to the summer season, when PM deposition was higher at 31.9 μg/cm², the results demonstrated less PM deposition in the spring season, totaling 20.2 μg/cm², with a high proportion for the fine fraction (2.5–10 μm). Most of the particulate matter (PM) deposition, accounting for 23.9% of the total, was deposited by the ultra-fine fraction (0.2–2.5 μm) [201].

8.3. Phytoremediation of Inorganic Air Pollutants

Smog contains inorganic air pollutants such as NOx, SO₂, and O_3, which have adverse effects on human health [202]. Phytoremediation uses plants to absorb volatile pollutants like nitrogen dioxide (NO₂), a major urban air pollutant linked to health issues and environmental impacts [203]. Nitrogen dioxide, NO₂, is prevalent in indoor environments due to outdoor sources and indoor combustion and exceeds safe levels, with chronic exposure risks outlined by health guidelines (Hasselblad et al. 1998). Mitigation typically involves costly systems like HEPA filters and ventilation. Plants offer a simpler alternative for NO₂ removal through stomatal uptake and leaf absorption [204].

The increasing urbanization of populations and the subsequent increase in the percentage of time spent indoors has made indoor environmental quality an increasingly pressing issue. Indoor air quality has been a big issue due to the release of volatile organic compounds (VOCs) from synthetic materials and carbon dioxide (CO₂) from human breathing. The effectiveness of Spathiphyllum wallisii “Verdi”, Dracaena fragrans “Golden Coast”, and Zamioculcas zamiifolia “Aroid Palm’’ in removing nitrogen dioxide (NO₂) from indoor environments has been studied. Dracaena fragrans achieved the highest removal rate, up to 3 ppb of NO₂ per m² of leaf area in a 150 L chamber. The removal rate in a modeled small office was 0.62 ppb per plant. These findings confirm that potted plants can significantly improve indoor air quality and are viable for NO₂ bioremediation [173].

One way to measure how vulnerable or resistant plants are to air pollutants is with the help of the air pollution tolerance index (APTI). The four parameters utilized are the ascorbic acid concentration, total chlorophyll concentration, relative water content, and pH of the leaf extract. The plant’s APTI is obtained by jointly determining and computing these parameters. Many researchers have been looking at the APTI of plants to create a green belt. Barjoee et al. (2023) studied Clerics siliquastrum, Melia azedarach, Caesalpinia gilliesii, Eucalyptus camaldulensis, Robinia pseudoacacia, and Morus alba for air pollution tolerance and bioremediation in Najafabad County, Iran. Melia azedarach and Eucalyptus camaldulensis showed high tolerance. Key APTI factors were ascorbic acid for Robinia pseudoacacia (88.1%) and Caesalpinia gilliesii (78.9%), pH for Eucalyptus camaldulensis (45.7%) and Melia azedarach (98.8%), chlorophyll for Clerics siliquastrum (56.1%), and water content for Morus alba (54.6%) [174].

8.4. Phytoremediation of VOCs

The effectiveness of various houseplants in improving indoor air quality through phytoremediation in Alexandria, Egypt, has been studied. Placing plants in 21 homes significantly reduced CO₂ and VOC levels. Nephrolepis exaltata, Dracaena marginata, and Spathiphyllum wallisii were particularly effective. The findings support using specific plants to reduce indoor air pollutants and enhance residential air quality [176]. The plant–microbe system uses Tradescantia zebrina, Aloe vera, and Vigna radiata to enhance formaldehyde removal. Adding microbes to plant rhizospheres increased formaldehyde removal from 6.7% to 90.5%, with light intensity affecting efficiency. The approach effectively reduces indoor pollutants and addresses sick building syndrome [177].

A vegetation biofilter with an air handling unit (AHU) achieved a 20% removal efficiency for isobutylene at 5000 ppm. This method aligns with commercial air purifier standards and shows potential for improving indoor air quality in large spaces [168]. Out of the 67 plant species in Trivandrum for air pollution tolerance, Agave americana and Cassia roxburghii were highly tolerant and consistent across seasons, while Ficus elastica and Mangifera indica showed high performance for urban planting. The research guides the selection of plants to improve urban air quality [179].

Plants like Spathiphyllum wallisii and Chlorophytum comosum cut formaldehyde by 60–80%. Sansevieria trifasciata and Chlorophytum comosum lower benzene by up to 70%, while both reduce toluene by over 60%. Aloe vera and Gerbera jamesonii decrease xylene by 50–60%. These plants are effective in bioremediation, offering natural solutions for improving indoor air quality [180]. The 12 studies on potted plants’ ability to remove indoor VOCs showed a median clean air delivery rate (CADR) of 0.023 m³/h per plant, underscoring the impracticality of using plants alone to match typical outdoor-to-indoor air exchange rates. The results suggest limited effectiveness of individual plants for VOC removal in typical indoor settings without a high density of plants [141]. During the Black Summer wildfires (2019–2020), green walls in Sydney achieved pollutant removal efficiencies of 63.17% for NO₂, 38.79% for O₃, and 24.84% for PM_2.5. Clean air delivery rates were 558.9 m³/h for NO₂, 343.2 m³/h for O₃, and 219.8 m³/h for PM_2.5 per 5 m² wall. This study shows green walls can reduce wildfire pollutants, indicating their potential for urban air quality management [205].

In the study by Pettit et al. (2020), the 12 chamber experiments demonstrated that potted plants have a median CADR of 0.023 m³/h, far below the air exchange rate of typical building ventilation systems. Achieving effective VOC removal would require 10–1000 plants per square meter. Future research should focus on VOC uptake mechanisms, alternative biofiltration technologies, and the broader impacts of plant emissions for improved indoor air quality management [205].

Bandara et al. (2021) studied the air pollution tolerance index (APTI) of five roadside tree species in Colombo, Sri Lanka, based on pH, ascorbic acid content, relative water content, and total chlorophyll content. Madhuca longifolia demonstrated the highest APTI, indicating superior air pollution tolerance, followed by Peltophorum pterocarpum, Terminalia catappa, Cassia fistula, and Pongamia pinnata. This research is vital for selecting effective tree species for air pollution mitigation and improving roadside green spaces in humid tropical regions lacking scientific selection guidelines [182].

Cyperus, Brachiaria, Nephrolepis, Syagrus, Mimosa, Schinus, and Eryngium for phytoremediation were observed in Brazil. Cyperus and Brachiaria are the most effective, with Nephrolepis being useful for organic contaminants. It highlights the need for a specific understanding of processes and native species to maintain ecological balance [183], as well as the health and environmental risks from petroleum waste hydrocarbons (PWHCs), with oil emulsions containing 48% oil and other forms ranging from 23 to 42%. It advocates for biological remediation (bio-stimulation and bioaugmentation) and phytoremediation (phytodegradation, rhizoremediation, phytovolatilization, rhizome-filtration) as effective and eco-friendly methods. Key enzymes and plants like Acacia and Chloris enhance PWHC degradation, offering sustainable solutions for mitigating environmental contamination [184]. At landfills, phytoremediation effectively reduces volatile contaminants, achieving a 50–78% removal of Dieldrin and a 19.5–28% removal of benzopyrene. This method demonstrates phytoremediation’s potential as a sustainable eco-friendly approach for soil decontamination, utilizing plants to degrade or volatilize pollutants and restore soil health [185].

8.5. Phytoremediation and CRISPR-Cas9

The genome editing of plants for phytoremediation using CRISPR systems is a promising venture to increase their remedial capacity. Model phytoremediators, such as Noccaea caerulescens, Arabidopsis halleri, Pteris vittata, Hirschfeldia incana, and Brassica juncea, have been sequenced, and their genomes have been edited for increased tolerance to pollutants like Cd, Zn, An, and Pb. The manipulation of genomic sequences can facilitate the identification and characterization of key genetic determinants in phytoremediation processes, such as phytoextraction, phytostabilization, phytovolatilization, phytodegradation, or phytodesalination. Sequence data information from these plants can be used to establish CRISPR systems for phytoremediation by the targeted engineering of mechanisms involved in the accumulation, complexation, volatilization, and degradation of pollutants. CRISPR is a programmable next-generation method for high-throughput genetic manipulation compared to low-throughput ZFNs and TALENs [206].

8.6. Efficacy of Plants in Urban Phytoremediation and Major Challenges

Phytoremediation is involved in the removal of pollutants from the air; for this purpose, different species of plants are being used. In this strategy, different plants show different efficiencies of pollutant absorption. Some species can absorb significant percentages of air pollutants; some absorb a moderate amount and some absorb very low amounts. The major objective is to remove pollutants from the urban environment from air pollution by using phytoremediation; having a large population in urban areas faces high risks to health. The suggestion is implanting specific species of plants to remove the maximum amount air pollutants from urban areas [207].

The pollen allergenic potential of species Ambrosia artemisiifolia, its protein Bet v1, was shown to exhibit raised allergenicity in polluted environments, especially when exposed to elevated NO₂ levels [208,209]. Despite having a similar allergen load, urban birch pollen specimens showed a greater allergenic capability than their rural counterparts [210]. Nitrogen is essential for synthesizing plant proteins; plants always use nitrogen in the form of ammonium, and research indicates that certain species can also absorb nitrogen from NO₂ [204].

Several species, including Robinia pseudoacacia, Populus nigra, Eucalyptus viminalis, and Magnolia kobus, have been observed as be effective NO₂ consumers and strong heavy metal phytoremediants [211]. Carbon monoxide (CO) is poisonous to humans; investigations on 35 species have indicated that different plants can tolerate and even accumulate (CO) [212]. These findings have a significant effect because they have shown how plants react to air pollution and also highlight the role of some species in urban forestry and environmental restoration projects [212].

When dealing with urban pollution, phytoremediation is effective and environmentally friendly, especially when pollutants like heavy metals and oxides of nitrogen are involved. Different plant species have varying efficiencies to absorb and remove contaminants; Magnolia kobus, Eucalyptus viminalis, Populus nigra, and Robinia pseudoacacia are among the species that are the most efficient in utilizing NO₂. These plants have demonstrated a significant role in nitrogen uptake from NO₂, with studies showing that they can thrive in high concentrations of NO₂ [204,211].

On the other hand, plant species like Erechtites hieracifolia and Nicotiana tabacum indicate a moderate level of absorbing NO₂. While discussing heavy metal phytoremediation, it has been identified that plant species like Lemna minor, Ipomoea aquatica, and Brachiaria mutica are more efficient in eliminating pollutants from surface waters, involving heavy metals such as lead (Pb), chromium (Cr), and cadmium (Cd). Certain plant species are more effective than others at collecting heavy metals in arid urban regions; Pennisetum divisum and Brassica tournefortii have shown exceptional capacities in this remediation, improving the condition of the soil and urban ecology [213].

The proper choice and management of the plant species play a vital role in phytoremediation; efforts are necessary to obtain positive results, including the time it takes for the plants to remediate soils and air successfully and the possibility of toxins accumulating in the food chain. Even if certain plants are effective in phytoremediation, it is essential to adopt a calculated strategy to optimize advantages and reduce the hazardous effects of urban air pollution [214].

8.7. Major Challenges in Scaling up Phytoremediation for Urban Air Pollution

Phytoremediation is used to remove air pollutants by using plants; a lot of challenges have been faced in scaling up phytoremediation. In this modern era, every country is trying to create a strong economy, and economic growth depends on urbanization and industrialization. As a result, there is severe air pollution, serious health issues, and environmental problems. Natural sources such as (volcanic eruptions, decomposition, and forest fires) and man-made sources (industrial activities and the burning of fossil fuels by automobiles) can cause air pollution [172]. It is estimated that about 7 million deaths per year are due to air pollution; diseases include respiratory diseases, cardiovascular diseases, nervous disorders, and cancers. These are due to PM, heavy metals, and VOCs. The best remediation techniques, including the air filtration process, adsorption, and the photocatalysis of particulate matter, heavy metals, and volatile organic compounds, cost a lot, increase problems, and can introduce the production of different secondary pollutants [202].

The best new emerging strategies, such as phytoremediation, provide environmentally friendly and better substitutes by using different plants to remove pollutants [215]. This effective methodology allows air pollution in urban areas to be removed easily without any cost and secondary pollutants. Phytoremediation is very significant in urban areas; it influences several critical factors, particularly physiological traits, ecological and environmental conditions, and plant species variability [172].

Urban vegetation plays a significant role in reducing NOx by 13.9 to 36.2%, SO₂ by 20.5 to 47.8%, and PM by 16.5 to 26.7%. One of the critical points is that these plant species are less effective in removing ground-level ozone (O₃), which may rise by 5.1 to 25.9% due to the complex interaction of the dynamics in urban air pollution [216]. The overall efficiency of phytoremediation is limited by the unequal geographical distribution of different plant species in urban areas, specifically in populated areas with high levels of pollution [217]. Politicians and environmental managers frequently fail to recognize these benefits of phytoremediation, which makes it difficult to implement phytoremediation in urban areas [218].

According to the economic estimates from the Center for Research on Energy and Clean Air (CREA), air pollution caused by burning fossil fuels, mainly automobiles and industries, resulted in USD 2.9 trillion in medical expenses and economic losses in 2018, equal to 3.3% of the world’s gross domestic product [219]. Long-term phytoremediation concerns plant health maintenance and soil fertility [215].

Bioavailability contaminants present further difficulties since low concentrations may restrict the ability of different plant species; the endophytes attach symbiotically with them to absorb and break down toxins [220]. The evapotranspiration of volatile organic molecules from soil or groundwater plants may further reduce the efficiency. Plant science, microbiology, engineering, and urban planning must be combined to overcome these issues and improve the application of plant–endophyte relationships in urban areas [172].

Using scientific research with different perspectives, phytoremediation plants can be optimized and be more responsive to seasonal fluctuations and the local environmental conditions of urban areas. Ultimately, using modern research that gives proper guidelines for managing urban vegetation to raise the stakeholder knowledge of the potential advantages of phytoremediation is necessary for successfully scaling up this technique for reducing urban air pollution [221].

9. Phylloremediation of Smog

Though phytoremediation has an effective role in mitigating smog constituents, the process is restricted to underground VOC remediation. Subsequent studies found that plants, particularly those in very contaminated environments, engaged in phylloremediation, a distinct type of phytoremediation. Phylloremediation is a process in which the remediation of contaminants is performed through the aid of bacteria associated with the leaf surface. It is not uncommon for plants to naturally evolve ways to convert toxic chemicals into less dangerous forms when they thrive in contaminated environments. In this case, the leaves and the bacteria that live on or in them are responsible. When bacteria and leaves work together, they can regulate air pollution even more effectively than when they work alone. Soil and water contaminants might be addressed by phytoremediation, whereas air pollutants are the focus of phylloremediation. When leaves treat air pollutants and the bacteria associated with those leaves—rather than just by those leaves or the microbes themselves—a process known as phylloremediation occurs. Over the past three decades, the expansion of the economy and cities has led to a dramatic rise in air pollution. Despite implementing several legislations and procedures to reduce air pollution, the problem persists. The phyllosphere, or the surface of a plant’s leaves, is home to a vast microbial community that might be the key to unlocking a world of biotechnology, agriculture, medicine, and other fields’ untapped potential via developing novel products, techniques, and instruments [43].

Microbials that repair foliar contamination with airborne contaminants, residual pesticides, or plastics are one example; probiotics and fermented foods that benefit human health are another. Another example is Phyllobacteria, which both promote plant development and inhibit diseases. Microbes in the phyllosphere aid in the transformation of plant biomass into useful products, including compost, renewable energy, feed for animals, and fiber. Their food products include thickeners, sugar replacements, industrial grade biosurfactants, enzymes for phytoremediation, new antibiotics, and cancer medications, and these microbes also contribute to the removal of air pollutants [222].

The air we breathe has become increasingly polluted due to various harmful pollutants released into the atmosphere by human activities like farming, rapid urbanization and industrialization, vehicles, and other human-caused activities. These pollutants include radionuclides, organic and inorganic compounds, agrochemicals, oil spills, heavy metals (HMs), and metalloids. In addition to causing serious health problems, exposure to these contaminants is a leading cause of mortality on a worldwide scale. Consequently, a critical issue humans face is the need to remediate air pollution to preserve ecological processes and functions while improving human health and well-being [223].

Ozone depletion is caused by particulate matter, hazardous heavy metals and metalloids, ionizing radiation, oxidized and reduced gasses (CO₂, CO, CH₄, NO₂, NO, N₂O₄, NH₃, NH⁴⁺, SO₂, O₃, C₆H₆ vapors, and VOCs: volatile organic compounds), coarse and fine particulate matter, and ultra-fine particles. Air pollution can inflict obvious harm to plants, stunt their growth, or reduce their output even when no outward signs of harm are present. Damage to plant cell membranes, stomatal closure, restriction in photosynthesis, the generation of reactive oxygen species in plant cells, soil acidification, eutrophication, and changes in soil physicochemical and biological characteristics are common in polluted environments [224].

There needs to be a full evaluation of all possibilities to lessen the impact of pollutants on plants because air pollution has grown into a critical issue on a global scale. For the bioremediation of air pollution, one approach is to use phylloremediation and phytoremediation techniques. This entails choosing and studying bacteria and plant species that can withstand pollution and remove contaminants from the air [225].

Among the most dangerous air pollutants is PM. Numerous plant species can significantly decrease air pollution, making them ideal for use in green belts that offer pristine outdoor areas that promote human health. However, plants experience physiological changes and stress when exposed to excessive amounts of PM. Kończak et al. (2024) examined the involvement of phyllospheric bacteria in air bioremediation processes, particularly concerning plants that thrive in moderate climates. Their research findings suggested that phyllosphere bacteria can metabolize air contaminants, but their effectiveness depends on the interaction between the bacteria and the plants in the phyllosphere. The European tree species most frequently utilized for this purpose are also showcased. The gathered data addressed the lack of a practical application of tree species in air bioremediation within a moderate climatic zone [226].

Mokarram-Kashtiban et al. (2019) studied a novel mix of heavy metal remediation strategies, including phytoremediation and soil amendment using rhizosphere microorganisms and nano-sized zero-valent iron (nZVI). The results demonstrated that inoculation with PGPR and AMF, particularly dual inoculation, improved plant development physiological and biochemical parameters of white willow and increased the bioconcentration factor (BCF) of Pb, Cu, and Cd. The low dose of nZVI dramatically boosted seedling root length and leaf area while also increasing Cd BCF [227].

Dharmasiri et al. (2023) thoroughly examined how aerobic bacterial strains biodegrade phenanthrene and what byproducts they produce. In ornamental plants cultivated in urban polluted environments, Bacillus spp. lived in the phyllosphere. The HPLC results showed that four species of Bacillus with a specific growth rate could break down over 88% of the phenanthrene in the first two days of incubation. The degradation percentages were 95%, 90%, 91%, and 93%, respectively. Many health problems are associated with the phyllosphere being deposited with PAHs from oil refineries and vehicles, which lowers the quality of food items derived from leaves [228].

However, many endophytes in the tea phyllosphere are capable of breaking down polyaromatic hydrocarbons, pyrene, and anthracene [220]. Phyllosticta capitalensis, Colletotrichum gloeosporioides, Colletotrichum siamense, Pseudopestalotiopsis chinensis, and Daldinia eschscholtzii all showed that their PAH degradation kinetics followed the first-order kinetic model and that Phyllosticta sp. had the highest pyrene degradation and anthracene degradation, respectively, according to the HPLC results. Because of the researcher’s ability to genetically modify phyllobacteria, they could speed up how plants and microorganisms work together to reduce air pollution [229].

Theoretically, colonized leaves should be able to biodegrade more contaminants than uncolonized leaves. Since the phyllosphere is usually a low-carbon zone, bacteria living there have a high chance to biodegrade organic contaminants. It has been previously shown that greenhouse plants typically do not have a well-established, natural, and diversified phyllosphere microbiome because of restricted bacterial sources like soil. It is reasonable to assume that most plants cultivated in containers or biofilters do not have an adapted phyllosphere microbiome, variety, or bacterial population because they are subjected to the same conditions. Nanotechnology has recently provided new answers to this problem, expanding the reach and efficiency of phytoremediation procedures. One approach that can show promise as a long-term, cost-effective substitute for conventional on-site and off-site cleanup methods is phytoremediation coupled with nanotechnology. The consideration of the contamination’s type and location informs the selection of nanomaterials and nanotools for phytoremediation. Both the direct removal of toxins and the stimulation of plant growth are ways that nanomaterials can aid in phytoremediation. Because nanoparticles can act as both a stimulant and a poisonous substance for microbes, choosing the right nanoparticle for phytoremediation is crucial. For bioremediation to be sustainable, more research into the principles, methods, potential, regulatory issues, obstacles, and future of nano-mediated phytoremediation is required [230].

Li et al. (2024) investigated the composition of the bacterial population in the rhizocompartments of plants, which is influenced by changes in soil enzymes caused by ZnO NPs. The results indicate notable alterations in plant-based outcomes, especially at low concentrations, by enhancing the ability of plants to remove pollutants from the environment (phytoremediation potential), general plant health, and the structure of bacterial communities associated with the roots (rhizocompartments) [231]. Shi et al. (2023) created a root colonization strategy combining artificial functional bacteria and magnetic nanoparticle assistance to enhance the phytoremediation of heavy metals in rhizospheres. A synthetic heavy metal-capturing protein was made visible by grafting iron oxide magnetic nanoparticles onto Escherichia coli, and magnetic nanoparticles worked to reduce metal levels by increasing plant weight and the heavy metal-removing capability. This novel method provided a fresh perspective on altering the rhizosphere microbiome of metal-accumulating plants and enhancing phytoremediation’s effectiveness [232].

10. Cost-Effectiveness and Challenges of Bioremediation

It has recently been determined that environmental contamination caused by xenobiotics and other associated resistant substances poses a serious concern to human health as well as the environment. Worldwide, there is a growing awareness of the crucial importance of finding an efficient treatment technique that can contribute to waste management while being both cost-effective and environmentally benign. For the remediation of wastewater, a range of sophisticated, traditional, developing, and biological treatment technologies are available [233].

It can be difficult to develop effective methods for controlling air pollution because of the complexity of the sources and characteristics of these pollutants. The fact that different microenvironments have varying levels of a given pollutant’s prevalence adds to the complexity of the situation and calls for either site-specific remediation for individual pollutants or groups of pollutants [234]. Bioremediation is a sustainable non-invasive method that is acceptable to the environment and the economy. The main disadvantage is the adaptability of microorganisms or their stimulation in the polluted area. Nevertheless, it is useful for the in situ bioremediation of polluted soil, water, and air because it does not produce harmful byproducts. Nevertheless, it is often not possible to achieve high efficiency with a single approach in a short time; instead, combining two or more procedures concurrently or consecutively is advised. A hybrid system that combines two or more physical, biological, or chemical treatment processes could provide an answer. This processing technique ought to be more effective, particularly when eliminating the intricate makeup of various contaminants. For instance, it seems that a viable environmental restoration strategy is combining biological treatment with the physical confinement of pollutants [233].

For the bioremediation of air pollution, some of these strategies include phytoremediation and phylloremediation, which entail choosing and assessing appropriate plant species and microorganisms that are tolerant of pollution and capable of eliminating one or more air pollutants. There is great potential to purify indoor and outdoor settings when combined with various microbes and plant species. On the other hand, using plants to clean up air pollution is still relatively new in the business world. The scientific community and the public know the many benefits of planting and growing trees. However, there are unknowns regarding the capacity and suitability of particular species for particular contaminants. There are restrictions on the quantity of vegetation and the type of plant species needed to remove various air pollutants. Modeling the phytostabilization process is another uncharted subject, particularly in terms of cost, practicality, and the safe disposal of contaminants. The fact that indoor and outdoor settings differ greatly from one another only serves to increase the complexity. In order to fully utilize plants, distinct plant species and agronomic techniques need to be used [235].

An indoor plant is expected to be regularly exposed to specific pollutants at high concentration levels. However, an outdoor plant must address the synergistic effect of several air pollutants and environmental stress and be able to adapt to fluctuating climate conditions. This suggests that each environment should have a very diverse plant–soil–microbe system chosen. One more obstacle in the widespread use of phytoremediation is its gradual elimination process, which essentially permits contamination to build up inside the limited space. Therefore, further research is needed on the phytoremediation of air contaminants, particularly in utilizing the full plant–soil–microbe interaction. In particular, for the rapidly urbanizing cities of the developing world, better instruments and procedures must be created with a fresh perspective to connect urban forestry with city planning [236].

However, bioremediation approaches can encounter limitations because of the slow degradability of bacteria and the high accumulability of plants, respectively. To work around these problems and promote sustainable development, genetic engineering techniques are crucial for creating transgenic plants and microorganisms that can biodegrade and detoxify toxins more effectively. Genetically modified organisms (GMOs) have great remediation potential [237].

10.1. Limitations of Bacteria

Bioremediation, while promising, faces several challenges that limit its effectiveness. One major limitation is the bioavailability of contaminants. Many pollutants, especially hydrophobic ones like hydrocarbons, are not easily accessible to microorganisms due to their poor solubility and the tendency to bind with soil particles. This significantly reduces the degradation rate of such contaminants [238].

Certain bacteria are only active at their optimum pH, temperature, and osmotic pressure, so their applications in remediating pollutants that require stringent conditions become a challenge. Using solitary microbes to remove hard-to-deal-with air pollutants is less effective than using a microbial consortium [239]. Using physicochemical methods for the remediation of air pollutants is considered more prolific than bacteria, as the latter leads to generating more recalcitrant pollutants or residual toxic compounds. Microbes are specific for their degradation range and rely on natural electron donors and acceptors, limiting their efficiency. To deal with this limitation, microbial electrochemical approaches are better at providing an external source of electrons to degrade pollutants efficiently [240]. Furthermore, using microbes for the remediation of industrial effluents offers less mineralization and a higher generation of secondary pollutants, so microbial bio-machines are preferred to enhance mineralization and efficiency with a considerable reduction in cost [241].

10.2. Limitations of Fungi

Fungal bioremediation is an emerging approach that holds promise in addressing various environmental pollutants, including air pollution. Fungi are known for their unique enzymatic systems that allow them to break down complex pollutants such as hydrocarbons, heavy metals, and even some organic pollutants. However, the application of fungi for the bioremediation of smog and other air pollutants presents several challenges. One major limitation is the diversity of air pollutants. Smog, for example, consists of multiple harmful components, such as nitrogen oxides, sulfur dioxide, particulate matter, and volatile organic compounds (VOCs). Fungi may be effective at degrading certain compounds, but their ability to efficiently break down the wide range of chemicals present in air pollution is not fully understood [242].

Another challenge involves the environmental conditions necessary for fungal growth. Fungi typically thrive in moist nutrient-rich environments, whereas air pollution occurs in open dry conditions. This mismatch complicates the deployment of fungi in large-scale outdoor environments like cities. Additionally, maintaining and controlling fungal growth in such settings can be difficult, which raises concerns about how effectively fungi can be applied to air pollution remediation [243].

Moreover, there is the risk that the degradation of pollutants by fungi could result in secondary toxic byproducts. These byproducts might necessitate further treatment, potentially undermining the environmental benefits of fungal bioremediation [242].

10.3. Limitations of Nano-Remediation

The nano-remediation of smog presents several limitations and challenges despite its promise as an innovative approach for addressing air pollution. One of the key challenges is the potential toxicity of nanomaterials. Many nanoparticles, while effective in breaking down pollutants, can themselves pose environmental and health risks. The use of toxic agents in synthesizing nanoparticles can introduce harmful substances into ecosystems, undermining the very goal of remediation [244].

Another significant issue is the flocculation of nanoparticles. Nanoparticles tend to aggregate due to their increased surface area, reducing their effectiveness in capturing and degrading pollutants. This aggregation not only diminishes their reactive properties but also complicates their dispersion in the environment, making the remediation process less efficient [245].

The high energy input required for producing and deploying nanomaterials is another limitation. Nano-remediation processes often require substantial amounts of energy, particularly when targeting large-scale pollution like smog. This can make the method economically and environmentally unsustainable in the long run [246].

Finally, scalability is a challenge. While nano-remediation has shown promise in controlled environments, applying it to larger open areas affected by smog is complex and costly. Monitoring and managing nanoparticle dispersion in the air remains a significant obstacle to the widespread application of this technology [136].

10.4. Limitations and Challenges for Phytoremediation

In the current era, several methods are used to remove air pollutants. One of these methods is phytoremediation, but there are many challenges in phytoremediation. First, inadequate funding is the main challenge; regular funding is essential to proceeding and driving the economic constraints of research, which the government and other higher officials are neglecting. Some environmental projects are costly, so most developing countries’ governments depolarize them, while developed countries respond urgently [247].

Second, the complexity of environments in different regions and the variability in the soil mixed with contaminants creates challenges, as effective phytoremediation needs proper information on microbial activities and soil properties, which is lacking. Third, limited plant knowledge and phytoremediation expertise hinder progress. Fourth, there is a lack of organized regional environmental legislation; unlike developed countries, there is a need to prioritize environmental management by creating regional regulations. Fifth, research is needed to find the genetic traits of native plants, focusing on pollutant accumulation and removal [247].

One of the main challenges is selecting suitable plant species, as environmental factors such as temperature, pH, and soil composition slow the remediation process, which may take many years to achieve effective remediation results. A high concentration of certain contaminants stops the growth of plants, which may lead to public doubt, restrictions on genetically modified organisms (GMOs), and further complications [248]. Some other challenges, such as phytoremediation, are slow and site-specific, leading to the transfer of pollutants to the food chain, the risk of biomagnification, economic viability, a complex regulatory process, long-term maintenance (specifically for genetically modified organisms), low public awareness, incomplete pollutant removal, and more difficulties. Collaborative efforts of the government, technical experts, and the enforcement of proper policies should provide effective results [248].

10.5. Ethical and Ecological Concerns of GMOs

Interbreeding between GMOs and either wild-type or other genetically suitable relatives is a potential outcome of their introduction into the field. Unless the bearer of the novel trait benefits from natural selection, it may vanish in wild kinds. Unfortunately, native species’ ecological relationships and behaviors can be changed when the tolerance abilities of wild types also increase. As they mature more quickly, GMOs may be able to outcompete their native counterparts. Because of this, they have a chance to become invasive, expand into other areas, and harm ecosystems and economies. Both target and nontarget species may experience an uptick in adaptation pressure, similar to that experienced by geological changes or natural selection, leading to the evolution of separate populations resistant to the introduced alterations. Changes in one species can have far-reaching consequences for the ecosystem. Due to individual impacts, there is always the chance of harm or annihilation to ecosystems. Problems caused by GMOs cannot be eradicated after they have been released into the environment [249].

The possibility of horizontal gene transfer (HGT) is one major concern with GMOs. Various organisms in different environments can acquire foreign genes through heterologous gene transfer (HGT), which includes transformation, transduction, and conjugation. Organisms, particularly prokaryotes, gain access to genes that cannot be passed down via generations when this process takes place, particularly in reaction to changing environmental conditions [250].

11. Conclusions

Environmental pollution, particularly air pollution, poses severe risks to both ecosystems and human health, with smog being a major consequence of industrial emissions and fossil fuel combustion. Bioremediation has shown potential in mitigating air pollution through microorganisms and plants. However, significant research gaps remain. Future studies must investigate the potential of nanoparticle- and microbe-assisted phytoremediation, as well as genetic engineering, for enhanced absorption and degradation capabilities. Additionally, enzyme identification and optimization, scalability and field applications, microbe–plant interactions, air pollution monitoring and detection, the bioremediation of emerging pollutants, and cost-effective and sustainable solutions must be explored. By addressing these gaps, we can develop more effective bioremediation solutions. This study’s findings provide a foundation for future research, and it is essential to continue exploring bioremediation’s potential to address the growing concern of air pollution. Future research must prioritize several key areas to maximize the potential of bioremediation. A major focus should be the synergistic use of plants, microbes, and nanoparticles, exploring how these elements can work together to enhance pollutant absorption and degradation. Additionally, the genetic engineering of plants and microbes may unlock more efficient pollutant removal processes. The development of advanced enzyme optimization techniques to accelerate degradation also demands attention. Researchers must also address the scalability and real-world application of bioremediation techniques, ensuring that these methods can be applied effectively in diverse environments.