Advancing Wastewater Treatment: A Comparative Study of Photocatalysis, Sonophotolysis, and Sonophotocatalysis for Organics Removal

Abstract

Clear and sanitarily adequate water scarcity is one of the greatest problems of modern society. Continuous population growth, rising organics concentrations, and common non-efficient wastewater treatment technologies add to the seriousness of this issue. The employment of various advanced oxidation processes (AOPs) in water treatment is becoming more widespread. In this review, the state-of-the-art application of three AOPs is discussed in detail: photocatalysis, sonophotolysis, and sonophotocatalysis. Photocatalysis utilizes semiconductor photocatalysts to degrade organic pollutants under light irradiation. Sonophotolysis combines ultrasound and photolysis to generate reactive radicals, enhancing the degradation of organic pollutants. Sonophotocatalysis synergistically combines ultrasound with photocatalysis, resulting in improved degradation efficiency compared to individual processes. By studying this paper, readers will get an insight into the latest published data regarding the above-mentioned processes from the last 10 years. Different factors are compared and discussed, such as degradation efficiency, reaction kinetics, catalyst type, ultrasound frequency, or water matrix effects on process performance. In addition, the economic aspects of sonophotolysis, photocatalysis, and sonophotocatalysis will be also analyzed and compared to other processes. Also, the future research directions and potential applications of these AOPs in wastewater treatment will be highlighted. This review offers invaluable insights into the selection and optimization of AOPs.

1. Introduction

Environmental pollution refers to the introduction of various compounds (gas, liquid, or solid) or forms of energy into the environment at a rate that exceeds the environment’s capacity to eliminate them. Due to environmental pollution, a wide spectrum of harmful effects can be observed, such as health issues or biodiversity loss [1]. All natural resources (soil, water, and air) are exposed to the dreadful effects of environmental pollution; however, water pollution and pure water scarcity are globally the most threatening ones. Water pollution is a severe problem both in urban and rural areas. Interaction with contaminated water can result in various infections and parasitic illnesses, and it can multiply the number of industrial effluents, heavy metals, or algal toxins [2]. Based on a UN report from 2023, 26% of the global population is facing pure water scarcity, while 46% of the population has a lack of access to adequately managed sanitation. It is expected that by 2050, 2.4 billion people will be facing water scarcity. In addition, extreme and prolonged droughts are also stressing ecosystems, with severe consequences for the living world [3].

Water pollution is mainly the result of various human activities and facilities (e.g., farming, industry, healthcare). The level of organic pollution, for instance in rivers, results from two opposing processes: pollutant input and natural purification. Urban and agricultural wastewater discharges are the primary sources of organic pollutants, expected to rise with urbanization and intensified farming. While pollution originates at discharge points, its effects propagate downstream, affecting both communities and ecosystems as pollutants travel through river networks. The severity of downstream impacts hinges on the river’s ability to self-purify through dilution from runoff and degradation by microorganisms [4,5,6]. The present persistent organics are also known as emerging organic pollutants (EOPs). Several sources of EOP emissions have been identified; however, effluent discharges from wastewater treatment plants are the primary contributors. These substances are consistently detected in wastewater samples, both pre- and post-treatment, typically in a concentration range from ng/L to µg/L. The composition and concentration of EOPs in raw wastewater is largely influenced by the socioeconomic demographics of the contributing population. Conversely, the concentration of EOPs in treated effluents depends on the initial pollution level of incoming waters and the efficiency of the purification process [7,8]. Furthermore, the possible bioaccumulation and biomagnification of the mentioned EOPs makes them extremely hazardous to the environment [9]. Namely, as one ascends the food chain, concentrations of EOPs typically increase. Consequently, species occupying the upper levels of the food chain, such as fish, predatory birds, mammals, and humans, exhibit the highest levels of these chemicals, placing them at a heightened risk of experiencing acute and chronic adverse effects [10]. Thus, the appearance of EOPs results in significant health issues for human beings, fauna, flora, and the marine ecosystem [9]. EOPs encompass a wide array of compounds from various chemical classes, while the most common ones include pharmaceuticals (e.g., antibiotics, hormones), personal care products (ingredients found in cosmetics, soaps, fragrances, etc.), pesticides and their metabolites, plasticizers, and organic colors [9,11,12].

Taking all the negative impacts into account, it is obvious that EOPs must be removed from the environment, and their discharge to the environment has to be discontinued. There are various conventional water treatments available to ensure safe drinking water and a pure aquatic environment for the global community [13]. In response to the adverse impacts of polluted water, numerous nations globally have implemented increasingly stringent regulations and restrictions on effluent emissions. The approach to water treatment varies among countries, with each employing unique combinations of processes based on regulatory constraints, the utilization of the best available technology, and the characteristics of the treated water [14]. In general, there are three types of usual water treatment lines: (i) simple physical treatment and disinfection; (ii) normal treatment, chemical and disinfection; and (iii) physical treatment, advanced chemistry, refining and disinfection [15]. The simple disinfection techniques include ultraviolet (UV) irradiation, chemical methods that include the usage of chlorine, chloramines, and unscented bleach, then distillation and ozonation, along with boiling, in times of emergency. Shock chlorination stands out as the prevalent sanitizing treatment for wells [15]. The simple physical treatment targets high-quality surface waters with minimal turbidity fluctuations and applies to all groundwater, particularly karstic waters with low turbidity. While these waters share similarities with the previous category, their suspended matter concentrations necessitate prior physical treatment. Coagulation occurs before filtration (coagulation in the filter). Typically, closed-type filters are utilized, due to their compact dimensions and ability to facilitate high filtration rates. Presently, an effective substitute approach is the utilization of ultrafiltration membranes, accompanied by disinfection [15]. If the normal treatment, chemical and disinfection, is being observed, this treatment targets surface waters abundant in suspended particles and organic substances, surpassing the standards for drinking water due to the presence of micropollutants. Additionally, it is suitable for waters with elevated levels of iron and manganese. The pre-oxidation stage serves to oxidize ammoniacal nitrogen (when chlorine is the oxidant), dissolved iron, and manganese, leading to the formation of precipitates. It also aids in algae removal and safeguards settling tanks. Pre-oxidation plays a vital role in enhancing the coagulation–flocculation process [15]. In addition, the physical treatment, advanced chemistry, and refining and disinfection techniques are thoroughly developed and applicable to surface waters, rich in organic matter, dyes, and micropollutants. However, the above-mentioned EOPs cannot be effectively removed by these techniques [15].

Thus, in the quest for sustainable solutions to environmental challenges, advanced oxidation processes (AOPs) have emerged as powerful tools in the purification of water, air, and soil. AOPs represent a cutting-edge approach to (waste)water treatment and pollution remediation, harnessing the potent reactivity of reactive oxygen species, such as hydroxyl radicals (HO^•), to degrade and transform a wide array of organic and inorganic contaminants [16]. Over the years, various AOPs have been introduced and studied for degrading numerous EOPs found in (waste)water. These processes involve cavitation induced by ultrasonic irradiation [17,18,19,20], photocatalytic oxidation [21,22,23,24,25,26], and Fenton chemistry that relies on the reaction of iron(II) sulfate with hydrogen peroxide (H₂O₂), i.e., Fenton’s reagent [27,28]. Additionally, the synergy of AOPs leads to the development of hybrid methods, including ultrasound-assisted Fenton [29], sonophotocatalysis [30,31], photo-Fenton [32,33], and ozone/H₂O₂ [34,35,36], to achieve enhanced oxidation efficiency. These hybrid approaches are designed to overcome the limitations and challenges associated with individual AOPs, particularly in targeting specific pollutants [37]. Previous research by Elkacmi and Bennajah presented a literature review on AOPs applications for detoxifying olive mill wastewater, covering chemical, photochemical, sonochemical, and electrochemical technologies, along with their combination with biological and other techniques. It also addressed the influence of operating parameters on these processes [38]. Additionally, the study by Babu Ponnusami et al. focused on integrating AOPs and biological processes in wastewater treatment, evaluating their advancements, feasibility, and practicality, and recommending their combined use to achieve treatment goals, while considering economic and operational factors [39]. Furthermore, Yu et al. demonstrated recent advancements in using encapsulated transition-metal nanoparticle catalysts for AOPs targeting organic pollutant degradation. The authors discussed the structure, composition, synthesis methods, applications in various AOPs, and highlighted major challenges and opportunities associated with these catalysts, providing insights for future research and development [40].

This review aims to give readers an overview about the state-of-the-art application and efficiency of photocatalysis, sonophotolysis, and their combination, i.e., sonophotocatalysis, in (waste)water treatment for organic pollutants removal. By exploring these innovative methods, it provides valuable insights for researchers and professionals dedicated to purifying water contaminated with various organic pollutants.

2. Advanced Oxidation Processes

In the Introduction, it was mentioned that AOPs are advanced and efficient techniques, which are being used, for instance, in organic pollutants removal. AOPs are new green chemical technologies that are promising and sustainable alternatives to conventional techniques. AOPs are based on the formation of highly reactive oxygen species like HO^•, hydroperoxyl radicals (${\mathrm{HO}}_2^{•}$), sulfate radicals (${\mathrm{SO}}_4^{•–}$), and superoxide radical anions (${\mathrm{O}}_2^{•–}$) that degrade hazardous pollutants by ISO 9001:2015 standards by utilizing renewable and clean solar energy [41]. These techniques minimize the formation of sludge and other hazardous byproducts, reducing disposal costs and environmental impact. The economic feasibility of AOPs presents challenges due to high capital and operational costs, while their effectiveness and potential for integration into sustainable treatment systems make them a valuable tool for advanced wastewater management. By optimizing costs, improving energy efficiency, and leveraging policy support, AOPs can play a crucial role in achieving sustainable water treatment and protecting environmental and public health.

AOPs are typically categorized according to the radical formation method, including chemical, photochemical, radiation-induced, cavitation, and electrochemical processes [42]. The classification of conventional and advanced AOPs techniques is presented in Figure 1.

This review is focusing on AOPs techniques where the reactive species are generated by cavitation and photochemical routes, as well as on their synergistic combination. The input data (article title, abstract, keywords) were extracted from academic papers indexed in Scopus with the search terms “photocatalysis/sonophotolysis/sonophotocatalysis” + “advanced oxidation processes” + “water” + “organics OR organic pollutant *”. The search was set to include only articles/reviews papers (excluding book chapters and proceedings). The number of publications about photocatalysis, sonophotolysis, and sonophotocatalysis is continuously increasing. In the last 10 years (the time period was set from 2014 to 2024), 688 research studies were published in the case of photocatalysis, 3 studies regarding the application of sonophotolysis, and 8 articles about the employment of sonophotocatalysis in the degradation of organic pollutants in (waste)water. The annual distribution of these articles is shown in Figure 2.

2.1. Heterogeneous Photocatalysis

Photocatalysis, another type of AOPs, is the combination of photochemistry and catalysis. Photocatalysis hinges on the necessity of both light and a catalyst to drive or expedite a chemical conversion. Essentially, it can be characterized as the “enhancement of a photoreaction in the presence of a catalyst”. This definition encompasses photosensitization, wherein a chemical species undergoes photochemical modification due to the initial absorption of radiation by another chemical species referred to as the photosensitizer [43]. Photocatalysis can be divided into two types: homogeneous and heterogeneous. However, due to easier separation of catalyst particles from the reaction mixture, heterogeneous photocatalysis is more popular and practical. Thus, the further discussion in this review will cover this photocatalysis type [44].

The very first application of heterogeneous photocatalysis is dated to 1972, when Fujishima and Honda conducted photoelectrochemical water splitting using natural radiation [45]. Following this, in the late 1970s, intensive research in the field of heterogeneous photocatalysis commenced based on Fujishima and Honda’s study, but in the absence of an electrochemical cell [46]. One of the numerous advantages of heterogeneous photocatalysis is the possibility of the complete mineralization of present pollutants in a gaseous or liquid phase into safe compounds such as water, carbon dioxide, and inorganic ions [47].

Heterogeneous photocatalysis revolves around photoreactions taking place specifically at the catalyst’s surface. If the adsorbate undergoes photoexcitation initially and then interacts with the catalyst in its ground state, this process is termed a “sensitized photoreaction”. Conversely, if the catalyst is photoexcited first, and then interacts with the adsorbate molecule in its ground state, it is labeled as a “catalyzed photoreaction”. In most instances, when discussing heterogeneous photocatalysis, we are referring to semiconductor photocatalysis or semiconductor-sensitized photoreactions [43].

In an ideal photocatalytic process, organic pollutants undergo mineralization, transforming into carbon(IV) oxide (CO₂), water (H₂O), and mineral acids in the presence of catalysts and reactive oxidizing species. The initiation of photocatalytic reactions occurs as the catalyst particles absorb photons with energies higher than their bandgap energy, under illumination. Consequently, photo-induced electrons are transformed from the valence band (VB) to the conduction band (CB), generating positively charged holes (${\mathrm{h}}_{\mathrm{VB}}^{+}$) and free electrons (${\mathrm{e}}_{\mathrm{CB}}^{–}$) on the surface of the applied catalyst (Figure 3). It is crucial to mention that the photo-generated holes in the VB tend to recombine with the photo-excited electrons in the CB, dissipating energy as heat. Hence, the presence of oxygen as electron scavengers extends the recombination of ${\mathrm{e}}_{\mathrm{CB}}^{–}$–${\mathrm{h}}_{\mathrm{VB}}^{+}$ pairs, while producing ${\mathrm{O}}_2^{⦁–}$. The interaction of ${\mathrm{O}}_2^{⦁–}$ with OH⁻ may result in the generation of $\mathrm{HO}⦁$. Notably, the $\mathrm{HO}⦁$ radicals serve as an immensely potent, non-selective oxidant, facilitating the partial or complete mineralization of organic compounds. Additionally, the substantial oxidative potential of the ${\mathrm{h}}_{\mathrm{VB}}^{+}$ in the photocatalyst enables the direct oxidation of organic substances to reactive intermediaries [43]. The mentioned processes can be described as follows (Equations (1)–(6)):

$$\mathrm{Catalyst} + \mathit{hv}\to {\mathrm{h}}_{\mathrm{VB}}^{+}+{\mathrm{e}}_{\mathrm{CB}}^{–}$$

(1)

$${\mathrm{h}}_{\mathrm{VB}}^{+}+{\mathrm{e}}_{\mathrm{CB}}^{–}\to \mathrm{heat}$$

(2)

$${\mathrm{e}}_{\mathrm{CB}}^{–}+{\mathrm{O}}_2\to {\mathrm{O}}_2^{⦁–}$$

(3)

$${\mathrm{h}}_{\mathrm{VB}}^{+}+\mathrm{OH}–\to \mathrm{HO}⦁$$

(4)

$$\mathrm{HO}⦁+\mathrm{R}-\mathrm{H} \to \mathrm{R}⦁+{\mathrm{H}}_2\mathrm{O}$$

(5)

$${\mathrm{h}}_{\mathrm{VB}}^{+}+\mathrm{R} \to \mathrm{R}+\to \mathrm{Intermediates}$$

(6)

Heterogeneous photocatalysis with different photoactive materials can be efficiently employed in the removal of selected emerging organics, such as pharmaceutically active ingredients [22,23,24,25,49,50], pesticides [51,52,53,54,55], dyes [56,57,58], etc., from aquatic environments using simulated solar irradiation (SSI) or UV irradiation. Heterogeneous photocatalysis, while offering several advantages in environmental remediation and pollutant degradation, also comes with certain disadvantages: limited light absorption [59], catalyst recycling and separation challenges [60], surface recombination of charge carriers [61], potential toxicity of byproducts [62], and catalyst stability and durability [63]. Thus, it is necessary to improve the above-mentioned drawbacks.

2.2. Sonophotolysis

Since the early 20th century, ultrasound (US) has garnered significant attention for its ability to enhance chemical and physical effects in various processes. In the 1930s, it was observed that sonication lead to the breakdown of polymers [64]. Basically, ultrasound can be divided into three groups by intensity: low (20–100 kHz); medium (200–1000 kHz); and high (5000–10,000 kHz). In general, high-intensity US waves that come into contact with gases dissolved in liquid medium encourage acoustic cavitation, i.e., the formation, expansion, and implosive collapse of bubbles. US waves have cycles of compression and expansion. During expansion, bubbles can be generated if the waves have an intensity that can exceed the molecular forces of the liquid. These bubbles continually absorb energy from the alternating ultrasonic cycles of compression and expansion. Thus, bubbles grow (by diffusion of vapor or gas from the liquid medium) until they attain a critical size, at which point they collapse. This implosion creates a localized “hot spot” with extreme conditions of temperature (~5000 K), pressure (~1000 atm), and short life. Hence, water molecules and gases are being broken [64]. Additionally, the process can be further improved by introducing O₃, which thermally degrades within the microbubbles [42]. The mentioned processes can be described using the following Equations (7)–(14):

$${\mathrm{H}}_2\mathrm{O}+\mathrm{ultrasound} \to {\mathrm{H}}^{⦁}+\mathrm{HO}⦁$$

(7)

$${\mathrm{O}}_2+\mathrm{ultrasound} \to 2 \mathrm{O}⦁$$

(8)

$${\mathrm{H}}_2\mathrm{O}+\mathrm{O}⦁\to 2 \mathrm{HO}⦁$$

(9)

$${\mathrm{O}}_2+\mathrm{H}⦁\to \mathrm{O}⦁+\mathrm{HO}⦁$$

(10)

$$\mathrm{Organic} \mathrm{compounds}+\mathrm{HO}⦁\to \mathrm{products}$$

(11)

$${\mathrm{O}}_3+{\mathrm{H}}_2\mathrm{O} \to {\mathrm{O}}_2+2 \mathrm{HO}⦁$$

(12)

$${\mathrm{O}}_3+\mathrm{HO}⦁+\mathrm{e}–\to {\mathrm{HO}}_2^{–}+{\mathrm{O}}_2$$

(13)

$${\mathrm{O}}_3+{\mathrm{HO}}_2^{–}\to \mathrm{HO}⦁+{\mathrm{O}}_2^{•–}+{\mathrm{O}}_2$$

(14)

There are various factors affecting the efficiency of sonochemical reactions: (a) ultrasound frequency—influences the cavitation process by altering the size of the bubbles and collapse time of the cavity [65]; (b) dissolved gas—the characteristics of dissolved gases, such as thermal conductivity or water solubility, have an impact on sonochemical activity (e.g., implosion temperature, heat transfer, etc.) [66]; (c) power input—an increase in ultrasonic power leads to higher levels of sonochemical activity in the liquid medium; regardless of the type of substance (volatile, hydrophobic, or hydrophilic), the response to ultrasonic power remains consistent [64]; (d) effect of bulk temperature—the highest temperature reached during bubble collapse is contingent upon the bulk temperature of the liquid [64]; and (e) pollutant concentration—since the pollutant concentration is dependent on the water source, it is an important factor. This influence is dependent on the type of organic pollutants (volatile or nonvolatile, hydrophilic or hydrophobic) [64].

The combination of ultrasound and light irradiation—sonophotolysis—is recognized as a highly effective reagent-free method for inactivating pathogenic microorganisms present in aqueous media, as well as for removing EOPs. While UV treatment has become a prevalent technology for water and wastewater disinfection, its efficacy can be significantly impacted by the quality of water and various constraints (light scattering and absorbance, cell shading and reactivation effects). In contrast, ultrasound disinfection is less affected by such factors. Therefore, ultrasonication can be regarded as a substitute standalone method, or be used in conjunction with UV radiation to enhance the overall treatment performance [67]. Moreover, the combination of these two techniques presents a more economically attractive option for wastewater treatment compared to using either technique individually. To reduce operational costs associated with ultrasound and UV treatment, researchers have explored combining these techniques with other AOPs, such as sono-hybrid and photo-hybrid processes [68]. The processes during sonophotolysis are represented in Figure 4. However, the number of studies is limited, i.e., there were three research articles published about organic pollutant removal using sonophotolysis in the last 10 years.

2.3. Sonophotocatalysis

The combination of sonolysis with photocatalysis facilitates a notably improved degradation of emerging pollutants. The heightened efficacy of sonophotocatalytic processes in eliminating toxic contaminants primarily stems from the synergistic interaction between photocatalysis and sonolysis. The fusion of these methodologies offers several advantages, including the enhanced formation of cavitation bubbles through ultrasound waves in the presence of solid catalysts, along with an increased production of ultrasonic cavitation bubbles and free radicals via the separation of ${\mathrm{e}}_{\mathrm{CB}}^{–}$–${\mathrm{h}}_{\mathrm{VB}}^{+}$ pairs in the semiconductor photocatalyst. Additionally, ultrasound waves consistently cleanse the surface of the photocatalyst, thereby boosting and sustaining its performance over an extended period (Figure 5). Moreover, the integration of both techniques facilitates the degradation of both hydrophobic and hydrophilic organic pollutants [69].

Apart from the irradiation sources, namely, ultrasound and light, the overall effectiveness of sonophotocatalysis is heavily reliant on the characteristics of the materials involved, known as sonophotocatalysts, and their proficiency in generating an adequate amount of reactive oxygen species. Hence, suitable sonophotocatalysts must possess the capability to responsively interact with both ultrasound irradiation and light, while preserving favorable chemical- and photo-stability, commendable electronic properties, and efficiency, along with minimal toxicity and cost-effectiveness. Semiconductor materials are frequently favored due to their pivotal role in reducing the energy barrier for the generation of cavitation bubbles in the process of sonocatalysis, alongside their ability to harvest light, facilitating the photogeneration of ${\mathrm{e}}_{\mathrm{CB}}^{–}$–${\mathrm{h}}_{\mathrm{VB}}^{+}$ pairs during the photocatalytic process [71].

Even though there is a limited number of articles about the employment of sonophotocatalysis in the removal of organic pollutants, the published studies showed promising results. Namely, various organics were effectively removed from the environment, for instance, methylene blue [72], phenol [17], phthalocyanine pigment [73], and 17β-estradiol [74].

The aim of this review is to give readers a comprehensive revision guide about the up-to-date achievements of photocatalysis, sonophotolysis, and sonophotocatalysis, with the most recent published studies.

3. State-of-the-Art Application of AOPs

The following provides the latest research data on photocatalysis, sonophotolysis, and the synergistic effects observed in the combined process known as sonophotocatalysis. A summary of the reviewed studies is represented in

Table 1. Summary of efficiency of sonophotolysis, photocatalysis, and sonophotocatalysis.

Organic Pollutant	Type of AOPs	Removal Efficiency (%)	Reference
	Photocatalysis
Alprazolam	Heterogeneous photocatalysis with ZnO and TiO2 P25, under UV irradiation	100	[75]
Phenol	Heterogeneous photocatalysis with ZnO under UV irradiation	55	[17]
Mesotrione	Heterogeneous photocatalysis with ZnO and TiO2 P25 under UV	100	[76]
Alprazolam	Heterogeneous photocatalysis zinc–tin oxide nanocrystalline powders (both ternary and coupled binary) under UV irradiation	90–100 depending on catalyst type	[77]
Amitriptyline	Heterogeneous photocatalysis with ZnO/SnO2, ZnO, and TiO2 P25 under UV irradiation	80	[78]
Direct blue 71	Heterogeneous photocatalysis with Cu-doped ZnO under visible light	96	[79]
Alprazolam	Heterogeneous photocatalysis with ZnO and TiO2 Degussa P25 under UVA, visible, and solar irradiation	100 (UV/ZnO)	[80]
Cefotaxime	Heterogeneous photocatalysis with ZnO under SSI	90.6	[81]
Atrazine	Heterogeneous photocatalysis with mixed oxide catalysts (ZnO/TiO2) using various ZnO concentrations, under UV irradiation	97.5 (5% ZnO/TiO2) 94 (10% ZnO/TiO2)	[82]
Mesotrione	Heterogeneous photocatalysis with ZnO and TiO2 under UV irradiation	40 (ground water and ZnO) 50 (river water and ZnO)	[83]
Acid red 18	Heterogeneous photocatalysis with ZnO-Ag-Nd under UV irradiation	100	[84]
Ibuprofen	Heterogeneous photocatalysis with TiO2 and ZnO under UV irradiation	99	[85]
Two fungicides (vinclozoline and fenarimol) and four insecticides (malathion, fenotrothion, quinalphos, and dimethoate)	Heterogeneous photocatalysis with ZnO in pilot plant under artificial light	92 (DOC)	[86]
Methylene blue	Heterogeneous photocatalysis with reduced graphene oxide-N-ZnO, under visible irradiation	98.5	[87]
Selected organic compounds (tetrachlorethylene; chlorodifluoroacetamide; amphetamine; dibutyl phthalate; (2,3-dihydroxypropyl Z)-9-octadecenoate; fluoxetine; furazan; phenylpropanolamine; phthalic acid; 1,2-benzisothiazol-3-amine; 2,3-dihydroxypropyl elaidate)	Heterogeneous photocatalysis with ZnO/TiO2 and TiO2 catalysts, under SSI	85.5	[88]
Wastewater with various organics	Heterogeneous photocatalysis with ZnO immobilized on polystyrene pellets under UV irradiation	61.7 (TOC)	[89]
Two antibiotics (ciprofloxacin and ceftriaxone) and two herbicides (tembotrione and fluroxypyr).	Heterogeneous photocatalysis with TiO2, ZnO, and MgO under UV and SSI	90–100 (depending on pollutant, under UV) 70–100 (depending on pollutant, under SSI)	[90]
Amitriptyline hydrochloride	Heterogeneous photocatalysis under UV and SSI using ZnO, TiO2 P25, and TiO2 Hombikat	30 (TOC)	[50]
4-bromophenol and diethyl phthalate	Heterogeneous photocatalysis with nanocomposites composed of reduced graphene oxide and ZnO under UV irradiation	99 (4-bromophenol) 98.6 (diethyl phthalate)	[91]
Phenol	Heterogeneous photocatalysis with CdO/ZnO/Yb2O3 under SSI	71.5 97.8 (with H2O2)	[92]
p-nitrophenol	Heterogeneous photocatalysis under UV irradiation using ZnO	80	[93]
Penicillin G	Heterogeneous photocatalysis with graphitic carbon nitride–calcium or magnesium co-doped cobalt ferrite–zinc oxide nanocomposite under natural sunlight	74	[94]
Various organics	Heterogeneous photocatalysis with ZnO nanoparticles under natural sunlight	17.1 (COD) 71.7 (BOD)	[95]
Clomazone, amitriptyline, and sulcotrione	Heterogeneous photocatalysis with ZnO under SSI	20–70% (depending on the pollutant)	[96]
Phenol, o-cresol, toluene, and xylene	Heterogeneous photocatalysis with green ZnO under SSI	51 52 88 93	[97]
Methylene blue dye	Heterogeneous photocatalysis with Ag/ZnO-ZnS/PANI under UV irradiation	95	[31]
Methylene orange and nitrophenol	Heterogeneous photocatalysis with chromium-doped ZnO under UV irradiation	99 98.2	[98]
Phenol	Heterogeneous photocatalysis with ZnO under UV and solar irradiation	>25	[99]
Rhodamine B	Heterogeneous photocatalysis with ZnO under natural sunlight and UV irradiation	91 (sunlight) 99 (UV)	[100]
Docosane	Heterogeneous photocatalysis with ZnO nanorods under natural sunlight	68.5	[101]
Clomazone, ciprofloxacin, and 17α-ethynilestradiol	Heterogeneous photocatalysis with ZnO/MeOx nanopowders (ZnO/MgO, ZnO/CeO2, and ZnO/ZrO2) under SSI	77 86 71	[25]
Acid blue 25	Heterogeneous photocatalysis with TiO2-ZnO/coal fly ash under UV irradiation	98	[102]
	Sonophotolysis/sonocatalysis
Ibuprofen Sulfamethoxazole	Sonolysis	97 92	[103]
Ciprofloxacin	Sonocatalysis	61	[104]
	Sonophotocatalysis
Acid red 17	Sonophotocatalysis with Pt/CeO2	90	[105]
Amoxicilin	Sonophotocatalysis with N-doped TiO2	37	[19]
Flonicamid	Sonophotocatalysis with CuO, ZnO, and TiO2	98.36 (COD, for TiO2)	[106]
Tetracycline	Sonophotocatalysis with TiO2 decorated on magnetic activated carbon	93	[107]

, while the main strengths and weaknesses of each individual process are given in Figure 6.

3.1. Photocatalysis

Heterogeneous photocatalysis is a highly researched topic among scientists. By the application of an adequate semiconductor as a photocatalyst, promising results can be achieved in the removal of various organics from the environment, such as drugs, pesticides, or dyes. In this section, the latest results will be highlighted.

For instance, Ivetić et al. [75] synthesized Mg-doped ZnO nanocrystallites via a conventional solid-state reaction and used them in the degradation of the anxiolytic drug alprazolam, under UV irradiation. The impact of annealing temperature on the structural and optical properties of the resulting nanomaterial was examined with X-ray powder diffraction (XRD), scanning electron microscope (SEM), mercury intrusion porosimetry (MIP), Raman spectroscopy (Raman), and ultraviolet–visible diffuse reflectance spectroscopy (UV–Vis DRS). Moreover, the effectiveness of Mg-doped ZnO in the photocatalytic degradation of alprazolam was compared with the efficiency of pure ZnO and TiO₂ Degussa P25 photocatalysts. The obtained results showed a complete removal efficiency of alprazolam, both in the presence of the newly synthesized Mg-doped ZnO, annealed at 700 °C, and in the presence of commercially available ZnO and TiO₂ Degussa P25, after 60 min of UV irradiation.

Jyothi et al. [17] examined the effectiveness of various AOPs, including ZnO-mediated sono-/photocatalysis, for removing phenol from water at ambient conditions. Their findings indicated that phenol degradation followed the sequence sonophotocatalysis > photocatalysis > sonocatalysis > sonolysis > photolysis. Specifically, the employment of photocatalysis resulted in a 55% removal of phenol, under UV irradiation.

In the research carried out by Šojić et al. [76], the photocatalytic degradation of the herbicide mesotrione was investigated in aqueous suspensions of TiO₂ Degussa P25 and ZnO, considering various operational parameters, such as the type of catalyst and light source, catalyst loading, and initial pH. The findings clearly demonstrated that photocatalytic treatment utilizing TiO₂ Degussa P25 and ZnO with UV irradiation effectively removed mesotrione from water. Namely, complete removal was accomplished after 15 min of UV irradiation, while the mineralization of mesotrione was completed after about 4 h. Applying the liquid chromatography–electrospray ionization–tandem mass spectrometry (LC–ESI–MS/MS) method, several intermediates were identified, suggesting that the degradation pathways with TiO₂ Degussa P25 and ZnO may not be identical.

Additionally, Ivetić et al. [77] introduced the preparation and characterization of zinc–tin oxide nanocrystalline powders, both ternary and coupled binary types, via a straightforward solid-state mechanochemical method. The structural and optical properties of the resulting powder samples were investigated using XRD, SEM, Raman, and reflectance spectroscopy. The efficacy of the synthesized nanomaterials in the photocatalytic degradation of alprazolam under UV irradiation was assessed and compared with pure ZnO and SnO₂. The photocatalytic investigation revealed that mixed binary zinc–tin oxide catalysts displayed superior photocatalytic activity compared to their ternary counterparts.

Furthermore, Ivetić et al. [78] conducted a study involving the preparation of a ZnO/SnO₂ heterojunction through a straightforward three-step mechanochemical solid-state method. The phase composition, crystalline structure, morphology, and bandgap of the resulting coupled catalyst were extensively examined. The efficiency of these prepared materials was evaluated in the photocatalytic removal of amitriptyline under SSI and UV irradiation. The findings of this investigation demonstrate that ZnO/SnO₂ nanoparticles possess superior photocatalytic activity in comparison with both ZnO and the widely used TiO₂ Degussa P25 catalyst. Namely, 100% of amitriptyline was removed after 60 min of UV irradiation, while a slightly lower degradation efficiency (80%) was reached in the case of SSI.

In the study conducted by Thennarasu and Sivasamy [79], the detailed synthesis and characterization of Cu-doped ZnO were described. The synthesis involved the co-precipitation method, followed by a thorough characterization using various techniques, including XRD, field-emission scanning electron microscope (FE-SEM), transmission electron microscope (TEM), Fourier transform infrared spectroscopy (FT-IR), and UV–Vis DRS. The photocatalytic performance of the Cu-doped ZnO catalyst was evaluated in the degradation of direct blue 71 dye in an aqueous environment under visible light. Several factors, such as the pH of the aqueous suspension, duration of irradiation, quantity of photocatalyst used, and initial dye concentration, influenced the extent of degradation. The optimal pH for achieving maximum photocatalytic degradation was determined to be 6.8. Furthermore, it was found that 3 mg/mL of Cu-doped ZnO was the ideal amount for complete degradation of the investigated dye at a concentration of 0.01 g/L. The photocatalyst reusability was assessed, revealing that up to 96% of direct blue 71 dye was degraded even after three cycles of usage, employing visible light. Additionally, chemical oxygen demand (COD) and electrospray ionization–mass spectrometry analyses confirmed the complete mineralization of direct blue 71 dye molecules.

Finčur et al. [80] conducted a comprehensive investigation into the photocatalytic degradation of alprazolam, using heterogeneous photocatalysis, with ZnO and TiO₂ Degussa P25 as photocatalysts. The study explored various factors, including the type of irradiation (UVA, visible light, solar), photocatalyst type, photocatalyst loading, pH value, and the presence of HO^• radical and ${\mathrm{h}}_{\mathrm{V}\mathrm{B}}^{+}$ scavengers. The findings clearly demonstrate that the UV/ZnO system was more efficient in alprazolam removal, since a complete removal efficiency was found after 10 min of UV irradiation. A detailed analysis of degradation intermediates using LC–ESI–MS/MS suggested that the degradation pathways with ZnO and TiO₂ Degussa P25 might differ. Additionally, reutilization experiments with ZnO showed that the photocatalyst stayed effective after three runs.

León et al. [81] presented a study of the photocatalytic removal of the antibiotic cefotaxime from aqueous solutions utilizing SSI in the presence of ZnO. Photocatalytic experiments on cefotaxime (20.0 mg/L) were conducted in a sunlight simulator equipped with a xenon lamp. A central composite circumscribed experimental design was employed to evaluate the impact of the catalyst loading and initial pH value on cefotaxime removal efficiency. The acquired results established that the optimized conditions that led to the highest pollutant removal (90.6%) after 8 min of phototreatment were pH = 7.5, ZnO loading = 1.45 g/L, under SSI. In the scavenger experiments, it was concluded that both HO^• radicals and h⁺ participate in cefotaxime removal using ZnO. Lastly, a dissolved organic carbon (DOC) analysis confirmed that it was possible to reduce the organic matter present in the systems by photocatalytic treatment.

Silva et al. [82] carried out a study to assess the efficacy of mixed oxide catalysts (ZnO/TiO₂) with varying ZnO concentrations (5, 8, 10, and 15 wt. %) in the photocatalysis of atrazine in aqueous solutions under UV irradiation. The catalysts underwent preparation via wet impregnation and were subjected to characterization via parameters such as specific surface area (Brunauer–Emmett–Teller (BET) method), pore diameter, pore volume, XRD, thermogravimetric analysis (TGA), and SEM. Photocatalytic experiments were carried out in a batch-type reactor, and the resulting products were scrutinized using high-performance liquid chromatography (HPLC) and UV–Vis spectrophotometry at λ_max = 221 nm to track the atrazine’s behavior throughout the process. The findings unveiled that the catalyst labeled as 5% ZnO/TiO₂ and 10% ZnO/TiO₂ exhibited the highest photocatalytic performance, achieving a complete degradation of atrazine and partial mineralization of the byproducts. Specifically, after 10 min of the process, the percentages of removal were recorded as 97.5 and 94% for the 5% ZnO/TiO₂ and 10% ZnO/TiO₂ catalysts, respectively.

The study of Šojić Merkulov et al. [83] sought to investigate how ground and river water matrices influence the photocatalytic removal of mesotrione, the active ingredient of Callisto^®. The results showed that under UV irradiation, ZnO exhibited a higher efficiency in degrading mesotrione compared to TiO₂, in both ground and river water, whilst a comparable degradation efficiency was observed for both photocatalysts in distilled deionized water. The ZnO/SSI system reduced the concentration of mesotrione by approximately 40% and 50% in the ground and river water, respectively. Furthermore, ground and river water as the matrix significantly diminished the removal rate (approximately by 1.5 and 4 times for ZnO and TiO₂, respectively) during mesotrione photocatalytic oxidation under UV irradiation.

The well-known role of heterogeneous photocatalysis in solving water pollution issues was put to use by Azadi et al. [84] to remove the organic dye acid red 18. The process was carried out for 180 min in a semi-batch photoreactor using UV light, applying a zinc oxide–silver–neodymium (ZnO-Ag-Nd) nanocomposite, synthesized via the combustion method, and later characterized using XRD, FT-IR spectroscopy, and SEM. The degradation kinetics were evaluated by altering several experimental parameters (photocatalyst loading, initial pH value, and substrate concentration). The treatment process efficiency was determined by spectrophotometric analysis, following the decolorization of the solution at λ_max = 507 nm. After the optimization of the treatment conditions (catalyst loading = 0.08 mg/mL, pH value = 6.2, and low concentration of dye solution), total organic carbon (TOC) measurements were performed to assess the mineralization degree, i.e., the degradation of the dye after heterogeneous photocatalysis. The obtained results showed that the degradation degree was always below the degree of decolorization. Namely, after 180 min of treatment in the presence of ZnO-Ag-Nd under UV irradiation, around 65% of organic matter was transformed into inorganic, while the decolorization of the acid red 18 dye was almost 100%.

Tanveer et al. [85] studied the degradation of ibuprofen under UV and solar irradiation using two photocatalysts, TiO₂ and ZnO. Various catalyst loadings, ranging from 0.1 to 2.0 mg/mL, were tested for both types of catalysts, and the effect of pH value on degradation rate was examined across acidic, neutral, and basic pH. In addition to a degradation rate analysis, TOC and COD analyses were conducted under optimized conditions including catalyst loading, pH, and radiation source. An optimal ZnO loading of 1.0 mg/mL was identified for ibuprofen degradation in an aqueous environment. In addition, ZnO-based experiments demonstrated higher degradation rates under neutral conditions (pH 7.0). Furthermore, the degradation performance and TOC, COD removal rates were higher under UV irradiation compared to solar setups, whether utilizing quartz or borosilicate assemblies. Overall, UV-based catalysis proved to be a more effective option for addressing pharmaceutical pollutants compared to solar photocatalysis. Namely, 99% of ibuprofen was removed after 15 min of UV irradiation with TiO₂, while the overall mineralization, based on TOC, after the same time was 32%.

In the work of Vela et al. [86], the photocatalytic removal of two fungicides (fenarimol and vinclozoline) and four insecticides (dimethoate fenotrothion, malathion, and quinalphos) under natural sunlight at a pilot plant scale was presented. The approach integrated HO^• radical- and sulfate radical-based AOPs, utilizing ZnO as a photocatalyst and Na₂S₂O₈ as an oxidant, respectively. Preliminary experiments involved optimizing catalyst loading, electron acceptor effects, and pH in a laboratory photoreactor under artificial light. Subsequently, the study included 200 mg/L of ZnO and 250 mg/L of Na₂S₂O₈ at a pH of about 7 for further experiments at pilot plant scale. The findings indicated a significant enhancement in the reaction rate of the targeted pesticides when utilizing the tandem ZnO/Na₂S₂O₈ system compared to photolytic testing. At the end of the treatment, the remaining DOC percentage was substantially below its initial amount, with an up to 92% reduction.

The study conducted by Suresh and Sivasamy [87] focuses on a novel nanocomposite photocatalyst, reduced graphene oxide–N-ZnO, which exhibits visible light activity. This photocatalyst was prepared using the hydrothermal technique, and characterization of the synthesized nanocomposite was carried out using various techniques including XRD, FT-IR, UV–Vis DRS, FE-SEM, atomic force microscopic (AFM), X-ray photoelectron spectroscopy (XPS), and BET analyses. The degradation of methylene blue under visible light supported the estimation of the reduced graphene oxide–N-ZnO nanocomposite’s photocatalytic performance. The nanocomposite exhibited significantly enhanced degradation, achieving the upper limit of methylene blue degradation (98.5%) after 120 min of irradiation with visible light.

Ángel-Hernández et al. [88] employed a heterogeneous photocatalytic process in order to reduce the initial concentrations of various organics present in wastewater treatment plant (WWTP) effluent using ZnO/TiO₂ and TiO₂ catalysts, synthesized via the sol–gel method. Newly synthesized materials were characterized by XRD and SEM techniques. A total of eleven organic compounds (tetrachlorethylene; chlorodifluoroacetamide; amphetamine; dibutyl phthalate; (2,3-dihydroxypropyl Z)-9-octadecenoate; fluoxetine; furazan; phenylpropanolamine; phthalic acid; 1,2-benzisothiazol-3-amine; 2,3-dihydroxypropyl elaidate), detected via gas chromatography–mass spectrometry (GC–MS) analysis of the water sample, were subjected to photocatalytic removal using both artificial UV and natural solar light. Namely, a batch reactor was equipped with a UV lamp, whereas the solar reactor was utilizing natural sunlight. A solar collector with ZnO/TiO₂ proved to be the best option for several reasons. Firstly, the amount of total suspended solids removed after heterogeneous photocatalytic process was 85.5%. Secondly, the production price of the ZnO/TiO₂ catalyst was rather low. And finally, this treatment combination is simple, low-cost, and environmentally friendly, given that it demands electrical energy only for the stirring of the reaction mixture.

Caracciolo et al. [89] studied the photocatalytic treatment of four different water samples coming from the WWTP of leather industries, using a commercially available ZnO immobilized on polystyrene pellets (ZnO/PS). Photocatalytic degradation experiments were carried out in a cylindrical Pyrex photoreactor surrounded with four UV lamps placed at the same distance from the external photoreactor surface. The reaction progress was examined with regard to the residual COD and TOC. After 7 h of UV irradiation in the presence of ZnO/PS with 0.5 mg/mL photocatalyst loading, the average removal of pollutants present in four different wastewater samples amounted to 63.4% COD and 61.7% TOC.

Finčur et al. [90] reported the synthesis of TiO₂, ZnO, and MgO nanoparticles using the sol–gel method, followed by an investigation into their structural and morphological properties through XRD, FTIR, UV–Vis DRS, BET, and SEM/EDX techniques. A comparative analysis was conducted on the photocatalytic performance of these nanopowders in degrading two antibiotics (ceftriaxone and ciprofloxacin) and two herbicides (fluroxypyr and tembotrione). TiO₂ emerged as the most effective nanopowder under UV and SSI. Specifically, the introduction of (NH₄)₂S₂O₈ in low concentrations was found to accelerate the removal of both antibiotics and the herbicide fluroxypyr in the presence of TiO₂. LC–ESI–MS analysis identified several degradation intermediates of ciprofloxacin, tembotrione, and fluroxypyr, suggesting that HO^• radicals primarily drive the transformations of selected pollutants.

Work carried out by Finčur et al. [50] was focused on the investigation of the kinetics, mineralization, and toxicological effects of the antidepressant drug amitriptyline hydrochloride in UV- or solar-illuminated aqueous suspensions of ZnO, TiO₂ Degussa P25 and TiO₂ Hombikat. The findings suggest that ZnO effectively catalyzed the photodegradation of amitriptyline in the presence of oxygen and under solar light. The degradation rate was influenced by various parameters, including catalyst loading, the initial substrate concentration, and the presence or absence of electron acceptors such as molecular oxygen, HO^• radicals, and h⁺ scavengers. The optimal loading of the photocatalyst was experimentally determined to be 1.0 mg/mL. Additionally, the degradation rate increased with higher initial concentrations of amitriptyline. The presence of H₂O₂ and (NH₄)₂S₂O₈ as electron acceptors initially decreased the reaction rate, while KBrO₃ slightly increased the degradation rate of amitriptyline. Moreover, ethanol inhibited the photodegradation of amitriptyline more than NaI, indicating that the reaction mechanism primarily relies on free HO^• radicals, with partial involvement of ${\mathrm{h}}_{\mathrm{V}\mathrm{B}}^{+}$. Generally, a ~30% level of TOC decrease was found after 240 min of SSI.

The positive qualities of photocatalysis were also harnessed by Kumar et al. [91]. In their study of 4-bromophenol and diethyl phthalate removal from an aqueous environment, nanocomposites composed of reduced graphene oxide and ZnO (rGZ), synthesized with a high-temperature refluxing method, were used as photocatalysts. Techniques including XRD, FE-SEM, UV–Vis DRS, Raman, and FT-IR were used to study the properties of the nanocomposites. Degradation studies of the aqueous solutions of selected pollutants (10 ppm) were performed for 180 min under UV light, while an HPLC analysis was employed to follow 4-bromophenol and diethyl phthalate photocatalytic degradation. Several rGZ nanocomposites were prepared with an altered wt. % ratio of graphene oxide and ZnO, and among them, the superior photocatalytic efficiency in the degradation of both pollutants was displayed by rGZ nanocomposite with 5% of ZnO (rGZ-5). Subsequent photocatalytic degradation experiments were performed using different loadings of rGZ-5, and in the light of the obtained results, it was established that 0.8 mg/mL was the optimal loading. This setting brought forth the elimination of 99 and 98.6% of 4-bromophenol and diethyl phthalate, respectively.

The research of Li et al. [92] was focused on the sustainable heterogeneous photocatalytic treatment of phenol, which is a common organic persistent pollutant in industrial wastewaters. For this purpose the authors developed a cadmium oxide/zinc oxide/ytterbium(III) oxide (CdO/ZnO/Yb₂O₃) composite through a simple one-pot hydrophile synthesis pathway. The synthesized materials were then described in detail by UV–Vis DRS, FTIR, XRD, FE-SEM, TEM, energy-dispersive X-ray spectroscopy (EDS), and XPS techniques. The degradation of phenol (5.0 mg/L) was carried out at the natural pH value in a custom-made photoreactor under SSI. The degradation samples were analyzed at λ_max = 271 nm using UV–Vis absorption spectroscopy. After 15 min of photocatalytic treatment, the degradation of the phenol by the CdO/ZnO/Yb₂O₃ photocatalyst (0.67 mg/mL) reached 71.5%. Afterwards, the authors managed to improve the removal efficiency of phenol up to 97.8% by conducting photocatalytic experiments with the addition of H₂O₂ and isopropyl alcohol.

In the research conducted by Mahy et al. [93] the effectiveness of the UV-assisted heterogeneous photocatalysis of p-nitrophenol was explored in the presence of ZnO nanoparticles, synthesized by a sol–gel method and characterized by XRD, BET, TEM, UV–Vis DRS, and XPS techniques. The photocatalytic degradation process of the selected pollutant (0.1 mmol/L) was conducted in a quartz sealed round-bottom flask that was exposed to two/four UV lamps after the addition of 10 mg of each ZnO nanomaterial. The reduction in the p-nitrophenol concentration was measured by UV–Vis spectroscopy at λ_max = 317 nm. The influence of the ZnO nanoparticles’ distinctive morphology (nanotubes, nanorods, and nanospheres) on the photocatalytic activity was put to test. The following conclusion was drawn: with the increase in the photocatalyst particle size, its catalytic activity decreased. Namely, in the presence of Z19, which is the ZnO nanomaterial with the smallest particle size (8.3 nm), a 52% removal of p-nitrophenol was achieved under the illumination of two UV lamps. To enhance the photocatalytic degradation of p-nitrophenol in the presence of the same nanopowder, authors irradiated p-nitrophenol solution with four UV lamps and reached a removal efficacy equal to 80% after 7 h of the process. Z19 photocatalyst’s reusability was inspected in three consecutive runs, and it was observed that the material kept its integrity for 21 h.

In the research by Baladi et al. [94], penicillin G was subjected to heterogeneous photocatalytic removal. The experiments were performed in an aqueous environment utilizing natural sunlight at room temperature, and in the presence of a graphitic carbon nitride–calcium or magnesium co-doped cobalt ferrite–zinc oxide nanocomposite (gCN-Ca, Mg doped CFO-ZnO), prepared via the hydrothermal method and characterized by powder XRD, FT-IR, Raman, FE-SEM, TEM, EDS, DRS, vibrating sample magnetometer (VSM), and photoluminescence (PL) techniques. By applying a magnetic field, the magnetic composites were effortlessly separated from the reaction mixture. Photocatalytic experiments aimed to investigate several experimental parameters such as irradiation time, initial pH value, and initial substrate concentration, as well as the ZnO ratio in the nanocomposites. The removal of the antibiotic was monitored using UV–Vis spectrometry. As expected, prolonged irradiation time led to an increase in the penicillin G degradation degree in the presence of all studied nanocomposites. Among the three examined novel materials with different ZnO content, the one with 33.3% of ZnO displayed the highest degradation efficiency of penicillin G, regardless of the photocatalytic treatment length. Next, the degradation of penicillin G in the presence of gCN-Ca, Mg-doped CFO-ZnO (33.3%) nanocomposite was favored in the acidic environment and at a lower initial antibiotic concentration. Therefore, the highest penicillin G degradation efficiency of 74% was accomplished under the following optimal conditions: time of irradiation = 120 min, pH value = 5, penicillin G concentration = 10 ppm, and gCN–CFO–ZnO (33.3%) loading = 1.0 mg/mL.

Tamashiro et al. [95], in their research study, represented the synthesis of ZnO nanoparticles, using the precipitation of zinc sulfate heptahydrate and sodium hydroxide. The synthesized photocatalyst was applied in heterogeneous photocatalysis for diminishing COD and biochemical oxygen demand (BOD), as well as for antimicrobial purposes. Microscopic analysis indicated the presence of Saccharomyces cerevisiae microorganisms in vinasse, with the minimum inhibitory concentration for ZnO nanoparticles determined as 1.56 mg/mL. Under sunlight exposure for 4 h, photocatalysis with 40 mg/L of ZnO nanoparticles led to a 17.1% and 71.7% reduction in COD and BOD, respectively. This research underscores the feasibility of ZnO nanoparticles utilization in vinasse treatment, promoting sustainable practices, and mitigating the environmental repercussions of fertigation.

Despotović et al. [96] showed the synthesis of ZnO nanoparticles via the precipitation method, using water and ethanol solutions of zinc acetate dihydrate and zinc nitrate hexahydrate as the corresponding metal precursors. As-synthesized catalysts were afterwards calcined at various temperatures, and their structure and morphology were characterized using diverse techniques such as XRD, BET, and SEM. Also, the photocatalytic performance of the newly synthesized nanoparticles under SSI was evaluated for the removal of the antidepressant drug amitriptyline and the pesticide clomazone. The results showed that ZnO nanoparticles prepared from the water solution of zinc acetate dihydrate and calcined at 500 °C exhibited the highest efficiency under SSI. Additionally, the impact of initial pH was investigated. It was observed that the initial pH had no effect on the removal of clomazone, while for amitriptyline, the reaction rate constant slightly increased within a pH range from approximately 7 to 10.

Solar-assisted heterogeneous photocatalysis was put to use in the study by El Golli [97], with the intention of removing four aromatic (toluene, phenol, o-cresol, and xylene) hydrocarbons, frequently found in refinery effluent. ZnO nanoparticles synthesized via a conventional chemical route (chem-ZnO) and green ZnO nanoparticles (green-ZnO), prepared via a green synthesis route using Moringa oleifera leaves extract, were applied as photocatalysts. Materials were characterized by UV–Vis, XRD, FE-SEM, TEM, BET, and Barrett–Joyner–Halenda (BJH) analyses. The sets of experiments were conducted in a borosilicate photoreactor equipped with a xenon lamp placed in a solar box to simulate sunlight. Following the removal of target pollutants, an HPLC analysis was employed. Through the catalyst adsorption and catalyst loading experiments, both conducted at three identical levels, the optimal catalyst loading was determined to be 0.25 mg/mL. The impact of the initial pH value on the removal efficiency of the target pollutants was studied at four levels in the presence of the catalyst at the optimal loading. It was concluded that a neutral pH value increased the degradation degree of all the studied aromatic hydrocarbons. And finally, in comparison with the chem-ZnO nanoparticles, the green-ZnO nanoparticles exhibited greater degradation effectiveness in the heterogeneous photocatalysis of o-cresol, phenol, toluene, and xylene, with a respective 51, 52, 88, and 93% removal efficiency after 180 min of the process.

As a promising AOP, photocatalysis was employed by Khan et al. [31] for the degradation of methylene blue dye, which has the status of a usual persistent organic pollutant in the textile industry. Used photocatalysts, silver-doped zinc oxide–zinc sulfide–polyaniline composites (Ag/ZnO-ZnS/PANI), were made through a simple and straightforward coprecipitation method, followed by ultrasonic-assisted Ag deposition and subsequent in situ oxidative polymerization. Different techniques like FTIR, XRD, SEM, EDS, and XPS were engaged to describe the properties of the new materials. The photocatalytic activity of several Ag/ZnO-ZnS/PANI photocatalysts was investigated using a digital UV light chamber with six UV lamps, whilst methylene blue removal was monitored at λ_max = 664 nm by a double-beam UV spectrophotometer. To optimize the photocatalytic process, the influence of several parameters, such as pH value, catalyst loading, dye concentration, and irradiation time, was studied. In light of the findings, the optimal process parameters were as follows: pH value = 6, methylene blue dye concentration = 10 mg/L, photocatalyst loading = 0.05 mg/mL, and reaction time = 90 min. To avoid the electron/hole recombination process, the effect of H₂O₂ addition in different concentrations was examined, and the optimal oxidant dose proved to be 3.0 mmol/L. The authors also compared the efficiency of both immobilized and suspended photocatalysts, and the first one demonstrated superior efficiency, removing 95% of methylene blue after the process of photocatalysis. Additionally, a reusability study of the various Ag/ZnO-ZnS/PANI composites was conducted under optimal conditions, and the results showed that the photocatalytic activity was preserved even after five consecutive runs. And lastly, radical-scavenging tests in the presence of isopropyl alcohol, ascorbic acid, disodium ethylene diamine tetraacetate, and potassium dichromate revealed that HO^• radicals play the main role in the photocatalytic degradation of methylene blue.

The research conducted by Pavithra and Raj [98] explores the development of nanoporous chromium-doped ZnO as a sustainable, energy-efficient, and stable photocatalyst. The porous chromium-doped ZnO nanostructures were produced using an ultrasonic-assisted co-precipitation method, whereas techniques such as XRD, FT-IR, SEM, TEM, high resolution TEM, and BET were used for the characterization of the prepared nanostructures. Also, methylene orange and nitrophenol were removed up to 99% and 98.2%, respectively, with half-life degradation rate constants of 22 min and 26 min, under UV irradiation. An analysis of TOC in the treated water confirmed the successful degradation of xenobiotic molecules, indicating the potential for reusing the treated water.

Sarvothaman et al. [99], in their work, investigated the efficiency of different AOPs, a cavitation-mediated pre-treatment of ZnO, and cavitation photocatalysis peroxide-based hybrid processes. The pre-treatment method resulted in a >25% enhancement in phenol oxidation compared to conventional ZnO photocatalysis alone. Additionally, hydrodynamic cavitation photocatalysis with a peroxide-based system exhibited a cavitational yield five times greater than its acoustic cavitation counterpart. In the case of photocatalysis, the effect of radiation type (UV and sunlight) was investigated, and the results showed that the rate constant obtained for phenol conversion demonstrates an approximately five-fold increase when exposed to UV light compared to natural solar light.

Furthermore, a Thongam and Chaturvedi [100] study addressed the primary hurdles encountered in deploying photocatalytic water treatment. This was achieved by modifying the slurry system, removing the need for external triggering sources, and repurposing face mask fabric coated with ZnO to serve as a floating photocatalyst. The face mask fabric was dip-coated with ZnO photocatalysts (with the variations in synthesis medium, diethylene glycol, N,N-dimethylformamide, or water, and methods, precipitation or solvothermal), to serve as a floating agent for rhodamine B degradation under natural sunlight. The results revealed that distinct morphological structures resembling cauliflower, hydrangea, and petals can be obtained through variations in the synthesis medium and methods. These variations were found to be contingent upon the solvent properties. In addition, the obtained results showed that the degradation efficiency of rhodamine B was approximately 91% after 100 min of treatment, comparable to the 99% efficiency observed in the UV light-illuminated slurry system.

In the study by Alsharyani and Muruganandam [101], photocatalytic degradation of organic docosane solution was carried out in the presence of hexagonal-shaped ZnO nanorods, obtained via a microwave-assisted hydrothermal synthesis method and later grown on substrates made of glass. SEM, XRD, UV–Vis, and PL techniques were utilized to characterize novel material. The photocatalytic degradation of docosane (10 ppm) was performed in a solar simulator equipped with a xenon lamp, whilst the effectiveness was monitored by GC-MS/MS analysis. It was found that after 5 h of irradiation, 68.5% of the docosane was removed from the aqueous solution, and that the initial pH value gradually decreased towards the end of the process. The beneficial presence of ZnO nanorods was later confirmed with a TOC analysis, when 60.5% more docosane was removed by photocatalysis compared to photolysis.

The research of Bognár et al. [25] aimed to explore the effectiveness of newly synthesized binary coupled ZnO/MeO_x nanomaterials (ZnO/MgO, ZnO/CeO₂, and ZnO/ZrO₂) in the photocatalytic degradation of the pesticide clomazone and two pharmaceutically active compounds, 17α-ethynilestradiol and ciprofloxacin, under varying experimental conditions, such as catalyst loading and initial pH. Nanopowders were prepared by a three-step mechanochemical-assisted calcination method and characterized using XRD, SEM-EDS, TEM, DLS, Raman, UV–Vis DRS, and zeta potential analysis. Chemometrics helped in defining optimal photocatalytic experimental conditions. Under optimal experimental conditions, the presence of ZnO/ZrO₂ resulted in the highest degradation efficiency, with 71% of EE2, 77% of CLO, and 86% of CIP removed after 120 min of SSI.

Yusuff et al. [102] showed the synthesis of different TiO₂-ZnO/coal fly ash (CFA) photocatalysts (ZnO/CFA, TiO₂/CFA, and TiO₂-ZnO/CFA) using sol-gel and impregnation techniques, and their application for degrading acid blue 25 dye from an aquatic environment, in a photoreactor that provides UV light. In addition, the chemical structure, functional groups, texture, and optical properties of the binary and ternary composites were assessed using various spectroscopic techniques (XRF, XRD, BET, FTIR, UV–Vis DRS, TEM, and EDS). The results indicated that TiO₂-ZnO/CFA exhibited a faster degradation of acid blue 25 (98%) compared to ZnO/CFA and TiO₂/CFA, due to its dualistic photocatalytic sites. In addition to the evaluation of TiO₂-ZnO/CFA reusability, the optimal experimental conditions (initial acid blue 25 concentration, flow rate, UV light intensity, and time of irradiation) required for achieving maximum removal efficiency were investigated.

3.2. Sonophotolysis

The first process which will be discussed in this review is sonophotolysis. However, in order to obtain a more detailed picture about the efficacy of ultrasound, some data will be given on sonolysis, as well.

First of all, in the study by Gao et al. [108], the inactivation of Enterobacter aerogenes in skim milk using both low-frequency (20 kHz) and high-frequency (850 kHz) ultrasonication was examined. Low-frequency acoustic cavitation inflicted lethal damage to the target, which proved more vulnerable to ultrasound in water than in skim milk due to the varying protein concentrations. However, even at powers as high as 50 W for 60 min, high-frequency US failed to inactivate E. aerogenes in milk. Interestingly, while the high-frequency US treatment of milk induced no physical alterations, low-frequency ultrasonication reduced particle size and milk viscosity.

Another antibacterial activity of US was observed in the study by Gemici et al. [109], who delved into the synergistic effects of US and UV processes in inactivating Escherichia coli in a simulated aqueous environment. Their findings showcased the potential of a hybrid antibacterial approach integrating silver columns with US and UV, offering an effective means of water disinfection, while mitigating environmental risks and overcoming individual limitations associated with traditional disinfection methods.

The first-time successful coupling of photo-Fenton processes and US was witnessed in the study by Giannakis et al. [110], for Escherichia coli inactivation in secondary treated effluent. They employed sequential high-frequency/low-power sonication, followed by mild photo-Fenton treatment under SSI. The study aimed to assess the individual contributions of Fenton reaction, US, and irradiation towards removal rates and long-term bacterial survival. The results highlighted a significant enhancement in treatment efficiency, with the coupled process achieving total inactivation within a 4 h treatment period. Furthermore, the study revealed the short-term disinfecting benefits of US and its adverse effects on long-term bacterial survival, along with the influence of irradiation.

On the other hand, Yasir et al. [103] investigated the degradation of ibuprofen and sulfamethoxazole using ultrasonic irradiation (1000 kHz) with and without single-walled carbon nanotubes (SWCNTs). In the absence of SWCNTs, the efficiency of degradation was minimal. However, when ultrasound was combined with SWCNTs, the degradation efficiency significantly improved. Under SWNTs (adsorption) reactions, removal rates for ibuprofen and sulfamethoxazole were 57% and 48%, respectively. Under US reactions alone, these rates increased to 77% and 70%. Remarkably, under US/SWNTs reactions, removal rates surged to 97% and 92%, respectively. The ultrasonic irradiation-assisted dispersion of SWCNTs bolstered oxidation and adsorption activities, contributing to enhanced pollutant removal. The synthesized TiO₂ and TiO₂/montmorillonite samples served as catalysts for the sonocatalytic degradation of ciprofloxacin.

One more successful drug removal was recognized in the research conducted by Hassani et al. [104]. The degradation efficiency of ciprofloxacin via sonocatalysis was influenced by solution pH, catalyst loading, initial pollutant concentrations, and ultrasonic power. Notably, the TiO₂/montmorillonite nanocomposite exhibited a higher degradation efficiency (61%) compared to the pristine TiO₂ sample. This enhancement can be attributed to the decrease in TiO₂ nanoparticle size facilitated by the immobilization process and the electron acceptor role of montmorillonite within the TiO₂/montmorillonite structure. Consequently, the TiO₂/montmorillonite nanocomposite emerges as a promising catalyst for the sonocatalytic degradation of ciprofloxacin.

3.3. Sonophotocatalysis

Bearing in mind the efficiency of sonophotolysis and photocatalysis, in this part of this review, readers will be informed about the triumphant application of sonophotocatalysis.

For instance, Khan et al. [105] examined the degradation of acid red 17 dye using a Pt/CeO₂ sonophotocatalyst through a photocatalytic and sonophotocatalytic process, under visible light. The remarkable sonophotodegradation of acid red 17 in the presence of the Pt/CeO₂ photocatalyst was attributed to the synergy of superoxides and additional free HO^• radicals produced by ultrasonic waves. Additionally, the Pt/CeO₂ sonophotocatalyst exhibited outstanding recyclability.

Next, Karim and Shriwastav [19] investigated the effectiveness of photocatalytic, sonocatalytic, and sonophotocatalytic oxidation methods for degrading amoxicillin under visible light, employing N-doped TiO₂ nanoparticles as the catalyst and low-frequency ultrasound. In a novel multifrequency reactor and under optimal conditions, photocatalysis and sonocatalysis achieved degradation efficiencies of 27% and 31%, respectively. However, the combination of visible light and ultrasound in the presence of N-doped TiO₂ resulted in better amoxicillin degradation, attributed to the catalyst’s reduced bandgap, enhanced cavitation effect, sonoluminescence phenomenon, and improved pollutant mass transfer. Consequently, sonophotocatalysis exhibited a greater degree of degradation (37%). The degradation of amoxicillin across these three studied oxidation processes followed a pseudo-first-order kinetics model.

After that, combination approaches incorporating sonolysis and photolysis, in the presence of CuO, TiO₂, and ZnO catalysts, have been utilized to treat commercial flonicamid solutions in the research of Ayare and Gogate [106]. At optimal loading, an extent of COD reduction was reached with a combination of ultrasound and the mentioned catalysts, and the following values were determined: 62.07%, 73.73%, and 77.59% for the CuO, ZnO, and TiO₂ catalysts, respectively. The sonocatalytic treatment process efficiency surpassed the efficiency of sonolysis, with a COD removal rate of 77.59% observed for TiO₂ catalysts compared to 39.23% for sonolysis operated individually. A similar trend was observed when comparing photocatalysis with photolysis. Sonophotocatalytic oxidation, with a maximum COD reduction of 98.36% in the case of TiO₂, was identified as the most effective process in comparison with sonocatalysis and photocatalysis.

Moreover, a combined system comprising photocatalysis and sonocatalysis was utilized in the degradation of tetracycline antibiotic, employing TiO₂ decorated on magnetic activated carbon (MAC@T) in conjunction with UV and US irradiations. This hybrid system, MAC@T/UV/US, integrating adsorption, photocatalytic, and sonocatalytic processes, proved effective in efficiently degrading tetracycline. MAC@T demonstrated strong catalytic activity when combined with both US and UV irradiation. Under optimal conditions, during a 180 min treatment, 93% of tetracycline was removed, whilst a 50.4% of TOC removal efficiency was reached. In summary, the synergy of MAC@T composite and US/UV for improved catalytic removal efficiency represents a prosperous and promising technique, which enjoys excellent catalytic activity, easy recovery, high adsorption capacity, and notable durability and photocatalyst recyclability [107].

4. Cost Estimation and Constraints of Different AOPs

The comprehensive cost analysis of a treatment technology encompasses its capital, operational, and maintenance expenses. In the realm of industrial-scale systems, operational costs stem from diverse factors, including initial investments, equipment and installation costs, amortization, raw materials procurement, energy consumption, parts replacements, labor, and more [111,112]. For example, in environmental remediation practices, expenditure is typically benchmarked against a fixed reference level of pollutant removal, often measured by metrics such as chemical oxygen demand or total organic carbon [111]. Notably, AOPs have emerged as highly effective techniques for degrading a broad spectrum of organic compounds, often ensuring their thorough mineralization. In this section, the cost-effectiveness of heterogeneous photocatalysis, sonocatalysis, and sonophotocatalysis will be compared mutually and also to other AOPs, including UV photolysis, UV/H₂O₂ treatment, Fenton reaction, ozonation, and sonolysis.

Certainly, it is imperative to weigh both the effectiveness and limitations of AOPs, particularly regarding the operational pH range. Notably, heterogeneous systems exhibit efficacy across a wider pH spectrum, encompassing the typical pH range found in natural waters and wastewaters (pH 2–9). This stands in contrast to treatments like Fenton and ozonation, which require specific acidic or alkaline conditions. From an industrial and practical standpoint, this broad operational pH range is a crucial prerequisite [111].

Although photocatalysis, especially with TiO₂, is recognized as a relatively expensive AOP, it remains highly effective in removing pollutants [113]. In recent decades, photocatalysis has garnered significant attention for several compelling reasons. These include its cost-effectiveness regarding its remarkable capacity for photocatalyst reuse and recyclability, the generation of highly active radicals, minimal sludge production, and rapid mineralization rates [114]. However, limitations such as equipment costs, maintenance requirements, and scalability challenges need to be addressed for widespread implementation. In general, various factors contribute to the overall costs of sono-/photocatalytic processes. For instance, the reusability of catalysts across multiple cycles can significantly reduce material consumption. Moreover, harnessing natural sunlight irradiation instead of energy-intensive lamps (UV photolysis, UV/H₂O₂ treatment) holds promise for cost reduction in terms of electricity expense. However, when UV radiation is integrated into other AOPs, removal efficiencies usually increase, though this comes with higher operational costs.

Furthermore, while sonochemistry effectively degrades numerous organic compounds, it often achieves low levels of mineralization (<10%) due to the hydrophilic and nonvolatile nature of the generated products, hindering their complete elimination. Additionally, the operation of ultrasound entails high electrical energy consumption, leading to elevated economic costs. Consequently, there is a pressing need for short treatment times. However, these challenges can largely be addressed by integrating ultrasound with other AOPs or biological systems [64].

Additionally, the efficacy of sono-/photocatalytic processes may vary depending on factors like pollutant type, concentration, and environmental conditions, necessitating the careful consideration of application-specific factors. Ahmad et al. [115] investigated the ZnO-decorated multiwall carbon nanotubes-assisted degradation of rhodamine B using photocatalysis, sonocatalysis, and sonophotocatalysis. Sonophotocatalytic processes exhibited relatively shorter reaction times and higher reaction rate constants compared to individual processes.

Moreover, several studies have conducted analyses on the electrical energy consumption of photocatalytic and sonocatalytic degradation systems. Moradi et al. [116] determined the electrical energy consumption for selected previous investigations and provided a summary of fundamental parameters and data, taking account of the consumption of energy translated into cost values. It is noteworthy that, across all cases, the electrical energy output of sonophotocatalytic systems is lower than the combined costs of photocatalytic and sonocatalytic systems. This observation confirms the efficacy and cost-effectiveness of the integrated system. The decrease in electrical energy output is due to the synergistic interaction between the two radical-producing processes, which enhances degradation events and shortens the time needed for the reaction.

5. Conclusions and Future Perspectives

As the global population constantly grows and the necessity for pure and sanitarily adequate water increases, various innovative techniques are being applied to remove organic pollutants from the aqueous environment.

This review aimed to give readers an insight into up-to-date data about the application of AOPs, with special emphasize on sonophotolysis, photocatalysis, and their synergistic approach, sonophotocatalysis. These techniques are very popular and highly investigated, which is proved by the number of publications in the last 10 years. The application of sonophotolysis is a relatively new approach for organic pollutant removal. Even though this AOP can be efficiently applied for organic pollutant removal, there are some drawbacks which should be addressed in the future, such as energy consumption, complexity of operation, cavitation-induced damage, and environmental (noise pollution) and safety (thermal runaway due to the freed-up heat) concerns. Putting photocatalysis under the magnifying glass, it can be concluded that promising results can be achieved. According to the reviewed data, it can be assumed that a wide spectrum of organic pollutants can be efficiently removed under various experimental conditions. Consider, for instance, that 100% of mesotrione and 95% of atrazine pesticides can be degraded under UV and employing ZnO and TiO₂ as photocatalysts. Promising results are achieved in the field of pharmaceutical removal, as well. Namely, 80–100% of antibiotics and/or different endocrine disruptors can be successfully degraded depending on the operational conditions. According to the published data, a milestone has been achieved in the degradation of organic dyes too. To be exact, 90–100% of several dyes (e.g., methylene blue, methylene orange, acid blue 25, acid red 17, etc.) can be successfully removed by choosing the optimal photocatalytic conditions (catalyst type, irradiation source). Meanwhile, although the employment of freely available sunlight energy makes this process eco-friendly, the utilization of various semiconductors can also be harmful to the environment. Namely, the traditional synthesis of different nanomaterials that are being used as photocatalysts is increasing our footprint in the environment and making this process less favorable. Furthermore, the activity of the newly synthesized materials is often low under sunlight irradiation. Hence, in the future, advanced and green, possibly plant-based, catalysts should be developed with high photocatalytic activity under natural conditions. In addition, heterogeneous photocatalytic process optimization and revolutionized reactor designs with scale-up and commercialization can spread the application of this technique. Considering sonophotocatalysis, it can be stated that this synergistic approach can result in great removal efficiency for various organic pollutants. Specifically, employing sono-based AOPs, promising findings are achieved in the field of pharmaceutical removal. Widespread drugs, such as ibuprofen, ciprofloxacin, or amoxicillin, can be degraded in the range of 30–97% applying optimal experimental conditions. Furthermore, pesticides (e.g., flonicamid) can also be eliminated using sonophotocatalysis with different semiconductors. However, some drawbacks should be addressed to realize its full benefits and make it a more practical and sustainable technology, such as energy consumption, equipment cost, limited penetration depth, limited scalability, and environmental concerns.

Lastly, based on the economy aspects of these processes, it can be noted that the mentioned AOPs are economically more feasible if they are coupled. For instance, lower energy consumption can be observed for sonophotocatalysis than for sonophotolysis and photocatalysis, separately. Also, the investigated AOPs in this review possess lighter operational parameters, compared to other AOPs, such as photo-Fenton or UV/H₂O₂. However, different key factors should be discussed and solved in the future, such as scale-up, integration with other technologies, application in emerging fields, or resource recovery, in order to obtain a completely sustainable technique. Additionally, the future integration of electricity from renewable resources, such as solar cells, is expected to diminish the operational costs of sonophotocatalytic systems.