photoelectro-Fenton Treatment Research Articles

A sustainable activated carbon fiber/TiO2 cathode for the photoelectro-Fenton treatment of pharmaceutical pollutant enalapril

In this work, a TiO2-decorated electrode was fabricated by dip coating activated carbon fibers (ACF) with TiO2, which were then used as a cathode for the photoelectro-Fenton (PEF) treatment of the pharmaceutical enalapril, an angiotensin-converting enzyme inhibitor that has been detected in several waterways. The TiO2 coating was found to principally improve the electrocatalytic properties of ACF for H2O2 production via the 2-e- O2 reduction, in turn increasing enalapril degradation by PEF. The effect of the current density on the mineralization of enalapril was evaluated and the highest TOC removal yield (80.5% in 3 h) was obtained at 8.33 mA cm−2, in the presence of 0.5 mmol L−1 of Fe2+ catalyst. Under those conditions, enalapril was totally removed within the first 10 min of treatment with a rate constant k = 0.472 min−1. In contrast, uncoated ACF only achieved 60% of TOC removal in 3 h at 8.33 mA cm−2. A degradation pathway for enalapril mineralization is proposed, based on the degradation by-products identified during treatment. Overall, the results demonstrate the promises of TiO2 cathodes for PEF, a strategy that has often been overlooked in favor of photoelectrocatalysis (PEC) based on TiO2-modified photoanodes.

Assessing the electrochemical degradation of reactive orange 84 with Ti/IrO2–SnO2–Sb2O5 anode using electrochemical oxidation, electro-Fenton, and photoelectro-Fenton under UVA irradiation

Today, water shortage problems around the world have forced the search for new treatment alternatives, in this context, electrochemical oxidation technology is a hopeful process for wastewater treatment, although it is still needed exploration of new efficient and economically viable electrode materials. In this way, mixed metal oxide anodes look like promising alternatives but their preparation is still a significant point to study, searching for finding low-cost materials to improve electrocatalytic efficiencies. In an exploration of this kind of highly efficient materials, this work presents the results obtained using an MMO Ti/IrO2–SnO2–Sb2O5 anode. All the prepared anodes exhibited excellent physical and electrochemical properties. The electrochemical oxidation of 100 mL and 200 mg L−1 Reactive Orange 84 (RO 84) diazo dye was studied using 3 cm2 of such synthesized anodes by applying current densities of 25, 50, and 100 mA cm−2. Faster and more efficient electrochemical oxidation occurred at 100 mA cm−2 with 50 mM of Na2SO4 + 10 mM NaCl as supporting electrolyte at pH 3.0. The degradation and mineralization processes of the above solution were enhanced with the electro-Fenton process with 0.05 mM Fe2+ and upgraded using photoelectron-Fenton with UVA light. This process yielded 91% COD decay with a low energy consumption of 0.1137 kWh (g COD)−1 at 60 min. The evolution of a final carboxylic acid like oxalic was followed by HPLC analysis. The Ti/IrO2–SnO2–Sb2O5 is then an efficient and low-cost anode for the photoelectro-Fenton treatment of RO 84 in a chloride and sulfate media.

Continuous H2O2 production sustained by anodic O2 for the destruction of the antibiotic ampicillin by photoelectro-Fenton process in a rotating cylinder electrode reactor

In this work, the experimental characterization of a rotating cylinder electrode (RCE) reactor employed for the complete degradation of the antibiotic ampicillin (AMP) by photoelectro-Fenton (PEF) process has been addressed for the first time. Once produced from water oxidation at six Ti|IrO2 anodic plates, O2 was quickly transported by forced convection toward the central RCE, which consisted of a 316 stainless-steel cylinder covered with a (C-PTFE)-coated carbon cloth, thus ensuring the continuous production of H2O2 from the two-electron O2 reduction reaction (ORR). The accumulated H2O2 reached a concentration of 83.3 mg L−1 H2O2 after 60 min in a 50 mM Na2SO4 solution at pH 3, operating at an RCE peripheral velocity U = 79.6 cm s−1 and fixed cathodic potential of Ecath = −0.45 V vs. SHE. Furthermore, the optimum PEF conditions led to the complete destruction of 10 mg L−1 AMP in only 10 min upon addition of 0.4 mM Fe2+ as catalyst under UVA light irradiation, with a low electrolytic energy consumption of 0.211 kWh (g TOC)−1. In addition, the evolution of final carboxylic acids and inorganic ions over the electrolysis time was monitored by chromatographic and spectrophotometric techniques. PEF treatment clearly outperformed the anodic oxidation with (AO-H2O2) and the electro-Fenton (EF) processes, which opens the door to a sustainable and powerful electrochemical technology with no need for an air compressor for H2O2 production and viable under limitless sunlight irradiation.

Characterization of a flow-through electrochemical reactor for the degradation of ciprofloxacin by photoelectro-Fenton without external oxygen supply

This work reports the experimental characterization of an innovative electrolyzer designed for self-sustained H2O2 production and synchronized degradation of the antibiotic ciprofloxacin (CIP) by photoelectro-Fenton (PEF) process tested in recirculation operation mode. The flow-through electrochemical reactor comprises an arrangement of two alternated anodes and cathodes, using the anodic O2 generated from water oxidation at Ti|IrO2-Ta2O5 anodes to produce H2O2 at graphite-felt cathodes from the oxygen reduction reaction (ORR). The influence of the most relevant operation conditions such as the initial Fe2+ concentration, applied current density, liquid flow rate (Q) and initial CIP concentration was systematically examined during the electrolytic trials by analyzing the CIP concentration decay, solution mineralization, and time course of Fe2+ content. It is demonstrated that the H2O2 concentration attained with this reactor configuration is sufficient to run the PEF treatment without needing an air pump or compressor. Complete elimination of CIP was reached at 40 min, with mineralization current efficiency and electrolytic energy consumption of 42% and 0.25 kWh (g TOC)-1, respectively. Oxalic and oxamic acids were the final organic by-products of the destruction of CIP, which also yielded nitrate, ammonium, and fluoride ions. PEF outperformed other processes like anodic oxidation with cathodic H2O2 and electro-Fenton (EF), as revealed by comparing the degradation and mineralization profiles.

Replacing carbon cloth by nickel mesh as substrate for air-diffusion cathodes: H2O2 production and carbenicillin degradation by photoelectro-Fenton

The development of air-diffusion electrodes (ADEs) that electrocatalytically produce H2O2 for its decomposition by Fenton’s reaction is gaining relevance. However, attention has been paid to improve the catalytic layer using standard carbonaceous substrates like Ccloth. This work clearly proves the superior endurance of 3 cm2 ADEs fabricated with Nimesh as substrate, achieving stable H2O2 accumulation up to 60 mM at 100 mA for 48 h in an undivided cell. Consequently, the antibiotic carbenicillin could be totally degraded in 13 min by photoelectro-Fenton (PEF) process using the Nimesh|C-PTFE ADE at 20 mA in an acidic model solution. This high performance was confirmed by PEF treatment in urban wastewater, reaching total drug disappearance in 18 min with >80 % TOC abatement. The time course of OH and toxicity was monitored, whereas chromatographic analysis of treated solutions revealed the formation of 32 aromatic and 16 aliphatic byproducts, including 7 linear carboxylic acids.

Degradation of commercial glyphosate-based herbicide via advanced oxidative processes in aqueous media and phytotoxicity evaluation using maize seeds

Glyphosate is a herbicide that acts as a broad-spectrum, non-selective, post-emergence systemic pest controller. Its continuing, increasing, and excessive use in many countries in recent years poses a significant threat to the environment and human health due to the prevalence of this herbicide in water bodies and its impact on non-target organisms. In this context, it is essential to develop processes aimed at the non-selective degradation of glyphosate and its by-products. In this study, various advanced oxidative processes were applied: Fenton, electro-Fenton, photoelectro-oxidation, and photoelectro-Fenton, with the objective of oxidizing glyphosate in the commercial product Roundup®. The resultant oxidation products and the phytotoxicological effect on maize seed germination were also analyzed. Following each treatment, chemical oxygen demand (COD), total organic carbon (TOC), glyphosate degradation, and oxidation by-product formation were analyzed. The treated solutions were used to germinate maize seeds for 7 days in a germination chamber applying a photoperiod of 12 h at 24 °C. The % of germination, protein and hydrogen peroxide (H2O2) content, lipid peroxidation extent (MDA), and superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) activities were determined. The photoelectro-Fenton treatment was the most effective in degrading glyphosate, operating synergistically to break glyphosate bonds, thereby generating non-toxic short-chain molecules. Maize seed germination was satisfactory (> 50 %), but the persistent formation of reactive oxygen species (ROS) led to increased antioxidant activities of SOD, CAT, and POD enzymes acting in a compensatory manner against ROS, thus sustaining the photosynthetic apparatus. Hormesis, a stimulatory effect of glyphosate, was also observed in the presence of low concentrations of glyphosate.

Bisphenol-S removal via photoelectro-fenton/H2O2 process using Co-porphyrin/Printex L6 gas diffusion electrode

The present work reports the development and application of gas diffusion electrode (GDE) based on carbon Printex L6 modified with 5% Co-Porphyrin/CPL6 for the removal of Bisphenol S (BPS) in wastewater using different electrochemical advanced oxidation processes (EAOPs). The application of the modified GDE led to the in-situ generation of 333 mg L−1 of H2O2 in only 90 min; this represents an increase of 88% in H2O2 generation in relation to the unmodified GDE. The process involving bisphenol S degradation fitted well into a pseudo-first order kinetic reaction for all the treatment techniques evaluated; the treatment techniques investigated recorded an increase in kinetic rate constant in the following order: anodic oxidation (0.004 min−1) < UV-C (0.006 min−1) < H2O2 (0.008 min−1) < H2O2/UV-C (0.016 min−1) < electro-Fenton (0.063 min−1) < photo electro-Fenton (0.154 min−1). The photo electro-Fenton (PEF) process exhibited the best degradation efficiency; BPS was completely removed after only 30 min of treatment and 78% mineralization was recorded after 360 min of treatment. The remaining organic matter under the PEF treatment process corresponded to short-chain carboxylic acids; this confirms the efficiency of the process applied for BPS decontamination. The results obtained show that the proposed treatment mechanism can be successfully combined with cheaper biological treatments for a total remediation of wastes.

Advanced Oxidation Processes as sustainable technologies for the reduction of elderberry agro-industrial water impact

World is facing an environmental crisis and industrial awareness regarding their wastewaters is gaining strength. By treating them, water footprint can be reduced allowing their recycle and reuse, as well as recovering substances with added value. This work reports the pre-treatment of non-biodegradable elderberry agro-industrial wastewater at pilot plant scale by different oxidative processes (solar photo-Fenton, solar electrochemistry and ozonation). An economical approach has been addressed for improving biodegradability for a subsequent lower-cost biological treatment. Solar photo-Fenton process attained biodegradability in 30 min entailing 0.22 € m−3 of operation costs. Solar photoelectro-Fenton treatment turned industrial wastewater biodegradable after 20 min costing 0.11 € m−3, while solar-assisted anodic oxidation required 60 min and 0.12 € m−3. Solar processes combined with electrooxidation showed to be more promising options than ozonation for reducing water footprint of biorecalcitrant industrial wastewaters and moderate organic content, if an engagement between treatment time and operating costs is prioritized.

Photoelectro-Fenton treatment of pesticide triclopyr at neutral pH using Fe(III)-EDDS under UVA light or sunlight.

One of the main challenges of electrochemical Fenton-based processes is the treatment of organic pollutants at near-neutral pH. As a potential approach to this problem, this work addresses the use of a low content of soluble chelated metal catalyst, formed between Fe(III) and ethylenediamine-N,N'-disuccinic (EDDS) acid (1:1), to degrade the herbicide triclopyr in 0.050 M Na2SO4 solutions at pH 7.0 by photoelectro-Fenton with UVA light or sunlight (PEF and SPEF, respectively). Comparison with electro-Fenton treatments revealed the crucial role of the photo-Fenton-like reaction, since this promoted the production of soluble Fe(II) that enhanced the pesticide removal. Hydroxyl radicals formed at the anode surface and in the bulk were the main oxidants. A boron-doped diamond (BDD) anode yielded a greater mineralization than an IrO2-based one, at the expense of reduced cost-effectiveness. The effect of catalyst concentration and current density on the performance of PEF with BDD was examined. The PEF trials in 0.25 mM Na2SO4 + 0.35 mM NaCl medium showed a large influence of generated active chlorine as oxidant, being IrO2 more suitable than RuO2 and BDD. In SPEF with BDD, the higher light intensity from solar photons accelerated the removal of the catalyst and triclopyr, with small effect on mineralization. A plausible route for the herbicide degradation by Fe(III)-EDDS-catalyzed PEF and SPEF is finally proposed based on detected byproducts: three heteroaromatic and four linear N-aliphatic compounds, formamide, and tartronic and oxamic acids.

Mechanism and stability of an Fe-based 2D MOF during the photoelectro-Fenton treatment of organic micropollutants under UVA and visible light irradiation

This work reports the novel application of an Fe-based 2D metal−organic framework (MOF), prepared with 2,2′-bipyridine-5,5′-dicarboxylate (bpydc) as organic linker, as highly active catalyst for heterogeneous photoelectro-Fenton (PEF) treatment of the lipid regulator bezafibrate in a model matrix and urban wastewater. Well-dispersed 2D structures were successfully synthesized and their morphological, physicochemical and photocatalytic properties were assessed. UV/Vis PEF using an IrO2/air-diffusion cell with an extremely low catalyst concentration (0.05 g L−1, tenfold lower than reported 3D MOFs) outperformed electro-oxidation with electrogenerated H2O2, electro-Fenton and visible-light PEF. Its excellent performance was explained by: (i) the enhanced mass transport of H2O2 (and organic molecules) at the 2D structure, providing active sites for heterogeneous Fenton’s reaction and in-situ Fe(II) regeneration; (ii) the ability of photoinduced electrons to reduce H2O2 to •OH, and Fe(III) to Fe(II); and (iii) the enhanced charge transfer and excitation of Fe–O clusters, which increased the number of electron-hole pairs. LC-QToF-MS and GC-MS allowed the identification of 16 aromatic products of bezafibrate. The complete removal of four micropollutants mixed in urban wastewater at pH 7.4 revealed the great potential of (Fe−bpydc)-catalyzed PEF process.

Electrochemical advanced oxidation discoloration and removal of three brown diazo dyes used in the tannery industry

Three different advanced oxidation processes, electrochemical oxidation (EO), electro-Fenton (EF) and photoelectro-Fenton (PEF) were tested for the treatment of individual and mixed solutions of Bismark Brown G, Bismark Brown R and Brown DGI dyes. The experiments were performed in a 500 mL stirred undivided cell with a 25 cm2 BDD anode, using 0.050 M Na2SO4 as electrolyte and pH 3.0. The cathodes used for the different processes were a stainless-steel plate in EO and another BDD electrode in EF and PEF. In the two latter processes, pumped air was bubbled for H2O2 generation and 0.50 mM Fe2+ was added as the Fenton's reagent. For PEF, the solution was irradiated using an UVA lamp. The oxidation power of the processes increased in the order: EO < EF < PEF. This trend is explained by the prevalent formation of hydroxyl radicals at the anode surface from water oxidation and in the bulk from Fenton's reaction. The best performance of PEF is considered to be related with the photolytic action of UVA. In all cases, the discoloration presents pseudo-first order kinetics. The performance of PEF treatment of individual dye solutions was improved by raising diazo dye concentration and current density. When PEF was tested with a mixture with 60 mg L−1 of each dye, 80%, 92% and 97% discoloration, and 58%, 76% and 95% chemical oxygen demand decay were found after 60 min using 20 mA cm−2. Small concentrations of succinic, malic and oxalic acids were detected at the end of the PEF process.

Expanding the application of photoelectro-Fenton treatment to urban wastewater using the Fe(III)-EDDS complex

This work reports the first investigation on the use of EDDS as chelating agent in photoelectro-Fenton (PEF) treatment of water at near-neutral pH. As a case study, the removal of the antidepressant fluoxetine was optimized, using an electrochemical cell composed of an IrO2-based anode an air-diffusion cathode for in-situ H2O2 production. Electrolytic trials at constant current were made in ultrapure water with different electrolytes, as well as in urban wastewater (secondary effluent) at pH 7.2. PEF with Fe(III)–EDDS (1:1) complex as catalyst outperformed electro-Fenton and PEF processes with uncomplexed Fe(II) or Fe(III). This can be explained by: (i) the larger solubilization of iron ions during the trials, favoring the production of •OH from Fenton-like reactions between H2O2 and Fe(II)–EDDS or Fe(III)–EDDS, and (ii) the occurrence of Fe(II) regeneration from Fe(III)–EDDS photoreduction, which was more efficient than conventional photo-Fenton reaction with uncomplexed Fe(III). The greatest drug concentration decays were achieved at low pH, using only 0.10 mM Fe(III)–EDDS, although complete removal in wastewater was feasible only with 0.20 mM Fe(III)–EDDS due to the greater formation of •OH. The effect of the applied current and anode nature was rather insignificant. A progressive destruction of the catalytic complex was unveiled, whereupon the mineralization mainly progressed thanks to the action of •OH adsorbed on the anode surface. Despite the incomplete mineralization using BDD as the anode, a remarkable toxicity decrease was determined. Fluoxetine degradation yielded F− and NO3− ions, along with several aromatic intermediates. These included two chloro-organics, as a result of the anodic oxidation of Cl− to active chlorine. A detailed mechanism for the Fe(III)–EDDS-catalyzed PEF treatment of fluoxetine in urban wastewater is finally proposed.

Removal of Orange II (OII) dye by simulated solar photoelectro-Fenton and stability of WO2.72/Vulcan XC72 gas diffusion electrode

The objectives of this study were to determine the viability of removing Orange II (OII) dye by simulated solar photoelectro-Fenton (SSPEF) and to evaluate the stability of a WO2.72/Vulcan XC72 gas diffusion electrode (GDE) and thus determine its best operating parameters. The GDE cathode was combined with a BDD anode for decolorization and mineralization of 350 mL of 0.26 mM OII by anodic oxidation with electrogenerated H2O2 (AO-H2O2), electro-Fenton (EF) and photoelectro-Fenton (PEF) at 100, 150 and 200 mA cm−2 and SSPEF at 150 mA cm−2. The GDE showed successful operation for electrogeneration, good reproducibility and low leaching of W. Decolorization and OII decay were directly proportional to the current density (j). AO-H2O2 had a reduced performance that was only half of the SSPEF, PEF and EF treatments. The mineralization efficiency was in the following order: AO-H2O2 < EF < PEF ≈ SSPEF. This showed that the GDE, BDD anode and light radiation combination was advantageous and indicated that the SSPEF process is promising with both a lower cost than using UV lamps and simulating solar photoelectro-Fenton process. The PEF process with the lowest j (100 mA cm−2) showed the best performance-mineralization current efficiency.

On the positive effect of UVC light during the removal of benzothiazoles by photoelectro-Fenton with UVA light

Benzothiazole (BTH) and 2-hydroxybenzothiazole (2-OH-BTH) are ubiquitous pollutants in aquatic ecosystems. This article reports their photoelectro-Fenton (PEF) treatment, either alone or mixed, in sulfate medium at pH 3.0 using an IrO2-based/air diffusion cell that generates H2O2 under UVA and/or UVC irradiation. UVC-PEF was more effective than UVA-PEF to remove the target pollutants, which suggests a positive impact of OH formed via Fenton’s reaction and photo-induced homolysis of H2O2 in the former method. In addition, BTH disappeared more quickly than 2-OH BTH. Full-time UVA-/UVC-PEF outperformed UVC-PEF and UVA-PEF to mineralize the mixtures, although requiring a much higher energy consumption. The evolution of generated H2O2 and homogeneous OH confirmed the positive contribution of UVC photolysis in UVA-PEF. Part-time use of UVC radiation in UVA-PEF yielded a similar total organic carbon removal, with much lower energy consumption. BTH was oxidized to 2-OH-BTH, which was subsequently transformed into other five heteroaromatics.

TiO2/Au/TiO2 multilayer thin-film photoanodes synthesized by pulsed laser deposition for photoelectrochemical degradation of organic pollutants

Most commonly employed anodes for photoelectrochemical degradation of organic contaminants in water are too thick, resulting in a low degradation efficiency due to the excessive electron-hole recombination. Multilayer-type TiO2/Au/TiO2 nanocomposites have been prepared as photoanode thin films by pulsed laser deposition (PLD). The films were composed of six TiO2 and five Au alternate layers, with total thickness of about 212 nm, aimed to optimize the absorption of photons and minimize the recombination. The influence of gold on the optical, structural, and chemical properties of the semiconductor nanocomposites was investigated. The band gap determined from Tauc model was 3.22 eV, close to that obtained by spectroscopic ellipsometry and lower than that of the TiO2 film, revealing an enhancement of visible light absorption. The catalytic performance of PLD films was evaluated by using them as anodes in electro-oxidation (EO) and photoanodes in photoelectrocatalysis (PEC) to degrade 39–157 mg L−1 paracetamol in sulfate medium. The drug removal was very slow in EO, due to the low ability to form OH on the anode surface, which was significantly enhanced upon UVA irradiation in PEC. The presence of Cl− allowed a faster degradation by produced active chlorine. Finally, a hybrid process involving PEC + photoelectro-Fenton (PEF) with an air-diffusion cathode yielded total paracetamol decay in 4–5 min at an anodic potential of +4.0 V because of the efficient OH generation from Fenton’s and photo-Fenton reactions. Hydroquinone, p-benzoquinone and hydroxylated products were detected during EO and PEC. The generation of active chlorine was confirmed by identifying a chlorinated derivative, N-(4-chloro-2-hydroxyphenyl)acetamide, in PEC + PEF treatment.

A hybrid photoelectrocatalytic/photoelectro-Fenton treatment of Indigo Carmine in acidic aqueous solution using TiO2 nanotube arrays as photoanode

This article reports the synthesis of TiO2 nanotube arrays (TiO2 NTs), grown by Ti anodization, and their use as photoanode in a hybrid photelectrocatalytic (PEC)/photoelectro-Fenton (PEF) treatment of Indigo Carmine solutions in sulfate medium at pH3.0. The anode was combined with an air-diffusion cathode that ensured continuous H2O2 electrogeneration. Comparative trials by electrochemical oxidation with electrogenerated H2O2 (EO-H2O2), electro-Fenton (EF) and PEF with Pt anode were made. The photoanode was stable operating up to 3mAcm−2 with irradiation from a 36-W UV LED lamp, showing photoelectroactivity from an anodic potential (Ean) of +0.20V, as determined by cyclic voltammetry. At 3mAcm−2, color removal by EO-H2O2 with Pt and PEC with TiO2 NTs was very slow, being much faster in EF, PEF and PEC/PEF due to main role of OH formed from Fenton's reaction upon addition of Fe2+. The absorbance and dye concentration decays agreed with a pseudo-first-order kinetics, yielding a slightly lower rate constant for decolorization because of the formation of colored products. The mineralization ability increased as: EO-H2O2≪EF≪PEF<PEC/PEF. The holes photogenerated at the TiO2 NTs surface had higher oxidation ability than OH formed at the Pt surface from water discharge. In PEC/PEF, a slower mineralization was found at 2mAcm−2, although the final mineralization percentage was similar to that attained at 3mAcm−2. Both, SO42− and NH4+ ions were released during the treatments, along with isatin-5-sulfonic and formic acids as main products.

Assessment of 4‐Aminoantipyrine Degradation and Mineralization by Photoelectro‐Fenton with a Boron‐Doped Diamond Anode: Optimization, Treatment in Municipal Secondary Effluent, and Toxicity

Abstract4‐Aminoantipyrine (4‐AA), a persistent metabolite of dipyrone found in natural water, has been treated in 100 mL of aqueous 0.050 M Na2SO4 solutions at pH 3.0 by photoelectro‐Fenton (PEF) with a 4 W UVA lamp. The assays were performed in a cell equipped with a boron‐doped diamond (BDD) anode and an air‐diffusion cathode for H2O2 generation. The formation of an unstable Fe(III)‐4‐AA complex with 1 : 2 molar ratio was evidenced. A 24 central composite design was used to assess the effect of four independent variables on PEF performance. The optimized conditions for 62.5 mg L−1 4‐AA were: current density of 77.5 mA cm−2 and 47.75 mg L−1 Fe2+, yielding 99 % 4‐AA degradation at 7 min, and 45 % 4‐AA mineralization with 3.2 % mineralization current efficiency at 130 min. Slightly slower degradation and similar mineralization were obtained under these conditions when 4‐AA was spiked into a municipal secondary effluent, showing a low influence of natural organic matter on PEF. The initially high acute toxicity determined using Artemia salina was largely diminished upon PEF treatment.

Enhanced electrocatalytic production of H2O2 at Co-based air-diffusion cathodes for the photoelectro-Fenton treatment of bronopol

(Co, S, P)-decorated multiwalled carbon nanotubes (MWCNTs) have been synthesized following a hydrothermal route as electrocatalysts to manufacture large surface area air-diffusion cathodes with carbon cloth as substrate. The enhanced electrocatalytic H2O2 production as compared with Co-free MWCNTs cathodes was demonstrated in a 2.5-L pre-pilot plant with either a RuO2-based or boron-doped diamond (BDD) anode, accumulating between 2- and 3-fold greater H2O2 contents with the catalyzed cathode. The good stability of this new material was ensured from the low Co leaching, with less than 9% Co released to solutions upon repeated usage. Aqueous solutions of the brominated organic preservative bronopol with 0.050 M Na2SO4 at pH 3.0 were comparatively treated by electro-oxidation (EO-H2O2), electro-Fenton (EF), UVA-assisted photoelectro-Fenton (PEF) and solar PEF (SPEF) at constant current density. SPEF with BDD anode and the catalyzed cathode showed the best performance, with total bronopol removal at 210 min and 94% mineralization after 360 min at 40 mA cm−2, thanks to the action of OH, BDD(OH) and sunlight. Formic acid was identified as main reaction by-product, whereas Br and N atoms were mainly converted to Br–, BrO3– and NO3–. Some unidentified organic by-product containing Br and N was formed as well.

Effect of surface properties of activated carbon fiber cathode on mineralization of antibiotic cefalexin by electro-Fenton and photoelectro-Fenton treatments: Mineralization, kinetics and oxidation products

Solutions of 200 mg L−1 cefalexin (CLX), an antibiotic with high usage frequency and biodegradation resistance, have been comparatively degraded by electro-Fenton (EF) and photoelectro-Fenton (PEF) processes using two kinds of activated carbon fiber (ACF) cathodes with different physical properties. These two ACFs shared similar pore volumes and pore diameters but varied BET surface areas, which were confirmed to be 0.5210 cm3 g−1, 2.26 nm and 921 m2 g−1 for ACF1, while 0.6508 cm3 g−1, 2.16 nm and 1206 m2 g−1 for ACF2, respectively. Their oxidation abilities were comparatively assessed in terms of degradation kinetics and mineralization rates, which increased in the order: ACF1-EF < ACF2-EF < ACF1-PEF < ACF2-PEF. These results confirmed the superiority of ACF with higher surface area, which was correlated to faster H2O2 and OH accumulation in more reaction sites provided. After 120 min electrolysis, ACF1 exhibited 1510 μM H2O2 and 37 μM OH accumulation, while ACF2 generated 1934 μM H2O2 and 85 μM OH. Moreover, ACF cathode with more developed pore structure also revealed faster formation of degradation by-products like inorganic ions (NH4+ and NO3− ions) and short-chain carboxylic acids (acetic, formic, oxamic and oxalic acids), as well as enhanced removal for partial acids. In order to gain a deeper understanding of degradation mechanisms for ACF2-PEF system, evolutions of six aromatic by-products generated from sulfoxidation, hydroxylation and decarboxylation were confirmed by UPLC-QTOF-MS/MS determination. Based on the above identifications of the degradation intermediates, a plausible reaction pathway for CLX removal was proposed.

Photoelectro-Fenton as post-treatment for electrocoagulated benzophenone-3-loaded synthetic and urban wastewater

The removal of benzophenone-3 (BP-3), a ubiquitous pollutant in municipal wastewater treatment facilities, was optimal by means of a sequential electrocoagulation (EC)/UVA photoelectro-Fenton (PEF) treatment. Overall mineralization was attained upon combination of EC (Fe/Fe cell, 15 mA cm−2, 20 min) with PEF (boron-doped diamond/air-diffusion cell, 33.3 mA cm−2, 720 min), being superior to EC/electro-Fenton (EF) and requiring shorter time than single PEF. In EC, an Al/Al cell yielded the largest removal of BP-3 in a simulated matrix at pH 11.0 due to precipitation of its neutral form caused by a substantial pH drop, with optimum current density of 15 mA cm−2. EC of BP-3-loaded urban wastewater at natural pH was quite effective also with a Fe/Fe cell, being preferred since it provided the required metal catalyst for subsequent treatment. Among the electrochemical advanced oxidation processes tested, PEF was superior to electrochemical oxidation with electrogenerated H2O2 (EO-H2O2) and EF, especially when using the boron-doped diamond instead of a RuO2-based anode, due to the oxidation of generated active chlorine and hydroxyl radicals, along with the photolytic action of UVA irradiation. GC-MS revealed the formation of 14 cyclic products in PEF treatment, two of them being also formed during EC.