Recent Advances in Biochar Production, Characterization, and Environmental Applications

Abstract

Biochar has gained a lot of attention due to its numerous applications and environmental benefits. It is a specialized form of charcoal derived from various types of organic materials such as wood chips, agricultural waste, and other biomass feedstock. It is produced through a process called pyrolysis, resulting in a highly porous material with a large surface area, making it an excellent material. Biochar has several unique properties that make it a promising tool for mitigating climate change and improving soil fertility and crop yields, among other things, making it an attractive option for sustainable agriculture. In addition, biochar can be used to filter contaminants from water, improve water quality, and reduce the risk of pollution-related health problems. Furthermore, biochar has the potential to be used as a fuel or catalyst for renewable energy production. Its multifunctional nature makes biochar a compelling tool for sustainable agriculture and a viable strategy in the fight against global warming. In the present review, we discuss the synthesis, characterization, and numerous applications of biochar in a detailed manner.

1. Introduction

Biochar, a carbon-rich, porous, and black solid material produced from the pyrolysis of biomass, has garnered significant attention due to its multifaceted applications and environmental benefits. The synthesis of biochar involves the thermal decomposition of biomass under oxygen-limited conditions, resulting in the formation of a stable carbonaceous residue [1,2,3]. There are a variety of feedstocks that can be used to create biochar, namely agricultural residues, forestry waste, and organic by-products, offering a sustainable solution for waste management and resource utilization. The versatile nature of biochar enables its utilization in a wide range of applications, including catalysts, catalyst support, soil amendment, water treatment, carbon sequestration, and renewable energy production [4,5,6]. In agriculture, it can boost soil fertility, improve water retention, and mitigate greenhouse gas emissions, thereby contributing to sustainable agricultural practices and climate change mitigation efforts. Additionally, biochar-based adsorbents demonstrate promising potential for removing pollutants from air and water, addressing environmental pollution challenges.

The need for sustainable solutions to environmental and agricultural problems is increasing day by day. Therefore, it has become essential to understand the synthesis, characterization, and applications of biochar in order to address global sustainability challenges. It highlights the significance of biochar and its diverse applications in numerous fields. The review also aims to comprehensively overview the methods of biochar synthesis, characterization techniques, and their applications in various fields such as catalysis, wastewater treatment, soil remediation, storage devices, and supercapacitors.

2. History of Biochar

The history of biochar spans millennia, with evidence of its use dating back thousands of years across various civilizations worldwide [7]. Although contemporary interest in biochar focuses on its environmental benefits and sustainable agriculture, the origins of this practice stem from ancient soil amendment and waste management techniques. One of the earliest documented uses of biochar dates back to the Amazon basin, where indigenous communities practiced a technique known as “terra preta” (dark earth) agriculture [8,9]. This involved enriching their soils with charcoal and organic matter. The result was highly productive agricultural lands in an area where the tropical soils were otherwise nutrient-poor. The longevity of these enriched soils, lasting for centuries despite intense weathering and erosion, underscores the effectiveness of biochar as a soil amendment. Similarly, ancient civilizations such as the Egyptians, Greeks, and Romans were aware of the benefits of using charcoal and burnt organic materials to improve soil fertility and agricultural productivity. Historical records suggest that biochar was employed for soil enhancement and medicinal and filtration purposes across different cultures and time periods. During the 18th and 19th centuries, advancements in agriculture and soil science led to a deeper understanding of the role of organic matter in soil fertility. Early agricultural scientists, such as Liebig and von Liebig, recognized the importance of organic carbon in soil health. They proposed various methods for incorporating organic materials into farming practices. Recently, there has been renewed interest in biochar as a carbon-negative technology and soil amendment. This resurgence can be traced back to the early 21st century when researchers began exploring its potential. The publication of scientific studies showing the positive effects of biochar on soil fertility, carbon sequestration, and climate change mitigation sparked renewed interest from the scientific community, policymakers, and environmentalists.

Nowadays, biochar is acknowledged as a promising tool for sustainable agriculture, carbon sequestration, and environmental remediation. Current research efforts continue to explore its potential uses in diverse fields, including waste management, renewable energy production, and water treatment. The history of biochar reflects a long-standing tradition of harnessing the beneficial properties of charred organic materials for soil improvement and resource management. From ancient agricultural practices to contemporary sustainable solutions, biochar remains an enduring symbol of humanity’s ingenuity in harnessing nature’s potential to benefit both ecosystems and societies. According to PubMed, a total of 12,162 articles were published on biochar within the last two decades. Since 2015, there has been a significant increase in research on biochar (as shown in Figure 1). Many of these articles explore the utilization of biochar derived from agricultural residues in various fields such as soil application, as a catalyst, wastewater treatment, energy storage, and supercapacitor applications.

3. Synthetic Protocols of Biochar

Biochar is a porous, carbon-rich material derived from organic biomass decomposition. Various feedstocks and production techniques were employed to obtain a variety of biochars, and the quality was evaluated using biochar’s physicochemical and surface properties [10]. The char product is classified into three groups, namely biochar, hydrochar, and charcoal [11]. This section discusses in detail the production of biochar using various methods such as pyrolysis, torrefaction, gasification, hydrothermal liquefaction, and solvothermal liquefaction (Figure 2).

3.1. Pyrolysis

Pyrolysis is an ancient and well-developed technique for producing bio-oil and biochar. It involves the decomposition of biomass in the absence of oxygen, resulting in the production of biochar, also known as pyrochar. This process uses heat and converts the biomass into solid, liquid, and gaseous products, and the yields depend mostly on the type of biomass and operating conditions. Pyrolysis occurs in a closed reactor system at higher temperatures above 300 °C (Table 1). Based on the heating sources, pyrolysis is categorized into four types: thermal pyrolysis, solar pyrolysis, infrared pyrolysis, and microwave radiation pyrolysis. Pyrolysis is also classified based on operating conditions, including temperature, heating rate, and residence time, which can result in slow, intermediate, or fast pyrolysis. During pyrolysis, several simultaneous and parallel reactions occur, including depolymerization, isomerization, dehydration, aromatization, decarboxylation, and charring of biomass. The biomass is typically made up of cellulose, hemicellulose, and lignin, which have varying chemical structures and differ from one another, making biochar production challenging. In general, the pyrolysis reaction mechanism is of primary decomposition and secondary decomposition. Primary decomposition is responsible for breaking down chemical bonds through thermal heating, resulting in the significant degradation of biomass into volatile compounds and biochar. At high temperatures and a slower rate of vapor condensation, the unstable volatiles and solid matrix undergo secondary reactions [12]. As the temperature rises, secondary reactions result in a higher amount of non-condensable gases being produced [13]. Here, we discuss the recent literature on biochar production using pyrolysis.

Biochar obtained from lignocellulosic biomass may be more appropriate for solid fuels, while non-lignocellulosic biomass-derived biochar might be more suitable for soil application amendments [14]. Suo et al. performed the co-pyrolysis of corn straw and seaweed (Enteromorpha prolifera) and found it to cause a synergistic interaction between these two feedstocks, and the resulting biochar (co-pyrolysis of ratios of Enteromorpha prolifera and corn straw of 1:1 and 7:3) had a higher water-soluble N/P ratio in comparison to corn straw, which is helpful for soil amendment [15]. Various researchers have synthesized biochar using pyrolysis of lignocellulosic biomass like rice straw, rice husk, and wheat straw, employing temperatures around 500 °C. This biochar was effectively employed for adsorbing heavy metals, including copper, lead, cadmium, and other heavy elements [16,17,18]. Numerous researchers have generated biochar through pyrolysis and utilized it for energy production and storage, as shown in Table 1 [19,20,21,22].

Apart from conventional pyrolysis, biochar can be generated through microwave pyrolysis, characterized by efficient, rapid, even, and volumetric heating compared to conventional pyrolysis. This method entails the consistent penetration of microwave radiation through the biomass, leading to heating rates ranging from 0.1 °C/s to over 1000 °C/s [23]. Consequently, the biochar generated through microwave pyrolysis exhibits superior quality with increased surface area and pore volume. Microwave-assisted pyrolysis can be a time-saving technique for converting biomass into value-added products like biochar. The biochar yield from this method varies from 12 to 75% under different operating conditions.

Table 1. Biochar from different feedstocks via pyrolysis.

S. No.	Biomass	Pyrolysis Condition				Biochar Yield, %	Application	References
		Heating Source	Temperature, °C	Heating Rate, °C/min	Residence Time, Minute
1	Rice husk	Thermal	300	20	90	37.71	-	[24]
2	Rape straw	Thermal	500	10	120	-	Pb removal	[16]
3	Litchi seeds	Thermal	700	10	120	-	Fabrication of supercapacitor	[22]
4	Cashew nutshell	Thermal	500	Fast	Minimal	26–28	CO2 adsorbent	[25]
5	Sewage sludge and bamboo waste (4:1)	Thermal	700	10	30	-	Ciprofloxacin adsorption	[26]
6	Straw	Thermal	500	10	-	-	Co-combustion with coal	[19]
7	Pinewood	Thermal	500	Fast	Minimal	-	A catalyst for green needle coke production	[27]
8	Wood biomass + Coal	Thermal	350	10	20	-	Fuel cell	[21]
9	Corn straw + seaweed	Thermal	400	-	120	31.6	Soil amendment	[15]
10	Rice husk	Thermal	500	10	60	-	Pb2+ and Cu2+ adsorption	[17]
11	Cotton textile waste	Thermal	450	5	120	-	Activated carbon	[28]
12	Citrus peel fruit waste	Thermal	300–700	5	60	53.62–22.01	Solid biofuel	[20]
13	Wheat straw	Microwave (100 to 600 W)	-	-	-	24.25–74.66	Adsorption of heavy metals (Cu2+, Cd2+, Pb2+)	[18]
14	Sugarcane bagasse	Thermal	600	10	60	-	Phenol adsorption	[29]
15	Lignin	Thermal	750	10	120	-	Methyl orange adsorption	[6]
16	Canola straw	Microwave	300–500	-	30	41.9–29.8	-	[14]
17	Wheat straw	Microwave	300–500	-	30	43.3–31	-	[14]
18	Corn stalk	Microwave	400–600	-	45	36.4–24.8	-	[30]
19	Pinewood	Microwave	400–600	-	45	33.1–19.3	-	[30]
20	Algae	Microwave	400–600	-	45	13.4–10.8	-	[30]

Table 1. Biochar from different feedstocks via pyrolysis.

S. No.	Biomass	Pyrolysis Condition				Biochar Yield, %	Application	References
		Heating Source	Temperature, °C	Heating Rate, °C/min	Residence Time, Minute
1	Rice husk	Thermal	300	20	90	37.71	-	[24]
2	Rape straw	Thermal	500	10	120	-	Pb removal	[16]
3	Litchi seeds	Thermal	700	10	120	-	Fabrication of supercapacitor	[22]
4	Cashew nutshell	Thermal	500	Fast	Minimal	26–28	CO2 adsorbent	[25]
5	Sewage sludge and bamboo waste (4:1)	Thermal	700	10	30	-	Ciprofloxacin adsorption	[26]
6	Straw	Thermal	500	10	-	-	Co-combustion with coal	[19]
7	Pinewood	Thermal	500	Fast	Minimal	-	A catalyst for green needle coke production	[27]
8	Wood biomass + Coal	Thermal	350	10	20	-	Fuel cell	[21]
9	Corn straw + seaweed	Thermal	400	-	120	31.6	Soil amendment	[15]
10	Rice husk	Thermal	500	10	60	-	Pb2+ and Cu2+ adsorption	[17]
11	Cotton textile waste	Thermal	450	5	120	-	Activated carbon	[28]
12	Citrus peel fruit waste	Thermal	300–700	5	60	53.62–22.01	Solid biofuel	[20]
13	Wheat straw	Microwave (100 to 600 W)	-	-	-	24.25–74.66	Adsorption of heavy metals (Cu2+, Cd2+, Pb2+)	[18]
14	Sugarcane bagasse	Thermal	600	10	60	-	Phenol adsorption	[29]
15	Lignin	Thermal	750	10	120	-	Methyl orange adsorption	[6]
16	Canola straw	Microwave	300–500	-	30	41.9–29.8	-	[14]
17	Wheat straw	Microwave	300–500	-	30	43.3–31	-	[14]
18	Corn stalk	Microwave	400–600	-	45	36.4–24.8	-	[30]
19	Pinewood	Microwave	400–600	-	45	33.1–19.3	-	[30]
20	Algae	Microwave	400–600	-	45	13.4–10.8	-	[30]

3.2. Hydrothermal Liquefaction

In the previous section, we discussed thermochemical methods that are highly effective for breaking down dry biomass [31]. Hydrothermal liquefaction (HTL) uses a variety of wet materials, such as human and municipal waste, sewage sludge, and algal residue, in addition to lignocellulosic biomass (Table 2). It is a thermochemical process that converts biomass into bio-oil, gas, and biochar products in water at elevated temperatures and pressures. The process of producing hydrochar through hydrothermal carbonization involves several key steps. These include feedstock preparation, heating and pressurization, liquefaction, and separation. Biochar produced by HTL is called hydrochar. It is formed when water is used to produce solid carbon-rich residue through hydrothermal carbonization, which occurs under subcritical conditions. Although hydrochar is similar to biochar, it differs in terms of its production methods, feedstock, and physicochemical properties [32]. The process of creating hydrochar slightly differs from the traditional method used to make biochar. Researchers have conducted multiple studies on hydrothermal carbonization (HTC) to transform carbohydrates, cellulose, biomass, and other materials into hydrochar [33,34,35]. During the process, various chemical reactions may occur, like hydrolysis, dehydration, decarboxylation, aromatization, and depolymerization. These reactions typically occur in a specific order, but they do not necessarily happen in succession. Instead of this, there is a network of interconnected reactions where each one affects the others [36].

Harisankar et al. [33] conducted experiments using various types of water—including milli-Q water, tap water, seawater, and industrial wastewater during the hydrothermal liquefaction of rice straw biomass. They concluded that the highest quality hydrochar was produced using industrial wastewater due to its high levels of organic content and used this hydrochar as fertilizer in soil amendment. Several researchers produced hydrochar through hydrothermal liquefaction (HTL) and utilized it as a solid fuel, as shown in the table. For example, Wang et al. [37] generated hydrochar from sunflower stalks via the HTL process and used in supercapacitor applications. Hydrochar made from hydrothermal liquefaction of cotton, wood dust, and pinecone is able to absorb a significant amount of metal from water [38,39,40]. Rodriguez et al. made activated carbon from beech wood under hydrothermal conditions at 220 °C, 45 bar internal pressure, and 5 h of residence time [41]. In conclusion, hydrochar is mostly used in solid fuel applications. The yield of hydrochar varies from 30 to 60% depending on the conditions.

Table 2. Biochar from different feedstocks via hydrothermal liquefaction (HTL).

S. No.	Biomass	HTL Condition				Biochar Yield, %	Application	References
		Heating Source	Temperature, °C	Pressure	Residence Time, Minute
1	Rice straw	Thermal	350	18 MPa	30	36.4	Fertilizer	[33]
2	Corn stalk	Thermal	190–240	-	30	56.96–42.31	Solid fuel	[34]
3	Green waste	Microwave	190	-	60	-	Solid fuel and adsorbent	[42]
4	Sunflower stalk	Thermal	230	-	1440	-	Supercapacitor	[37]
5	Corncob residue	Thermal	230	-	60	48	Solid fuel	[43]
6	Olive tree	Thermal	200	-	60	58.2	Solid fuel	[44]
7	Beech wood	Thermal	220	45 bars	300	56	Activated carbon	[41]
8	Cotton	Thermal	250	-	180	-	Heavy metal Adsorption (Pb2+, Cd2+)	[38]
9	Wood dust	Thermal	180	-	600	-	Metal Adsorption (Cr, Sb)	[39]
10	Pinecone	Thermal	200	-	300	-	Adsorption (Pb, Cd)	[40]

Table 2. Biochar from different feedstocks via hydrothermal liquefaction (HTL).

S. No.	Biomass	HTL Condition				Biochar Yield, %	Application	References
		Heating Source	Temperature, °C	Pressure	Residence Time, Minute
1	Rice straw	Thermal	350	18 MPa	30	36.4	Fertilizer	[33]
2	Corn stalk	Thermal	190–240	-	30	56.96–42.31	Solid fuel	[34]
3	Green waste	Microwave	190	-	60	-	Solid fuel and adsorbent	[42]
4	Sunflower stalk	Thermal	230	-	1440	-	Supercapacitor	[37]
5	Corncob residue	Thermal	230	-	60	48	Solid fuel	[43]
6	Olive tree	Thermal	200	-	60	58.2	Solid fuel	[44]
7	Beech wood	Thermal	220	45 bars	300	56	Activated carbon	[41]
8	Cotton	Thermal	250	-	180	-	Heavy metal Adsorption (Pb2+, Cd2+)	[38]
9	Wood dust	Thermal	180	-	600	-	Metal Adsorption (Cr, Sb)	[39]
10	Pinecone	Thermal	200	-	300	-	Adsorption (Pb, Cd)	[40]

3.3. Gasification

Gasification is a thermochemical process that uses heat to transform organic biomass like wood, crop residues, and agricultural waste into a gaseous fuel known as syngas. This process is an eco-friendly and renewable method that produces energy and chemicals as it uses abundant organic materials that are often considered waste. Gasification of biomass has numerous benefits, such as reduced greenhouse gas emissions and the ability to use a diverse range of feedstock. The biomass feedstock is collected and processed for gasification through shredding, chipping, drying, or size reduction to ensure uniformity. The processed biomass is introduced into the gasifier, which produces two main products: syngas and biochar (

Table 3. Biochar from different feedstocks via gasification.

S. No.	Biomass	Gasification Condition		Application	Reference
		Heating Source	Temperature, °C
1	Mallee wood Gasification	Thermal	880	As catalyst for tar reforming	[50]
2	Pinewood	Thermal	1200	Soil amendment	[45]
3	White oak	Thermal	600–710	Additive in anaerobic digestion	[51]
4	Woodchips	Thermal	900	Additives in cement	[52]
5	Rice straw	Microwave	550		[53]
6	Beech wood	Thermal	670–750	Soil amendment	[10]
7	Greenhouse waste	Thermal	670–750	Soil amendment	[10]
8	Pinewood chips	Thermal	700–750	Carbon sequestration and soil amendment	[46]
9	Cereal straw	Thermal	700–750	Carbon sequestration and soil amendment	[46]
10	Cotton crops	Thermal	695–834	Soil amendment	[47]
11	Wood chips	Thermal	850	CO2 capture	[49]
12	Oak	Thermal	850	Soil mineralization	[48]
13	Corn stover	Thermal	850	Soil mineralization	[48]

). Syngas is used for energy and chemical production, while biochar is utilized in various applications, including as an adsorbent and catalyst carrier. During the gasification process, very minimal biochar formation occurs due to the partial oxidation of biomass. The biochar obtained from the gasification process consists of high ash content, which is rich in minerals and mainly used for soil amendment purposes. Numerous researchers have used gasification biochar in soil amendment to increase fertility and mineralization, improve water retention, etc. [10,45,46,47,48]. Due to its porous structure and the presence of basic functional groups, biochar has the potential to be a sustainable material for CO₂ capture [49]. Zhang et al. used gasification biochar as a catalyst in tar reforming [50]. Using microwave heating instead of conventional heating reduced the reaction temperature by 200 °C while achieving the same results.

3.4. Torrefaction

Torrefaction is a process that involves mild pyrolysis, which is a thermochemical procedure that aims to reduce the moisture and volatile components in biomass (

Table 4. Biochar from different feedstocks via torrefaction.

S. No.	Biomass	Torrefaction Condition		Biochar Yield, %	Application	References
		Heating Source	Temperature, °C
1	Rice husk	Thermal	290	67.0	Solid fuel	[56]
2	Rice husk	Microwave	220	39.71	Energy	[55]
3	Sugarcane residues	Microwave	320	32.90	Energy	[55]
4	Cotton stalk	Thermal	300	61.0	Solid fuel	[57]
5	Sugarcane bagasse	Thermal	300	54.0	Solid fuel	[57]
6	Prosopis	Thermal	300	73.0	Solid fuel	[57]
7	Pine needles	Thermal	350	44.19	Energy	[58]
8	Wheat straw	Microwave	392	66.3	Energy	[59]
9	Barley straw	Microwave	282	80.9	Energy	[59]
10	Sweet sorghum bagasse	Thermal	250–300	43–65	Energy	[60]
11	Peanut shell	Solar thermal	200–300	61.9–96.2	Solid fuel	[61]
12	Soybean straw	Solar thermal	200–300	43.2–92	Solid fuel	[61]
13	Pine wood	Solar thermal	200–300	53.4–97.8	Solid fuel	[61]

). This process improves the fuel characteristics of biomass, such as increased energy density, hydrophobic behavior, elimination of biological reactivity, improved grindability, and more uniform composition, among others. Unlike pyrolysis, torrefaction uses lower temperatures (200–350 °C) and occurs under an inert or reducing atmosphere without oxygen [54]. During torrefaction, the different components of lignocellulosic materials, including hemicellulose, cellulose, and lignin, undergo degradation. This process usually leads to the emission of CO₂ and H₂O, causing a decrease in the levels of hydrogen and oxygen present in the resulting torrefied residue. However, the carbon content in the torrefied biomass increases as compared with the feedstock. Among these lignocellulosic components, hemicellulose is the most prominently affected by degradation during torrefaction. The majority of studies demonstrate that biochar produced from torrefaction is used for energy and heat production, as presented in Table 4. In comparison, microwave-induced torrefaction is an effective and promising technology that reduces the oxygen/carbon ratio of biomass when compared to that of thermal torrefaction [55].

3.5. Solvothermal Liquefaction

Solvothermal liquefaction (STL) is a thermochemical process that is similar to hydrothermal liquefaction. The only difference between them is that water is used in hydrothermal liquefaction, while solvents are used in solvothermal liquefaction. This process involves similar steps to that of HTL: feedstock preparation, heating and pressurization, liquefaction, and separation. The solvents are chosen based on their ability to interact with the biomass components. Common solvents for this process include water, alcohol, and other organic solvents, depending on the nature of the biomass and the desired products. The combination of temperature, pressure, and solvent interactions leads to the breakdown of complex biomass components such as cellulose, hemicellulose, and lignin into simpler compounds, resulting in the production of biochar and bio-oil (

Table 5. Biochar from different feedstocks via solvothermal liquefaction (STL).

S. No.	Biomass	STL Condition				Biochar Yield, %	Application	References
		Heating Source	Temperature, °C	Solvent	Residence Time, Minute
1	Corn stalk	Thermal	250	Ethanol	75	39	Energy	[62]
2	Corn stalk	Thermal	200	Ethylene glycol	600		Magnetic biochar	[63]
3	Orange peels	Thermal	230	Ethanol/ acetone	15	40.71	-	[64]
4	Rice husk with iron precursors	Thermal	180	Ethanol	120	41.61	Magnetic biochar composite	[65]
5	Rice husk with iron precursors	Microwave	180	Ethanol	120	33.59	Magnetic biochar composite	[65]

).

4. Characterization of Biochar

Understanding biochar’s properties is crucial for predicting its impact on the environment and its potential applications. This characterization process involves analyzing its structure, surface chemistry, and elemental makeup [66,67,68]. pH is a significant factor that affects biochar’s effectiveness. Depending on the surrounding pH, biochar’s behavior and its interaction with metal contaminants change [69]. This characteristic makes biochar a powerful biosorbent, capable of removing significant amounts of pollutants like caffeine and diclofenac.

Various techniques are utilized to characterize biochar (Figure 3). These methods include Scanning Electron Microscopy (SEM), Fourier Transform Infrared Spectroscopy (FTIR), X-ray Diffraction (XRD), Nuclear Magnetic Resonance (NMR), Brunauer–Emmett–Teller (BET) Surface Area Analysis, Thermogravimetric Analysis (TGA), elemental analysis, and Raman Spectroscopy. For instance, Veiga et al., used a combination of FTIR, pKa determination, elemental analysis, Raman spectroscopy, zeta potential measurements, SEM, and Electron Paramagnetic Resonance (EPR) to characterize biochar [70].

4.1. Fourier Transform Infrared (FTIR) Spectroscopy

Fourier Transform Infrared (FTIR) Spectroscopy is a valuable tool for analyzing the functional groups present on biochar’s surface [71,72]. Studies have shown significant changes in biochar’s composition within a mixture as the temperature increases. A non-destructive FTIR instrument is ideal for monitoring these transformations effectively [73]. The analysis typically reveals a decrease in aromatic groups at higher temperatures (around 650–800 °C).

An alternative approach, Diffuse Reflectance Infrared Fourier Transform (DRIFT) spectroscopy, requires pelleting the sample with potassium bromide (KBr) before analysis. Within this approach, the sample interacts with an attenuated total reflectance (ATR) crystal, enabling the detection of functional groups [73]. Compared to traditional FTIR, DRIFT is preferred by many researchers due to its user-friendliness and efficiency [74].

Alfattani et al. [75] employed FTIR to analyze biochar derived from various walnut shells. Their findings showed distinct peaks around 3411–3465 cm⁻¹, indicating the presence of phenolic hydroxyl groups. Similarly, Adekanye et al. [76] investigated the functional groups of maize cob biochar produced at different temperatures (300, 400, and 500 °C). Their findings showed an expansion of the O-H group (3383–3402 cm⁻¹) with increasing temperature. These studies, along with others like Sahoo et al. [77], highlight the effectiveness of FTIR in understanding how temperature affects biochar’s functional groups during pyrolysis (300–750 °C).

4.2. Raman Spectroscopy

Raman spectroscopy is a popular technique for analyzing the structure of carbon-based materials such as biochar. It is favored for several reasons, including minimal sample preparation, clear resolution, high sensitivity, and the ability to analyze samples without causing damage [78,79]. This technique is effective for characterizing biochar derived from various agricultural waste materials, such as rice husk, sugarcane bagasse, and pine wood.

Raman spectroscopy can reveal changes in biochar’s structure as the pyrolysis temperature increases. This technique identifies changes in the position and width of Raman bands, which indicates a more ordered carbon structure at higher temperatures. It is important to remember that biochar’s source material significantly impacts its chemical composition [80]. Studies have shown a consistent trend: as pyrolysis temperature increases, the degree of carbon order in biochar also increases. A report by Veiga et al. [70] compared batch and continuous flow pyrolysis of sugarcane bagasse. Their findings, based on elemental analysis and Raman spectroscopy, demonstrated a higher degree of carbonization and a more ordered structure in the biochar produced using the continuous flow process.

Numerous researchers have utilized Raman spectroscopy to understand biochar’s structure and properties. Tsaneva et al. [81] studied structural changes in biochar using Raman spectroscopy and observed a correlation between spectral changes and the biochar’s structure. Similarly, Yu et al. [82] investigated structural changes in biochar from pyrolyzed beechwood. They observed a consistent shift in the intensity ratio of specific Raman bands with increasing temperature, indicating an increase in aromatic rings and carbon crystallites within the biochar.

Raman spectroscopy has also been used to analyze the structural transformation of chars during biomass pyrolysis. Smith et al. [83] found evidence for the development of polyaromatic hydrocarbons, reduced oxygen content, and the growth of non-hexagonal ring systems as the pyrolysis temperature increased. In a more recent work, Guizani et al. [84] proposed a new method combining Raman spectroscopy with elemental analysis. The authors observed a correlation between the intensity ratio of Raman bands and the atomic ratio of oxygen and hydrogen to carbon. However, it is important to note that their findings were limited to a specific type of biomass pyrolyzed at high temperatures (above 600 °C).

4.3. Scanning Electron Microscopy (SEM)

Scanning Electron Microscopy (SEM) is a valuable tool for examining the surface structure and morphology of biochar [70,85]. It reveals how temperature and processing methods can significantly impact the shape and size of biochar particles. Several studies have shown that increasing pyrolysis temperature can enhance the pore properties of biochar. The SEM technique provides a detailed account of the distribution and characteristics of pores within biochar particles.

The surface morphology of biochar can be predicted before and after it adsorbs pollutants using SEM [86]. Furthermore, combining SEM with Energy-Dispersive X-ray (EDX) analysis allows researchers to investigate the composition and properties of biochar’s surface [87]. SEM-EDX investigation is helpful for finding different elements present on the biochar’s surface. This technique is commonly used in biochar research to assess changes in surface characteristics after pollutant adsorption [88].

4.4. Nuclear Magnetic Resonance (NMR) Spectroscopy

Nuclear Magnetic Resonance (NMR) spectroscopy is a powerful tool for investigating the structural composition of biochar molecules [89,90]. This technique uses radio waves to measure the resonance of specific atomic nuclei within the biochar. NMR spectroscopy, especially solid-state NMR, helps to detect the types of carbon functional groups existing in biochar. It offers insights into the molecule’s structure, including the extent of aromatic ring formation. In addition, NMR can analyze the composition of both aromatic and aliphatic hydrocarbons within biochar. Recent reports have even utilized NMR to compare the degree of carbonization, which influences biochar’s stability [89].

However, NMR spectroscopy has limitations. The presence of ferromagnetic minerals in biochar can interfere with the NMR signal, making analysis difficult. Additionally, biochar produced at high pyrolysis temperatures often exhibits a low signal-to-noise ratio in NMR measurements [91]. Despite these limitations, solid-state NMR techniques remain valuable for measuring the total number of carbon functional groups, estimating the degree of aromatic ring formation, and revealing the overall structure of biochar molecules [86].

4.5. X-Ray Diffraction (XRD)

X-ray Diffraction (XRD) is a widely used technique for analyzing the crystallinity and structure of biochar [92]. Pioneering work by Franklin used XRD to categorize various carbon materials into two main groups based on their diffraction patterns [93]. Graphitizing carbons exhibit sharp, narrow peaks in the (002) region after thermal treatment, while non-graphitizing carbons show broad peaks in the same region. Studies by Zoroufchi et al. have confirmed the reliability of XRD in identifying the formation of amorphous materials in biochar at temperatures exceeding 350 °C [94].

The presence of sharp and well-defined peaks in the XRD pattern indicates the formation of nanocrystals within the biochar. Interestingly, research suggests that the size of these nanocrystals increases over time [95,96]. By analyzing the XRD pattern, researchers can efficiently develop high-quality biochar with a strong sorption capacity, all in a non-destructive manner.

4.6. Brunauer–Emmett–Teller Analysis

Biochar production significantly increases the surface area compared to the original biomass. This is due to the creation of more pores within the biochar. The Brunauer–Emmett–Teller (BET) method, a well-established technique, is used to measure the specific surface area of these particles. Many studies have employed BET to investigate the surface area of biochar [97]. It is crucial parameter for any adsorbent material, as it directly affects its ability to remove pollutants from soil and water.

During biochar formation, the release of volatile materials creates a highly porous structure with diverse pore sizes and a lower overall density. Compared to raw biomass, biochar exhibits a significant increase in BET surface area after pyrolysis. Natural biomass typically lacks microscopic pores. However, during pyrolysis, biochar develops new micropores. Studies have shown that for both biomass and biochar, the BET surface area and micropore volume increase with higher power levels (e.g., from 2100 W to 2400 W). This can be attributed to the rapid release of volatile components and the enhanced formation of micropores at higher heating rates [98,99].

4.7. X-Ray Photoelectron Spectroscopy (XPS)

X-ray Photoelectron Spectroscopy (XPS) is a powerful method for analyzing the elemental composition of biochar, particularly focusing on the surface elements [100]. Unlike bulk analysis techniques, XPS provides information specific to the biochar’s surface, allowing researchers to identify and quantify functional groups and other constituents present there. Changes in oxygen-containing functional groups, as detected by XPS, can be an indicator of biochar’s short-term stability. Additionally, XPS can measure the elemental oxygen-to-carbon (O/C) ratio, which serves as a general indicator of biochar’s overall stability [101]. However, it is important to note that XPS cannot differentiate certain types of functional groups, such as alcohols and phenols. Moreover, XPS also requires expensive instrumentation. Both XPS and FT-IR spectroscopy can be used for preliminary characterization of surface functional groups. However, they have limitations in distinguishing between different types of hydroxyl groups (aliphatic vs. aromatic) [102].

Despite these limitations, XPS data can be helpful in understanding the mechanisms by which biochar removes metals. Researchers suggest that complexation between oxygen-containing groups and metals plays a significant role. XPS analysis has also revealed the formation of MnO and cation exchange as additional mechanisms for manganese removal by biochar [103].

4.8. Thermogravimetric Analysis (TGA)

Thermogravimetric Analysis (TGA) is a thermal analysis technique that is used to understand how a material’s physical and chemical properties change with increasing temperature [71]. It is an essential tool for studying the thermal behavior of biomass at various temperatures. TGA is known for being reliable, simple, and fast, making it a great choice for analyzing the rate of biomass decomposition during pyrolysis (heating in an oxygen-limited environment). TGA measures change in mass of a sample over time as the temperature increases [104,105]. This method is widely used to characterize and analyze the thermal behavior of different materials. For instance, Labiadh and Kamali employed TGA to assess the ease of ignition (ignitability) of biochar and biochar–biomass mixtures [105].

TGA, combined with Mass Spectrometry (TGA-MS), can provide detailed information about the gases released and the amount of char remaining (char yield) during the pyrolysis of biomass such as sugarcane bagasse. Studies have shown that these factors can vary significantly depending on the heating rate used [106]. Another example is the use of TGA to analyze biochar derived from various sources, like bamboo. In this study, the biochar was heated at different rates (10, 20, and 30 °C per minute) in an inert (oxygen-free) environment. The study also mentions that vacuum pyrolysis was used to produce the bamboo biochar [107].

5. Applications of Biochar

The socio-economy of India is strongly dependent on agriculture and agriculture-based activities like crop production and processing [108]. As the population continues to rapidly increase, crop production in India is also increasing to meet the demand for food. This has led to an increase in crop residues as well. According to the Ministry of New and Renewable Energy, about 500 million tons of annual crop residues have been generated in India. At present, a large portion of these residues is being used for animal feeding, roof material for rural houses, fuel for cooking, and industrial purposes [109]. However, about 140 million tonnes were reported as unused and mostly being burnt out in the open field, resulting contributions to global warming, degrading soil fertility, and increasing the particulate matter in air [110]. Hence, there is a strong need for proper crop residue utilization to avoid the above-mentioned issues. Pyrolysis is a promising technology to convert these crop residues into solid residues known as biochar by thermal degradation in the absence of air. Biochar is considered a potentially attractive material for various applications (Figure 4) due to its low cost, large surface area, unique properties, etc.

5.1. Biodiesel Synthesis

The rapid depletion of fossil fuels and the release of greenhouse gases have caused energy and environmental challenges. As a result, carbon-neutral renewable energy sources are urgently needed to replace fossil fuels and reduce carbon emissions. Transportation fuels are the prime energy-consuming sector with about 28% of total energy consumption [111,112,113]. Hence, biodiesel has been considered one of the most suitable liquid fuels for an internal combustion engine, with almost comparable fuel quality to conventional diesel [114,115]. Transesterification is the well-known process for manufacturing biodiesel using homogeneous and heterogeneous catalysts (Figure 5) [116]. In general, KOH, NaOH, NaOMe, etc., are widely used as homogeneous catalyst for biodiesel manufacturing due to their high reactive efficiency. However, homogeneous catalysts suffer from issues like not being reusable and the extensive effort required for their purification from reaction products [117,118]. Heterogeneous catalysts have been widely employed for manufacturing biodiesel due to their easy separation from products, good stability, fast recycling, and long lifetime catalysts [119]. Metal-based heterogeneous catalysts such as CaO, MgO, Amberlyst-15, TiO₂/ZrO₂ and Al₂O₃/ZrO₂, WO₃/ZrO₂, and various carbon-based acids or bases were employed for biodiesel production [117,120,121]. In recent years, biochar has received a huge amount of attention as a catalyst for biodiesel production due to its low cost, large surface area, tailored surface functional groups, etc.

Generally, biochar is employed as a catalyst in two ways for biodiesel production: (a) acid-functionalized and (b) base-functionalized biochar catalysts. Acid-functionalized, i.e., sulfonated, biochar catalysts are considered the most commonly used solid catalysts for biodiesel production. Sulfonated biochar catalysts are prepared by impregnating the biochar with conc. H₂SO₄ or exposing the biochar to the gaseous SO₃. For example, Kastner et al. conducted esterification of a free fatty acid (FFA) blend with vegetable oil (VO) and animal fats (AFs) using peanut hull-derived biochar (PHC) sulfonated catalysts using H₂SO₄ and SO₃ [122]. Studies showed that H₂SO₄ was able to increase the surface area and pore volume of the biochar matrix, whereas gaseous SO₃ generated higher SO₃H compared to H₂SO₄, shown in

Table 6. Biochar utilization for biodiesel production.

Biochar	Feed	T, °C	A/C	CX, %	X, %	Key Findings	Ref.
PHC	FFA blend with VO and AF	50–60	6:1		97	SAPHC: 1–4 m2/g SAPHC-SO4: 242 m2/g PVPHC: ND PVPHC-SO4: 0.13 cm3g -SO3H Density of PHC: 0 -SO3H Density of PHC-SO4: 0.62 mmol/g	[122]
OH	WCO	100 and 140	10:1	10	90 Y	SAOH: 49.3 m2 g−1 SAOH: 30.6 m2 g−1 @100 SAOH: 5.4 m2 g−1 @140	[123]
Cork	WCO	65	25:1	1.5	98	Pore size of cork char at 600 °C = 2.3 cm decreased to 2.10 at 800 °C.	[124]
CS	PFAD	40–60	6:1 to 12:1	7	87		[125]
Fir wood	Microalgal oil	80–120	5:1–30:1	3–7	99Y	CSFFA content: <0.5 Amberlyst-15FFA content = 2.8	[126]
WM	Canola oil	65	15:1	5	24.5Y, 44.2 Y	(TAD and SA)450: 2.6 mmol/g and 1.88 m2/g (TAD and SA)675: 1.2 mmol/g and 640 m2/g (TAD and SA)875: 0.43 mmol/g and 1411 m2/g	[127]
PKS	SO	60	9:1	5	-	PKS is good source for CaO based catalyst to produce biodiesel.	[129]
PKS	SO	65	9:1	3	99 Y		[130]

A/C: alcohol to oil ratio; C_X: catalyst concentration (w/v); X: conversion; Y: yield of biodiesel; TAD: total acid density.

. Sulfonated biochar catalysts thus exhibited high catalytic activities with 90–100% conversion of fatty acids in 30–60 min at 50–60 °C, shown in Table 6. Biochar produced from various biomass resources was employed for biodiesel production after sulfonation. For example, sulfonated oat hull (OH)-derived biochar catalysts were first prepared with H₂SO₄ at different temperatures and employed as catalysts to produce biodiesel from waste cooking oil (WCO) in a microwave reactor [123]. The surface area of biochar catalysts was significantly reduced from 49.3 m² g⁻¹ to 30.6 and 5.4 m² g⁻¹ during sulfonation with H₂SO₄ at 100 and 140 °C, respectively. This result is found to be the opposite of Kashner et al.’s study. However, the total acidity of the catalysts was much higher at 140 °C compared to 100 °C. The catalysts sulfonated at 140 °C showed very high conversion of WCO to biodiesel although had a low surface area. This shows that total acidity is the key parameter for the high catalytic activity of sulfonated biochar to produce a high yield of biodiesel close to 90%, as shown in Table 6. Bhatia et al. studied the transesterification of WCO in the presence of cork-derived biochar for the production of biodiesel [124]. They synthesized biochar from cork by pyrolysis at 400, 600, and 800 °C. The biochar synthesized at 600 °C was reported as the most active catalyst for biodiesel production due to its large pores compared to those synthesized at 400 and 800 °C. High-temperature-synthesized biochar showed a collapsed porous structure, shown in Table 6. Palm fatty acid distillate (PFAD) was esterified using coconut shell (CS) biochar catalyst by Hidayat et al. and reported as a promising catalyst for biodiesel production [125]. The catalyst was sulfonated by conc. H₂SO₄ for acid functionalization of the biochar catalyst to improve the catalytic activity. The esterification of PFAD was studied at different catalyst to oil ratios, methanol to oil ratios, and reaction temperatures; 7 wt%, 12:1, and 60 °C were reported as the optimal conditions for the production of biodiesel from PFAD. Fir wood (FW) chip-derived biochar was also used for biodiesel production from microalgal oil by Dong et al. [126]. The biochar was prepared from Auger pyrolysis of FW at 600 °C followed by sulfonation. The catalyst showed very high catalytic activity compared to conventional Amberlyst-15 with 99% yield of biodiesel. The biochar catalyst was observed to be superior to Amberlyst-15 for reducing free fatty acid content, as shown in Table 6. Yu et al. studied the transesterification of canola oil to produce biodiesel using a wood mixture (WM)-derived biochar [127]. The catalyst carbonized at 675 °C was reported as the most active among catalysts carbonized at three different temperatures: 450, 675, and 875 °C. The high catalytic activity of the catalyst carbonized at 675 °C was due to high acid density despite having a low surface area compared to that carbonized at 875 °C, shown in Table 6. This shows that total acidity is the most important parameter for transesterification reactions using biochar catalysts. The yield of biodiesel was improved by using an iron-incorporated biochar catalyst compared to conventional acid catalysts [128].

CaO/biochar, KOH/biochar, and K₂CO₃/biochar were recently employed as base-functionalize biochar catalysts to produce biodiesel. For example, Fatty Acid Methyl ester (FAME) production from sunflowers was studied in the presence of a CaO-based palm kernel shell (PKS) biochar catalyst by Bazargan et al. [129,130]. A CaO-based biochar catalyst was prepared from the residue of palm kernel shell gasification and used to produce biodiesel. Studies show that the calcium carbonate present in the PKS biochar is the source of CaO-based catalysts. They studied the esterification reaction using commercial CaO and the PKS-derived CaO-based biochar catalyst and reported the same catalytic activity at the same dose and reaction conditions, shown in Table 6 [129]. Kostic et al. also studied the effect of various parameters on biodiesel production from sunflower oil (SO) in the presence of the PKS biochar-derived CaO-based catalyst [130]. The significant effect of reaction temperature and methanol to oil ratio on biodiesel production was observed. The reaction temperature of 65 °C and 9:1 methanol to oil ratio is reported as the optimal condition to produce 99% yield of biodiesel in the presence of 3 wt% of CaO-based biochar catalyst.

5.2. Catalyst Support

Biochar has been widely employed as a support for metal supported catalysts for various processes (Figure 6), shown in

Table 7. Biochar as support for various kinds of reactions.

Metal	Biochar	Feed	Reaction Condition Detail			Yield, %	Ref.
			Reaction	Reactor	T, °C; P, MPa
Ni	Microalgae	MABO	HDO	Batch	300	80 n-heptadecane	[131]
MgO	Wood waste	Glucose	Isomerization	100 mL Microwave	100	80 S	[132]
Ru	Lauan	Bio-syngas	Methanation	Fixed bed	360–420	92 S	[133]
FeCo2O4		Glucose	Oxidation	Fuel cell	250; 2	PD: 35.91 W/m2	[135]
NiMo2	Saw dust	Lignin	Hydrogenation			61.3 X	[136]
Cu, Ni, Zn, Fe, Co	Lignin-derived	Methane	Oxidation	Fixed bed	240–400	686.92 Y	[141]
Cu, Co, Mn, and Ni oxide	Pruning waste	4-nitrophenol	Reduction	-	-	-	[138]
Ni	Western red cedar	CO2	Methanation	Fixed-bed reactor	400–600	58 in 1 h	[139]
Co	Rice husk, Coconut shell, algae	Syn gas	FTS	Fixed-bed micro reactor	220	67 XCO	[140]

X: conversion; S: selectivity; PD: power density; Y: µmol/g_Cu/h.

. For example, a microalgae biomass-derived biochar-supported Ni catalyst was employed for hydrodeoxygenation (HDO) of bio-oil to produce diesel-range hydrocarbons by Naguyen et al. [131]. The pyrolysis of microalgae biomass was conducted at 400 °C for 2 h to produce bio-oil and biochar. The Ni precursor was impregnated on the produced biochar to prepare biochar-supported Ni catalysts. The same bio-oil was further employed to conduct HDO using synthesized Ni/biochar catalyst in a high-pressure batch reactor. The Ni/biochar was reported to be a very active catalyst for HDO of microalgae bio-oil (MABO) to produce an 80% yield of n-heptadecane as a key hydrocarbon for diesel at given reaction conditions. Glucose isomerization is considered the key reaction step in biomass valorization. Chen et al. studied the glucose-to-fructose isomerization reaction over biochar-supported MgO catalysts [132]. The 80% selectivity of fructose was observed at 100 °C temperature in only 30 min of reaction time, as shown in Table 7. The high surface area, chemical stability, and easily modified surface properties of biochar make it a potential support for a methanation reactions. Wang et al. studied the methanation reaction of bio-syngas over Ru/activated biochar and reported 92% selectivity of CH₄ [133]. The high catalytic surface area of biochar leads to the high metal dispersion of the biochar-supported metal catalysts. Secondly, the increased stability, improved activity, and low cost of biochar are also considered for its use as a potential support material for direct methanol fuel cells [134]. For example, Dong et al. used FeCo₂O₄-modified activated carbon as an electrocatalyst for direct glucose alkaline fuel cells [135]. A very high power density of 35.91 W/m² was observed, which was 151% higher than the control. Catalytic hydrogenation of lignin was conducted over a biochar-supported Ni-Mo₂ catalyst to produce chemical-range hydrocarbons [136]; 61.3% conversion of lignin was reported under mild reaction conditions, as shown in Table 7. This shows their high catalytic activity. The five-fold recycling of the catalyst was also active in converting around 60% of lignin. It again showed the stability of the catalyst for the hydrogenation of lignin. Wang et al. studied the oxidation of methane to methanol over lignin-derived biochar (LBC)-supported Cu, Ni, Zn, Fe, and Co catalysts [137]; 686.92 µmol/g_Cu/h is reported to be the highest yield of methanol over Cu/LBC catalyst in optimal reaction conditions. Biochar-supported Cu showed the highest catalytic activity for 18 h during oxidation of methane to methanol compared to Ni, Zn, Fe, and Co. Biochar-supported metal oxide catalysts were employed to study the reduction of 4-nitrophenol to 4-amino phenol by Ramirez et al. [138]. The catalyst showed the same catalytic activity after five reaction cycles. Frainetti et al. also reported an efficient biochar-supported Ni catalyst for CO₂ methanation, shown in Table 7 [139]. A biochar-supported Ni catalyst showed quite good performance for the catalytic conversion of CO₂ to methane in terms of methane yield compared to an alumina-supported catalyst. Rice husk-, coconut shell-, and algae-derived biochar-supported Co was employed for the Fischer–Tropsch synthesis (FTS) process and the results were compared with alumina-supported industrial catalysts [140]. The algae-derived biochar-supported catalysts showed the maximum FTS rate of 0.345 g HC/(g cat. h) and 67% CO conversion, as shown in Table 7.

5.3. Soil Remediation

Biochar has been used as a biomass-derived carbon-rich material for soil remediation due to its contaminant removal ability and for the improvement of soil quality [142]. It has also been proven to be a very efficient material in immobilizing heavy metals in polluted soils. The performance of biochar strongly depends on its characteristics such as porosity, composition, and pyrolysis temperature [143]. The usage of biochar and modified biochar is a realistic way of quickly improving the organic matter content present in the soil by lowering the level of heavy metal contaminants, which enhances crop production and its quality [144].

Several researchers and scientific communities around the globe have been applying biochar as an efficient renewable adsorbent for bioremediation enhancement. It is a promising approach over conventional technologies to mitigate heavy metals (HMs) and/or polycyclic aromatic hydrocarbons (PAHs) in co-contaminated soils. In order to improve HM/PAH co-remediation efficiency, Anae and co-workers explored all possible preparation techniques and mechanisms for the removal of pollutants. For example, ion exchange, electrostatic attractions, complexation, and volatilization are the possible mechanisms for elimination of polyaromatic hydrocarbons (PAHs) through immobilized by biochar (Figure 7) [145]. The development and implementation of a PGPR (plant growth-promoting rhizobacteria)-biochar-based remediation system can aid in the management of hazardous PAH-contaminated soil. Saeed and team offer three main contributions: (i) an outline of the PGPR mechanism for hydrocarbon degradation; (ii) a discussion of the contaminants that biochar can absorb and its properties; and (iii) a critical analysis of the joint action of PGPR and biochar in reducing the mobility and bioavailability of hydrocarbons [146]. Using hydroxyapatite (HAP) and Phanerochaete chrysosporium (P. chrysosporium, PC), which had a high phosphate-solubilizing capacity and HM tolerance, He et al. proposed a novel combination approach for the treatment of various HM-contaminated acidic mine soils. Adsorption and ion exchange were shown to be significant factors in the remediation process based on the findings of the characterization [147].

5.3.1. Adsorption and Ion Exchange Mechanism

Organic pollutants, insecticides, and heavy metals may all be adsorbed and retained by biochar because of their porous nature. By binding to these pollutants specifically, biochar can stop them from migrating across the soil profile through ion exchange. In order to remove heavy metals (HMs) from soil and water habitats, a new corn straw biochar-loaded calcium–iron layered double hydroxide composite (CaFe-LDH@CSB) was created using the co-precipitation technique [148]. The results demonstrated that a process involving monolayers complexed with functional groups by chemical endothermic adsorption was involved in the HM adsorption mechanism of CaFe-LDH@CSB in the aquatic phase. Two types of bio-based and coal-based humic acid materials (BHA and CHA) were synthesized industrially using aged coal and rice husk [149]. The Pb(II) adsorption and immobilization properties and processes were also examined in soil and water [149]. In addition to ion exchange and surface complexation, the three primary adsorption processes that were also involved were electrostatic contact, precipitation reaction, and π–π interaction. Environmental contamination may be quite harmful when it comes to arsenic-contaminated soil and wastewater. Ca-mPG, a novel phosphogypsum-based passivate, was created by partially mechanically activating calcium oxide with phosphogypsum. Its remediation efficiency and microbiological response were assessed on arsenic-contaminated soil [150].

The global scientific community has also focused a great deal of attention on chromium (Cr)-contaminated soils. Sulfur-modified biochar (SBC) has the potential for the remediation of Cr due to the combined benefits of biochar with the sulfur element. Chen et al. sought to investigate the modes of Cr immobilization of sulfur-modified biochar (SBC) and virgin wheat straw biochar (BC), as well as the Cd immobilization consequences of SBC and BC in soil polluted with Cr.

They investigated the process of cadmium sorption using sulfur-modified wheat straw biochar and its use in soil that has been polluted with cadmium. The effective loading of sulfur onto the pure biochar was validated by elemental and SEM analysis [151]. The biochar’s surface area had a negative correlation with the Cr leaching concentration. Greater surface area, increased porosity, and organic matter content of biochar were better for the variety of soil microbes. In order to simultaneously remediate Cr-contaminated soil and groundwater, they set out to demonstrate the efficacy and viability of an integrated approach. After remediation, the local rainfall and the average transit rate of Cr in the soil profile were both only 0.420 cm [151]. In order to investigate the potential of biochar as a remediation agent in wetlands, including its mechanism and efficacy, Annisa and co-workers intend to qualitatively synthesize all study results. The primary process of biochar-based soil remediation is not metal removal; rather, metals are accumulated into hydroxide or carbonate deposits with the assistance of pre-existing microbes. In the jointing stage, in which the time demand for organic matter is the highest, organic matter content increased by 35.4% when biochar was applied at 50 t/ha [152]. A three-dimensional surface correlation equation was developed by Cen and colleagues using the synergistic interactions between organic matter content, water-stable aggregates, and biochar. It offered proof that biochar has a strong chance of being used for soil rehabilitation and irrigation with less water [153]. An innovative iron–zinc oxide composite-modified corn straw (Fe/Zn-YBC) was synthesized and used to treat acidic and alkaline farming soils polluted with cadmium [154]. This unique functional biochar’s effective remediation ability and related mechanism offer insights for better remediation of heavy metal-contaminated farming soil. The physicochemical, electrochemical, and biological remediation methods are compared and thoroughly analyzed by Lin et al., offering guidance and references for the selection of future remediation technologies in agricultural settings [155]. Because it can be remedied quickly and contains reusable metals and minerals, magnetic biochar is a great substance for soil remediation. Researchers produced a unique layered double hydroxide (LDH) biochar composite that may be used to remediate soil and water habitats that are jointly polluted with heavy metals. The multivariate link between soil environmental variables and enzyme activity is explained by Zhang et al. using biological data. O-containing functional groups, such as Mn-O/As and Fe-O/As, developed on the surface of the biochar during the remediation process, lowering the phytotoxicity of As and encouraging the transformation of As from the mobile fraction to the residual fraction. This remediation ability of biochar was observed to be greater than that of the Fe-modified biochar [156].

Li et al. investigated the dual impacts of biochar addition on the accessibility of Cd in the soil and Cd hyperaccumulation under various Cd contamination levels. The addition of 5% biochar enhanced the urease activity by 41.18% as compared to the addition of 1% biochar in Cd-contaminated soil at a concentration of 50 mg/kg. Several scientific experts believe that biochar is a potential and effective renewable adsorbent with improved bioremediation ability compared to traditional solutions of contaminated soils [157]. In order to improve the efficacy of HM/PAH co-remediation by lowering their mobility and bioavailability, Anae et al. intend to (i) give an overview of biochar preparation and its use and (ii) critically evaluate and examine the possibilities of (bio)engineered biochar. The study examines the possibilities of combining biochar with hydrogel and bioaugmentation to create biochar composites in terms of (bio)engineered biochar. Research on the long-term rehabilitation of Cd-contaminated soil using Fe–Mn-modified biochar has been lacking [145]. During a three-year laboratory investigation, Liu and colleagues reported on the efficacy of biochar generated from coconut shells that were treated with ferromanganese in connection to soil Cd stability and rice Cd bioaccumulation. When using biochar to remediate soil, dissolved organic matter (DOM) released from the soil (SDOM) and biochar (BDOM) have a significant influence in the destiny of contaminants [158]. The importance of the fluorescence feature for DOM is emphasized by Deng et al. This property can expand our understanding of the fate of contaminants for biochar utilization. Huge efforts have been undertaken to enhance the effectiveness of biochar in the hydrocarbon-based remediation process by altering its characteristics before its use and application in conjunction with other bioremediation approaches [159]. The possibility of using biochar in conjunction with other well-known bioremediation methods, including bioaugmentation, phytoremediation, and bio-stimulation, was evaluated by Dike and colleagues [160]. The detrimental effects of persistent organic pollutants on the ecosystem and soil biota are emphasized by Haider et al. It is important to assess the physicochemical characteristics of both the biochar and the soil/wastewater, as well as the types of organic pollutants present before the use of biochar in soil or wastewater. The methods and use of biochar in the restoration of mixed HM-contaminated areas have not been extensively studied [161]. In order to remove heavy metals (HMs) from soil and water habitats, a new corn straw biochar-loaded calcium–iron layered double hydroxide composite (CaFe-LDH@CSB) was created using the co-precipitation technique [148]. Applying biochar to remediate contaminated soils might provide an innovative solution to problems with soil pollution. Murtaza and colleagues offer specialized expertise for creating an economical, environmentally friendly biochar that might be applied to large-scale decontamination of certain contaminated soils [162].

5.3.2. Microbial Activity Enhancement

It has been demonstrated that biochar enhances microbial activity in soil remediation, particularly in the context of heavy metal contamination. Its porous nature acts as a habitat for microbes, promoting their growth and activity. With an improved microbial community, pollutants are broken down, and soil fertility is generally improved. According to Wang et al., pig dung-derived biochar can encourage the growth of both local and alien microorganisms, which increases plant growth and immobilize heavy metals [163]. These results were corroborated by Gregory and colleagues, who demonstrated that biochar may boost soil microbial activity and encourage the breakdown of organochlorines [164]. All of these investigations point to the possibility that biochar might increase the microbial activity in soil remediation by altering soil characteristics, adsorbing pollutants, and promoting electron transfer compared to other processes [165].

Verma and colleagues investigated the impact of process factors and contemporary methods on microbial technology used in heavy metal cleanup. Microbial bioremediation provides an affordable option for treating environmental areas contaminated with heavy metals (HMs). Biosorption, bioleaching, biomineralization, biotransformation, and intracellular accumulation are some of the mechanisms that lead to the microbial breakdown of heavy metals. Compared to untreated soil, biochar-treated soil had a 41% increase in microbial diversity. When comparing soil incubated with biochar to control under oil stress, the microbial respiration rate increased by 33.67% [166]. Gas Chromatography–Mass Spectrometry (GC-MS) assessment revealed that the soil treated with biochar possessed a higher concentration of low-molecular-weight hydrocarbons (C-C) than the untreated soil [167]. Char materials (such as biochar and hydrochar) have received a lot of interest as soil additives to help rebuild deteriorated soil. A microcosmic experiment was utilized to examine the effect of biochar and hydrochar from cow dung (CBC, CHC) and reed straw (RBC, RHC) on the growth of lettuce in an acidic soil [168]. Li and colleagues investigated bacterial extracellular polymeric substances, particularly their effect on soil microbial community composition and possible involvement in heavy metal-contaminated soil. Microbial electrochemical technology (MET) is one of the soil remediation technologies. It has various challenges, such as poor mass transfer, restricted electro-activity of anodes, low bioavailability of hydrocarbons, limited activity of helpful microorganisms, and inefficient electron transport [169]. Ambaye et al. studied the function of adding rhamnolipid as an analyte solution in order to improve the effectiveness of MET. The electric fields can alter soil parameters, which influences the microbial activity and community composition [170]. Li et al. established the optimal electric field for electro-bioremediation of PAH-contaminated soil by studying the effects of electric fields on soil parameters and microbiological populations [169].

For red mud restoration, a unique technique based on natural biomass and without the use of chemicals was proposed. Wang et al. reported the neutralization of red mud using bio-acid created by hydrothermal carbonization of waste biomass for its possible soil application. Hydrothermal carbonization (HTC) of local fallen leaves was carried out to obtain a solution containing bio-acids (SBAs) and hydrochar [171]. Atienza-Martíne and their collaborators have studied one-step and multi-step slow pyrolysis of digested dairy cattle dung (DM) at temperatures of 250–600 °C. A balance between the pH and electrical conductivity of char was possible by pyrolysis at 400–550 °C for possible application as a soil supplement [172]. Fan et al. assessed how the bio-electrokinetic (BIO-EK) remediation method interacted with total petroleum hydrocarbons (TPHs), soil characteristics, and microbial populations. This was carried out on saline–alkali soil polluted with petroleum and injected with bacteria that degrade petroleum and have a high resilience to saline–alkali. Because bio-amelioration uses microorganisms like bacteria, it is a viable remediation technique for saline–sodic and calcareous–sodic soil because of its low cost, high efficiency, and environmental friendliness [173]. Bacillus subtilis BSN-1, a salt-resistant bacterium isolated from an arid area in Xinjiang, China, was studied for its impacts on salt crystallization during the evaporation crystallization of saline–alkali soil solution. It was shown that the fermentation products of B. subtilis BSN-1, such as glutamic acid, dramatically decreased the pH of salty soil solution due of the ionization of carboxyl [171]. Haider et al. underlined the negative impact of persistent organic pollutants on the ecosystem and soil biota. Biochar application in the soil has a considerable influence on the biodegradation, leaching, and sorption/desorption of organic pollutants [161].

5.3.3. Improvement in Soil Structure

Biochar enhances soil structure by promoting aggregation, reducing compaction, and increasing water retention. It facilitates better root development and nutrient uptake by plants. He and colleagues conducted a pot experiment in a greenhouse to assess the influence of biochar amendment on the development of Miscanthus lutarioriparius, a potential bioenergy crop, in a coastal saline–alkali soil during a 92-day period. The soil bacterial community structure and diversity were significantly altered by the biochar addition, affecting soil N and P cycling and improving the soil physicochemical parameters [174]. Amoah-Antwi et al. conducted a study on a variety of soil amendments, such as biochar and brown coal waste (BCW). Although biochar is commonly used, it is unclear how it affects bacterial responses to PAH stress when combined with plant roots. The study found a strong connection between the soil’s characteristics, bacterial members, and metabolites in both biochar-amended and rhizosphere soils. This connection enhanced bacterial resilience to PAH stress and improved the elimination of PAH [175]. In light of the above results, Li and colleagues suggest that biochar application can efficiently improve bacterial functions in rhizosphere soil. This can help to develop in situ remediation programs in soil contaminated with PAHs [176]. The ramifications of biochar aging are also discussed in terms of its prospective development and application as a soil amendment. Wang and co-workers suggest that for improved simulation and prediction, artificial aging methods must shift from qualitative to quantitative approaches [177]. A study conducted by Huang and his team in 2020 revealed that applying biochar (40 g kg⁻¹ soil) to rice plants can help reduce the impact of heat stress. The experiment was conducted in a pot and involved 6 consecutive days (6–11 days after transplanting) of daily mean temperatures above the critical high temperature (33 °C) for rice tillering. The results demonstrated that applying biochar enhanced the root-zone environment of rice plants by lowering soil bulk density, increasing soil organic matter content, and changing the soil bacterial community structure by raising the Proteobacteria to Acidobacteria ratio. Understanding the reaction of the microbial community structure to biochar-based fertilizer application is very significant in karst soil rehabilitation [178]. To examine the impact of applying fertilizer based on biochar on the structure of the microbial community in karst soil, a high-throughput sequencing technique was used in a field experiment conducted in southwest China [179]. Typically, biochar is added to agricultural soil as a soil supplement to enhance the microbial ecology and availability of soil nutrients. A soil column leaching simulation experiment was used to assess the microbial community structure, nitrogen retention capacity, and migratory dispersion properties of nitrogen fertilizer. Biochar-treated soil demonstrated a considerable improvement in the soil pore structure and rice production after 6-year treatment [180]. An and colleagues found that replacing a moderate amount of NPK fertilizer with biochar could be an effective approach to augment soil quality, increase the yield and growth of rice, and decrease the quantity of chemical fertilizers required for rice cultivation [181]. The biochar was derived from different straw types (wheat, rice and maize) by pyrolysis at 500 °C and applied to soil to improve organic carbon, nitrogen, phosphorus, and enzyme activity, particularly in paddy soil. The results suggested that the organic carbon content increased with the addition of biochar [182]. The impact of straw mulch and biochar application was investigated, and the researchers noted the enhanced growth of maize crops [183].

5.3.4. Long-Term Soil Stability

Biochar is a sustainable solution that can have long-lasting benefits and serve as a reliable remedy for soil improvement. Unlike other soil additives that decompose over time, biochar is highly stable and its positive impact on soil quality can persist for a considerable period. The long-term stability of soil can be significantly improved by the application of biochar, a carbon-rich material produced from biomass. Applying biochar to paddy soil can improve carbon sequestration; the amount of organic carbon in the soil affects how stable the carbon in the biochar is [184]. Similar to this, Sun et al. found that adding biochar to brown earth can improve soil structure by raising the amount of biological binding agents and stabilizing soil aggregates [185]. The stability of biochar carbon depends on the feedstock and pyrolysis temperature [186]. Additionally, Tian et al. highlighted that applying biosolids over an extended period can help to stabilize soil organic matter, which is beneficial for soil carbon sequestration [187]. These studies collectively underscore the significant role of biochar and biosolids in enhancing the long-term stability of soil.

5.3.5. Soil Carbon Sequestration

Organic waste can efficiently trap carbon in the soil and mitigate climate change by being converted into biochar. This makes the use of biochar aligned with broader sustainability goals. Under ideal circumstances, when biochar breaks down slowly, it has shown potential to act as a long-term carbon sink. In order for biochar to be successful and suitable for a range of applications, its physicochemical properties—such as the H/C atomic ratio and the lowering of O and H levels by heat treatment—are crucial [188]. By retaining carbon in the soil and improving soil absorption, biochar plays a key role in reducing emissions of methane (CH₄) and nitrous oxide (N₂O) and thus helps mitigate the effects of climate change [189].

Applying biochar to soil remediation is a viable path toward environmentally responsible and sustainable methods. Its numerous advantages, which range from improving soil structure to immobilizing contaminants, make it an invaluable instrument for tackling environmental problems. Because of its high carbon content, biochar sequesters carbon in soil for a long time. By taking carbon dioxide out of the atmosphere and depositing it in the soil, this helps slow down climate change. Because of its porous nature, water may be absorbed and held by biochar. This increases the soil’s ability to retain water, which lessens the need for regular watering. More water retention is especially helpful in dry or drought-prone areas. A variety of organic waste products, including forestry waste, organic municipal garbage, and agricultural leftovers, can be used to make biochar. This promotes the efficient use of biomass and helps manage organic waste. Biochar can enhance nutrient-use efficiency, minimizing the release of harmful gases into the atmosphere. It supports microbial communities involved in nutrient cycling and promotes a healthy soil ecosystem. Biochar has the ability to buffer soil pH. It can neutralize both acidic and alkaline soils, creating a more balanced and suitable environment for plant growth. Overall, the positive effects of biochar on soil properties contribute to improved resilience and crop productivity. As research in this field continues, the full potential of biochar in mitigating soil pollution and promoting sustainable land use is likely to unfold, ushering in a new era of soil remediation practices. This encourages the effective use of biomass and aids in the handling of organic waste. By improving nutrient-use efficiency, biochar helps reduce the amount of hazardous gasses released into the atmosphere. It fosters a robust soil ecosystem and supports microbial populations engaged in the cycling of nutrients. The pH of soil can be buffered by biochar. Both acidic and alkaline soils can be neutralized by it, resulting in a more balanced and ideal growing environment for plants. Overall, increased resilience and agricultural yield are facilitated by biochar’s beneficial impacts on soil characteristics. The entire potential of biochar in reducing soil pollution and encouraging sustainable land use is likely to be realized as this field of study progresses, bringing in a new age of soil remediation techniques.

5.4. Wastewater Treatment

Human perceptions about wastewater have changed over the past few decades due to the scarcity of water resources, moving from trash to a useful resource. In this scenario, biochar has been playing a crucial role in wastewater management. The purpose is to evaluate the functions of biochar in recovering resources from wastewater (Figure 8) [190]. Overall, the results are promising for the material in treating wastewater that contains high levels of Cd(II) and provide an obvious image of its use. Zhang and associates provide an explanation of the fixed-bed filtration procedure and further advocate for its use in the treatment of wastewater [191]. Biochar is a highly promising heterogeneous catalyst for treating wastewater, and adding iron oxide to it can increase its effectiveness. Chu and colleagues employed food waste as a starting material for the production of magnetic charcoal (MC), and a pre-pyrolysis procedure was effective in producing MC that included maghemite. Produced by pyrolyzing biomass waste, biochar is a carbon-rich substance that works well for water treatment, soil amendment, and climate improvement [192]. A thorough analysis of the existing techniques for in situ biochar amendments tailored to contaminated sediments is given by Yang and colleagues [193]. According to Hossain and colleagues, rice husk (RH) can be used as a potential feedstock for producing useful goods from agricultural waste biomass. Rice husk biochar is a substance that has the potential to remediate wastewater and produce bioenergy [194]. The objective of Sashidhar and colleagues is to methodically evaluate the applications of biochar as a possible carrier material for the transfer of microorganisms and agrochemicals. One of the most effective ways to remove color from water bodies is through adsorption, which is a popular water treatment method [195]. In a study conducted by Srivastav and colleagues, the focus was on how biochar can be a cost-effective and environmentally friendly adsorbent for eliminating harmful dyes from aquatic environments. This method has proven to be promising in removing colorants from water bodies [196].

5.4.1. Odor Control

Although wastewater contains a wide range of odorants, only a few specific ones have been targeted with biochar for removal. This section deals with ammonium cations, dimethyl sulfide, and volatile organic chemicals, such as butyric acid, acetic acid, propionic acid, and phenyl mercaptan. There is a chance that ammonium cations, which smell strongly like urine, is a precursor to ammonia [197]. With unpleasant odors and an extremely low olfactory threshold of between 0.6 and 40 parts per billion, dimethyl sulfide is a potent hazardous pollutant that causes intense odors [198]. Industrial effluents may include volatile organic chemicals at relatively high concentrations of 1200–3600 ppm, including volatile fatty acids like acetic and propionic acid [199]. Dimethyl sulfide and phenyl mercaptan concentrations were less than 0.03 parts per million in a number of wastewater samples from urban and industrial sources.

Different manufacturing and activation techniques are used to make biochar, which increases the effectiveness of removing the chemicals that cause odors. Khotsena and associates employed seven distinct food waste items as starting points for their biochar production process: starchy staples, green-stemmed vegetables, nut husk, fruit pericarp, bean dregs, tea leaves, meat, and bone. Additionally, biochar made from walnut shells was made to reduce dimethyl sulfide in a liquid sample used in petrochemical processes [199]. In another study, Fan and associates reduced the ammonium cations in wastewater by using biochar made from bamboo that had high Brønsted acidity. Comparable outcomes were seen using biochar derived from the leftovers of distillers’ grains through anaerobic digestion. Kotsana and colleagues investigated the decrease in dimethyl sulfide in petrochemical liquid using a biochar generated from walnut shell. Ag⁺, Fe³⁺, Cu⁺, and Cu⁺² were the four distinct metal ions that were impregnated into the biochar. With respect to those metal ions, the biochar that was impregnated with silver had the maximum potential to reduce dimethyl sulfide. To produce biochar, many raw materials have been employed, with the goal of removing volatile organic molecules from aqueous media. For the purpose of eliminating volatile fatty acids such as acetic, propionic, and butyric acid, a biochar made from vineyard leftovers was developed [200]. The acetic acid in an aqueous media was extracted using three distinct biochars that were derived from melon seeds, hazelnut shells, and orange peel. To obtain the activated biochar, phosphoric acid or zinc chloride were used [201]. These biochars can therefore be used to reduce a variety of odor-causing compounds that may be present in wastewater (such as ammonium ions, dimethyl sulfide, and volatile organic compounds), which strengthens their synergistic role in reducing odorants in wastewater and addressing problems related to the disposal of biowaste.

5.4.2. Biochar Filters

Although there is not enough evidence to support their appropriateness for on-farm testing, current research suggests that biochar filters are a suitable option for removing microbes from wastewater using the hydraulic loading rate (HLR) intended for sand filters. Biochar filters are being investigated by Perez-Mercado and associates as an on-farm treatment to lower pathogens while irrigating with wastewater-polluted sources. Removal of 2 log bacteria and 1 log virus at 3x HLR was revealed by correlation analysis [202]. De Jesus and colleagues evaluated the effects of biochar (BC) combined with sugar cane filter cake (FC) on soil physical and hydraulic characteristics in humid tropical conditions. The combination of biochar and filter cake increases micropores, aggregate stability, and plant-available water content after 18 months [203]. In a lab-modified bio sand filtration system, Guan and co-workers examined the potential of biochar in E. coli elimination. The research findings suggest that the removal of E. coli and its adsorption are different, which could indicate the importance of biochar straining. Furthermore, the study examined the adsorption kinetics and mechanisms of biochar’s ability to remove Pb(II) from water using a fixed-bed continuous flow adsorption system. However, it is noted that the lack of fine biochar particles may have affected the results [204]. The adsorption data were subjected to the Thomas, Yoon–Nelson, and Adams–Bohart models [205]. By employing “Biologically Enhanced Biochar”, or BEB, to treat biological water, Jayakumar and colleagues assessed the extent, possible advantages (economic and environmental), and difficulties associated with this practice. In order to reduce the environmental effect of BEB filters, “sequential biochar systems” are presented as specifically created end-of-life approaches. Examples of their integration into biological water treatment that can meet BEBs’ zero-waste standards have been shown [206]. Pritchard and team have developed a unique method for producing synthetic stormwater that is reusable and contains materials from catch basins and dissolved organic carbon produced from straw. With larger biochar and regenerated activated carbon (RAC) content filters and lower ambient stormwater pollutant concentrations, performance in the field is probably going to be considerably better. As a result of hydrogen bonding and electrostatic interactions between the ions and the active sites on the biochar, DFT simulations show that chlorine (Cl) is strongly adsorbed to the surface of the material [207]. In order to sequester carbon and stop deicing salt pollutants from leaking into water bodies, Pahlavan and colleagues showed the possibility of supplementing soils with algal biochar [208].

5.4.3. Adsorption of Heavy Metals

For the purpose of eliminating heavy metals from wastewater, biochar has been found to be a viable and affordable adsorbent [209]. Factors like feedstock composition and modification technique affect biochar activity [210]. Although activated carbon is more effective in adsorbing heavy metals than biochar, surface alternations can maximize the potential of biochar [211]. These changes can increase biochar’s recyclability and adsorption capacity, making it a more viable and efficient method of removing heavy metals from wastewater. By employing a hybrid post-pyrolysis magnetization, a double-layer magnetized/functionalized biochar composite was developed, maintaining and even greatly enhancing the adsorption capability of microporous carbonaceous biochar (BC) [212]. By virtue of its high amino group chelation capacity toward metal ions, 3-aminopropyltriethoxysilane (TES) contributes to water purification while stabilizing the magnetic nanoparticles on the surface of biochar. Zn(II)- and Pb(II)-polluted water has been tested for treatment using raw jujube seeds (RJSs) treated with sulfuric acid and ultrasonic treatment, such as ultrasonically assisted jujube seed (UAJS)-based biochar [213]. Additionally, UAJS-based biochar was used to treat the industrial effluent from electroplating in order to eliminate metal ions including copper, nickel, chromium, and zinc. A MgAl-LDH rice husk biochar composite (MgAl-LDH@RHB) with a regular hydrotalcite structure was created using a straightforward hydrothermal process, and it was then utilized to extract Cd(II) and Cu(II) from water. The results show how MgAl-LDH@RHB removes heavy metals and offer a theoretical framework for managing water contamination and disposing of agricultural waste [214]. Metals and other pollutants can potentially be removed from aqueous solutions through the use of nanomaterials derived from various environmental wastes. The study examined the potential low-cost and environmentally friendly absorbents for the removal of heavy metals (HMs) from aqueous solutions, specifically the mono- and multi-adsorption systems of cadmium (Cd), chromium (Cr), and nickel (Ni) on biochar and nano biochar of water hyacinth (BWH and NBWH) and black tea waste (BTW and NBTW) [215]. Chen and colleagues investigated the possible use of chemically modified engineered biochar made from fruit waste to extract chromium (Cr) from an aqueous solution [216]. Positive relevance is presented by Chen and colleagues in the design and optimization of waste fruit peel-derived adsorption materials for water purification. In another work, activated alumina biochar composites (γ-Al₂O₃/BCs) were synthesized by Zhou and colleagues using the sol–gel method. This solved the issue that the γ-Al₂O₃ surface charge did not facilitate the removal of heavy metal cations in a neutral solution. The study also investigated the viability of utilizing γ-Al₂O₃/BC to remove Pb(II) and the recycling of Pb-laden waste sludge to remove phosphorus (P) and its micro-adsorption mechanisms [217].

5.4.4. Removal of Dye and Phosphorous Compounds

Adsorption is the most commonly used method of water treatment to remove color from water. The study of Srivastav and co-workers aimed to give a general review of the use of biochar as an affordable and environmentally acceptable adsorbent for the removal of harmful colorants (dyes) from aquatic environments. One kind of sustainable carbonaceous material is biochar, which is made by thermally treating organic waste streams, such as agricultural wastes, without the presence of oxygen [196]. Nobaharan and team examined the possible applications of biochar as well as the major determinants influencing wastewater P reduction [218]. According to research conducted by Barman and colleagues, the KOH-modified biochar made from Sterculia foetida shells that can absorb acenaphthene and naphthalene from water has been made simple and environmentally friendly [219]. They contend that endothermic and spontaneous adsorption occurred [219]. Song and team demonstrated the adsorption behaviors of microcystin-LR (MC-LR) on virgin biochars (BCs) made from different waste biomass of microalgae (Spirulina sp.) and Kentucky bluegrass (KB) (Poa pratensis L.). During two hours of pyrolysis at 350, 550, and 750 °C, BCs made from waste biomass showed various physicochemical properties. Cherry kernels, a plentiful biomass waste source, were used to create a sulfur-functionalized microporous biochar that was used to selectively remove Pb(II), a typical heavy metal, from landfill leachate [220]. Using sulfur-doped microporous biochar, Pap and co-workers studied the remediation of landfill leachate using a circular economy approach. A Freundlich model proved the most effective in explaining the adsorption process [221]. The adsorption of norfloxacin from an aqueous solution on biochar made from spent coffee grounds was studied by Nguyen and associates. The adsorption process was improved using master variables and the response surface approach. Norfloxacin (NOR) was absorbed by biochar made from spent coffee grounds (SCGB) in water. Since biochar made from coffee grounds uses solid waste, reduces costs, and produces adsorbents to cope with new contaminants like antibiotics, it seems like a viable and environmentally friendly alternative [222]. Zhang and colleagues used pyrolyzing PMS waste containing ferric salt to extract anionic P from water in order to create paper mill sludge (PMS) biochar (PMSB) in a single step. By using a one-step pyrolysis technique to transform waste PMS into metal/metal oxide-embedded biochar with good P removal capacity and straightforward magnetic separation features, they were able to implement a waste-to-wealth approach [223]. Wang et al. employ nitric-acid washing biochar (BCN) and acetone washing biochar (BCA) made from bagasse to extract tetracycline (TC) and sulfamethoxazole (SMX) from water. Bagasse biochar, which is made from agro-waste, encourages waste recycling and has the capacity to remove antimicrobial pollutants from multi-interference situations. This leads to a harmonious coexistence of materials and the natural environment [224]. The sludge biochar’s function in persulfate activation is yet unknown, which restricts the amount of organic contaminants it may remove from water bodies. A two-step process of pyrolytic carbonization (400–800 °C) and subsequent KOH activation was utilized to create activated sludge charcoal (ASC) from metal-rich petrochemical sludge (abbreviated as ASC 400–800) [225].

Engineered biochar has demonstrated increased adsorption capability. Biochar has also been utilized as an adsorbent to extract nutrients, organic contaminants, and hazardous metals from wastewater [226]. Biochar has been used in biofiltration in on-site wastewater treatment systems to eliminate pathogens, nutrients, and pharmaceutical and personal care items. The efficacy of biochar is influenced by its manufacturing methods, source, and surface modification [227]. The combined findings of these studies demonstrate the potential of biochar in several wastewater treatment applications, including biofiltration and pollutant removal.

5.5. Storage Devices and Supercapacitors

The unique qualities of biochar, namely its high porosity and surface area, make it suitable as a medium for thermal energy storage. Carbon black has many applications in thermal energy storage, which involves storing heat energy for future use. When phase-change materials (PCMs) are embedded or coated with biochar, their ability to absorb and release heat energy can be enhanced by the porous structure of the biochar. Heat loss during storage can be reduced by using biochar as an insulator. Electrochemical energy storage systems with high power and energy density are essential for intermittent renewable energy storage and significantly reduce reliance on fossil fuels. In this setting, research on biomass—a cheap, plentiful, and renewable source of biochar—has increased significantly to address environmental concerns and advance the creation of sustainable energy storage applications. This section discusses recent developments in the conversion and effective use of biomass and its biochar as electrode materials for energy storage devices, such as supercapacitors and batteries (Li-ion, Na-ion, Li–S, and metal–air) [228]. While biochar is widely considered a versatile material, converting it into high-performance carbon compounds for environmental and energy-related applications is still difficult. A planned carbonization/activation process was utilized to create hierarchically porous carbon compounds from Torreya grandis inner shell, a common lignocellulosic biomass waste [229]. As the most crucial component of electrochemical energy storage (EES) devices made from carbon obtained from fruit, Ehsani and colleagues studied the electrochemical energy storage electrode (EESE). Supercapacitors, batteries, and hybrid devices—all with a variety of conventional and cutting-edge applications—are among the EES devices. Compared to other carbon compounds, biochar materials have garnered particular interest since they are effective, affordable, and active materials for energy storage [230]. Gao and co-workers provided a potentially useful synthesis for using biochar in the realm of energy storage [229]. Lang and colleagues investigated the selection and assessment of hydrochars and biochars made from agricultural waste for application as materials for energy storage and adsorbents. Rice husks, cocoa pod husks, and banana rachis were all used to make biochars and hydrochars at 600 °C for two hours while a nitrogen flow was applied. Osman and colleagues examined the application of biochar-based carbon sinks in agronomy, animal farming, anaerobic digestion, composting, environmental remediation, building, and energy storage. Finally, creating biochar-based materials for energy storage applications necessitates further functionalization. Pyrolysis is a mature and promising technique for converting biomass-derived polymers to functional biochar materials, which may be used in various applications, including carbon sequestration, power generation, environmental remediation, and energy storage [231]. Yang and associates investigated the supercapacitor’s energy storage methods in order to provide readers with a general understanding of the char material made of biological polymers used in electrochemical energy storage. Mishra and team highlighted the use of biochar-based materials in various energy storage and conversion sectors. These sectors include different kinds of conversion technologies, biochar formation mechanisms, methods for modifying the surface chemistry and functionality of biochar, catalysts, biochar application in energy storage devices like nanotubes and supercapacitors, bio-based composite materials, and inorganic-based composite materials (Figure 9) [232].

5.5.1. Thermal Energy Storage

Biochar may efficiently absorb and retain heat due to its high porosity and surface area. That makes it appropriate for uses where storing and releasing heat is necessary. Phase-change materials (PCMs), which collect and release latent heat during solid-to-liquid or liquid-to-solid phase transitions, can be combined with biochar. Das and associates have worked on a new form-stable bio-composite material based on PCMs for solar thermal energy storage applications. They have created and tested a new, inexpensive latent thermal energy storage material called biochar–PCM hybrid. They also proposed the ideal ratio of PCM and biochar for the hybrid thermal energy storage material [233]. Using a one-step hydrothermal process, Antifau and co-workers created hybrid materials based on multi-walled carbon nanotubes (CNTs) and widely accessible, ecologically friendly biochar made from bamboo. For PCM encapsulation and energy storage capacity in the composite PCMs, the hybrid material offers advantageous morphological and linked framework structures [234]. To investigate “green” composite PCMs for effective thermal energy storage applications, Antifao and co-workers have used commercially available biochar materials that are generated from two distinct biomaterials, wheat straw and softwood, at pyrolysis temperatures of 550 °C and 700 °C, as well as organic PCM, n-eicosane (ES) [235]. Rajan and co-workers have examined how well a cold storage system works when combined with inexpensive phase-change materials (PCMs) based on surface-functionalized biochar nanoparticles for space cooling. They also conducted some experimental research to improve the thermal performance of a refrigeration system that uses a subcooled cold storage unit. The cold storage room with PCM is shown to have annual energy savings of 691.1 kWh and an energy saving potential of 9% compared to a system without a thermal storage unit [236]. According to Liu and colleagues, the energy storage capacity of biochar-based ss-BCPCMs largely depends on the pore structure and surface activity of the biochar. The resulting composite has shown excellent form stability, leakage resistance, and high latent heat storage up to 100.2 J g, indicating its high thermal storage stability, which is advantageous for thermal energy storage applications [237]. In order to achieve encapsulation and improve the thermal conductivity of phase-change materials (PCMs), biochar for phase-change energy storage fields was prepared using water hyacinth (WH) [238]. The energy storage efficiency of LMPA/LWB900 was 99.1%, and its enthalpy was 105.16 J/g, which was 25.79% more than that of LMPA/VWB900. To improve the state of the global energy scene, environmentally friendly and renewable energy infrastructure architecture is greatly desired. Antamoi and team have investigated the energy storage and critical factors influencing the thermal stability of octadecane during phase transition assembly using biochar produced from animal dung. Using octadecane and biochar generated from biowaste by vacuum impregnation, a green phase transition composite was created [239].

5.5.2. Biochar-Based Batteries

The efficacy of lithium–sulfur (Li-S) and lithium-ion (Li-ion) batteries has been enhanced by biochar. Salimi, Gu, and colleagues demonstrated the potential of biochar in Li-S batteries [240]. A further expansion of biochar application in Li-ion batteries was performed by Salimi, Gu, and co-workers using biochar derived from Cladophora glomerata and metal-free tannery waste [240,241]. This research emphasizes the potential of biochar-based batteries in providing long-term and cost-effective energy storage options. A three-dimensional (3D) self-standing NiFeP@NC/BC (BC = biochar) electrode is developed by deposition of Prussian blue analogs (PBAs) on biomass, followed by one-step phosphating using red phosphorous as the phosphorous source [242]. The rational design of this 3D self-standing cathode integrates the advantages of an efficient electron transport network of biochar and sufficiently accessible reaction sites of transition metal phosphide (TMP) catalysts. A first-of-its-kind Mn-Zn ferrite/biochar composite (MZF-BC) was manufactured by a green two-step bioleaching and hydrothermal approach, employing waste batteries and pine sawdust. According to the Langmuir sorption isotherm, at 298 K, Pb²⁺ has a maximum adsorption capacity of 99.5 mg g⁻¹, and the adsorption process is accurately described by a pseudo-second-order model [243]. Kalinke and colleagues discussed the use of biochar, a low-cost substance derived from renewable resources, for the creation of electrochemical devices. They have given some of the major biochar-based devices used for various techniques, such as sensors and biosensors, supercapacitors, fuel cells, and batteries [244]. Sawdust-based biochar designed for lithium-ion battery (LIB) anodes can efficiently maximize sawdust value. TiO₂/C-880 composites are made by one-step carbonization of TiO₂ nanoparticles (NPs) and sawdust at 880 °C previously dissolved in 1-butyl-3-methylimidazolium ([Bmim]H₂PO₄)/dimethyl sulfoxide (DMSO) [245].

5.5.3. Biochar-Based Supercapacitors

Many studies have explored the potential of biochar as a supercapacitor material, with promising results. The authors of [246] established the superior performance of porous biochar obtained from apricot shells, whereas [247] emphasized the benefits of N self-doped biochar in boosting electrochemical performance [248]. They increased specific capacitance and energy density by embedding polypyrrole into magnetic biochar and offered a detailed study of the elements impacting biochar’s electrochemical performance, such as surface area, pore structure, and conductivity [249]. The production of biochar-based composites from biomass and the promise of generating carbon-based electrodes has sparked widespread interest in the electrochemistry world. Norouzi and team compare their electrochemical properties in energy storage devices. Despite growing interest in utilizing biochar to make supercapacitors for eco-friendly energy modules, most biochar-based supercapacitor manufacturing procedures are neither flexible nor simple [250]. Kim and associates used pen lithography to create an all-solid-state flexible MnO₂/HNO₃ pretreatment biochar (NBC)/Poly(3,4-ethylenedioxythiophene) (PEDOT) composite microsupercapacitor (MSC) [251]. The electrochemical performances of carbons derived from three different types of fungus waste (Flammulina velutipes, Pleurotus ostreatus, and Shiitake mushroom stipe) were investigated to gain a better understanding of the electrochemical properties of various fungus-derived carbon materials as electrodes for supercapacitors [252]. Kalinke and associates discussed the use of biochar in the creation of electrochemical devices. Many of the most common biochar-based technologies, including fuel cells, batteries, supercapacitors, sensors, and biosensors, were demonstrated in the study. Two cutting-edge electrodes were successfully manufactured to construct symmetric and asymmetric supercapacitors. The asymmetric supercapacitor apparatus maintained 100.9% of its original capacitance even after 4000 cycles at a current density of 4 A g [244]. Geng and colleagues presented an easily scalable and straightforward method for producing in situ N, O-dually doped biochar for ultra-high-power supercapacitors using waste algal biomass [253]. Research has indicated that biochar has the potential to be used in supercapacitor electrodes, as evidenced by studies showing that the specific capacitance can reach up to 200 Fg⁻¹ [254,255]. Additional investigation has delved into augmenting the electrochemical characteristics of biochar by surface functionalization and graphitization, culminating in a noteworthy rise in capacitance [256]. These results demonstrated biochar’s potential as an effective and sustainable energy storage medium. Though the idea of utilizing biochar for energy storage is intriguing, it is important to remember that research in this field is still in its early stages, and large-scale practical applications could necessitate more refinement and development. However, due to its distinct qualities, biochar is a promising contender for environmentally friendly energy storage systems.

6. Future Prospects

Soil infertility poses a widespread challenge in numerous global regions, a hurdle that could be surmounted through the future application of biochar. Biochar holds promise in enhancing soil water retention, which is particularly vital for fostering robust vegetation in arid climates. Moreover, its synergistic utilization with compost offers added advantages, as biochar can adsorb nutrients from compost, gradually releasing them to plants, potentially diminishing reliance on chemical fertilizers. Anticipating forthcoming developments, various agro-industries may pioneer commercial biochar-based products for farmer accessibility. Biochar’s adsorptive capabilities also present opportunities beyond agriculture, potentially revolutionizing water purification technologies by removing microorganisms and suspended and dissolved solids. Its cost-effectiveness and efficacy compared to conventional carbon filters could prompt widespread adoption. Moreover, biochar’s selective adsorption properties could be helpful in dye industries. Furthermore, biochar holds substantial promise in climate change mitigation, particularly in carbon capture, storage, and sequestration efforts. Its versatility extends to pharmaceuticals, where it could effectively detoxify contaminants. Envisioning the future, biochar emerges as a prospective commodity across diverse sectors, including food, agriculture, fertilizers, and pharmaceuticals.

7. Conclusions

A comprehensive analysis of recent advancements in biochar production techniques and their diverse applications has been conducted. Biochar’s utility spans across various sectors, serving as an adsorbent, nutrient source, and soil enhancement agent capable of mitigating plant diseases. The characteristics and applications of biochar are heavily influenced by production methods and the type of organic material used as feedstock. Furthermore, the efficacy of biochar in improving soil quality and promoting plant growth is subject to variations in factors such as pyrolysis temperature, feedstock composition, and biochar aging. Assessing biochar applications’ field performance and economic viability is crucial for determining their potential benefits, presenting a significant challenge that necessitates further research to provide certainty.