Abstract
This research emphasized the importance of removing organic pollutants from wastewater discharges. In this review, different advanced oxidation processes are discussed. A broad classification of advanced oxidation processes was used for wastewater treatment. An overview of TiO2-based photocatalysis, the Fenton process, and photocatalytic ozonation has been done. The mechanism of different methods has been studied. The advantages and limitations of these processes are also discussed. Various kinds of catalyzed were used in TiO2-based photocatalysis for various categories of organic contaminants, and several factors with crucial effects on TiO2-based photocatalytic degradation were examined. The typical treatment scheme of Fenton's method was reviewed. Also similarly, a review of photocatalytic ozonation: mechanism of the reaction, its applications with different catalysts, and economic aspects of photocatalytic ozonation were done.
HIGHLIGHTS
Overview of advanced oxidation processes (AOPs) and their potential for wastewater treatment.
Photocatalysis: using catalysts and light to produce reactive oxygen species (ROS) for oxidation.
Ozonation: using ozone to generate ROS for oxidation.
AOPs effective at removing organic compounds that are resistant to biological treatment.
INTRODUCTION
In today's world, finding freshwater is a major problem. As industries grow, they produce a huge amount of untreated or partially treated discharge in freshwater reservoirs (Akar & Uysal 2010). Synthetic colours were released from industrial effluents such as fabric, paper, dyeing, printing, food, and others, posing serious risks to both human health and the environment. Drinking water of poor quality that is contaminated with pathogens and chemical contaminants is related to several adverse short- and long-term health effects, for example, diarrhea. On the other hand, the need for freshwater is increasing day by day as the population is growing in the world. There are so many traditional techniques available for wastewater treatment such as filtration, coagulation, and precipitation but these techniques are not sufficient for the removal of organic and inorganic impurities. Therefore, advanced oxidation processes (AOPs) have caught the interest of academics and industry experts, and they have been recommended for use in wastewater remediation. AOPs are generally understood to be water phase oxidation techniques that rely on the intermediary action of extremely reactive species like hydroxyl radicals (OH) (Tsydenova et al. 2015), the potent oxidant species known as the hydroxyl radical (•OH) can oxidize and mineralize almost any molecular substance, producing the ecologically friendly gases CO2 and inorganic ions (Malato et al. 2009). Rate constants (kOH, r = kOH [•OH] C) for most reactions involving hydroxyl radicals in an aqueous solution are usually in the order of 106–109 M−1 s−1. By targeting intracellular structures, cytoplasmic membranes, and cell walls, free radicals can also harm microbial organisms.
According to Tsydenova et al. (2015), one major area of research is water treatment using AOPs.
Treatment of industrial effluents, such as those from distilleries, agrochemical plants, kraft-bleaching plants, pulp and paper plants, textile dyehouses, oil fields, and metal-plating plants;
Treatment of harmful effluents, such as waste from hospitals and slaughterhouses; removing pathogens and pharmaceutical residues that persist and disrupt the endocrine system from municipal wastewater treatment plant (WWTP) effluents (after secondary treatment) removing heavy metals like arsenic and chromium from water and organic micropollutants like pesticides;
Maintenance and acclimatization of biological sludge from WWTPs.
Clearly, when properly developed, chemical destruction methods provide a complete solution to the pollutant abatement issue in contrast to phase separation methods, which pose the issue of final disposal.
The hydroxyl radical is the most highly oxidizing species that is available, as shown in Table 1. According to Carey (1992), the generation of •OH to initiate oxidations is the foundation of the majority of AOPs for the treatment of waste water.
Oxidant . | Oxidation potential (V) . |
---|---|
OH | 2.8 |
O3 | 2.070 |
H2O2 | 1.770 |
1.670 | |
ClO2 | 1.5 |
Cl2 | 1.360 |
Oxidant . | Oxidation potential (V) . |
---|---|
OH | 2.8 |
O3 | 2.070 |
H2O2 | 1.770 |
1.670 | |
ClO2 | 1.5 |
Cl2 | 1.360 |
- (1)
catalytic advanced oxidation process (c-AOP) and
- (2)
physical advanced oxidation process (p-AOP) AOPs.
However, it is important to note that this classification system should not be taken too seriously because a number of processes involve various technologies and could therefore be placed in a variety of categories. But all these AOPs are not frequently used. So in this review study, we mainly focused on ozone-based photocatalytic ozonation, the Fenton process, and TiO2 photocatalyst because these are used frequently for the degradation of organic contaminants from wastewater on industrial as well as at the laboratory scale.
AOPs based on ozone
O3/H2O2
O3/H2O2-based AOPs are methods that use the combination of O3 and H2O2 as a means of removing contaminants from water. These methods rely on the production of hydroxyl radicals, that are extremely volatile (•OH), which are very effective at purifying water by eliminating a variety of organic contaminants, viruses, and other toxins.
A combination of hydrogen peroxide and ozone is used in this kind of ozone-based process, and the hydrogen peroxide serves as a reagent and speeds up the breakdown of the ozone into the hydroxyl radical. According to research by Staehlin et al. (1982), H2O2 interacts with ozone very slowly at an acidic pH but quickly dissociates into HO2 at a high pH. This demonstrated that ozone decomposition is an efficient process both in potable water purification and reuse of water and is more efficient than OH–O3/H2O2. When manufactured dyehouses were treated with peroxide (H2O2/O3) at 10 mM H2O2, 74% of the ozone was absorbed at pH 11.5 while only 11% was absorbed at pH 2.5. Acar (2004)'s studies on ozone decomposition in the presence of H2O2 also showed that the decomposition of ozone is increased at the following pH values investigated: pH = 2.5, pH = 7, and pH = 10; however, it is more evident at pH levels of 7 and 10. Additionally, statistical analysis revealed that original dissolved ozone content and pH are key factors that have a big impact on how much COD and color are removed. Studies have shown that the advantages for its utilization in wastewater are constrained because of strong competitive reactions and already effective radical formation with O3 alone (Hübner et al. 2015).
Photocatalytic ozonation
Ozone (O3) has very high oxidation potential as shown in Table 1; so, it is considered as a strong oxidant that can interact with a variety of inorganic and organic compounds (Preis et al. 1995; Mehrjouei et al. 2014a, 2014b). With no sludge and residual ozone, ozonation methods are a hopeful technique for disposing of effluent because the ozone breaks down into oxygen and water.
Organic pollutants are reacted with ozone in water either directly using molecular O3 or indirectly using •OH. The use of photocatalytic ozonation to remove biodegradable contaminants from water is not monetarily feasible because it is still one of the highly expensive treatment methods. In a two-step process (Equations (12) and (13), the light involved which initiated the homolysis of O3 and the following generation of •OH by the reaction of O(°D) with H2O has been suggested by Rajeswari & Kanmani (2009).
The high expense of O3 generation and incomplete degradation of organic waste found in water are two drawbacks of ozonation in water purification. According to Legube & Karpel Vel Leitner (1999) and Agustina et al. (2005), many scholars use alternative methods to ozonation to increase the generation of •OH, such as the use of a catalytic system of O3 and boosting •OH formation by using photo-Fenton or TiO2 (Mehrjouei et al. 2012). Since organic compounds are anticipated to breakdown more rapidly and completely in the presence of ozone to CO2, water, a mixture of photocatalysis and ozone (O3), a powerful oxidizer, is sensible for the treatment for degradation of organic compounds. Photocatalytic ozonation was found to be the most efficient technique for completely mineralizing 4-chloronitrobenzene, degrading aniline (Ochiai et al. 2013), dibutyl phthalate (Huang et al. 2015; Wang et al. 2018), and acid (Hammad Khan et al. 2013).
In the beginning of a photocatalytic process, UV–Vis light is used to excite the electrons from the surface of the photocatalyst, which can supply the necessary band gap energy to produce photoactivated electron at CB and hole pairs at VB. Physical adsorption, weak hydrogen bond formation with surface hydroxyl groups, and molecular or dissociative adsorption into Lewis acid sites are three different interactions that ozone molecules can engage concurrently to adsorb on the surface of the photocatalyst. Each interaction results in the generation of active oxygen radicals (•O2). These active oxygen radicals interact with water molecules to create (•OH), which are essential for photocatalytic ozonation reactions.
Application of photocatalysts to photocatalytic ozonation reactors
Mare et al. (1999) and used the photocatalysts in different–different hydroxyls. Some of them mixed the photocatalysts with wastewater. Also, Araña et al. (2002; Addamo et al. (2005); Hur et al. (2005) used immobilized photocatalysts on an inert support material. When 2-chlorophenol is photocatalytically oxidized, immobilized TiO2 particles have a 50% lower mineralization and dichlorination rate than suspended TiO2 particles under the same testing circumstances. In suspension systems, the recycling of photocatalysts is very expensive so it is not used on an industrial scale and is limited to laboratory use alone.
Hur et al. (2005); Černigoj et al. (2007); Zou & Zhu (2008) immobilized photocatalysts and used them in photocatalytic ozonation processes. A lot of study has been done on them. As a result, different designs have been documented. For example, the use of an annular flow reactor with a Pyrex glass tube's interior side coated with an immobilized TiO2 layer as the reactor wall; immobilized TiO2 on alumina balls and they were placed in four reaction tubes inside the batch photoreactor (Ochiai et al. 2013); for their photocatalytic study, TiO2 nanoparticles were used as adapted Ti-mesh sheets. However, these photocatalytic ozonation designs have some drawbacks, so some scholars come up with the best designs where the photocatalysts are fixed to the surface plates and used in a multiphase falling film reactor (Mehrjouei et al. 2013, 2014a, 2014b). In this arrangement, under carefully controlled circumstances, wastewater samples (i.e., liquid phase) pass over immobilized TiO2 particles (i.e., solid phase) near the movement of O3/O2 molecules (gaseous phase). The key benefit of using falling film reactors is that these designs offer a large wastewater volume-to-active photocatalyst surface ratio, resulting in improved oxidation system mass transfer properties.
Reduced toxicity in wastewater is achieved by photocatalytic ozonation in addition to the breakdown and mineralization of organic contaminants in wastewater. A lot of research scholars studied this and got success in reducing the toxicity of wastewater with the help of photocatalytic ozonation. Ochiai et al. (2013) demonstrated that the removal rate of pathogens in effluent will nearly double if they merely switch from ozonation to photocatalytic ozonation. Similarly, Beltrán et al., (2008) performed Danphnia experiments and discovered that photocatalytic ozonation reduced the toxicity of sulfamethoxazole solutions from 60 to 10%. The removal of Galaxolide and Tonalide (as pollutants) from wastewater using oxidative and photochemical methods is one of the most successful and provides the greatest oxidation effectiveness. The creation and buildup of toxic transformation products in wastewater in this instance caused the toxicity readings to first indicate a small decline (within the first 10 min) before increasing.
Economic benefits of photocatalytic ozonation
The cost of photocatalytic ozonation devices is higher than that of simple photocatalysis, and they might not be commercially viable. However, determining the exact energy usage for each oxidation system, where the energy expended during the oxidation process is assigned to the amount of decomposed materials, could lead to a more accurate cost assessment of these oxidation techniques for water filtration. Oxalic acid decomposition by photocatalytic oxidation and catalytic ozonation was about 2 and 9 times less cost-effective than that by photocatalytic ozonation, respectively, according to research by Mehrjouei et al. (2014a, 2014b), which looked at using three distinct kinds of AOPs to degrade oxalic acid and dichloroacetic acid. In comparison to photocatalytic ozonation systems, catalytic ozonation and photocatalytic oxidation were 2 and 15 times more costly for the removal of dichloroacetic acid, another molecule.
Similar to this, research by Kopf et al. (2000) has demonstrated the precise energy requirements for mineralizing monochloroacetic acid through photocatalytic ozonation. The findings of this research demonstrate that ozonation without photocatalyst and well-known photocatalytic oxidation with O2 are two qualitatively and numerically distinct processes. The photocatalytic decay of ozone brought on by the interaction of TiO2 and UV-A rays is most likely the cause of the greater oxidation rate. Photocatalytic ozonation consumes considerably less specific energy than the other processes when taking into account the overall mineralization of the compounds, according to a comparison of the electric energy usage during the tests. Gilbert (2002) compared four different AOPs with four different pollutant compounds and found that photocatalytic ozonation has high energy consumption than simple ozonation.
Based on these studies, it appears that photocatalytic ozonation may be more economically beneficial than photocatalysis and ozonation in the removal of some organic pollutants under ideal operating conditions, depending on factors like the concentration of O3 and pollutant, experimental variables, pollutant properties, etc. However, this conclusion cannot be applied universally. Table 2 summarizes recent work on photocatalytic ozonation.
S. No. . | Removing compound . | Experimental conditions . | Outcomes . | References . |
---|---|---|---|---|
1 | Metoprolol | TiFec + TiO2 + Fe and solar light | Its showed maximum 85% mineralization in 5 h | Quiñones et al. (2014) |
2 | Reactive red 198 (RR198) and Direct green 6 (DG6) | UV/O3, O3/MWCNT and UV/O3/MWCNT | UV/O3/MWCNT showed the highest efficiency of the removal of dye. | Mahmoodi .(2013) |
3 | 1-amino-4-bromoanthraquinone-2-sulfonic acid (ABAS) | Use of TiO2 thin film on a glass and UV-A 39w lamp | TiO2/UV/O3 showed <90% TOC removal and it is more efficient method for the treatment of ABAS wastewater | Wang et al. (2013) |
4 | Reactive red 198 (RR198) and Reactive red 120 (RR120) | Use of copper ferrite nanoparticle catalyst | Copper ferrites nanoparticle enhanced the photocatalytic ozonation and its removed dyes without O2 and heating. | Mahmoodi .(2011) |
5 | Pentachlorophenol, atrazine, chlorfenvinfos, diuron, alachlor, and isoproturon | Use of Degussa P25 catalyst | It showed strong TOC removal except for atrazine | Farré et al. (2005) |
6 | Tetracycline | UV/TiO2/O3 Conc.: 1–100 mg/L | Removal of 90% TOC | Wang et al. (2003) |
7 | Acetamiprid and atrazine | UV-A/TiO2/O3 Conc.: 100 ug/L | photocatalytic ozonation enhanced containment removal by 105–127% | Silva et al. (2019) |
8 | 1,4-dioxane | O3/UV/TiO2/ZnO/Mg (OH)2 = 100 mg/L, V = 400 ml, gas = 200 ml−min, photocatalyst = 0.3 g/L and pH = 3 | It help to degrade it by 100% and remove TOC by 84.37% | Wang et al. (2020a, 2020b) |
9 | Terephthalic acid | vanadium oxide (VxOy) with ZnO | Photocatalytic ozonation enhanced its by 310% | Fuentes et al. (2020) |
10 | Aqueous micropollutants | Magnetite and titania with graphene | Photocatalytic ozonation enhanced degradation efficiency | Chávez et al. (2020) |
11 | Primidone | GO/TiO2 | GO enhanced mineralization upto 82% compared to alone TiO2 | Checa et al. (2019) |
12 | Methylene blue | UV/Fe-PSA/O3 peanut shell ash (PSA) | 94% decolorization and 72.7% COD removal | Ikhlaqa et al. (2020) |
S. No. . | Removing compound . | Experimental conditions . | Outcomes . | References . |
---|---|---|---|---|
1 | Metoprolol | TiFec + TiO2 + Fe and solar light | Its showed maximum 85% mineralization in 5 h | Quiñones et al. (2014) |
2 | Reactive red 198 (RR198) and Direct green 6 (DG6) | UV/O3, O3/MWCNT and UV/O3/MWCNT | UV/O3/MWCNT showed the highest efficiency of the removal of dye. | Mahmoodi .(2013) |
3 | 1-amino-4-bromoanthraquinone-2-sulfonic acid (ABAS) | Use of TiO2 thin film on a glass and UV-A 39w lamp | TiO2/UV/O3 showed <90% TOC removal and it is more efficient method for the treatment of ABAS wastewater | Wang et al. (2013) |
4 | Reactive red 198 (RR198) and Reactive red 120 (RR120) | Use of copper ferrite nanoparticle catalyst | Copper ferrites nanoparticle enhanced the photocatalytic ozonation and its removed dyes without O2 and heating. | Mahmoodi .(2011) |
5 | Pentachlorophenol, atrazine, chlorfenvinfos, diuron, alachlor, and isoproturon | Use of Degussa P25 catalyst | It showed strong TOC removal except for atrazine | Farré et al. (2005) |
6 | Tetracycline | UV/TiO2/O3 Conc.: 1–100 mg/L | Removal of 90% TOC | Wang et al. (2003) |
7 | Acetamiprid and atrazine | UV-A/TiO2/O3 Conc.: 100 ug/L | photocatalytic ozonation enhanced containment removal by 105–127% | Silva et al. (2019) |
8 | 1,4-dioxane | O3/UV/TiO2/ZnO/Mg (OH)2 = 100 mg/L, V = 400 ml, gas = 200 ml−min, photocatalyst = 0.3 g/L and pH = 3 | It help to degrade it by 100% and remove TOC by 84.37% | Wang et al. (2020a, 2020b) |
9 | Terephthalic acid | vanadium oxide (VxOy) with ZnO | Photocatalytic ozonation enhanced its by 310% | Fuentes et al. (2020) |
10 | Aqueous micropollutants | Magnetite and titania with graphene | Photocatalytic ozonation enhanced degradation efficiency | Chávez et al. (2020) |
11 | Primidone | GO/TiO2 | GO enhanced mineralization upto 82% compared to alone TiO2 | Checa et al. (2019) |
12 | Methylene blue | UV/Fe-PSA/O3 peanut shell ash (PSA) | 94% decolorization and 72.7% COD removal | Ikhlaqa et al. (2020) |
Catalyst-based AOPs
A ‘photocatalyst’ is a ‘catalyst that accelerates the solar photo reaction,’ and the following are the minimum requirements for a catalyst to qualify for photocatalyst: The photocatalyst should (i) not be immediately consumed or involved in the reaction and (ii) other mechanisms from already-existing photoreactions must be provided, as well as an increased reaction rate.
TiO2 photocatalysis
To improve TiO2-based photocatalysts for wastewater organic compounds, various parameters were controlled. These key boundaries including intensity of light (Blake et al. 1991; Nasirian & Mehrvar 2016), TiO2 design and structure (Bagbi et al. 2017; Saquib & Muneer 2003), substrate type (Akhavan et al. 2011; Fernandez-Ibanez et al. 2015), pH value (Chiang et al. 2004; Huang et al. 2008), and doping type impacted the productivity of the photocatalysis interaction. Due to its strong oxidation power it decomposes recalcitrant substances and it is significantly more cost-effective and friendly to the environment than chlorine that is in the water purification process. The proper techniques are being used to certify the TiO2 photocatalytic device. A overview of recent studies on TiO2 photocatalysis can be found in Table 3.
S. No. . | Removing compound . | Experimental conditions . | Outcomes . | Reference . |
---|---|---|---|---|
1 | 2-chlorophenol (CP) | Graphene oxide based TiO2 catalysed was used and Solar light, catalyst = 25 mg/L, time = 4 h, | This Ni(OH)2/GO/TiO2 enhanced removal efficiency of CP to 80% | Barakat et al. (2020) |
2 | Rhodamine B | Graphene oxide + TiO2 +UV, H2O2, pH 3–11 | Rhodamine B degraded to 100% | Munikrishnappa et al. (2019) |
3 | Congo red | rGO + TiO2 + Visible light, 2.5,5,7.5,and 10% concentration of rGO based TiO2, at 120 min | Congo red degraded to 92% | Brindha & Sivakumar .(2017) |
4 | Methyl orange (MO) and Methylene blue (MB) | TiO2 + visible light, time = 120 min, Pd-TiO2 concentration = 0.25, 0.5, 0.75, and 1.0 wt.% | The removal efficiency of MB and MO was 94.4 and 92.6%, respectively. | Nguyen et al. (2018) |
5 | Azo dye | ZnO + TiO2 + UV–Vis light; time = 180 min; TiO2 = 0.5–1.5 gL−1 | Almost 99% azo dye removal was reported | Çalışkan et al. (2017) |
6 | Crystal violet dye (CV) | N-TiO2 + UV, time = 180 min | Almost 100% dye removal was reported | Vaiano et al. (2019) |
7 | Reactive red 76 (RR76) and Reactive blue 19 (RB19) | TiO2, C-TiO2, S-TiO2 under Visible light, time = 120 min | C-TiO2 reported 100% dye removal after 60 min, S-TiO2 reported 100% dye removal after 120 min, TiO2 reported only 20% dye removal after 120 min. | Hsing et al. (2007) |
8 | Opaque dye | Pr-Co co-doped TiO2 catalysed was used. 300–1,100 nm light; 120 min | The removal efficiency of opaque dye was reported more than 90% | Yu et al. (2020) |
9 | Remazol dye | Photo-Fenton Fe2O3/TiO2 + UV light, time = 180 min | Remazol dye degraded to 90.57% | Singh et al. (2019) |
10 | Methyl orange | Photo-Fenton-TiO2 catalysed was used. visible light; 2 h | Methyl orange degraded to 98% | Zhang et al. (2019) |
11 | RhB, bMO, MB,Cr (VI)ions | Bi2WO6 particles + TiO2 Photocatalysis | The removal efficiency of rhodamine B, methyl orange, methylene blue and Cr4+ ions was 80.58, 77, 99 and 94%, respectively. | Wang et al. (2020a 2020b) |
12 | Ofloxacin | MnFe2O4@rGO@TiO2 pH = 5.4 | Best degradation at neutral pH 5.4 | Abdel-Wahed et al. (2020) |
13 | Copper and tetracycline (TC) | formic acid-assisted photocatalysis process with TiO2 | Formic acid with TiO2 enhanced degradation properties | Nguyen et al. (2021a, 2021b) |
14 | Methylene red and Methylene blue | TiO2@ZnO heterojunction + sunlight | The removal efficiency of methylene blue and Methylene red was 25 and 13%, respectively. | Mousa et al. (2021) |
S. No. . | Removing compound . | Experimental conditions . | Outcomes . | Reference . |
---|---|---|---|---|
1 | 2-chlorophenol (CP) | Graphene oxide based TiO2 catalysed was used and Solar light, catalyst = 25 mg/L, time = 4 h, | This Ni(OH)2/GO/TiO2 enhanced removal efficiency of CP to 80% | Barakat et al. (2020) |
2 | Rhodamine B | Graphene oxide + TiO2 +UV, H2O2, pH 3–11 | Rhodamine B degraded to 100% | Munikrishnappa et al. (2019) |
3 | Congo red | rGO + TiO2 + Visible light, 2.5,5,7.5,and 10% concentration of rGO based TiO2, at 120 min | Congo red degraded to 92% | Brindha & Sivakumar .(2017) |
4 | Methyl orange (MO) and Methylene blue (MB) | TiO2 + visible light, time = 120 min, Pd-TiO2 concentration = 0.25, 0.5, 0.75, and 1.0 wt.% | The removal efficiency of MB and MO was 94.4 and 92.6%, respectively. | Nguyen et al. (2018) |
5 | Azo dye | ZnO + TiO2 + UV–Vis light; time = 180 min; TiO2 = 0.5–1.5 gL−1 | Almost 99% azo dye removal was reported | Çalışkan et al. (2017) |
6 | Crystal violet dye (CV) | N-TiO2 + UV, time = 180 min | Almost 100% dye removal was reported | Vaiano et al. (2019) |
7 | Reactive red 76 (RR76) and Reactive blue 19 (RB19) | TiO2, C-TiO2, S-TiO2 under Visible light, time = 120 min | C-TiO2 reported 100% dye removal after 60 min, S-TiO2 reported 100% dye removal after 120 min, TiO2 reported only 20% dye removal after 120 min. | Hsing et al. (2007) |
8 | Opaque dye | Pr-Co co-doped TiO2 catalysed was used. 300–1,100 nm light; 120 min | The removal efficiency of opaque dye was reported more than 90% | Yu et al. (2020) |
9 | Remazol dye | Photo-Fenton Fe2O3/TiO2 + UV light, time = 180 min | Remazol dye degraded to 90.57% | Singh et al. (2019) |
10 | Methyl orange | Photo-Fenton-TiO2 catalysed was used. visible light; 2 h | Methyl orange degraded to 98% | Zhang et al. (2019) |
11 | RhB, bMO, MB,Cr (VI)ions | Bi2WO6 particles + TiO2 Photocatalysis | The removal efficiency of rhodamine B, methyl orange, methylene blue and Cr4+ ions was 80.58, 77, 99 and 94%, respectively. | Wang et al. (2020a 2020b) |
12 | Ofloxacin | MnFe2O4@rGO@TiO2 pH = 5.4 | Best degradation at neutral pH 5.4 | Abdel-Wahed et al. (2020) |
13 | Copper and tetracycline (TC) | formic acid-assisted photocatalysis process with TiO2 | Formic acid with TiO2 enhanced degradation properties | Nguyen et al. (2021a, 2021b) |
14 | Methylene red and Methylene blue | TiO2@ZnO heterojunction + sunlight | The removal efficiency of methylene blue and Methylene red was 25 and 13%, respectively. | Mousa et al. (2021) |
Fenton process
The homogenous Fenton processes are widely applied because they are based on the iron catalysis that breaks the oxidant H2O2 into •OH, which nonselectively degrade organic pollutants. The effectiveness of these procedures may be increased when combined with other techniques, like UV light or ultrasound, that enhanced the conversion of the Fe2+ catalyst from Fe3+ and produce more •OH.
Depletion of iron ions over time and creation of solid sediment, both of which necessitate additional management, are the main issues with the Fenton process (Kishimoto et al. 2013; Ochando-Pulido et al. 2017). Second is the high cost of chemicals like H2O2 (Cañizares et al. 2009; Babuponnusami & Muthukumar 2014). The cost of homogeneous Fenton process ranges from 0.2 to 17.7 €m3 that affects the overall effectiveness of the method. Fenton sludge has been linked to a number of negative effects on the economy and the environment.
Several recent advancements to address some of these issues have been made such as the availability of the iron catalyst in the solid form: zero-valent Fe has been shown to be very effective in acidic conditions, where the catalyst particle's top layer is oxidized by the Fenton process to produce Fe2+in situ. The production of Fe-supporting catalysts, where the Fe of the catalyst is usually embedded or enclosed in a solid support and originates from Fe minerals or Fe salts, and has also attracted attention recently. Iron can also be found naturally in the form of minerals. Hydroxides may change in size, orientation, dimensions, and shape when dehydroxylated into their oxide products under specific circumstances. Utilizing waste is a hopeful option. Relatively inexpensive leftovers can be used to support catalysts or as a source of iron, decreasing the need for Fe ores and upholding the circular economy's guiding principles. Additionally, because of their abundant active sites, low diffusion resistance, high surface area, and proximity to reactants, nanomaterials are ideal for use as Fenton catalysts (Wang et al. 2012, 2016). Table 4 outlines some of the Fenton process types. The wastewater treatment by Fenton oxidation is summarized in Table 5.
S. No. . | . | . |
---|---|---|
1 | Homogenous | Photo-Fenton (PF) |
Sono-Fenton (SF) | ||
Electro-Fenton (EF) | ||
Photo-electro-Fenton (PEF) | ||
Sono-electro-Fenton (SEF) | ||
2 | Heterogeneous | Nanomaterials |
Synthesized Fe-supporting catalyst | ||
Fe minerals and waste-derived catalysts | ||
Zero-valent state metal catalyst |
S. No. . | . | . |
---|---|---|
1 | Homogenous | Photo-Fenton (PF) |
Sono-Fenton (SF) | ||
Electro-Fenton (EF) | ||
Photo-electro-Fenton (PEF) | ||
Sono-electro-Fenton (SEF) | ||
2 | Heterogeneous | Nanomaterials |
Synthesized Fe-supporting catalyst | ||
Fe minerals and waste-derived catalysts | ||
Zero-valent state metal catalyst |
S. No. . | Removing compound . | Experimental conditions . | Outcome . | Reference . |
---|---|---|---|---|
1 | Pulp-bleaching wastewater | [Fe2+] = 8.5 mM, [H2O2] = 177 mM; pH = 2; time = 10 min; Temp. = 60 ◦C | Maximum removal efficiency in this conditions was 85% | Ribeiro et al. (2019) |
2 | Benzene dye | Condition: pH of 4.13, [H2O2] = 1.0 M,[Fe2+] = 0.36 M. | Removal efficiency of COD, TOC and color was 85, 75 and 99.9%, respectively. | Guo et al. (2018) |
3 | Leachate | Conditions: H2O2/COD ratio (w/w) = 0.5/1–4, H2O2/Fe2+ w/w ratio of 5/1 and pH = 9 | BOD removal 99% and COD removal 94% | Trapido et al. (2017) |
4 | Sawmill | [Fe3+] = 0.45 mM, [H2O2] = 188.2 mM, pH = 3 time = 60 min, and Temp. = 120 °C | COD and TOC removal efficiency reported was 80 and 70%, respectively. | Ribeiro & Nunes (2021) |
5 | Reverse osmosis concentrate from the graphical industry | 1.2 mol Fe2+/mol H2O2, 0.2 g H2O2/gCOD and, pH = 3, time = 20 min. | This optimum conditions showed best degradation of organic pollutants. | Van Aken et al. (2013) |
6 | Containers and drum cleaning | H2O2 = 45 g/L, FeCl3 = 0.8 g/L, pH = 3 | COD was measured as about 1,500 mg/L which was highest | Güneş et al. (2019) |
7. | Petroleum refinery | [H2O2] = 1,008.4 mM and [Fe3+] = 686.0 mg, pH = 3 | The maximum TOC removal efficiency was reported 70% and COD was 98% | Hasan et al. (2012) |
S. No. . | Removing compound . | Experimental conditions . | Outcome . | Reference . |
---|---|---|---|---|
1 | Pulp-bleaching wastewater | [Fe2+] = 8.5 mM, [H2O2] = 177 mM; pH = 2; time = 10 min; Temp. = 60 ◦C | Maximum removal efficiency in this conditions was 85% | Ribeiro et al. (2019) |
2 | Benzene dye | Condition: pH of 4.13, [H2O2] = 1.0 M,[Fe2+] = 0.36 M. | Removal efficiency of COD, TOC and color was 85, 75 and 99.9%, respectively. | Guo et al. (2018) |
3 | Leachate | Conditions: H2O2/COD ratio (w/w) = 0.5/1–4, H2O2/Fe2+ w/w ratio of 5/1 and pH = 9 | BOD removal 99% and COD removal 94% | Trapido et al. (2017) |
4 | Sawmill | [Fe3+] = 0.45 mM, [H2O2] = 188.2 mM, pH = 3 time = 60 min, and Temp. = 120 °C | COD and TOC removal efficiency reported was 80 and 70%, respectively. | Ribeiro & Nunes (2021) |
5 | Reverse osmosis concentrate from the graphical industry | 1.2 mol Fe2+/mol H2O2, 0.2 g H2O2/gCOD and, pH = 3, time = 20 min. | This optimum conditions showed best degradation of organic pollutants. | Van Aken et al. (2013) |
6 | Containers and drum cleaning | H2O2 = 45 g/L, FeCl3 = 0.8 g/L, pH = 3 | COD was measured as about 1,500 mg/L which was highest | Güneş et al. (2019) |
7. | Petroleum refinery | [H2O2] = 1,008.4 mM and [Fe3+] = 686.0 mg, pH = 3 | The maximum TOC removal efficiency was reported 70% and COD was 98% | Hasan et al. (2012) |
Physical AOPs
Electron beam
In the electron beam AOP, an accelerated electron penetrates the water's surface, resulting in the formation of electronically excited species both reducing and oxidizing species (•OH, and •H) in the water that could facilitate highly efficient pollutant decomposition and water disinfection (Ponomarev & Ershov 2020). Nickelsen et al. (1994) recognized that the incoming energy immediately correlates with the accelerated electron which have greatest penetration depth. However, due to the high cost of an electron accelerator and the high danger of exposure to X-rays, this method has drawbacks. Table 6 shows the outline of ongoing work done in the space of electron beam AOP.
S. No. . | Removing compound . | Experimental conditions . | Outcomes . | References . |
---|---|---|---|---|
1. | Polyvinyl alcohol | Electron beam (EB) + MBR treatment | The removal of COD achieved to 10% | Sun et al. (2016) |
2. | Textile wastewater | Electron beam + activated sludge | COD removal was enhanced by upto 79% after introducing activated sludge. | Mohd Nasir et al. (2010) |
3. | Antidepressant drug fluoxetine (FLX) | Electron beam irradiation + acidic condition | FLX degradation efficiency was achieved by 100% | Shao et al. (2018) |
4. | Iopromide | Electron beam/H2O2 system | This system enhanced removal efficiency of iopromide. | Kwon et al. (2012) |
5. | Clopyralid | Electron beam/H2O2 system + basic medium | The removal efficiency reported was 99%. | Andreozzi et al. (1999) |
6. | 1,4-Dioxane | Electron beam | The maximum removal efficiency of 1,4-dioxane was reported 94–99%. | Pearce et al. (2022) |
7. | Dyeing wastewater | Electron beam + wastewater (30,000 m3/d) | The colour of dye present in water was decreased upto 10 times. | Wang et al. (2022) |
8 | Tricyclazole (TC) | Electron beam,pH = 7.0, and TC = 4 mg/L and H2O2 = 4 mM, absorbed dose = 3.5 kGy | This reported 96% degradation of tricyclazole | Nguyen et al. (2021a 2021b) |
9 | Leucomalachite green (LMG) | pH = 6, absorbed = 4 kGy, an LMG = 4 mg/L and an H2O2 = 8 mM | The maximum degradation efficiency of LMG was 98.2% at these optimal conditions | Nickelsen et al. (1994) |
10 | Benzothiazole (BTH) | BTH conc. 20, 40, 50, 60, and 80 mg L−1 and adsorbed conc. 0.5,1.0,1.5,2.0, and 5.0 kGy, respectively | Benzothiazole degraded efficiency upto 90% | Chen et al. (2022) |
11 | Atrazine | Atrazine concentrations = 2 mg/L, electron beam irradiation = 6 kGy, pH = 5, H2O2 = 3 mM | 100.1% degradation of atrazine | Van Luu et al. (2021) |
12 | 1,4-Dioxane | Adsorbed conc. = 2.3kGy | 90–94% degradation were observed | Pearce et al. (2022) |
13 | Tricyclazole (TC) | Absorbed dose = 3.5 kGy, pH = 7.0, TC = 4 mg/L and a H2O2 = 4 mM | 96.5% degradation were observed | Nguyen et al. (2021a, 2021b) |
14 | Salbutamol (SAL) | SAL conc. = 100 mg L−1 Absorbed conc. = 10 kGy | 95.1% removal were observed | Shao et al. (2023) |
15 | Reactive blue 21 (RB21) | RB21 conc. = 0.61 g.L−1 Absorbed conc. = 5 kGy | 63.51% removal were observed | Melo et al. (2021) |
16 | Pyrazinamide (PZA) | PZA conc. = 0.2 mM, Absorbed conc.: 5 kGy | 99% removal were observed | Zou et al. (2021) |
S. No. . | Removing compound . | Experimental conditions . | Outcomes . | References . |
---|---|---|---|---|
1. | Polyvinyl alcohol | Electron beam (EB) + MBR treatment | The removal of COD achieved to 10% | Sun et al. (2016) |
2. | Textile wastewater | Electron beam + activated sludge | COD removal was enhanced by upto 79% after introducing activated sludge. | Mohd Nasir et al. (2010) |
3. | Antidepressant drug fluoxetine (FLX) | Electron beam irradiation + acidic condition | FLX degradation efficiency was achieved by 100% | Shao et al. (2018) |
4. | Iopromide | Electron beam/H2O2 system | This system enhanced removal efficiency of iopromide. | Kwon et al. (2012) |
5. | Clopyralid | Electron beam/H2O2 system + basic medium | The removal efficiency reported was 99%. | Andreozzi et al. (1999) |
6. | 1,4-Dioxane | Electron beam | The maximum removal efficiency of 1,4-dioxane was reported 94–99%. | Pearce et al. (2022) |
7. | Dyeing wastewater | Electron beam + wastewater (30,000 m3/d) | The colour of dye present in water was decreased upto 10 times. | Wang et al. (2022) |
8 | Tricyclazole (TC) | Electron beam,pH = 7.0, and TC = 4 mg/L and H2O2 = 4 mM, absorbed dose = 3.5 kGy | This reported 96% degradation of tricyclazole | Nguyen et al. (2021a 2021b) |
9 | Leucomalachite green (LMG) | pH = 6, absorbed = 4 kGy, an LMG = 4 mg/L and an H2O2 = 8 mM | The maximum degradation efficiency of LMG was 98.2% at these optimal conditions | Nickelsen et al. (1994) |
10 | Benzothiazole (BTH) | BTH conc. 20, 40, 50, 60, and 80 mg L−1 and adsorbed conc. 0.5,1.0,1.5,2.0, and 5.0 kGy, respectively | Benzothiazole degraded efficiency upto 90% | Chen et al. (2022) |
11 | Atrazine | Atrazine concentrations = 2 mg/L, electron beam irradiation = 6 kGy, pH = 5, H2O2 = 3 mM | 100.1% degradation of atrazine | Van Luu et al. (2021) |
12 | 1,4-Dioxane | Adsorbed conc. = 2.3kGy | 90–94% degradation were observed | Pearce et al. (2022) |
13 | Tricyclazole (TC) | Absorbed dose = 3.5 kGy, pH = 7.0, TC = 4 mg/L and a H2O2 = 4 mM | 96.5% degradation were observed | Nguyen et al. (2021a, 2021b) |
14 | Salbutamol (SAL) | SAL conc. = 100 mg L−1 Absorbed conc. = 10 kGy | 95.1% removal were observed | Shao et al. (2023) |
15 | Reactive blue 21 (RB21) | RB21 conc. = 0.61 g.L−1 Absorbed conc. = 5 kGy | 63.51% removal were observed | Melo et al. (2021) |
16 | Pyrazinamide (PZA) | PZA conc. = 0.2 mM, Absorbed conc.: 5 kGy | 99% removal were observed | Zou et al. (2021) |
Ultrasound
S.No. . | Removing compound . | Experimental conditions . | Outcomes . | References . |
---|---|---|---|---|
1 | Paracetamol | Sonophotocatalysis and TiO2 is a photocatalyst | The maximum degradation rate was observed | Miklos et al. (2018) |
2 | Acid red 88 (AR88) | Ultrasound + TiO2, AR88 = 0.09 mM and photocatalyst = 1 g/L | Enhanced degradation activity | Madhavan et al. (2010a, 2010b) |
3 | Orange-G (OG) | Ultrasound (213KHz) + TiO2 pH = 5.8 | At these optimum conditions degradation rate of OG reported maximum | Madhavan et al. (2010a, 2010b) |
4 | Dye-contaminated wastewater | Probe-type sonicator (sonolysis with dipping), Optimal pH = 6, time = 90 min. | In sonolysis with dipping, 82.18% of COD removal was obtained | Nair & Patel (2014) |
5 | Coomassie brilliant blue (CBB) | Ultrasonic irradiation (350KHz) and time = 30 min | The degradation of CBB was reported 90% | Rayaroth et al. (2017) |
6 | Ulfamethoxazole (SUX) antibiotic | MgO + ZnO + graphene (MZG) + UV light + US, SUX conc. = 55 mg/L, MZG conc. = 0.8 g/L, time = 120 min,pH = 9 | complete degradation of SUX can be attained | Moradi et al. (2020a 2020b) |
7 | C.I. Acid Orange 7 | Ultrasound + goethite + H2O2, pH = 3, time = 30min | 90% decolorization efficiency was obtained | Zhang et al. (2019) |
8 | p-nitrophenol | Sonophotocatalysis (25KHz) + H2O2 + UV | Degradation efficiency of 94% was obtained | Mishra & Gogate .(2011) |
S.No. . | Removing compound . | Experimental conditions . | Outcomes . | References . |
---|---|---|---|---|
1 | Paracetamol | Sonophotocatalysis and TiO2 is a photocatalyst | The maximum degradation rate was observed | Miklos et al. (2018) |
2 | Acid red 88 (AR88) | Ultrasound + TiO2, AR88 = 0.09 mM and photocatalyst = 1 g/L | Enhanced degradation activity | Madhavan et al. (2010a, 2010b) |
3 | Orange-G (OG) | Ultrasound (213KHz) + TiO2 pH = 5.8 | At these optimum conditions degradation rate of OG reported maximum | Madhavan et al. (2010a, 2010b) |
4 | Dye-contaminated wastewater | Probe-type sonicator (sonolysis with dipping), Optimal pH = 6, time = 90 min. | In sonolysis with dipping, 82.18% of COD removal was obtained | Nair & Patel (2014) |
5 | Coomassie brilliant blue (CBB) | Ultrasonic irradiation (350KHz) and time = 30 min | The degradation of CBB was reported 90% | Rayaroth et al. (2017) |
6 | Ulfamethoxazole (SUX) antibiotic | MgO + ZnO + graphene (MZG) + UV light + US, SUX conc. = 55 mg/L, MZG conc. = 0.8 g/L, time = 120 min,pH = 9 | complete degradation of SUX can be attained | Moradi et al. (2020a 2020b) |
7 | C.I. Acid Orange 7 | Ultrasound + goethite + H2O2, pH = 3, time = 30min | 90% decolorization efficiency was obtained | Zhang et al. (2019) |
8 | p-nitrophenol | Sonophotocatalysis (25KHz) + H2O2 + UV | Degradation efficiency of 94% was obtained | Mishra & Gogate .(2011) |
Microwave
S.No. . | Removing compound . | Experimental Conditions . | Outcomes . | References . |
---|---|---|---|---|
1 | Pentachlorophenol (PCP) | Microwave (700W), adsorbents FeO and CoFe2O4,PCP conc. = 1,000 mg/L, | The removal efficiency of PCP at these optimum conditions was reported more than 99% | Jou (2008) |
2 | Cypermethrin | Microwave + Photo-Fenton, time = 4 min | Degraded efficiently more than 98% in just 4 min | Gromboni et al. (2007) |
3 | Cutting-oil in water emulsions (COWE) | Microwave (800W)-assisted Catalytic Wet Peroxide Oxidation (MW-CWPO), catalyst: graphite (10 g/L), COWE 0.5%w, pH: 9, H2O2: 15.7 g/L, | At these optimum conditions 82% TOC removal was reported | Garcia-Costa et al. (2021) |
4. | Antibiotic metacycline (MTC) | Microwave + Fenton reaction (MAFR) + CuCO2O4(10 mg), H2O2: = 500 μL,MTC = 50 mg/L, Temp. = 90 °C | This showed 86.4% removal efficiency even after 5th cycle | Qi et al. (2019) |
5 | Sodium dodecyl benzene sulfonate (SDBS) | Microwave (280 W)+ potassium persulfate (KPS), Concentration: SDBS = 25–100 mg/L, KPS = 2 g/L | The degradation of SBDS was reported to be 98.3% | Bhandari & Gogate (2019) |
S.No. . | Removing compound . | Experimental Conditions . | Outcomes . | References . |
---|---|---|---|---|
1 | Pentachlorophenol (PCP) | Microwave (700W), adsorbents FeO and CoFe2O4,PCP conc. = 1,000 mg/L, | The removal efficiency of PCP at these optimum conditions was reported more than 99% | Jou (2008) |
2 | Cypermethrin | Microwave + Photo-Fenton, time = 4 min | Degraded efficiently more than 98% in just 4 min | Gromboni et al. (2007) |
3 | Cutting-oil in water emulsions (COWE) | Microwave (800W)-assisted Catalytic Wet Peroxide Oxidation (MW-CWPO), catalyst: graphite (10 g/L), COWE 0.5%w, pH: 9, H2O2: 15.7 g/L, | At these optimum conditions 82% TOC removal was reported | Garcia-Costa et al. (2021) |
4. | Antibiotic metacycline (MTC) | Microwave + Fenton reaction (MAFR) + CuCO2O4(10 mg), H2O2: = 500 μL,MTC = 50 mg/L, Temp. = 90 °C | This showed 86.4% removal efficiency even after 5th cycle | Qi et al. (2019) |
5 | Sodium dodecyl benzene sulfonate (SDBS) | Microwave (280 W)+ potassium persulfate (KPS), Concentration: SDBS = 25–100 mg/L, KPS = 2 g/L | The degradation of SBDS was reported to be 98.3% | Bhandari & Gogate (2019) |
Merits and demerits of AOPs
After reviewing various AOPs, it could be visualized that all the processes are associated with one or the other limitation so these finding are summarized in Table 9.
Method . | Merits . | Demerits . |
---|---|---|
Ozone-based AOPs |
|
|
Fenton process |
|
|
TiO2 photocatalyst |
|
|
Electron beam |
|
|
Ultrasound |
|
|
Microwave |
|
|
Method . | Merits . | Demerits . |
---|---|---|
Ozone-based AOPs |
|
|
Fenton process |
|
|
TiO2 photocatalyst |
|
|
Electron beam |
|
|
Ultrasound |
|
|
Microwave |
|
|
CONCLUSION
Present review compiles the salient features of AOPs used for wastewater treatment. In all these processes, hydroxyl radical plays a major role due to its oxidation potential. Among all the methods, photo catalysis is found to be most efficient and most popular method for wastewater treatment. All the AOPs have limitations, and efforts are being made so that these limitations could be overcome. For instance, some doping in the materials brings the light source from UV to visible region. There is also a need to stop or reduce electron–hole recombination, this is being done by trapping of an electron or utilization of a hole. There is also a requirement to increase the stability of the material used in these auto oxidation processes. AOPs can be commercialized and used on an industrial scale with some modifications in the materials used for various oxidation reactions.
ACKNOWLEDGEMENTS
The authors are grateful for the support of Lovely Professional University for providing the basic infrastructure for this review.
FINANCIAL SUPPORT
No funding assistance is available.
DATA AVAILABILITY STATEMENT
All relevant data are included in the paper or its Supplementary Information.
CONFLICT OF INTEREST
The authors declare there is no conflict.