- Key Laboratory for Advanced Materials and Joint International Research Laboratory of Precision Chemistry and Molecular Engineering, Feringa Nobel Prize Scientist Joint Research Center, Frontiers Science Center for Materiobiology and Dynamic Chemistry, School of Chemistry and Molecular Engineering, East China University of Science and Technology, Shanghai, China
Over the past few decades, Fenton process has been widely studied in environmental remediation. Due to the low efficiency of iron ions recycling, the Fenton efficiency has been seriously impeded for practical application. On this condition, with combination of photocatalysts, it is expected to fully use photo-generated electrons, thus enhancing the photo-Fenton efficiency. This synergistic methodology has assisted many researchers to remove various categories of organic pollutants in discharged wastewater or achieve gas conversion. This comprehensive review describes some significant advances with respect to presentative semiconductors (e.g., TiO2, g-C3N4, graphene, BiVO4, ZnFeO4 and BiFeO3, etc.) including their preparation methods, characterization and applications in environmental remediation (e.g., organic removal, bacteria disinfection, membrane separation, and gas conversion). The mechanism is preferentially discussed. Possibly future development and its correlated potential challenges are specifically proposed and discussed in this review.
Introduction
In recent decades, the aqueous environment arising from a great variety of contaminants, for instance, emerging contaminants (ECs) including pharmaceuticals and personal care products (PPCPs), dyes, endocrine-disrupting compounds (EDCs), flame retardants (FRs), pesticides, and their metabolites pose a major concern to human beings (Esplugas et al., 2007; Klamerth et al., 2013). Although their concentration in real sewage is in the μg L−1-ng L−1 range, they unconsciously discharge into the environment and biologically accumulate in human body, thus posing a severe threat to receiving waters (e.g., rivers, aquifers and groundwater) (Zhang et al., 2014). To the best of our knowledge, conventional wastewater treatment plants (WWTP) are often unable to entirely degrade persistent organic pollutants (Jelic et al., 2011; Klamerth et al., 2013). Consequently, the pollutants and their metabolites accumulate in the aquatic environment, indirectly they may cause ecological risk. As a result, the alternatively advanced technique is of paramount importance to be developed.
Advanced oxidation processes (AOPs), which have been proposed as replaceable solutions for degrading persistent organic compounds, because the hydroxyl radicals (·OH) are unselectively promote organic substances oxidation at high reaction rates (Zhang et al., 2009; Sun et al., 2020). Besides this, AOPs are also called as versatile technologies owing to the various alternative ways in generation hydroxyl radicals (Lee and von Gunten, 2010). In comparison with conventional treatment techniques, AOPs are more efficient and capable of degrading recalcitrant pollutants. In AOPs, a variety of oxidants such as hydrogen peroxide (H2O2) (Dong C. et al., 2018; Xing et al., 2018a), persulfate (Matzek and Carter, 2016; Wang and Wang, 2018), peroxymonosulfate (Shao et al., 2017; Tan et al., 2017; Duan et al., 2018; Lei et al., 2019; Ma et al., 2019; Shen et al., 2019), permanganate (Guo et al., 2018; Chow and Sze-Yin Leung, 2019; Wang et al., 2019), and ozone (O3) (Khan et al., 2017; Ikehata and Li, 2018) have been employed to degrade organic matters. Among these oxidants, H2O2 has been widely recognized as economically and environmentally friendly oxidant, according to its high efficacy and environmental friendliness.
In short, the combination of Fe2+ and H2O2 are called conventional Fenton reaction, which belong to a type of classical AOPs. Since 1894, H.J.H Fenton proposed the concept of Fenton process, he found that Fe2+ and H2O2 could degrade tartaric acid. Thus, it has been widely used in environmental remediation in the following decades (Jain et al., 2018). As the merits of Fenton progress, it is normally operated under ambient temperature and pressure (Qian et al., 2017, 2018a), and generate strongly oxidizing radical species (primarily ·OH) for the complete decomposition of organic pollutants into non-toxic products, such as CO2, H2O and inorganic salts (Dong C. et al., 2018). Besides this, the advantages of Fenton process over other WWTPs are extremely superior, including of higher removal efficiency, no residues and wide region for treating substances as well as no need of special equipment. The conventional Fenton mechanism, which is involved with Fe2+/H2O2, is exhibited as the following equations:
However, the conventional decomposition efficiency of H2O2 in Fe2+/H2O2 system is constrained by the low Fe3+/Fe2+ cycle, owing to the low reaction rate constant of Equation (2). Therefore, the key to enhance the Fenton efficiency is accelerating the reaction constant of Equation (2); meanwhile, the iron sludge will be greatly reduced due to the high efficacy of Fe3+/Fe2+ cycle.
Besides Fenton process, photocatalytic oxidation has been considered as alternative and efficient AOP as well. Photocatalysis based AOPs initiate the complex chain reactions, and it may produce the colorless organic intermediates. Correspondingly, these colorless intermediates maybe more toxic than the parent molecular. Additionally, all of the Fenton reactions including conventional Fenton process and Fenton-like or photo-Fenton-like process easily produce iron mud, and it needs a large amount of manpower and chemicals to treat and remove iron mud. Thus, to achieve a thorough mineralization of organic compound is the main goal of AOPs in environmental remediation. After retrospection of the past few decades, it can be found that many studies have utilized the nano-photocatalysts for the degradation of organic pollutants; however, the utilization efficiency of photo-generated electrons and holes still remains a challenge for researchers in WWTPS. Based on this background information, many researchers have coupled Fenton reaction with photocatalysis as a novel technique to treat wastewater, which can be assigned to novel Fenton-like reaction. By means of this, it is able to realize a win-win situation. To be more specific, it can not only prolong the lifetime of photo-generated carries [i.e., photo electrons (e−) and hole (h+)], but also the photo-generated electrons will accelerate the cycle of Fe3+/Fe2+, leading to higher mineralization of pollutant and the reduction of iron sludge.
In this paper, we mainly review and summary several representative nanomaterials, which have been applied in photocatalytic Fenton reaction, including of TiO2-based, g-C3N4-based, reduced graphene oxide (RGO)-based and other semiconductors-based nanomaterials (e.g., Ag, BiVO4, ZnFeO4, and BiFeO3 based nanomaterials). This review mainly aims to review the nanomaterials based AOPs that are used for degradation of different organic pollutants in wastewater or volatile organic compounds (VOCs) removal and provide some effective information for developing latest solution of WWTPs.
TiO2-Based Photocatalytic Fenton Reaction
Removing Organic Pollutant
Since 1972, Fujishima and Honda (1972) found that titanium oxide (TiO2) could split water to produce hydrogen under UV light irradiation, thus opening up a new era of titanium oxide (TiO2) in photocatalysis. Up to now, TiO2 has been well-applied in the fields of energy and environment, including organic removal (Vaiano et al., 2015; Shayegan et al., 2018; Chen et al., 2019; Dong et al., 2019), hydrogen production (Zhang et al., 2012; Xi et al., 2014; Xing et al., 2015; Zhou Y. et al., 2016), CO2 reduction (Yu et al., 2014; Dong et al., 2018a; Xing et al., 2018b), nitrogen fixation (Comer and Medford, 2018; Li C. et al., 2018; Zhao et al., 2019), and methane conversion (Wang P. et al., 2017; Yu et al., 2017), etc. Attributed to the sufficiently high reduction potential, low economical cost and high stability, TiO2 has attracted great attention as one of the most potential and influential photocatalysts (Dong et al., 2018b). In terms of photocatalytic Fenton reaction, especially, TiO2 based nanomaterials have been developed. Consequently, Photo-Fenton oxidation process employed modified TiO2, Fe2+or Fe3+, and H2O2 under light irradiation, leading to significant improved generation of hydroxyl radicals and degradation efficiency of organic pollutants owing to synergy between photocatalysis and Fenton reaction. Some examples have been summarized in Table 1.
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Table 1. Summary of TiO2 based heterogeneous photo-Fenton and photo-Fenton like process under light irradiation.
In Table 1, we have listed some representative work in recent years. In 2012, Ortega-Liébana et al. (2012) developed Fe2+/TiO2/H2O2/UV system for the degradation of 40 ppm 3-chloropyridine. The results showed that photo-Fenton process is more efficient than sole TiO2 photocatalysis, which meant that this technique possessed more practical industrialized application than photocatalysis. As to some modified TiO2 nanomaterials, for instance, Tryba et al. (2006) prepared Fe-TiO2 and Fe-C-TiO2 nanomaterials, which could effectively degrade phenol. The highly efficient decomposition of phenol on Fe-C-TiO2 photocatalysts mainly attributed from several factors: (1) the produced hydroxyl radicals in Fenton process; (2) the reduction of Fe3+ back to Fe2+ under UV light; (3) the cocatalytic effect of hydroquinone. Additionally, Banić et al. (2011) prepared Fe/TiO2 materials with various loading amount of Fe2O3 nanoparticles. The 7.2Fe/TiO2/H2O2 system exhibited the highest activity for removing TCL among all the used AOPs (Figures 1A,B). In Figure 1A, it could be observed that 1.67 g L−1 7.2Fe/TiO2/45 H2O2 system achieved 100% removal efficiency after 25 min. However, in this case, TiO2 only acted as a good support, because the degradation efficiency of 7.2Fe/TiO2 is lower compared to the TiO2 support, indicating the presence of Fe under UV irradiation did not contribute to the catalytic activity of the material, and not facilitating the cycle of Fe3+/Fe2+. Moreover, the results indicated that the removal efficiency was influenced by some possible parameters including pH and TCL concentrations (Figures 1C,D). In Figure 1C, the optimal pH value was fixed at 2.8. As to TCL concentrations, when TCL concentration exceeded 0.12 mM, the reaction rate tended to slow down (Figure 1D), ascribed to that most of the 7.2Fe/TiO2 active sites were occupied, leading to reduced generation of reactive oxygen species.
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Figure 1. (A) Photodegradation efficiency of various systems of TCL under UV light irradiation: (a) photolysis, (b) TiO2 support, (c) PrecFe, (d) 7.2Fe/TiO2, (e) Degussa P25, (f) H2O2, (g) TiO2 support/H2O2, (h) PrecFe/H2O2, (i) 7.2Fe/TiO2/H2O2, and (j) Degussa P25/45 mM H2O2. (B) TOC removal efficiency for TCL in the presence of 7.2Fe/TiO2 and Degussa P25, respectively. (C) Impact of pH value on the kinetic of TCL photodegradation efficiency. (D) Impact of TCL concentration on the kinetic photodegradation efficiency (inset picture: the impact of the TCL concentration on the TCL degradation rate at 6 min light irradiation). Copied with permission (Banić et al., 2011). Copyright 2011, Elsevier.
When it comes to coupled semiconductor, ThanhThuy et al. (2013) prepared ZnSe/TiO2 nanocomposites. Under simulated solar light, the ZnSe sensitized TiO2 nanotube arrays exhibited remarkable capability for photocatalytic degradation of pentachlorophenol assisted with photo-Fenton system (i.e., Fe3+/H2O2/humic acid). The results showed that 99.0% of pentachlorophenol could be degraded in comparison with pure TiO2 NTAs (64.0%) after 2 h solar light irradiation. Further, Kim and Kan (2015) synthesized CdS/carbon nanotube-TiO2 (CdS/CNT-TiO2), which was applied in heterogeneous photo-Fenton process as well. The heterogeneous photo-Fenton oxidation cocatalyzed by CdS/CNT-TiO2 was mainly engaged in the combination of the photocatalytic, photo-Fenton and photosensitizing oxidation. Consequently, the photo-Fenton process achieved a high decolorization (98%) and mineralization rate (83%). To the best of our knowledge, the concentration of ferrous and ferro ferric ion as well as hydrogen peroxide is of great necessary for enhancing treatment efficiency and minimizing operating costs. In Figure 2A, firstly, it was found that degradation and TOC removal efficiency of MB can be increased to 99 and 83%, respectively. Secondly, the ratio of [Fe3+]: [H2O2] increased from 0.05 to 0.5 led to the MB removal efficiency to 100%; meanwhile, the TOC removal efficiency could be increased from 61 to 84% (Figure 2B). The detailed mechanism has been proposed in Figure 2C. Specifically, the photo-induced electron () on the surface of TiO2 can be scavenged by oxygen, whereas the photo-induced hole () can react with OH− or H2O. Afterwards, the electrons transferred to these adsorb Fe3+ ions on TiO2 surface, leading to the cycle of Fe3+/Fe2+. Simultaneously, the electron on TiO2 surface would possibly impede the electron-hole recombination. Lastly, MB molecular acting as photosensitizer would form MB* under visible light irradiation. The electron transfer between the MB* and Fe3+ would be beneficial to regenerating Fe2+, which accelerated the kinetic rate of the photo-Fenton process.
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Figure 2. (A) Impact of the ratio of [H2O2]/[MB] on photo-Fenton degradation efficiency of MB. (B) Impact of the ratio of [Fe3+]/[H2O2] on photo-Fenton degradation efficiency of MB. (C) Proposed mechanism of CdS/CNT-TiO2 for photo-Fenton degradation of MB under visible light irradiation. Copied with permission (Kim and Kan, 2015). Copyright 2015, Elsevier.
Disinfection
Apart from the organic pollutant removal, photo-Fenton process serving for water disinfection has emerged in recent decades. Commonly, according to the rules, two main solutions are able to be adopted if researchers want to employ photo-Fenton process to bacterial inactivation, including of drinking water disinfection and secondary effluent from municipal wastewater treatment plants (MWWTPs) (Giannakis et al., 2016). In terms of the target microorganisms, especially E. coli K12, was mostly selected as studied target. Regarding TiO2 on disinfection, the earliest systematic study was reported mainly based on photocatalytic water disinfection for E. coli inactivation (Rincón and Pulgarin, 2004a,b). Afterwards, Rincón and Pulgarin (2006) compared Fe3+ and TiO2 assisted system. Several systems have been established (e.g., UV-vis/TiO2, UV-vis/TiO2/H2O2, UV-vis/Fe3+/H2O2, and UV-vis/H2O2). It was revealed that the disinfection rate of TiO2 could be accelerated if H2O2 existed. Herein, TiO2 itself can show disinfection efficiency of E. coli, acting as photocatalyst, which can produce hydroxyl radicals under UV-vis irradiation. After addition of H2O2, the photoinduced charge transfer will be effectively separated because H2O2 acting as electron acceptor, thus preventing the recombination of electron-hole pairs, and generating much more ·OH radicals. In Figure 3A, it could be clearly seen that TiO2/H2O2 system is more efficient than Fe3+/H2O2 system. Compared to the dark controls in Figure 3B, the irradiation of solar light could promote the bacterial inactivation. H2O2 alone did not obviously impede the E. coli survival, whereas TiO2 could increase its bactericide oxidation ability.
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Figure 3. (A) E. coli inactivation under different light driven AOPs systems including UV-vis/TiO2/H2O2, UV-vis/Fe3+/H2O2, UV-vis/Fe3+, UV-vis/H2O2, and only UV-vis. Dark controls: Fe3+ and TiO2. (B) E. coli survival under different dark systems including of TiO2/H2O2, Fe3+/H2O2, and H2O2 systems. Copied with permission (Rincón and Pulgarin, 2006). Copyright 2006, Elsevier.
In this section, TiO2 based nanomaterials have been briefly introduced in the field of organic pollutant removal and disinfection of bacteria in wastewater. Regarding of this, TiO2 based nanomaterial is expected to perform as well as in actual usage.
g-C3N4-Based Photocatalytic Fenton Reaction
Due to the simple synthesis method, good physical and chemical stability and earth abundant characteristics, graphite carbonitride (g-C3N4) has attracted scientists attention in terms of energy conversion and environmental remediation (Lan et al., 2019). As to homogeneous Fe2+/H2O2 Fenton process, the water-soluble Fe2+ catalyzed H2O2 to produce hydroxyl radical; however, traditional homogeneous catalysts will be constrained by the following drawbacks: (1) low decomposition efficiency of H2O2; (2) sludge residue; (3) limited pH value between 3.0 and 4.0 (Qian et al., 2018b). Overall, iron leaching issue has posed serious problem in acidic aquatic environment. Additionally, g-C3N4 nanomaterials have emerged as a promising visible-light-response photocatalyst in organic removal and water splitting (Dong H. et al., 2018; Safaei-Ghomi et al., 2019). In 2012, Cui et al. (2012) firstly reported that visible light excited of g-C3N4 catalyzed H2O2 to produce ·OH radicals. Successively, g-C3N4 coupled with various types of Fe-based materials were explored for removing recalcitrant organic pollutants via photo-Fenton process, including of iron minerals (He et al., 2017), Fe(III) (Hu et al., 2016), single molecule iron complex (Chen et al., 2010; Banić et al., 2011), and iron oxide (Zhou L. et al., 2016; Wang et al., 2018) etc. Moreover, carbon nitride-based nanomaterials could serve as photo-Fenton-like membranes (Yu et al., 2018; Lan et al., 2019). Except of above mentioned, Li et al. (2016) constructed solid-gas-interfacial Fenton reaction over alkalinized-C3N4 photocatalyst in isopropanol photodegradation. The specific illustration will be revealed in this section.
Removing Organic Pollutant
Regarding photo-Fenton degradation of refractory organic pollutants, notably, Zhou L. et al. (2016) has built a highly efficient visible-light-driven heterogeneous photo-Fenton system (i.e., Fe2O3/g-C3N4/H2O2), which demonstrated that Fe2O3 nanoparticles were uniformly dispersed onto the surface of g-C3N4 (Figure 4A) and formed a heterojunction with g-C3N4 to enhance the charge separation. It can be clearly observed that Fe2O3 nanoparticles uniformly dispersed onto g-C3N4 nanosheets in comparison with pristine g-C3N4 (Figure 4B). As to degradation efficiency, the result was clarified in Figure 4C. The photo-Fenton capacity of Fe2O3/g-C3N4 was investigated for removing MO aided by H2O2 under visible-light irradiation. From the figure curves, it can be obtained that all the Fe2O3/g-C3N4 exhibited superior degradation efficiency than pure Fe2O3 and g-C3N4 as well as g-C3N4+Fe2O3. The absorbance intensity of MO decreased gradually with time as shown in Figure 4D. Moreover, the kinetic constant of photo-Fenton degradation efficiency of Fe2O3/g-C3N4 was determined as shown in Figure 4E. Significantly, the k-value of Fe2O3/g-C3N4 is about 45.4 times higher than that of Fe2O3, 8.4 times higher than that of g-C3N4 and 7.2 times than that of g-C3N4+Fe2O3, respectively. Based on experimental analysis, a possible mechanism is proposed in Figure 4F. Once irradiated with visible light, the electron-hole pairs will be produced from both of Fe2O3 and g-C3N4; meanwhile, the interfacial charge transfer promoted by the contact between Fe2O3 and g-C3N4 will further enhance the charge separation efficiency. The highly efficient efficiency not only arose from the photo-generated holes reacting with water molecular to produce hydroxyl radicals, but also the photo-induced electrons will be utilized to produce hydroxyl radicals. The generated electrons in the CB of Fe2O3 and transferred from the CB of g-C3N4 will contributed the conversion between Fe3+ and Fe2+. Overall, the generation rate of hydroxyl radicals which produced during Fenton process will be accelerated, thus leading to the enhancement of degradation efficiency. In addition, the fully consumption of photo-induced electrons during Fenton process will ulterior promote the charge separation, introducing more holes devoting to degradation process. Lastly, the catalytic activity can be greatly enhanced if photocatalysis technique joint with Fenton process were employed in one system.
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Figure 4. (A) TEM images of Fe2O3/g-C3N4 and (B) g-C3N4. (C) Photo-Fenton degradation efficiency of various catalysts for 20 mg L−1 MO under visible light irradiation. (D) UV/Vis spectra of MO of Fe2O3/g-C3N4 at different time intervals. (E) Corresponding rate constant of various catalysts. (F) Proposed mechanism in the photo-Fenton reaction. Copied with permission (Zhou L. et al., 2016). Copyright 2016, Wiley.
Differentiate with the sole Fe2O3 nanoparticles, Guo et al. (2019) constructed a Z-scheme hetero-structured α-Fe2O3@g-C3N4 catalyst, which was obtained by calcination of melamine and Fe-based MOF. Commonly, MOFs are mainly used to prepare metal oxide nanoparticles, which can inherit the merits of metal oxide acting as sacrifice templates. The morphology characterization has been shown in Figure 5A. Regarding the TC degradation efficiency, by adjusting the addition amount of MIL-53 (Fe), the researchers obtained the optimized composite, which exhibited excellent photo-Fenton performance for the removal of TC (Figure 5B). The corresponding degradation kinetic rate was calculated in Figure 5C, the k-value of FOCN-0.45 (0.042 min−1) was 6, 7, and 14 times, compared to that of pristine MIL-53 (Fe) (k = 0.007 min−1), α-Fe2O3 (k = 0.006 min−1), and g-C3N4 (k = 0.003 min−1), respectively. As to reactive species, the scavenger results in Figure 5D demonstrated that ·OH was the dominant species, accompanied with and h+ co-participating in the degradation of TC. The specific mechanism of photo-Fenton reaction over Z-scheme hetero-structured α-Fe2O3@g-C3N4 was shown in Figure 5E. Generally, g-C3N4 and α-Fe2O3 can be both excited under visible light (420 nm) irradiation to produce photo-induced electron (e–) and hole (h+). Owing to the Z-scheme structure, a proportion of electrons of the CB of α-Fe2O3 would be prone to transferring to the VB of g-C3N4. The electron of CB for g-C3N4 would react with O2 to produce superoxide radicals to degrade pollutants. Because of the CB position of α-Fe2O3 more negative than the standard potential of Fe3+/Fe2+ (0.77 V vs. NHE), thus the electrons of α-Fe2O3 were able to participate in the cycling of Fe3+/Fe2+, leading to activating H2O2 to generate hydroxyl radical. As to the holes in the VB of α-Fe2O3, the VB position of α-Fe2O3 (1.98 V vs. NHE) was lower than the potential of OH−/·OH (1.99 V vs. NHE). Consequently, the accumulated holes in the VB of α-Fe2O3 directly degraded the organic pollutants.
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Figure 5. (A) SEM images of MIL-53 (Fe) (a), α-Fe2O3 (b), (c) FESEM image of FOCN-0.45 and its amplified area (d); (e) TEM and HRTEM (f) images of FOCN-0.45, (g) HAADF-STEM image; (h–k) corresponding EDX mapping of FOCN-0.45. (B) TC degradation efficiency of different samples. (C) Kinetic calculation of various samples at different conditions. (D) Scavenger test of FOCN-0.45 for removing TC. (E) Photo-Fenton reaction mechanism illustration of α-Fe2O3@ g-C3N4 heterojunction. Copied with permission (Palanivel et al., 2019). Copyright 2019, Elsevier.
Furthermore, some other novel g-C3N4 based nanomaterials have been progressed in photo-Fenton application. For instance, Palanivel et al. (2019) rationally designed an effective heterostructure (ZnFe2O4/g-C3N4) nanocomposite, which showed good photo-Fenton performance for degradation of 20 mg/L MB. Besides Fe2O3 nanoparticles, Sahar et al. (2017) prepared Fe3O4/g-C3N4 nanoparticles mainly throughout electrostatic self-assembly effect, exhibiting an obviously enhanced Fenton, photo-Fenton and peroxidase-like efficiency. To achieve the goal of easy access of recycling, Wang H. et al. (2017) synthesized magnetic BaFe12O19/g-C3N4 nanocomposites, which showed excellent capacity and recyclability for removing RhB. The enhanced photocatalytic performance was mainly attributed to the synergy of BaFe12O19 and g-C3N4, thus facilitating charge separation, and thereby promoted the photo-Fenton efficiency. For more examples, we have specially summarized many other excellent g-C3N4 based nanomaterials in Table 2.
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Table 2. Summary of C3N4 based heterogeneous photo-Fenton like process under visible light irradiation.
Volatile Organic Compounds (VOCs) Removal
Apart from the liquid-phase Fenton reaction, Li et al. (2016) constructed a solid-gas interfacial Fenton system, which is joint with an alkalized g-C3N4 based photocatalyst, converting the photogenerated H2O2 into reactive oxygen species (ROS). This powerful case contains light acting as driving force, alkalized g-C3N4-based photocatalyst as an in-situ H2O2 collector and surface-coated Fe3+ as a trigger for H2O2 conversion, thus achieving efficient activity of VOCs photodegradation. For instance, taking the photo-oxidation of isopropyl alcohol as a model. Specifically, Figure 6A showed the generation rate and apparent quantum yield (AQY) of acetone and CO2 at about 420 nm of each CNK-OH&Fe catalyst with various Fe3+ loadings. Notably, the CNK-OH&Fe samples exhibited an increasing photocatalytic capacity accounting for the loading amount of Fe3+. Consequently, when the loading amount 1.4 wt%, it exhibited the optimal efficiency, signifying the increasing amount of Fe3+ loading not only promoting the utilization rate of photoelectron as well as suppressing the light absorption of catalyst. The IPA photooxidation rate of the optimal CNK-OH&Fe sample achieved ca. 270 folds higher than that of pristine C3N4 (Figure 6B). In Figure 6C, the generating H2O2 was difficult to be observed over CNK-OH&Fe, attributing to the generated H2O2 quickly reacting with Fe2+ and the photoelectrons quickly reducing Fe3+ to form ·OH radicals. To confirm the truth of H2O2 converting into ·OH radicals over CNK-OH&Fe, EPR test were performed. It was found that apart from the direct ·OH formation of surface hydroxyl radicals inducing from holes, the CNK-OH&Fe indeed triggered the Fe2+ and H2O2 converting into ·OH. On one hand, the enhanced efficiency was arising from the conversion of electrons promoting the generation of reactive radicals (e.g., ·OH and · radicals) via the solid-gas interfacial Fenton process. On the other hand, the hydroxyl and Fe3+ species determined the in-situ protons production and Fe2+/Fe3+ formation. In Figure 6D, the amount of hydroxyl radicals over CNK-OH&Fe was ca. 4.0 times than that of CNK-OH. Figure 6E elaborately illustrated the possible mechanism over CNK-OH&Fe catalyzed isopropanol conversion. As to quantum yield, in Figure 6F, compared with the pristine C3N4, more than two orders of magnitude of conversion rate were obtained, which is equivalent to AQY of 49% around 420 nm. In addition, it is expected that this discovery is not only unique for C3N4-based nanomaterials, but provide a useful guidance for other semiconductors which can be alkalized on surface (e.g., WO3 and SrTiO3) should also provide the same light-driven Fenton process. It is expected that this will open a low economic cost and facile way to employ solar energy to effectively eliminate gaseous organic pollutants.
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Figure 6. (A) Product yield and AQY of CNK-OH&Fe as a function of Fe3+ loading. (B) Photooxidation efficiency of various C3N4-based photocatalysts. (C) Yield of H2O2 production over C3N4-based samples. (D) EPR spectra of DMPO-·OH of different samples. (E) Proposed mechanism of solid-gas interfacial Fenton reaction over CNK-OH&Fe. (F) Photooxidation performance of various C3N4-based photocatalysts. Copied with permission (Li et al., 2016). Copyright 2016, ACS.
Photo-Fenton-Like Membranes for Wastewater Treatment
Membrane separation, a potential water treatment technology that has caused global attention to energy shortages and environmental pollution crises. In the last few decades, a vast amount of polymer and inorganic membranes have been deeply developed. Particularly, reverse osmosis (RO), plays a pivotal role in the generation of high-standard purified water, and simultaneously removes a vast number of contaminants (e.g., total dissolved solids, pathogens and organic pollutants).
In membrane development history, the thin film composite (TFC) polyamide membranes, thin film nanocomposite (TFN) membranes, molecular layer-by-layer (mLBL) membranes and self-assembled artificial water channels, carbon nanotubes (CNTs), microporous metal-organic frameworks (MOFs), graphene, graphene oxide, and MoS2 incorporated membranes have been deeply developed and studied; however, to meet all the demands of high-quality membranes including high selectivity, evitable stability and anti-fouling capabilities as well as high permeation flux, is still a huge challenge (Yang et al., 2016, 2018; Tang et al., 2018).
Interestingly, carbonaceous materials are gradually recognized as promising candidates for designing high-performance membranes, including of 1D carbon nanotubes, 2D graphene as well as its derivatives. However, for the practical application of carbon materials, it is limited by the difficulty of manufacturing densely arranged nanostructures. In addition, as to graphene-based membranes, it is necessary to greatly enhance the permeability and structural stability of the membrane. Therefore, it is of paramount importance to constructing membranes with feasible mass transfer resistance and water permeation flux. It is worth noting that graphite carbon nitride (g-C3N4) is expected to establish a new platform for the development of innovative membranes with excellent separation and self-cleaning properties, especially ultra-thin g-C3N4 nanosheets. On this condition, for the first time, Lan et al. (2019) innovatively manufactured g-C3N4-based catalytic membrane by soft self-assembly method for water purification. This membrane is mainly based on the nanochannels in g-C3N4 nanosheets and heterogeneous catalysis engaging in Fe-containing polyoxometalates. As shown in Figure 7A, the synthetic method of photo-Fenton-like membrane has been briefly described. Continuous modification of large amounts of g-C3N4 is beneficial to forming g-C3N4 sol (step II). Afterwards, customized g-C3N4 sol molecules with abundant amino and hydroxyl functional groups, provide enough active sites for Fe-POM nucleation (step III). After that, the self-assembly of the dispersion is filtered through a polycarbonate filter membrane to perform a carbonitride-based membrane treatment (Step IV). As shown in Figures 7B–E, the performance of POM incorporated into a g-C3N4 membrane was evaluated by pumping wastewater into membrane. First, the effect of carbon nitride microstructure was studied. As shown in Figure 7B, MB molecules can be completely removed from the water by g-C3N4 membrane. Compared with the melamine-derived membrane, the urea-derived sample showed much better removal efficiency. Moreover, the versatility of Ug-C3N4/Fe-POMs membranes in water purification was tested by filtering various dye molecules, as shown in Figure 7C. The results indicated that the synthesized membranes are capable of intercepting methyl blue (MB), Congo red (CR), methyl orange (MO), and rhodamine B (RhB) is extremely effective. In Figure 7D, the unique nanochannels in Ug-C3N4/Fe-POM ensured that contaminant molecules are completely removed through each filtration process. Therefore, if coupled with light irradiation and H2O2, this membrane exhibited stable water flux and good repelling ability (Figure 7E). In addition, a possible mechanism was proposed in Figure 7F. It was revealed that periodic atomic vacancies and structural defects in tri-s-triazine are conducive to the nanopores on the surface of the g-C3N4 layer. It is convinced that the gap between layers of carbon nitride provided a shortcut to dyes molecular diffusion. The modification of Fe-POMs significantly improved the separation performance of carbon nitride. In actual sewage treatment, the membranes were verified by pumping actual textile wastewater (initial COD: 290 mg L−1) over U-g-C3N4/Fe-POMs membranes. Generally, the greater the amount of Fe-POM in the membrane, the higher the rejection rate, and the smaller the permeation flux of water (Figure 7G). As can be seen from Figure 7H that the addition of Fe-POMs posed a significant impact on the COD removal performance. Therefore, the U-gC3N4/Fe-POMs membrane maintained the same COD rejection rate and permeable flux after five batches of treatment. It well reveals the merits of self-assembled photo-Fenton membrane based on Fe-POM. Overall, this work provides a viable way to develop catalytic carbonitride-based membranes for water purification.
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Figure 7. (A) Schematic illustration of synthesis methodology of g-C3N4/Fe-POMs membranes. (B) Dye removal efficiency over different carbon-nitride-based membranes. (C) Filtration ability for U-g-C3N4/Fe-POMs membranes over various dye solutions. (D) Intermittent combination of membrane separation and photocatalytic oxidation process. (E) Treating MB solution by U-g-C3N4/Fe-POMs membranes with the aid of light and H2O2; (F) Possible mechanism of water purification over U-g-C3N4/Fe-POMs membranes. (G) Influence of Fe-POMs content on the water permeation flux. (H) COD removal rate of U-g-C3N4 and U-g-C3N4/1% Fe-POMs membranes in batch filtration experiments. Copied with permission (Lan et al., 2019). Copyright 2019, ACS.
In this section, graphite carbon nitride (g-C3N4) nanomaterials were reviewed for the application of photo-Fenton process in removing organic pollutant, volatile organic compounds (VOCs) removal and membrane separation for wastewater. If irradiated by visible light or solar light, the photo-generated electrons facilitated the cycle of Fe3+/Fe2+, leading to the enhanced efficiency of organic pollutant and VOCs removal, which is attributed to the simultaneous photocatalytic effect as well as photo-Fenton oxidation process. According to this line of thought, g-C3N4 can be a promising candidate not only in organic pollutant removal but also assist in membrane separation and self-cleaning.
Reduced Graphene Oxide (RGO)-Based Photocatalytic Fenton Reaction
Graphene has been recognized as an excellent candidate supporting material owing to its unique physical properties, such as a high electric potential density, electron mobility, and optical absorption (Nair et al., 2008; Mayorov et al., 2011). In the field of environmental remediation, for instance, graphene is usually employed as a good support for loading nanomaterials, such as TiO2, ZnO, Fe2O3, CdS, Co3O4, and CdSe, etc. (Xiang et al., 2012; Guo et al., 2013) With the assistance of graphene, these nanocomposites can be uniformly dispersed onto graphene nanosheets and suppress aggregation, thus enhancing the catalytic efficiency because of excellent specific area and electrical conductivity (Wang et al., 2016). In the broad sense of Fenton process, two-dimensional graphene nanosheets and three-dimensional graphene hydrogel or aerogel have been deeply investigated. In the last decade, graphene-based nanocomposites, such as Fe2O3/graphene oxide and Fe3O4/RGO composites have been widely studied in removing organic pollutants (Xiang et al., 2012; Qiu et al., 2016b). These reported graphene-based photocatalysts show excellent photocatalytic activity, which is due to the transfer of photogenerated charge from the NPs surface to the graphene surface, thereby promoting the separation of electron-hole pairs and the generation of additional ·OH radicals. In addition, the combination of the aromatic ring of rGO and organic pollutants mainly comes into being π-π interaction and electrostatic interaction, which is conducive to the degradation process (Zhang et al., 2009; Sun et al., 2020). Especially, Fe2O3, FeOOH, and Fe3O4 have been reported effective in Fenton reaction. Additionally, the three-dimensional graphene-based hydrogels, as well as aerogels have been investigated a lot in photo-Fenton like reaction. In this section, we will clarify them gradually.
Two-Dimensional Graphene-Based Nanomaterials
In 2016, Wang et al. (2016) prepared Fe3O4@HG with hydrophilic graphene (HG) and Fe3O4, which presented high removal efficiency of methyl orange (MO), owing to the rapid transference of photo-generated electrons on the surface of HG to promote the conversion efficiency of Fe3+/Fe2+. Further, Liu et al. (2017a) developed α-Fe2O3 loaded graphene oxide (GO) nanosheets through a facile hydrolysis process, which showed good Fenton-degradation efficiency for MB as well as rhodamine B, Orange II and Orange G, phenol, 2-nitrophenol and 17-estradiol (E2). An et al. (2013a) synthesized graphene-BiFeO3 composite for the photo-Fenton degradation of tetrabromobisphenol A. Moreover, Qiu et al. (2016b) innovatively adopted a Stöber-like method to synthesize ultra-dispersed Fe3O4 nanoparticles onto graphene, which was applied in photo-Fenton reaction and lithium-battery. As presented in Figure 8A, the in-situ grown Fe3O4 particles were uniformly dispersed on graphene layers via a Stöber-like method. The morphology and structure of the Fe3O4/RGO composites have been characterized by TEM and HRTEM, as shown in Figures 8B,C, respectively. TEM images showed that the thickness of the RGO plate with highly dispersed Fe3O4 nanoparticles is ultrathin (Figure 8B). The HRTEM image of the Fe3O4/RGO composite (Figure 8C) showed that all nanoparticles are well-dispersed on graphene, and the size distribution shows an average size of 3–8 nm. HRTEM images show clear crystal lattices with a pitch of 0.250 and 0.296 nm, corresponding to the (311) and (220) planes, respectively. The photo-Fenton ability was conducted in the presence of H2O2 under the irradiation of solar light. Correspondingly, the result showed that the Fe3O4/RGO catalyst possessed superior photo-Fenton activity (98%) (Figure 8D). The possible mechanism has been illustrated in Figure 8E. Once irradiated by solar light, these electrons generating from dyes and Fe3O4 NPs transferred to the graphene; meanwhile, Fe3+ ions can easily be reduced by photo-electrons back to Fe2+. Consequently, Fe2+ continue to react with H2O2 to produce more ·OH radicals on the surface of graphene (Figure 8E). As a result, the Fe3O4/RGO composites possessed an excellent photo-Fenton capacity for degradation of organic dyes (Figure 8F), which can be recycled by an extra magnetic (Figure 8G).
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Figure 8. (A) Synthetic route of Fe3O4/RGO nanomaterials; (B) TEM image and (C) HRTEM image of Fe3O4/RGO nanomaterials (Inset: particle size distribution of the loaded Fe3O4 nanoparticles); (D) Cycling test of Fe3O4/RGO and commercial Fe3O4 for degradation of MO irradiated by solar light; (E) Possible proposed photo-Fenton process mechanism of Fe3O4/RGO; (F) Photo-Fenton degradation of 10 mg/L MB and 10 mg/L RhB; (G) Magnetic recycle of Fe3O4/RGO photocatalyst. Copied with permission (Qiu et al., 2016b). Copyright 2016, Elsevier.
Three-Dimensional Graphene-Based Hydrogel or Aerogel
Besides the two-dimensional (2D) graphene-based heterogenous catalysts, to achieve the facile recycle of catalysts, three-dimensional (3D) graphene hydrogel or aerogel has been developed by many researchers, such as cobalt ferrite nanoparticles related graphene aerogel (Qiu et al., 2016a), graphene oxide (GO)-carbon nanotubes (CNTs)-FeOOH aerogel (Liu et al., 2017b), stretchable Fe2O3/graphene aerogels (Qiu et al., 2015), reduced graphene oxide nanosheet/FexOy/nitrogen-doped carbon layer aerogel (Yao et al., 2019), Fe3O4/RGO/PAM hydrogel (Dong et al., 2018c), etc.
Among these 3D graphene-based materials, especially, the graphene oxide aerogels have attracted tremendous attention in recent years, which is a new category of porous 3D framework, owing to its large surface-to-volume ratio, good electron mobility and conductivity, superior mechanical stability, and good adsorption capacity (Leary and Westwood, 2011). In a broad sense, 2D layered structures equipped with sp2- and sp3-hybridized carbon atoms are assembled in hexagonal rings, making graphene a useful building block for matrix self-assembly, whereas graphene aerogel can be effectively prevent graphene sheets from stacking. Generally, doping a metal oxide into porous 3D structure can effectively suppress stacking; meanwhile, these 3D aerogels possess huge surface area, fast mass and electron transfer kinetics, short diffusion paths in the matrix and good mechanical strength (Sui et al., 2012).
In addition, some previous studies have reported that nanoscale iron oxide in graphene aerogels, which possessed few defects, are especially effective for photo-Fenton degradation of phenols and more complex benzene ring compounds. For example, Qiu et al. (2015) reported three-dimensional (3D) Fe2O3 in-situ grown on graphene aerogels via Stöber-like method, as shown in Figure 9A. The obtained Fe2O3/GAs exhibited a 3D macroscopic appearance (Figure 9B, inset a) and an ultralight materials property. With the reaction volume increasing, the obtained Fe2O3/GAs increase in size (Figure 9B, inset b). The SEM image of Fe2O3/GAs indicated that the Fe2O3/GAs possessed a 3D macroporous structure (Figure 9B, inset c,d). Fe2O3 nanocrystals are well-embedded into graphene sheets (Figure 9B, inset e). TEM results further confirmed the highly dispersed Fe2O3 nanocrystals on the surface of graphene (Figure 9B, inset f,g). HRTEM image of Fe2O3/GAs revealed that d-spacing lattice fringes is ca. 0.25 nm, which corresponded to the (110) planes of Fe2O3 (Figure 9B, inset h). Subsequently, Fe2O3/GAs was investigated for the solar-light-driven Fenton reaction the in the presence of H2O2 for the degradation of 10 mg/L MO solution (Figures 9C–E). The highly photo-Fenton reaction activity was mainly caused by the conversion of Fe3+/Fe2+ instead of the adsorption. Unlike Fe2O3/GR, the 3D-GAs can prevent the Fe2+ from dissolving in the solution owing to its channels and confinement effect. Besides the acidic conditions, the pH value can be adapted to neutral condition at 7.0, and the corresponding Fenton efficiency of Fe2O3/GAs still keeps at a high level, which is much higher than pristine Fe2O3 (Figures 9D,E). It is well-known that Fe3+/Fe2+ cycle plays a crucial role in Fenton reaction. The excessive Fe3+ ions easily produce iron sludge in the aqueous solution, thus deactivating and poisoning the catalyst. In Fenton process, Fe3+ ions can capture the photo-induced electrons and be reduced to Fe2+, thus a high concentration of Fe2+ ions, which is complexed with phenanthroline can be detected in the solution (Figure 9F). The three-dimensional frame can effectively prevent the dissolution of iron. Once irradiated by solar light, the electrons can be both excited from dyes and Fe2O3 nanocrystals, which will transfer to the graphene (Figure 9G), leading to positive ions (e.g., Fe2+ and Fe3+) adsorbed onto the negative-charge graphene. These Fe3+ ions will subsequently capture photo-induced electrons and react with H2O2 to produce Fe2+ ions again.
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Figure 9. (A) Illustration of synthesis methodology of Fe2O3/Gas; (B) (a) photographs of as-prepared Fe2O3/GAs; (b) images of Fe2O3/GAs prepared via different reaction volumes; (c) SEM and (d,e) FE-SEM image of Fe2O3/GAs (f,g) TEM, and (h) HR-TEM images of Fe2O3/GAs; (C) cycling test for the photo-Fenton degradation of MO; (D) photo-Fenton degradation of MO by Fe2O3/Gas; (E) cycle test for the photo-Fenton degradation of MO on different catalysts (F) Fe2+ detection of Fe2O3 powders and Fe2O3/GAs in the presence of 1,10-phenanthroline; (G) Photo-Fenton reaction model of Fe2O3/GAs. Copied with permission (Qiu et al., 2015). Copyright 2015, RSC.
Additionally, Liu et al. (2018) used non-toxic sodium ascorbate as reductant to synthesize FeO(OH)/reduced graphene oxide aerogel (FeO(OH)-rGA) by means of facile and green-chemistry approach. The stable anchorage can ensure the circulation rate and reduce the leaching of iron, thereby ensuring the effective degradation of toxic phenolic compounds under visible light irradiation. Overall, the aerogel obtained not only has a stable structure, but also has significant catalytic activity for the degradation of phenolic organics at neutral pH. Besides this, Dong et al. (2018c) also developed a functional graphene hydrogel equipped with simultaneous photocatalytic Fenton reaction activity for the degradation of organic pollutants and adsorption for the heavy metal ions. These above-mentioned 3D graphene-based hydrogels or aerogels provide researchers a new pathway to process the wastewater treatment.
In a summary, reduced graphene oxide (RGO) based nanomaterials have been extensively employed in photocatalytic Fenton reaction, varying from two-dimensional (2D) nanosheets to three-dimensional (3D) hydrogels or aerogels. Especially, the 2D graphene nanosheets blocked the aggregation of iron oxide nanoparticles, benefiting the electron transferring on graphene and impeding the generation of iron sludge during photo-Fenton process. As to 3D graphene hydrogels and aerogels, it can effectively prevent the stacking of 2D graphene nanosheets by doping of metal ions. Moreover, its 3D-structured shape is suitable for multiple and simple recycling. It is expected that reduced graphene oxide will build a new platform for environmental remediation.
Other Semiconductors-Based Photocatalytic Fenton Reaction
Except of the TiO2-based, graphitic carbon nitride-based and graphene-based nanomaterials, some other semiconductor-based nanomaterials can work as effectively as well in photo-Fenton process, such as Ag (Chen et al., 2016; Zhu et al., 2018a,b), BiVO4 (Xu et al., 2017; Li X. et al., 2018; Gao et al., 2019), ZnO (Choi et al., 2015; Ojha et al., 2017; Saleh and Taufik, 2019), ZnFeO4 (Khadgi and Upreti, 2019; Palanivel et al., 2019), and BiFeO3 (Luo et al., 2010; An et al., 2013b; Jia et al., 2018), etc. In this section, we will make a brief description, including of Ag-based nanomaterials, BiVO4-based nanomaterials, ZnFeO4 and BiFeO3 based nanomaterials, which all perform well as photo-Fenton like catalysts.
Ag-Based Photo-Fenton Catalysts
In the past few decades, people have paid great attention to the research on the heterogeneous photo-Fenton process for the degradation of organic pollutants. As far as known, the high combination rate of photo-generated carriers will impede its photocatalytic efficiency. Thus, an idea is proposed that the joint of photo-Fenton catalysts with plasmonic materials, this issue might be solved.
Some researchers have reported that Ag/AgCl/Fe-S plasmonic catalyst could effectively degrade bisphenol A in photo-Fenton system under visible light irradiation (Liu et al., 2017c). Further, Zhu et al. (2018b) reported that a novel photo-Fenton catalyst of Ag/AgCl/ferrihydrite could degrade bisphenol A as well, which revealed that the loading of Ag/AgCl could accelerate the conversion of Fe3+/Fe2+ by the photo-generated electrons transferring from Ag nanoparticles owning to the surface plasmon resonance (SPR) effect. Moreover, Chen et al. (2016) prepared Ag/hematite mesocrystal, which displayed a high photo-Fenton activity in the oxidation of RhB, MO and glyphosate under visible light irradiation.
BiVO4-Based Photo-Fenton Catalysts
In recent years, BiVO4 has been well-considered as a typical photocatalyst with narrow band gap. Due to its non-toxicity, good stability and photochemical properties, it has been extensively studied in the treatment of photocatalytic wastewater. In addition, BiVO4 is a good candidate cocatalyst in photo-Fenton process; however, pure BiVO4 is constrained by several drawbacks, such as a narrow visible light response range, low specific surface area, and rapid recombination of charged charges. Thus, various modification strategies have been adopted to investigate its potential application including doping ions, heterostructure fabrication, and precious metal deposition.
Specifically, Liu et al. (2017c) synthesized Fe(III) grafted BiVO4 nanosheets, which was employed for the photodegradation of 2,4-dichlorophenol and antibiotics. The results showed that the Fe3+ species not only served as efficient electron scavengers, but also provided more active reaction sites. The detailed mechanism is briefly contained in Figure 10A. In Figure 10B, pure BiVO4 merely could degrade ca. 60% of TC under visible-light irradiation. When the mass content of Fe(III) was 7%, the degradation efficiency can be up to 80%. Correspondingly, in Figure 10C, the TOC removal efficiency for 7% Fe(III)-BVO could be up to 48% after 60 min. Moreover, quinolones antibiotics, including CIP, GAT, and LVX were selected to be degraded. As shown in Figures 10D–G, the photocatalytic degradation efficiency of three antibiotics for Fe(III)-BVO is higher than that of pure BVO. Meanwhile, as an intermediate for the synthesis of pesticides and typical toxic organic compounds, photocatalytic degradation of 2,4-DCP possessed important practical significance for the treatment of wastewater.
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Figure 10. (A) Systematic illustration of preparation method and mechanism; (B) photocatalytic degradation efficiency of TC using various contents of Fe(III)-BVO; (C) Total organic carbon (TOC) removal efficiency of TC by 7% Fe(III)-BVO photocatalyst and the photocatalytic activity of BVO and 7% Fe(III)-BVO for the degradation of (D) CIP, (E) GAT, (F) LVX, and (G) 2,4-DCP. Copied with permission (Liu et al., 2017c). Copyright 2019, Elsevier.
ZnFeO4 and BiFeO3 Based Fenton Catalysts
ZnFe2O4 is a novel semiconductor, which possesses a band gap of about 1.9 eV, showing a visible light response, thus becoming attractive in the field of visible light-driven photocatalysis (Sun et al., 2013; Zhu et al., 2016). Accordingly, both ZnFe2O4 and ZnFe2O4-based composites can act as photo Fenton-like catalysts to improve the degradation rate of organic pollutants in the presence of H2O2 (Su et al., 2012; Yao et al., 2014). For instance, Su et al. (2012) reported mesoporous ZnFe2O4 (meso-ZnFe2O4) prepared by a hydrothermal process. As to photocatalytic degradation of AOII, it showed that AOII almost completely removed in H2O2/visible light system after 2 h. It was revealed that high efficiency for AOII degradation was mainly attributed to the strong absorption of ZnFe2O4 in visible-light region and more generation amount of ·OH by H2O2. Further, Fu and Wang (2011) reported that they employed a feasible and facile one-step hydrothermal route to fabricate magnetic ZnFe2O4 graphene nanocomposites. The results showed that as prepared photocatalysts could serve as the photoelectrochemical catalysts for organic pollutants removal and the generator of hydroxyl radicals via the decomposition of H2O2 under visible light irradiation. Moreover, Chen H. et al. (2017) synthesized ZnO/ZnFe2O4 nanocomposite to degrade organic dye in the presence of H2O2 under near-infrared (NIR) irradiation. It was reported that ZnO/ZnFe2O4 nanocomposite performed well for the degradation of methyl orange under either UV, visible or NIR irradiation.
Besides ZnFe2O4-based nanocomposites, it is also discovered that BiFeO3 perform as well in photo-Fenton like process. Generally, BiFeO3 is recognized as one of the important semiconductors, which possess the ability for visible light response. Additionally, BiFeO3 is widely accepted to be a promising visible-light-response photocatalyst for organic pollutant removal and hydrogen production (Bharathkumar et al., 2016). In Fenton-like process, Di et al. (2019) constructed Z-scheme heterojunction Ag2S/BiFeO3. The morphology of Ag2S/BiFeO3 was characterized by DF-STEM (Figure 11A). The corresponding elemental maps displayed as well. It showed a uniform elemental distribution of Bi, Fe, O and elements Ag and S. The result suggested that the Ag2S nanoparticles have been successfully loaded onto the surface of BiFeO3. Further, the photocatalytic and photo-Fenton efficiency was tested in Figures 11B,C, respectively. It was found that MO degradation was significantly enhanced with the addition of catalysts and H2O2. The specific mechanism of photocatalytic process and photo-Fenton process have been explained in Figures 11D,E. As far as known, the improved efficiency by photo-Fenton process mainly originated from the reduction of Fe3+ ions by photo-generated electrons, thus promoting the decomposition rate of H2O2. In addition, An et al. (2013a) prepared a nanoscale composite of BiFeO3 and graphene, and used this composite to degrade tetrabromobisphenol A by photo-Fenton-like process, which showed that the graphene-BiFeO3 composite exhibiting higher catalytic ability. Moreover, some researchers have found that BiFeO3-based nanomaterials not only work on photo-Fenton process, but also perform well in sulfate radical based Advanced Oxidation Process (SR-AOP), revealing its vast potential in the field of wastewater treatment.
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Figure 11. (A) Dark-field scanning TEM image of 15% Ag2S/BiFeO3; (b–f) Corresponding elemental mapping images; (B) Photocatalytic activities of Ag2S, BiFeO3, and Ag2S/BiFeO3 composites for the degradation of MO under visible-light irradiation; (C) Photo-Fenton catalytic activities of BiFeO3 and Ag2S/BiFeO3 composites for the degradation of MO in the presence of H2O2; Possible degradation mechanism of Ag2S/BiFeO3 composites (D) Photocatalytic process; (E) Photo-Fenton catalytic process. Copied with permission (Di et al., 2019). Copyright 2019, MDPI.
As above mentioned, some other novel Ag-based nanomaterials, BiVO4-based nanomaterials, ZnFeO4 and BiF eO3 based nanocomposites were elucidated that effectively work on photo-Fenton reaction, stemming for their property acting as photocatalysts providing photo-electrons, which was involved in the transformation of Fe3+/Fe2+. On condition of this, there is no doubt that many other novel semiconductor photocatalyst can synergistically improve efficiency of the Fenton process if acting as the dual catalysts.
Conclusions and Outlook
In the last few decades, the photo-Fenton and photo-Fenton like process have been progressed a lot. The new developed photo-Fenton or photo-Fenton like process present higher efficiency than that of conventional ones. The key to solve the drawbacks of Fenton process is accelerating the transformation of Fe3+/Fe2+. Facing with this issue, the worldwide scientists have looked for many methodologies. Firstly, the TiO2 and modified TiO2 nanocomposites are able to perform as homogenous or heterogenous photo-Fenton cocatalysts, which focus on speeding up the cycle of Fe3+/Fe2+, leading to less iron sludge and higher removal efficiency for organic pollutant or bacteria. Secondly, to our best knowledge, graphitic carbon nitride is a new promising candidate in photocatalysis. Various types of iron oxide coupled g-C3N4 nanocomposites have been employed in wastewater treatment, VOCs removal and photo-Fenton like membrane separation. Thirdly, graphene is another excellent two-dimensional nanomaterial acting as good support for its good electron transferring capacity and large surface area. On premise that of good activity for 2D structure, three-dimensional graphene-based hydrogel or aerogel have been attempted in photo-Fenton and photo-Fenton like process. As to most of the 3D structural materials, the water-soluble Fe2+ ions can be easily anchored onto graphene, which explains why the photo-Fenton efficiency presents so good. In addition, some other semiconductors, such as Ag, BiVO4, ZnFeO4, and BiFeO3 based nanomaterials have been reviewed briefly as well. In summary, the existing semiconductors, which are equipped with suitable band gap, large surface area, well-separated photo-generated carriers, which no doubt can be promising candidates in photo-Fenton process. Moreover, in recent years, some other alternative nanomaterials, such as metal disulfides, metal carbides, perovskites, etc. are all expected to provide good reference and efficacy in AOPs. However, facing the actual usage in industrial treatment, the sophisticated engineers usually focus more on the nanomaterials whether can be applied in actual sewage treatment on a large scale. Hence, the economic cost, recycling issues and stability need to be paid much attention to in the future development. In the field of actual wastewater treatment, the input energy and economic cost are both factors that we should consider in advance. Due to the injurious to human body and limited wavelength region of UV light, developing visible-light-response photocatalysts is a promising way to rationally treat wastewater, so as it can run at ambient temperature and pressure, utilizing atmospheric oxygen or H2O2 as oxidant. So far, synthesizing nanomaterials is of high cost, it is of great necessary to synthesize and adopt magnetic nanocomposites, such as Fe3O4-based and cobalt ferrite-based nanocomposites etc. Moreover, catalyst immobilization, which is closely associated with catalyst recovery and agglomeration, as well as the design of photocatalytic reactor are both key research points in the coming years. One possible strategy is heterogenous photo-Fenton like process with membrane processes (PMRs), and the other one is trying to prolong the contact time between solution and immobilized photocatalyst. Many efforts still need to be made to actual application of photo-Fenton-like process in the future development. If feasible, the nanomaterials in photo-Fenton-like process is expected to make a huge progress in environmental remediation.
Author Contributions
CD and MX conceived the proposal of this manuscript. CD wrote the paper. MX and JZ gave suggestions on the writing. All authors contributed to the article and approved the submitted version.
Funding
This work was supported by the State Key Research Development Program of China (No. 2016YFA0204200), the National Natural Science Foundation of China (Nos. 21822603, 21677048, 21773062, 5171101651, and 21577036), the Shanghai Pujiang Program (No. 17PJD011), and the Fundamental Research Funds for the Central Universities (Nos. 22221818014 and 22A201514021).
Conflict of Interest
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
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Keywords: photocatalytic Fenton reaction, organic pollutant degradation, disinfection, volatile organic compound (VOC), membrance
Citation: Dong C, Xing M and Zhang J (2020) Recent Progress of Photocatalytic Fenton-Like Process for Environmental Remediation. Front. Environ. Chem. 1:8. doi: 10.3389/fenvc.2020.00008
Received: 08 July 2020; Accepted: 11 August 2020;
Published: 22 September 2020.
Edited by:
Qizhao Wang, Northwest Normal University, ChinaReviewed by:
Yanhui Ao, Hohai University, ChinaXufang Qian, Shanghai Jiao Tong University, China
Mingshan Zhu, Jinan University, China
Copyright © 2020 Dong, Xing and Zhang. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
*Correspondence: Mingyang Xing, mingyangxing@ecust.edu.cn; Jinlong Zhang, jlzhang@ecust.edu.cn