Photoelectrocatalytic/photoelectro-Fenton coupling system using a nanostructured photoanode for the oxidation of a textile dye: Kinetics study and oxidation pathway

Atrazine degradation through a heterogeneous dual-effect process using Fe-TiO2-allophane catalysts under sunlight

This study investigated the novel application of Fe-TiO2-allophane catalysts with 6.0 % w/w of iron oxide and two TiO2 proportions (10 % and 30 % w/w) for degrading atrazine (ATZ) using the heterogeneous dual-effect (HDE) process under sunlight. Comparative analyses with Fe-allophane and TiO2-allophane catalysts were conducted in both photocatalysis (PC) and HDE processes. FTIR spectra reveal the unique hydrous feldspathoids structure of allophane, showing evidence of new bond formation between Si-O groups of allophane clays and iron hydroxyl species, as well as Si-O-Ti bonds that intensified with higher TiO2 content. The catalysts exhibited an anatase structure. In Fe-TiO2-allophane catalysts, iron oxide was incorporated through the substitution of Ti4+ by Fe3+ in the anatase crystal lattice and precipitation on the surface of allophane clays, forming small iron oxide particles. Allophane clays reduced the agglomeration and particle size of TiO2, resulting in an enhanced specific surface area and pore volume for all catalysts. Iron oxide incorporation decreased the band gap, broadening the photoresponse to visible light. In the PC process, TiO2-allophane achieves 90 % ATZ degradation, attributed to radical species from the UV component of sunlight. In the HDE process, Fe-TiO2-allophane catalysts exhibit synergistic effects, particularly with 30 % w/w TiO2, achieving 100 % ATZ degradation and 85 % COD removal, with shorter reaction time as TiO2 percentage increased. The HDE process was performed under less acidic conditions, achieving complete ATZ degradation after 6 h without iron leaching. Consequently, Fe-TiO2-allophane catalysts are proposed as a promising alternative for degrading emerging pollutants under environmentally friendly conditions.

Multivariate optimization of methylene blue dye degradation using electro-Fenton process with self-doped TiO2 nanotube anode

This paper reports the optimization of the electro-Fenton (EF) process using different anode materials for the degradation of Methylene Blue (MB) dye as a model compound. The cathode used was an air-diffusion PTFE, while three different anode materials (Pt, DSA, and self-doped TiO2 nanotubes - SD-TNT) were tested individually. A full factorial design (FFD) with a central point combined with response surface methodology (RSM) was employed to optimize the experimental variables, including solution pH, applied current, and anode material. The optimized EF conditions involved a pH of 4.0, a current of 100 mA, and an SD-TNT anode for 120 min of electrolysis. Under these conditions, the MB solution achieved complete decolorization and 45% of total organic carbon (TOC) removal after 120 min of EF treatment. The findings indicate that the hydroxyl radical (•OH) plays a crucial role as the primary oxidizing agent in the EF process. The decay of MB followed pseudo-first-order kinetics, reflecting a consistent formation of •OH radicals that effectively attacked the MB dye and its subproducts during mineralization. Moreover, the EF process exhibited superior performance in terms of energy consumption (EC) and mineralization current efficiency (ECM) in the initial treatment stages, while the presence of recalcitrant by-products and loss of anode self-doping impacted performance in the later stages. The optimized EF conditions and the understanding gained from this study contribute to the advancement of sustainable wastewater treatment strategies for the removal of organic dyes.

Disclosing the mutual influence of photocatalytic fuel cell and photoelectro-Fenton process in the fabrication of a sustainable hybrid system for efficient Amaranth dye removal and simultaneous electricity production

Photocatalytic fuel cell (PFC) was employed to provide renewable power sources to photoelectro-Fenton (PEF) process to fabricate a double-chambered hybrid system for the treatment of azo dye, Amaranth. The PFC-PEF hybrid system was interconnected by a circuit attached to the electrodes in PFC and PEF. Circuit connection is the principal channel for the electron transfer and mobility between PFC and PEF. Thus, different circuit connections were evaluated in the hybrid system for their influences on the Amaranth dye degradation. The PFC-PEF system under the complete circuit connection condition attained the highest decolourization efficiency of Amaranth (PFC: 98.85%; PEF: 95.69%), which indicated that the complete circuit connection was crucial for in-situ formation of reactive species in dye degradation. Besides, the pivotal role of ultraviolet (UV) light irradiation in the PFC-PEF system for both dye degradation and electricity generation was revealed through various UV light-illuminating conditions applied for PFC and PEF. A remarkable influence of UV light irradiation on the production of hydrogen peroxide and generation and regeneration of Fe²⁺ in PEF was demonstrated. This study provided a comprehensive mechanistic insight into the dye degradation and electricity generation by the PFC-PEF system.

Orange G degradation by heterogeneous peroxymonosulfate activation based on magnetic MnFe2O4/α-MnO2 hybrid

Wastewater containing an azo dye Orange G (OG) causes massive environmental pollution, thus it is critical to develop a highly effective, environmental-friendly, and reusable catalyst in peroxymonosulfate (PMS) activation for OG degradation. In this work, we successfully applied a magnetic MnFe₂O₄/α-MnO₂ hybrid fabricated by a simple hydrothermal method for OG removal in water. The characteristics of the hybrid were investigated by X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy, Fourier transform infrared spectroscopy, Brunauer-Emmett-Teller method, vibrating sample magnetometry, electron paramagnetic resonance, thermogravimetric analysis, and X-ray photoelectron spectroscopy. The effects of operational parameters (i.e., catalytic system, catalytic dose, solution pH, and temperature) were investigated. The results exhibited that 96.8% of OG degradation was obtained with MnFe₂O₄/α-MnO₂(1:9)/PMS system in 30 min regardless of solution pH changes. Furthermore, the possible reaction mechanism of the coupling system was proposed, and the degradation intermediates of OG were identified by mass spectroscopy. The radical quenching experiments and EPR tests demonstrated that SO₄^•̶, O₂^•̶, and ¹O₂ were the primary reactive oxygen species responsible for the OG degradation. The hybrid also displayed unusual stability with less than 30% loss in the OG removal after four sequential cycles. Overall, magnetic MnFe₂O₄/α-MnO₂ hybrid could be used as a high potential activator of PMS to remove orange G and maybe other dyes from wastewater.

Fundamentals and applications of photoelectrocatalysis as an efficient process to remove pollutants from water: A review

Water pollution is an environmental problem in constant raising because of population growing, industrial development, agricultural frontier expansion, and principally because of the lack of wastewater treatment technology to remove organic recalcitrant and toxic pollutants from industrial and domestic wastewater. Recalcitrant compounds are a serious environmental and health problem mainly due to their toxicity and potential hazardous effects on living organisms, including human beings. Conventional wastewater treatments have not been able to remove efficiently pollutants from water; however, electrochemical advanced oxidation processes (EAOPs) are able to solve this environmental concern. One of the most recent EAOPs technology is photoelectrocatalysis (PEC), it consists in applying an external bias potential to a semiconductor film placed over a conductive substrate to avoid the recombination of photogenerated electron-hole (e^-/h⁺) pairs, increasing h⁺ availability and hydroxyl radicals' formation, responsible for promoting the degradation/mineralization of organic pollutants in aqueous medium. This review summarizes the recent advances in PEC as a promising technology for wastewater treatment. It addresses the fundamentals and kinetic aspects of PEC. An analysis of photoanode materials and of the configuration of photoelectrochemical reactors is also presented, including an analysis of the influence of the main operational parameters on the treatment of contaminated water. Finally, the most recent applications of PEC are reviewed, and the challenges and perspectives of PEC in wastewater treatment are discussed.

A comparison of visible-light photocatalysts for solar photoelectrocatalysis coupled to solar photoelectro-Fenton: Application to the degradation of the pesticide simazine

Three different visible-light photocatalysts (hematite (α-Fe₂O₃), bismuth vanadate (BiVO₄) and Mo-doped bismuth vanadate (BiMoVO₄)) deposited on transparent fluorine-doped SnO₂ (FTO) were evaluated for the solar-driven photoelectrocatalytic treatment of emerging pollutants. BiMoVO₄ was found to be the most effective photoanode, yielding the fastest degradation rate constant and highest mineralization efficiency using phenol as the oxidation probe. The BiMoVO₄ photoanode was then used to degrade the herbicide simazine in a photoelectrolytic cell combining photoelectrocatalysis (PEC) with photoelectron-Fenton (PEF) under solar light (SPEC-SPEC). Total simazine removal was achieved within 1 min of treatment (k_app = 4.21 min^-1) at the optimum electrode potential of 2.5 V vs Ag/AgCl, with complete TOC removal in 2 h. The analysis of anionic species in solution during treatment showed that most of the nitrogen heteroatoms in the simazine structure were converted into NO₃^- following ^•OH addition to organic N. This innovative process combining BiMoVO₄-PEC with PEF using solar light as a sustainable source of energy (SPEC-SPEF) achieved the highest degradation/mineralization efficiency ever reported for simazine treatment. Besides, this is the first work reporting the photo(electrochemical) degradation of this toxic herbicide.

Efficient activation of peroxymonosulfate and degradation of Orange G in iron phosphide prepared by pickling waste liquor

In this study, the low-cost preparation of iron phosphide by using pickling waste liquor as the initial material was performed through a two-step reaction. The degradation of Orange G was evaluated using iron phosphide coupled with peroxymonosulfate to construct a catalytic system. The removal efficiencies of Orange G and total organic carbon reached 97.4 and 58.4% at 60 min, respectively. Iron phosphide has dual-catalysis centers for the activation of PMS. Multiple free radicals (e.g., SO₄^•-, HO^•, SO₅^•-, and O₂^•-) and singlet oxygen were involved in the pollutant degradation, of which sulfate radicals played the main role. The iron phosphide catalyst exhibited excellent recycling stability, and its catalytic efficiency reached 95% after five cycles. In summary, the Fe₂P/PMS system-as a Fenton-like catalytic system-has certain advantages, including low cost, high efficiency, sufficient reusability, and good stability, all of which are favorable for its practical application.

Degradation of 2,4-dichlorophenoxyacetic acid by a novel photoelectrocatalysis/photoelectro-Fenton process using Blue-TiO2 nanotube arrays as the anode

2,4-dichlorophenoxyacetic acid (2,4-D)'s removal was studied in the photoelectrocatalysis/photoelectro-Fenton (PEC-PEF) process with Blue-TiO₂ nanotube (Blue-TNTs) and modified carbon felt as the anode and cathode, respectively. Polytetrafluoroethylene and carbon black were used to modify the carbon felt to improve the efficiency of H₂O₂ production. The impact factors of 2,4-D degradation in the PEC-PEF process were investigated, including Fe²⁺ dose, bias potential, light intensity and the concentration of 2,4-D. It was found that the removal of 2,4-D increased firstly and then decreased with the increase of Fe²⁺ dose. Bias potential and light intensity played a positive role on 2,4-D removal, while the opposite was right for the impact of 2,4-D initial concentration. Compared with stainless steel, the modified carbon felt was found more efficient for 2,4-D removal as it could generate more H₂O₂. Reactive species for 2,4-D degradation was studied and it was proved that •OH radical rather than holes was mainly responsible for the removal. Such PEC-PEF process offered a promising alternative for herbicide-containing wastewater treatment.

A novel photocatalytic reactor for the extended reuse of W-TiO2 in the degradation of sulfamethazine

In this study, a photocatalytic reactor with a novel engineering design has been used for the extended degradation of sulfamethazine (SMZ). The reactor employed four consecutive stainless-steel plates immobilized by tungsten-dope TiO₂ (W-TiO₂) using polysiloxane. The characterization of W-TiO₂ by X-ray diffraction (XRD), Raman spectroscopy, and energy-dispersive X-ray (EDX) denoted successful doping of tungsten in the lattice of anatase crystals of TiO₂ suggesting a high photocatalytic activity under UV and visible light. A Box-Behnken experimental design was employed for the optimization of the operating parameters such as solution pH, flow rate, and the initial SMZ concentration. The residual SMZ concentration was below the detection limit after 30 min of the photocatalytic reaction under the optimum operating conditions. A highly remarkable degradation of SMZ was observed in five consecutive cycles, which reveals an extended stable photocatalytic activity offered by the reactor design. The transformation products were identified by tandem mass spectrometry, and they were employed to propose the degradation pathway. These results highlight the importance of using the photocatalysts in retained forms and open additional avenues for the practical application of photocatalysis in wastewater treatment.

A review on the photoelectro-Fenton process as efficient electrochemical advanced oxidation for wastewater remediation. Treatment with UV light, sunlight, and coupling with conventional and other photo-assisted advanced technologies

Wastewaters containing recalcitrant and toxic organic pollutants are scarcely decontaminated in conventional wastewater facilities. Then, there is an urgent challenge the development of powerful oxidation processes to ensure their organic removal in order to preserve the water quality in the environment. This review presents the recent development of an electrochemical advanced oxidation process (EAOP) like the photoelectro-Fenton (PEF) process, covering the period 2010-2019, as an effective treatment for wastewater remediation. The high oxidation ability of this photo-assisted Fenton-based EAOP is due to the combination of in situ generated hydroxyl radicals and the photolytic action of UV or sunlight irradiation over the treated wastewater. Firstly, the fundamentals and characteristics of the PEF process are described to understand the role of oxidizing agents. Further, the properties of the homogeneous PEF process with iron catalyst and UV irradiation and the benefit of sunlight in the homogeneous solar PEF one (SPEF) are discussed, supported with examples over their application to the degradation and mineralization of synthetic solutions of industrial chemicals, herbicides, dyes and pharmaceuticals, as well as real wastewaters. Novel heterogeneous PEF processes involving solid iron catalysts or iron-modified cathodes are subsequently detailed. Finally, the oxidation power of hybrid processes including photocatalysis/PEF, solar photocatalysis/SPEF, photoelectrocatalysis/PEF and solar photoelectrocatalysis/SPEF, followed by that of sequential processes like electrocoagulation/PEF and biological oxidation coupled to SPEF, are analyzed.

Study of TiO2/Ti4O7 photo-anodes inserted in an activated carbon packed bed cathode: Towards the development of 3D-type photo-electro-Fenton reactors for water treatment

In this work, commercially available Polymethyl-meta-acrylate (PMMA) spectroscopy cells were modified on the external walls with films of TiO₂, Ti₄O₇ or TiO₂/Ti₄O₇ mixtures. Film characterization was carried out using SEM and UV–vis spectroscopy. The results of photocatalytic (PC), electro-oxidation (EO), and photoelectrochemical (PEC) experiments on the decolorization of a methyl orange (MO) model dye solution showed that while anatase provides better photocatalytic properties and the partially reduced Ti₄O₇ larger electronic conductivity, the TiO₂/Ti₄O₇ composite film behaves as a semiconductor substrate that combines the advantages of both materials (for PEC experiments for instance, decolorization values for the model dye solution using TiO₂, Ti₄O₇ and a TiO₂/Ti₄O₇ mixed film, corresponded to 35%, 46% and 53%, respectively). In order to test this film as an effective photoanode material in a 3-D type reactor for water treatment processes, a TiO₂/Ti₄O₇ modified PMMA spectroscopy cell was inserted in an activated carbon (AC) bed so that the semiconductor material could be illuminated using an external UV source positioned inside the PMMA cell. The connected AC particles that were previously saturated with MO dye were used as cathode sites for the oxygen reduction reaction so that the photoelectrochemical reactions that take place in the anode could be complemented with coupled electro-Fenton processes in the cathode. As expected, the combination resulted in an effective decolorization of the dye solution that results from a complex combination of processes. The experimental decolorization data was successfully fitted to a pseudo-first order kinetic model so that a deeper understanding of the contribution of each process in the reactor could be obtained.

A dual TiO2/Ti-stainless steel anode for the degradation of orange G in a coupling photoelectrochemical and photo-electro-Fenton system

A dual-anode consists of stainless steel and TiO₂/Ti electrodes is used to study the kinetics of the degradation of hazardous chemicals exemplified by azo dye orange G (OG) using a coupling photoelectrochemical catalytic and photoelectro-Fenton (PEC/PEF) system. Concurrent generation of hydroxyl radicals on the TiO₂/Ti photocatalyst and in-situ generation of Fenton reagents on the stainless steel electrode greatly enhances the performance of the PEC/PEF electrodes over that of the PEC and the PEF alone process. The efficiency of the PEC/PEF process is a function of Fe²⁺ and H₂O₂ concentration OH⋅ in the solution bulk, which promotes the oxidative degradation of OG and its byproducts. The mean carbon oxidation state (COS) is estimated to reflect the degree of mineralization. Based on the pseudo first-order kinetics with respect to OH, OG, Fe²⁺, the corresponding reaction rates is established. UV-Vis spectrometry reveals the presence of four major intermediates, which helps establish the OG degradation pathways.

Application and stability of cathodes with manganese dioxide nanoflowers supported on Vulcan by Fenton systems for the degradation of RB5 azo dye

This work describes the electrochemical degradation of Reactive Black 5 (RB5) by two methods: electrochemical and photo-assisted electrochemical degradation with and without a Fenton reagent. Two anodes were used, Pt and boron-doped diamond (BDD, 2500 ppm), and the cathode was 3% MnO₂ nanoflowers (NFMnO₂) on a carbon gas diffusion electrode (GDE). An electrochemical cell without a divider with a GDE with 3% w/w NFMnO₂/C supported on carbon Vulcan XC72 was used. The decolorization efficiency was monitored by UV-vis spectroscopy, and the degradation was monitored by Total Organic Carbon (TOC) analysis. For dissolution monitoring, aliquots (1 mL) were collected during the degradation. After 6 h of H₂O₂ electrogeneration, the manganese concentration in the RB5 solution was only 23.1 ± 1.2 μg L^-1. It was estimated that approximately 60 μg L^-1 (<0.2%) of manganese migrated from the GDE to the solution after 12 h of electrolysis, which indicated the good stability of the GDE. The photoelectro-Fenton-BDD (PEF-BDD) processes showed both the best color removal percentage (∼93%) and 91% of mineralization. The 3% NFMnO₂/C GDE is promising for RB5 degradation.

A novel magnetic nanoscaled Fe3O4/CeO2 composite prepared by oxidation-precipitation process and its application for degradation of orange G in aqueous solution as Fenton-like heterogeneous catalyst

In this work, magnetic nanoparticles (MNPs) Fe₃O₄/CeO₂ were firstly synthesized using three different preparation methods, including coprecipitation, impregnation oxidation-precipitation and impregnation reduction-precipitation, respectively. The catalytic activities of Fe₃O₄/CeO₂ MNPs, prepared by above three methods, were compared systematically in terms of the degradation of orange G (OG). The impregnation oxidation-precipitation process was economical and maneuverable due to the usage of air, no need of nitrogen protection and higher utilization efficiency of iron. Response surface methodology based on central composite design were used to investigate the individual and interactive effects of three process parameters on the OG degradation, i.e. the initial pH of the solution, the dosage of H₂O₂ and the initial concentration of OG. Under the optimal conditions of pH 2.5, H₂O₂ 30 mM, OG 50 mg L^-1, catalyst 2.0 g L^-1 and 35 °C, the degradation percentage of OG was 98.2% within 120 min, which agreed well with the modeling prediction (R² = 0.9984, and Adj-R² = 0.9969). And the degradation reaction well followed the first-order kinetic with R² = 0.9969. The Fe₃O₄/CeO₂-OX MNPs showed high catalytic activity, stability and reusability in the degradation of OG.

Sequential two-column electro-Fenton-photolytic reactor for the treatment of winery wastewater

The high amount of winery wastewaters produced each year makes their treatment a priority issue due to their problematic characteristics such as acid pH, high concentration of organic load and colourful compounds. Furthermore, some of these effluents can have dissolved pesticides, due to the previous grape treatments, which are recalcitrant to conventional treatments. Recently, photo-electro-Fenton process has been reported as an effective procedure to mineralize different organic contaminants and a promising technology for the treatment of these complex matrixes. However, the reactors available for applying this process are scarce and they show several limitations. In this study, a sequential two-column reactor for the photo-electro-Fenton treatment was designed and evaluated for the treatment of different pesticides, pirimicarb and pyrimethanil, used in wine production. Both studied pesticides were efficiently removed, and the transformation products were determined. Finally, the treatment of a complex aqueous matrix composed by winery wastewater and the previously studied pesticides was carried out in the designed sequential reactor. The high removals of TOC and COD reached and the low energy consumption demonstrated the efficiency of this new configuration.

Analysis of bacterial diversity and efficiency of continuous removal of Victoria Blue R from wastewater by using packed-bed bioreactor

The characteristics of a packed-bed bioreactor (PBB) for continuously removing Victoria Blue R (VBR) from an aqueous solution were determined. The effects of various factors including liquid retention time (RT), VBR concentration, shock loading, and coexisting compounds on the VBR removal and bacterial community in a continuous system were investigated. The intermediates of degraded VBR and the acute toxicity of the effluent from PBB were analyzed. When the VBR concentration was lower than 400 mg/l for a two-day retention time (RT), 100% removal was achieved. During continuous operation, the efficiency initially varied with the VBR concentration and RT, but gradually increased in one to two days. Furthermore, the acute toxicity of the effluent reduced by a factor of 21.25-49.61, indicating that the PBB can be successfully operated under turbulent environmental conditions. VBR degradation involved stepwise demethylation and yielded partially dealkylated VBR species. Phylogenetic analysis showed that the dominant phylum in the PBB was Proteobacteria and that Aeromonas hydrophila dominated during the entire operating period. The characteristics of the identified species showed that the PBB is suitable for processes such as demethylation, aromatic ring opening, carbon oxidation, nitrification, and denitrification.