Sequential use of a continuous-flow electrocoagulation reactor and a (photo)electro-Fenton recirculation system for the treatment of Acid Brown 14 diazo dye

The decolorization and TOC removal of solutions of Acid Brown 14 (AB14) diazo dye containing 50 mg L-1 of total organic carbon (TOC) have been first studied in a continuous-flow electrocoagulation (EC) reactor of 3 L capacity with Fe electrodes of ∼110 cm2 area each. Total loss of color with poor TOC removal was found in chloride, sulfate, and/or hydrogen carbonate matrices after 18 min of this treatment. The best performance was found using 5 anodes and 4 cathodes of Fe at 13.70 A and low liquid flow rate of 10 L h-1, in aerated 39.6 mM NaCl medium within a pH range of 4.0-10.0. The effluent obtained from EC was further treated by electro-Fenton (EF) using a 2.5 L pre-pilot flow plant, which was equipped with a filter-press cell comprising a Pt anode and an air-diffusion cathode for H2O2 electrogeneration. Operating with 0.10-1.0 mM Fe2+ as catalyst at pH 3.0 and 50 mA cm-2, a similar TOC removal of 68 % was found as maximal in chloride and sulfate media using the sequential EC-EF process. The EC-treated solutions were also treated by photoelectro-Fenton (PEF) employing a photoreactor with a 125 W UVA lamp. The sequential EC-PEF process yielded a much higher TOC reduction, close to 90 % and 97 % in chloride and sulfate media, respectively, due to the rapid photolysis of the final Fe(III)-carboxylate complexes. The formation of recalcitrant chloroderivatives from generated active chlorine limited the mineralization in the chloride matrix. For practical applications of this two-step technology, the high energy consumption of the UVA lamp in PEF could be reduced by using free sunlight.

Insights into antibiotic cefaclor mineralization by electro-Fenton and photoelectro-Fenton processes using a Ti/Ti4O7 anode: Performance, mechanism, and toxic chlorate/perchlorate formation

A comparative degradation of antibiotic cefaclor (CEC) between Ti/Ti4O7 and Ti/RuO2 anodes, in terms of degradation kinetics, mineralization efficiency, and formation of toxic chlorate (ClO3-) and perchlorate (ClO4-), was performed with electrochemical-oxidation (EO), electro-Fenton (EF), and photoelectro-Fenton (PEF) processes. Besides, CEC degradation by EF with boron-doped diamond (BDD) anode was also tested. Results showed CEC decays always followed pseudo-first-order kinetics, with increasing apparent rate constants in the sequence of EO < EF < PEF. The mineralization efficiency of the processes with Ti/Ti4O7 anode was higher than that of Ti/RuO2 anode, but slightly lower than that of BDD anode. Under the optimal conditions, 94.8% mineralization was obtained in Ti/Ti4O7-PEF, which was much higher than 64.4% in Ti/RuO2-PEF. The use of Ti/RuO2 gave no generation of ClO3- or ClO4-, while the use of Ti/Ti4O7 yielded a small amount of ClO3- and trace amounts of ClO4-. Conversely, the use of BDD led to the highest generation of ClO3- and ClO4-. The reaction mechanism was studied systematically by detecting the generated H2O2 and •OH. The initial N of CEC was released as NH4+ and, in smaller proportion, as NO3-. Four short-chain carboxylic acids and nine aromatic intermediates were also detected, a possible reaction sequence for CEC mineralization was finally proposed.

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.

Minerals as catalysts of heterogeneous Electro-Fenton and derived processes for wastewater treatment: a review

Advanced oxidation processes (AOPs) such as Fenton's reagent, which generates highly reactive oxygen species, are efficient in removing biorefractory organic pollutants from wastewater. However, Fenton's reagent has drawbacks such as the generation of iron sludge, high consumption of H₂O₂, and the need for pH control. To address these issues, Electro-Fenton (EF) and heterogeneous Electro-Fenton (HEF) have been developed. HEF, which uses solid catalysts, has gained increasing attention, and this review focuses on the use of mineral catalysts in HEF and derived processes. The reviewed studies highlight the advantages of using mineral catalysts, such as efficiency, stability, affordability, and environmental friendliness. However, obstacles to overcome include the agglomeration of unsupported nanoparticles and the complex preparation techniques and poor stability of some catalyst-containing cathodes. The review also discusses the optimal pH range and dosage of the heterogeneous catalysts and compares the performance of iron sulfides versus iron oxides. Although natural minerals appear to be the best choice for effluents at pH>4, no scale-up reports have been found. The need for further development in this field and the importance of considering the environmental impact of trace toxic metals or catalytic nanoparticles in the treated water on the receiving ecosystem is emphasized. Finally, the article acknowledges the high energy consumption of HEF processes at the lab scale and calls for their performance development to achieve environmentally friendly and cost-effective results using real wastewaters on a pilot scale.

A critical review on paracetamol removal from different aqueous matrices by Fenton and Fenton-based processes, and their combined methods

Paracetamol (PCT), also known as acetaminophen, is a drug used to treat fever and mild to moderate pain. After consumption by animals and humans, it is excreted through the urine to the sewer systems, wastewater treatment plants, and other aquatic/natural environments. It has been detected in trace amounts in effluents of wastewater plant treatments, sewage sludge, hospital wastewaters, surface waters, and drinking water. PCT can cause genetic code damage, oxidative degradation of lipids, and denaturation of protein in cells, and its toxicity has been well-proven in bacteria, algae, macrophytes, protozoan, and fishes. To avoid its harmful health problems over living beings, powerful Fenton and Fenton-based treatments as pre-eminent advanced oxidation processes (AOPs) have been developed because of the inefficient treatment by conventional treatments. This paper presents a comprehensive and critical review over the application of such Fenton technologies to remove PCT from natural waters, synthetic wastewaters, and real wastewaters. The characteristics and main results obtained using Fenton, photo-Fenton, electro-Fenton, and photoelectro-Fenton are described, making special emphasis in the oxidative action of the generated reactive oxygen species. Hybrid processes based on the coupling with ultrasounds, gamma radiation, photocatalysis, photoelectrocatalysis, zero-valent iron-activated persulfate, adsorption, and microbial fuel cells, are analyzed. Sequential treatments involving the initiation with plasma gliding arc discharge and post-biological process are detailed. Comparative results with other available AOPs are also described and discussed. Finally, 13 aromatic by-products and 9 short-linear aliphatic carboxylic acid detected during the PCT removal by Fenton and Fenton-based processes are reported, with the proposal of three parallel pathways for its initial degradation.

Comparative study of the removal of direct red 23 by anodic oxidation, electro-Fenton, photo-anodic oxidation and photoelectro-Fenton in chloride and sulfate media

This study aims to compare the efficiency of anodic oxidation with electrogenerated H₂O₂ (AO-H₂O₂), electro-Fenton (EF), and their association with UV irradiation (photo anodic oxidation (PAO), and photo electro-Fenton (PEF) for the removal of Direct Red 23 from wastewater using a BDD/carbon felt cell in chloride and sulfate medium and in their combination. The effect of the supporting electrolyte was investigated in AO-H₂O₂ and EF processes. High discoloration efficiency was obtained in chloride media while a higher mineralization rate was achieved in sulfate media. The EF process reached higher total organic carbon (TOC) removal efficiency than AO-H₂O₂. 90% TOC removal rate was achieved by the EF against 82% by AO-H₂O₂ in sulfate media. The influence of using the mixt supporting electrolyte formed of 75% Na₂SO₄ + 25% NaCl was found to have beneficial effect on TOC removal, achieving 89% and 97% by AO-H₂O₂ and EF, respectively. High currents led to higher mineralization rates while low currents yielded to a higher mineralization current efficiency (MCE%) and lower energy consumption (EC). UV irradiation enhanced process efficiency. Mineralization efficiency followed the sequence: AO-H₂O₂ < PAO < EF < PEF. The PEF process was able to remove TOC completely at 5 mA cm^-2 current density and 6 h of electrolysis with a MCE% value of 16.57% and EC value of 1.29 kWh g^-1 TOC removed.

Cost comparison of advanced oxidation processes for wastewater treatment using accumulated oxygen-equivalent criteria

Advanced oxidation processes (AOPs) have received a lot of attention over the years as advanced physico-chemical polishing wastewater treatments to remove biorefractory pollutants. Additionally, many studies report their excellent degradation and mineralization performance as stand-alone technologies too, demonstrating the versatility of these processes; however, there is a lack of suitable methods to compare the performance (in terms of removal efficiency and operating costs) of different AOPs in the same conditions. In this context, the goal of this paper is to propose a systematic investigation by introducing a novel criterion, namely the accumulated oxygen-equivalent chemical-oxidation dose (AOCD), to systematically compare the diverse AOPs available: ozonation, H₂O₂ photolysis, Fenton, photo-Fenton, electro-Fenton and photoelectro-Fenton (paired with anodic oxidation, for the latter two). For each of these, the cost efficiency was determined by optimizing the operating conditions for the removal of phenol, selected as a model pollutant (1.4 mM, equivalent to 100 mg-C L^-1). The operating costs considered sludge management, chemical use and electricity consumption. Among all AOPs, electro-Fenton was the most cost-effective (108 - 125 € m^-3), notwithstanding the mineralization target (50%, 75% and 99%), owing to its electrocatalytic behavior. Chemical Fenton proved competitive too up to 50% of mineralization, meaning that it could also be considered as a cost-effective pre-treatment solution. AOCD was the lowest for electro-Fenton, which could be attributed to its excellent faradaic yield, while UV-based processes generally required the highest dose. The AOCD criterion could serve as a baseline for AOP comparison and prove useful for the legislator to determine the "best available techniques" as defined by the Industrial Emissions European Union Directive 2010/75/EU.

Study of degradation of amitriptyline antidepressant by different electrochemical advanced oxidation processes

Amitriptyline (AMT) is the most widely used tricyclic antidepressant and is classified as a recalcitrant emergent contaminant because it has been detected in different sources of water. Its accumulation in water and soil represents a risk for different living creatures. To remove amitriptyline from wastewater, the Advanced Oxidation Processes (AOPs) stands up as an interesting option since generate highly oxidized species as hydroxyl radicals (OH) by environmentally friendly mechanism. In this work, the oxidation and mineralization of AMT solution have been comparatively studied by 3 Electrochemical AOPs (EAOPs) where the OH is produced by anodic oxidation of H₂O (AO-H₂O₂), or by electro-Fenton (EF) or photoelectro-Fenton (PEF). PEF process with a BDD anode showed the best performance for degradation and mineralization of this drug due to the synergistic action of highly reactive physiosorbed BDD (OH), homogeneous OH and UVA radiation. This process achieved total degradation of AMT at 50 min of electrolysis and 95% of mineralization after 360 min of treatment with 0.5 mmol L^-1 Fe²⁺ at 100 mA cm^-2. Six aromatic intermediates for the drug mineralization were identified in short time of electrolysis by GC-MS, including a chloroaromatic by-product formed from the attack of active chlorine. Short-chain carboxylic acids like succinic, malic, oxalic and formic acid were quantified by ion-exclusion HPLC. Furthermore, the formation of NO₃^- ions was monitored. Finally, the organic intermediates identified by chromatographic techniques were used to propose the reaction sequence for the total mineralization of AMT.

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.

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.

Treatment of antibiotic cephalexin by heterogeneous electrochemical Fenton-based processes using chalcopyrite as sustainable catalyst

The development of heterogeneous Fenton-based electrochemical advanced oxidation processes is important for the removal of organic pollutants at industrial level in the near future. This work reports the application of heterogeneous photoelectro-Fenton (HPEF) with UVA light as an enhanced alternative to the more widespread heterogeneous electro-Fenton (HEF) process. The treatment of the antibiotic cephalexin using chalcopyrite as a sustainable catalyst was studied using an undivided IrO₂/air-diffusion cell. XPS analysis showed the presence of Fe(III), Cu(I) and Cu(II) species on the surface. The amount of Fe²⁺ ions dissolved upon chalcopyrite exposure to continuous stirring and air bubbling was proportional to chalcopyrite content. In all cases, the occurrence of pH self-regulation to an optimum value near 3 was observed. The HEF and HPEF treatments of 100 mL of 50 mg L^-1 cephalexin solutions with 0.050 M Na₂SO₄ have been studied with 1.0 g L^-1 chalcopyrite at 50 mA cm^-2. Comparative homogeneous EF and PEF with dissolved Fe²⁺ and Cu²⁺ catalysts were also performed. HPEF was the most effective process, which can be mainly explained by the larger production of homogeneous and heterogeneous OH and the photodegradation of the complexes formed between iron and organics. The effect of applied current and catalyst concentration on HPEF performance was assessed. Recycling experiments showed a long-term stability of chalcopyrite. Seven initial aromatics and six cyclic by-products of cephalexin were identified, and a plausible degradation route that also includes five final carboxylic acids is proposed.

Photoelectro-Fenton system including electromagnetic induction electrodeless lamp and black carbon poly tetra fluoro ethylene air-diffusion cathode: Degradation kinetics, intermediates and pathway for azo dye

The role of illumination and cathode is important to improve the efficiency of photoelectro-Fenton (PEF) system. In this study, cathodes with black carbon-poly tetra fluoro ethylene (BC-PTFE) for increase the concentration of hydrogen peroxide in PEF. A new PEF system using EIEL and BC-PTFE air-diffusion cathode was established. The electrode performance was tested and the influence factors, degradation kinetics, intermediates, pathway and mechanism of the model compound methyl orange (MO) were studied. The capacities of concentration decays and total organic carbon (TOC) removals were compared between different electrochemical advanced oxidation processes. The experimental conditions were optimized for a current density of 20 mA cm^-2 with 0.5 mM Fe²⁺ and 100 mg L^-1 MO at 20 °C and pH 3.0 in an 8 L reservoir. The higher MO concentration was, the smaller pseudo-first-order kinetic constants of concentration decays and TOC removals were. Intermediate products were identified by gas chromatography-mass spectrometry and ion-exclusion high performance liquid chromatograph in EIEL-PEF. Combined with frontier electron density, the degradation pathway was deduced as follows: destruction of azo bond, substitution of ^•OH, dehydrogenation and oxidation, opening-ring and mineralization. In EIEL-PEF, the concentration of oxalic acid and oxamic acid reached the maximum value 9.2 and 1.5 mg L^-1 at 60 and 90 min, respectively. The photolysis of N-intermediates produced NH₄⁺-N was released in more proportion than NO₃^--N and oxamic acid-N. The study indicated that PEF system has the potential to remove organic pollutants in aquatic environments.

Electro-Fenton and photoelectro-Fenton degradation of sulfamethazine using an active gas diffusion electrode without aeration

A novel superhydrophobic gas diffusion electrode based on carbon black (CB)- polytetrafluoroethylene (PTFE) modified graphite felt cathode was prepared to increase oxygen mass transfer efficiency and produce hydrogen peroxide at the gas-liquid-solid three-phase interface without aeration. The gas diffusion electrode system was further tested for the degradation of sulfamethazine (SMT) by electro-Fenton (EF) and photoelectro-Fenton (PEF). In the EF process, SMT was removed effectively, but the mineralization degree was not high due to the generation of organic acids which were difficult to be further degraded. While in the PEF process, organic contaminant can be destroyed by the combined action of Fe²⁺/H₂O₂, UV/H₂O₂ and UV radiation, and more efficient mineralization (>83.5%) at low current (50 mA) was attained, which might be attributed to the high H₂O₂ utilization (70-90%), rapid regeneration of Fe²⁺ and photolysis of intermediates. In addition, it was verified that the PEF system had a good adaptability to pH and pollutant concentration. Compared with aeration system, the use of this active gas diffusion cathode in electrochemical advanced oxidation processes significantly reduced energy consumption.

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.

Mineralization of cefoperazone in acid medium by the microwave discharge electrodeless lamp irradiated photoelectro-Fenton using a RuO2/Ti or boron-doped diamond anode

The mineralization of 125 mL of 50-300 mg L^-1 cefoperazone (CFPZ) has been comparatively studied by electrochemical advanced oxidation processes (EAOPs) like anodic oxidation (AO), electro-Fenton (EF) and photoelectro-Fenton (PEF) with a RuO₂/Ti or boron-doped diamond (BDD) anode and an activated carbon fiber (ACF) cathode. A microwave discharge electrodeless lamp (MDEL) was used as the UV source in PEF process. CFPZ decays always followed pseudo-first-order kinetics and their constant rates increased in the order: AO < EF < MDEL-PEF, regardless of anode types. Higher mineralization was achieved in all methods using BDD instead of RuO₂/Ti, while the most potent BDD-MDEL-PEF gave 88% mineralization under its optimum conditions of 0.36 A, pH 3.0 and 1.0 mmol L^-1 Fe²⁺. The synergistic mechanisms were explored by quantifying the electrogenerated H₂O₂ and formed ^•OH, in which 2.27 and 2.58 mmol L^-1 H₂O₂ were accumulated in AO-H₂O₂ with RuO₂/Ti or BDD anode, respectively, while 92.0 and 263.5 μmol L^{-1 •}OH were generated in EF with RuO₂/Ti or BDD anode, respectively. The oxidation power of EAOPs with different anodes was also compared by measuring the evolutions of NO₃^- and NH₄⁺ as well as four generated carboxylic acids including oxalic, oxamic, formic and fumaric acids.

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.

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.

Antituberculosis drug isoniazid degraded by electro-Fenton and photoelectro-Fenton processes using a boron-doped diamond anode and a carbon-PTFE air-diffusion cathode

Solutions with 0.65 mM of the antituberculosis drug isoniazid (INH) in 0.050 M Na₂SO₄ at pH 3.0 were treated by electro-Fenton (EF) and UVA photoelectro-Fenton (PEF) processes using a cell with a BDD anode and a carbon-PTFE air-diffusion cathode. The influence of current density on degradation, mineralization rate, and current efficiency has been thoroughly evaluated in EF. The effect of the metallic catalyst (Fe²⁺ or Fe³⁺) and the formation of products like short-chain linear aliphatic carboxylic acids were assessed in PEF. Two consecutive pseudo-first-order kinetic regions were found using Fe²⁺ as catalyst. In the first region, at short time, the drug was rapidly oxidized by ^●OH, whereas in the second region, at longer time, a resulting Fe(III)-INH complex was much more slowly removed by oxidants. INH disappeared completely at 300 min by EF, attaining 88 and 94% mineralization at 66.6 and 100 mA cm^-2, respectively. Isonicotinamide and its hydroxylated derivative were identified as aromatic products of INH by GC-MS and oxalic, oxamic, and formic acids were quantified by ion-exclusion HPLC. The PEF treatment of a real wastewater polluted with the drug led to slower INH and TOC abatements because of the parallel destruction of its natural organic matter content.

On the performance of electrocatalytic anodes for photoelectro-Fenton treatment of synthetic solutions and real water spiked with the herbicide chloramben

The destruction of the herbicide chloramben in 0.050 M Na₂SO₄ solutions at natural pH has been studied by photoelectro-Fenton with UVA light (PEF). The trials were carried out in a cell equipped with an air-diffusion cathode for H₂O₂ generation and different electrocatalytic anodes, namely active IrO₂-based and RuO₂-based electrodes and non-active boron-doped diamond (BDD) and PbO₂ ones. Similar removal rates were found regardless of the anode nature because the herbicide was mainly oxidized by OH formed from Fenton's reaction, which was enhanced by UVA-induced photo-Fenton reaction. The use of an IrO₂-based anode led to almost total mineralization at high current density, as also occurred with the powerful BDD anode, since photoactive intermediates originated from OH-mediated oxidation were degraded under irradiation with UVA light. The good performance of the IrO₂-based anode in PEF was confirmed at different current densities and herbicide concentrations. The presence of Cl^- in the medium caused a slight deceleration of herbicide removal as well as mineralization inhibition, owing to the production of active chlorine with consequent formation of persistent chloroderivatives. Seven aromatic products along with oxalic and oxamic acids were identified in sulfate medium. Five aromatic derivatives were detected in Cl^--containing matrix, corroborating the generation of organochlorine compounds. In secondary effluent, larger mineralization was achieved by PEF with a BDD anode due to its high oxidation ability to destroy the chloroderivatives, although an acceptable performance was also obtained using an IrO₂-based anode.

Removal of tyrosol from water by adsorption on carbonaceous materials and electrochemical advanced oxidation processes

This work compares the ability of physical and chemical treatments, namely adsorption and electrochemical advanced oxidation processes, to remove tyrosol from aqueous medium. Adsorption on graphene nanoplatelets (GNPs) performed much better than that with a graphite intercalation compound. Adsorption isotherms were found to follow the Freundlich model (R² = 0.96), which is characteristic of a chemisorption process. Successful electrochemical regeneration enables 5 successive adsorption/regeneration cycles before corrosion of GNPs occurs. Other typical aromatic contaminants that may coexist with tyrosol can be also adsorbed on GNPs. Percentage of regeneration efficiency of GNPs showed a higher affinity towards Lewis acids group compounds and a lower one towards Lewis base. The treatment of 100 mL of 0.723 mM tyrosol solutions in non-chlorinated and chlorinated matrices at pH 3.0 was carried out by electrochemical oxidation with electrogenerated H₂O₂ (EO-H₂O₂), electro-Fenton (EF) and UVA photoelectro-Fenton (PEF). Trials were made with a BDD anode and an air-diffusion cathode at 10-30 mA cm^-2. Hydroxyl radicals formed at the anode from water oxidation and/or in the bulk from Fenton's reaction between added Fe²⁺ and generated H₂O₂, along with active chlorine produced in chlorinated medium, were the main oxidants. Tyrosol concentration always decayed following a pseudo-first-order kinetics and its mineralization rose as EO-H₂O₂ < EF < PEF, more rapidly in the chlorinated matrix. The potent photolysis of intermediates under UVA radiation explained the almost total mineralization achieved by PEF in the latter medium. The effect of current density and tyrosol content on the performance of all processes was examined.

Current advances and trends in electro-Fenton process using heterogeneous catalysts - A review

Over the last decades, advanced oxidation processes have often been used alone, or combined with other techniques, for remediation of ground and surface water pollutants. The application of heterogeneous catalysis to electrochemical advanced oxidation processes is especially useful due to its efficiency and environmental safety. Among those processes, electro-Fenton stands out as the one in which heterogeneous catalysis has been broadly applied. Thus, this review has introduced an up-to-date collation of the current knowledge of the heterogeneous electro-Fenton process, highlighting recent advances in the use of different catalysts such as iron minerals (pyrite, magnetite or goethite), prepared catalysts by the load of metals in inorganic and organic materials, nanoparticles, and the inclusion of catalysts on the cathode. The effects of physical-chemical parameters as well as the mechanisms involved are critically assessed. Finally, although the utilization of this process to remediation of wastewater overwhelmingly outnumber other utilities, several applications have been described in the context of regeneration of adsorbent or the remediation of soils as clear examples of the feasibility of the electro-Fenton process to solve different environmental problems.

Influence of chelation on the Fenton-based electrochemical degradation of herbicide tebuthiuron

This study describes the performance of electro-Fenton (EF) and photoelectro-Fenton (PEF) processes to degrade the herbicide tebuthiuron (TBH) in 0.050 M Na₂SO₄ at pH = 3.0. Experiments were performed in an undivided cell equipped with a boron-doped diamond (BDD) or Pt anode and an air-diffusion cathode that produces H₂O₂. Physisorbed hydroxyl radicals (M(OH)) generated from water oxidation at the anode and/or free OH formed from Fenton's reaction acted as main oxidants. All processes became much more effective using a BDD anode because of the higher oxidation power of BDD(OH). Sulfate and nitrate were the predominant ions released during TBH destruction. In both, EF and PEF treatments, two distinct kinetic regimes were observed, the first one corresponding to the oxidation of free TBH by OH and the second one to that of the Fe(III)-TBH complex by M(OH). The effect of Fe²⁺ and TBH concentrations on the kinetics of both regions has been examined. Moreover, a poor mineralization was reached with Pt anode, whereas almost total mineralization was attained by EF and PEF with BDD. Both processes showed analogous mineralization rates because the intermediates produced could not be photodegraded by UVA light. Gas chromatography-mass spectrometry analysis of electrolyzed solutions revealed the generation of eight heteroaromatics along with 1,3-dimethylurea, which have been included in a reaction pathway proposed for the initial degradation of TBH.

Treatment of olive oil mill wastewater by single electrocoagulation with different electrodes and sequential electrocoagulation/electrochemical Fenton-based processes

The treatment of olive oil mill wastewater (OOMW) by novel sequential processes involving electrocoagulation (EC) followed by electro-Fenton (EF) or photoelectro-Fenton (PEF) under UVA irradiation has been studied using a boron-doped diamond anode and an air-diffusion cathode for H₂O₂ electrogeneration. Their performance was monitored from the removal of total organic carbon (TOC), chemical oxygen demand, turbidity, total solids and total nitrogen, as well as from the energy consumption. Preliminary EC assays were performed with one pair of electrodes made of Al, Fe, AISI 304 or AISI 316L. The Fe/Fe cell showed the best performance, yielding 40% TOC decay in 20 min. Subsequent EF or PEF at natural pH 7.2 performed similarly, whereas PEF became superior at pH 3.0 due to the action of UVA photons. Comparison between EC/PEF and single EF or PEF at pH 3.0 and 25 mA cm^-2 with 0.50 mM Fe²⁺ revealed the positive outcome of the sequential process, attaining 97.1% TOC abatement after 600 min. GC-MS analysis of the raw wastewater allowed identifying 18 cyclic and 27 aliphatic compounds, most of which could not be removed by EC. The final solutions in EC/EF and EC/PEF contained a large plethora of persistent long-chain aliphatic acids and alkanes.

Application of electrochemical advanced oxidation to bisphenol A degradation in water. Effect of sulfate and chloride ions

Electrochemical oxidation with electrogenerated H₂O₂ (EO- H₂O₂), electro-Fenton (EF), photoelectro-Fenton (PEF) and solar PEF (SPEF) have been applied to mineralize bisphenol A solutions in 0.050 M Na₂SO₄ or 0.008 M NaCl + 0.047 M Na₂SO₄ at pH 3.0. The assays were performed in an undivided cell with a boron-doped diamond (BDD) anode and an air-diffusion cathode for continuous H₂O₂ production. The PEF and SPEF processes yielded almost total mineralization due to the potent synergistic action of generated hydroxyl radicals and active chlorine, in conjunction with the photolytic action of UV radiation. The higher intensity of UV rays from sunlight explained the superior oxidation ability of SPEF. The effect of applied current density was studied in all treatments, whereas the role of bisphenol A concentration was examined in PEF. Bisphenol A abatement followed a pseudo-first-order kinetics, which was very quick in SPEF since UV light favored a large production of hydroxyl radicals from Fenton's reaction. Eight non-chlorinated and six chlorinated aromatics were identified as primary products in the chloride matrix. Ketomalonic, tartronic, maleic and oxalic acids were detected as final short-chain aliphatic carboxylic acids. The large stability of Fe(III)-oxalate complexes in EF compared to their fast photomineralization in PEF and PEF accounted for by the superior oxidation power of the latter processes.

Advanced oxidation of real sulfamethoxazole + trimethoprim formulations using different anodes and electrolytes

A commercial sulfamethoxazole + trimethoprim formulation has been degraded in 0.050 M Na₂SO₄ at pH 3.0 by electrochemical oxidation with electrogenerated H₂O₂ (EO-H₂O₂), electro-Fenton (EF), photoelectro-Fenton with a 6-W UVA lamp (PEF) and solar photoelectro-Fenton (SPEF). The tests were performed in an undivided cell with an IrO₂-based, Pt or boron-doped diamond (BDD) anode and an air-diffusion cathode for H₂O₂ electrogeneration. The anode material had little effect on the accumulated H₂O₂ concentration. Both drugs always obeyed a pseudo-first-order decay with low apparent rate constant in EO-H₂O₂. Much higher values were found in EF, PEF and SPEF, showing no difference because the main oxidant was always OH formed from Fenton's reaction between H₂O₂ and added Fe²⁺. The solution mineralization increased in the sequence EO-H₂O₂ < EF < PEF < SPEF regardless of the anode. The IrO₂-based and Pt anodes behaved similarly but BDD was always more powerful. In SPEF, similar mineralization profiles were found for all anodes because of the rapid removal of photoactive intermediates by sunlight. About 87% mineralization was obtained as maximum for the powerful SPEF with BDD anode. Addition of Cl^- enhanced the decay of both drugs due to their quicker reaction with generated active chlorine, but the formation of persistent chloroderivatives decelerated the mineralization process. Final carboxylic acids like oxalic and oxamic were detected, yielding Fe(III) complexes that remained stable in EF with BDD but were rapidly photolyzed in SPEF with BDD, explaining its superior mineralization ability.

Efficient mineralization of antibiotic ciprofloxacin in acid aqueous medium by a novel photoelectro-Fenton process using a microwave discharge electrodeless lamp irradiation

In this study, a novel photoelectro-Fenton (PEF) process using microwave discharge electrodeless lamp (MDEL) as a UV irradiation source was developed for the removal of antibiotic ciprofloxacin (CIP) in water. Comparative degradation of 200mgL^-1 CIP was studied by direct MDEL photolysis, anodic oxidation (AO), AO in presence of electrogenerated H₂O₂ (AO-H₂O₂), AO-H₂O₂ under MDEL irradiation (MDEL-AO-H₂O₂), electro-Fenton (EF) and MDEL-PEF processes. Higher oxidation power was found in the sequence: MDEL photolysis < AO < AO-H₂O₂< MDEL-AO-H₂O₂< EF < MDEL-PEF. Effects of current density, pH, initial Fe²⁺ concentration and initial CIP concentration on TOC removal in MDEL-PEF process were examined, and the optimal conditions were ascertained. The releases of three inorganic ions (F^-, NH₄⁺ and NO₃^-) and two carboxylic acids (oxalic and formic acids) were qualified. Seven aromatic intermediates mainly generated from hydroxylation, dealkylation and defluorination of CIP were detected by UPLC-QTOF-MS/MS technology. Therefore, plausible degradation sequences for CIP degradation in MDEL-PEF process including all detected products were proposed.

Electrochemical Fenton-based treatment of tetracaine in synthetic and urban wastewater using active and non-active anodes

The electrochemical degradation of tetracaine hydrochloride has been studied in urban wastewater. Treatments in simulated matrix with similar ionic composition as well as in 0.050 M Na₂SO₄ were comparatively performed. The cell contained an air-diffusion cathode for H₂O₂ electrogeneration and an anode selected among active Pt, IrO₂-based and RuO₂-based materials and non-active boron-doped diamond (BDD). Electrochemical oxidation with electrogenerated H₂O₂ (EO-H₂O₂), electro-Fenton (EF) and photoelectro-Fenton (PEF) were comparatively assessed at pH 3.0 and constant current density. The pharmaceutical and its byproducts were oxidized by OH formed from water oxidation at the anode surface and in the bulk from Fenton's reaction, which occurred upon addition of 0.50 mM Fe²⁺ in all media, along with active chlorine originated from the anodic oxidation of Cl^- contained in the simulated matrix and urban wastewater. The PEF process was the most powerful treatment regardless of the electrolyte composition, owing to the additional photolysis of intermediates by UVA radiation. The use of BDD led to greater mineralization compared to other anodes, being feasible the total removal of all organics from urban wastewater by PEF at long electrolysis time. Chlorinated products were largely recalcitrant when Pt, IrO₂-based or RuO₂-based anodes were used, whereas they were effectively destroyed by BDD(OH). Tetracaine decay always obeyed a pseudo-first-order kinetics, being slightly faster with the RuO₂-based anode in Cl^- media because of the higher amounts of active chlorine produced. Total nitrogen and concentrations of NH₄⁺, NO₃^-, ClO₃^-, ClO₄^- and active chlorine were determined to clarify the behavior of the different electrodes in PEF. Eight intermediates were identified by GC-MS and fumaric and oxalic acids were quantified as final carboxylic acids by ion-exclusion HPLC, allowing the proposal of a plausible reaction sequence for tetracaine mineralization by PEF in Cl^--containing medium.

Removal of atrazine and its by-products from water using electrochemical advanced oxidation processes

Atrazine (ATZ) is one of the most common pesticides detected in surface water in Quebec (Canada). The present study was mainly focused on the degradation of ATZ and its by-products using electrochemical advanced oxidation processes such as photo-electro-Fenton (PEF), electro-Fenton (EF) and anodic-oxidation with simultaneous H₂O₂ formation (AO - H₂O₂). The comparison of these processes showed that PEF process was found to be the most effective process in removing ATZ and its by-products from both synthetic solution (ATZ₀ = 100 μg L^-1) and real agricultural surface water enriched with ATZ (ATZ₀ = 10 μg L^-1). Different operating parameters, including wavelength of the light, pH, current density and the presence of natural organic matter (humic acids) were investigated for PEF process using boron-doped diamond (BDD) anode and graphite cathode. The current density and the wavelength of the light were the most important parameters in the ATZ degradation efficiency. The best operating conditions were recorded for the synthetic samples at a current density of 18.2 mA cm^-2, a pH of 3.0 and treatment time of 45 min. Results showed that atrazine-desethyl-desisopropyl (DEDIA) was the most important by-product recorded. More than 99% of ATZ oxidation was recorded after 15 min of treatment and all the concentrations of major by-products were less than the limit of detection after 45 min of treatment. The PEF process was also tested for real surface water contaminated by ATZ: i) with and without addition of iron; ii) without pH adjustment (pH ∼ 6.7) and with pH adjustment (pH ∼ 3.1). In spite of the presence of radical scavenger and iron complexation the PEF process was more effective to remove ATZ from real surface water when the pH value was adjusted near to 3.0. The ATZ removal was 96.0% with 0.01 mM of iron (k_app = 0.13 min^-1) and 100% with 0.1 mM of iron (k_app = 0.17 min^-1).

Electrochemical destruction of trans-cinnamic acid by advanced oxidation processes: kinetics, mineralization, and degradation route

Acidic solutions of trans-cinnamic acid at pH 3.0 have been comparatively treated by anodic oxidation with electrogenerated H₂O₂ (AO-H₂O₂), electro-Fenton (EF), and photoelectro-Fenton (PEF). The electrolytic experiments were carried out with a boron-doped diamond (BDD)/air-diffusion cell. The substrate was very slowly abated by AO-H₂O₂ because of its low reaction rate with oxidizing ^•OH produced from water discharge at the BDD anode. In contrast, its removal was very rapid and at similar rate by EF and PEF due to the additional oxidation by ^•OH in the bulk, formed from Fenton's reaction between cathodically generated H₂O₂ and added Fe²⁺. The AO-H₂O₂ treatment yielded the lowest mineralization. The EF process led to persistent final products like Fe(III) complexes, which were quickly photolyzed upon UVA irradiation in PEF to give an almost total mineralization with 98 % total organic carbon removal. The effect of current density and substrate concentration on all the mineralization processes was examined. Gas chromatography-mass spectrometry (GC-MS) analysis of electrolyzed solutions allowed identifying five primary aromatics and one heteroaromatic molecule, whereas final carboxylic acids like fumaric, acetic, and oxalic were quantified by ion exclusion high-performance liquid chromatography (HPLC). From all the products detected, a degradation route for trans-cinnamic acid is proposed.

Degradation of the insecticide propoxur by electrochemical advanced oxidation processes using a boron-doped diamond/air-diffusion cell

A solution with 0.38 mM of the pesticide propoxur (PX) at pH 3.0 has been comparatively treated by electrochemical oxidation with electrogenerated H₂O₂ (EO-H₂O₂), electro-Fenton (EF), and photoelectro-Fenton (PEF). The trials were carried out with a 100-mL boron-doped diamond (BDD)/air-diffusion cell. The EO-H₂O₂ process had the lowest oxidation ability due to the slow reaction of intermediates with ^•OH produced from water discharge at the BDD anode. The EF treatment yielded quicker mineralization due to the additional ^•OH formed between added Fe²⁺ and electrogenerated H₂O₂. The PEF process was the most powerful since it led to total mineralization by the combined oxidative action of hydroxyl radicals and UVA irradiation. The PX decay agreed with a pseudo-first-order kinetics in EO-H₂O₂, whereas in EF and PEF, it obeyed a much faster pseudo-first-order kinetics followed by a much slower one, which are related to the oxidation of its Fe(II) and Fe(III) complexes, respectively. EO-H₂O₂ showed similar oxidation ability within the pH range 3.0-9.0. The effect of current density and Fe²⁺ and substrate contents on the performance of the EF process was examined. Two primary aromatic products were identified by LC-MS during PX degradation.

4-Hydroxyphenylacetic acid oxidation in sulfate and real olive oil mill wastewater by electrochemical advanced processes with a boron-doped diamond anode

The degradation of 4-hydroxyphenylacetic acid, a ubiquitous component of olive oil mill wastewater (OOMW), has been studied by anodic oxidation with electrogenerated H₂O₂ (AO-H₂O₂), electro-Fenton (EF) and photoelectro-Fenton (PEF). Experiments were performed in either a 0.050M Na₂SO₄ solution or a real OOMW at pH 3.0, using a cell with a boron-doped diamond (BDD) anode and an air-diffusion cathode for H₂O₂ generation. Hydroxyl radicals formed at the BDD surface from water oxidation in all processes and/or in the bulk from Fenton's reaction between added Fe²⁺ and generated H₂O₂ in EF and PEF were the main oxidants. In both matrices, the oxidation ability of the processes increased in the order AO-H₂O₂<EF<PEF. The superiority of PEF was due to the photolytic action of UVA radiation on photosensitive by-products, as deduced from the quick removal of Fe(III)-oxalate complexes. The effect of current density and organic content on the performance of all treatments was examined. 4-Hydroxyphenylacetic acid decay obeyed a pseudo-first-order kinetics. The PEF treatment of 1.03mM 4-hydroxyphenylacetic acid in 0.050M Na₂SO₄ allowed 98% mineralization at 360min even at low current density, whereas 80% mineralization and a significant enhancement of biodegradability were achieved with the real OOMW.

Degradation of trans-ferulic acid in acidic aqueous medium by anodic oxidation, electro-Fenton and photoelectro-Fenton

Solutions of pH 3.0 containing trans-ferulic acid, a phenolic compound in olive oil mill wastewater, have been comparatively degraded by anodic oxidation with electrogenerated H2O2 (AO-H2O2), electro-Fenton (EF) and photoelectro-Fenton (PEF). Trials were performed with a BDD/air-diffusion cell, where oxidizing OH was produced from water discharge at the BDD anode and/or in the solution bulk from Fenton's reaction between cathodically generated H2O2 and added catalytic Fe(2+). The substrate was very slowly removed by AO-H2O2, whereas it was very rapidly abated by EF and PEF, at similar rate in both cases, due to its fast reaction with OH in the bulk. The AO-H2O2 process yielded a slightly lower mineralization than EF, which promoted the accumulation of barely oxidizable products like Fe(III) complexes. In contrast, the fast photolysis of these latter species under irradiation with UVA light in PEF led to an almost total mineralization with 98% total organic carbon decay. The effect of current density and substrate concentration on the performance of all treatments was examined. Several solar PEF (SPEF) trials showed its viability for the treatment of wastewater containing trans-ferulic acid at larger scale. Four primary aromatic products were identified by GC-MS analysis of electrolyzed solutions, and final carboxylic acids like fumaric, acetic and oxalic were detected by ion-exclusion HPLC. A reaction sequence for trans-ferulic acid mineralization involving all the detected products is finally proposed.

Elimination of radiocontrast agent diatrizoic acid by photo-Fenton process and enhanced treatment by coupling with electro-Fenton process

The removal of radiocontrast agent diatrizoic acid (DIA) from water was performed using photo-Fenton (PF) process. First, the effect of H2O2 dosage on mineralization efficiency was determined using ultraviolet (UV) irradiation. The system reached a maximum mineralization degree of 60 % total organic carbon (TOC) removal at 4 h with 20 mM initial H2O2 concentration while further concentration values led to a decrease in TOC abatement efficiency. Then, the effect of different concentrations of Fenton's reagents was studied for homogeneous Fenton process. Obtained results revealed that 0.25 mM Fe(3+) and 20 mM H2O2 were the best conditions, achieving 80 % TOC removal efficiency at 4 h treatment. Furthermore, heterogeneous PF treatment was developed using iron-activated carbon as catalyst. It was demonstrated that this catalyst is a promising option, reaching 67 % of TOC removal within 4 h treatment without formation of iron leachate in the medium. In addition, two strategies of enhancement for process efficiency are proposed: coupling of PF with electro-Fenton (EF) process in two ways: photoelectro-Fenton (PEF) or PF followed by EF (PF-EF) treatments, achieving in both cases the complete mineralization of DIA solution within only 2 h. Finally, the Microtox tests revealed the formation of more toxic compounds than the initial DIA during PF process, while, it was possible to reach total mineralization by both proposed alternatives (PEF or PF-EF) and thus to remove the toxicity of DIA solution.

Comparative electrochemical degradation of salicylic and aminosalicylic acids: Influence of functional groups on decay kinetics and mineralization

Solutions of 100 mL with 1.20 mM of salicylic acid (SA), 4-aminosalicylic acid (4-ASA) or 5-aminosalicylic acid (5-ASA) have been comparatively degraded by anodic oxidation with electrogenerated H2O2 (AO-H2O2), electro-Fenton (EF) and photoelectro-Fenton (PEF). Trials were carried out with a stirred tank reactor with a BDD anode and an air-diffusion cathode for continuous H2O2 production. A marked influence of the functional groups of the drugs was observed in their decay kinetics, increasing in the order SA < 5-ASA < 4-ASA in AO-H2O2 and 5-ASA < SA < 4-ASA in EF and PEF, due to the different attack of OH generated at the BDD surface and in the bulk from Fenton's reaction, respectively. This effect was clearly observed when varying the current density between 16.7 and 100 mA cm(-2). The relative mineralization power of the processes always followed the sequence: AO-H2O2 < EF < PEF. The three drugs underwent analogous mineralization abatement up to 88% by AO-H2O2 at 100 mA cm(-2). The mineralization rate in EF and PEF grew in the order: 4-ASA < 5-ASA < SA. The most powerful process was PEF, attaining >98% mineralization for all the drugs at 100 mA cm(-2). Oxalic and oxamic acids were detected as final short-linear aliphatic carboxylic acids by ion-exclusion HPLC, allowing the fast photolysis of their Fe(III) complexes by UVA light to justify the high power of PEF.

Electrochemical incineration of the antibiotic ciprofloxacin in sulfate medium and synthetic urine matrix

The degradation of 100 mL of 0.245 mM of the antibiotic ciprofloxacin in 0.05 M Na2SO4 at pH 3.0 has been studied by electrochemical oxidation with electrogenerated H2O2 (EO-H2O2), electro-Fenton (EF), UVA photoelectro-Fenton (PEF) and solar PEF (SPEF). Electrolyses were performed with a stirred tank reactor using either a boron-doped diamond (BDD) or Pt anode and an air-diffusion cathode. In EF, PEF and SPEF, ciprofloxacin was rapidly removed due to its oxidation with (•)OH formed from Fenton's reaction between added Fe(2+) and H2O2 generated at the cathode. The larger electrochemical incineration of the antibiotic was achieved by SPEF with BDD with 95% mineralization thanks to the additional attack by hydroxyl radicals formed from water oxidation at the BDD anode surface and the photolysis of final Fe(III)-oxalate and Fe(III)-oxamate species from sunlight. Up to 10 primary intermediates and 11 hydroxylated derivatives were identified by LC-MS, allowing the proposal of a reaction sequence for ciprofloxacin mineralization. A different behavior was found when the same antibiotic concentration was oxidized in a synthetic urine matrix with high urea content and a mixture of PO4(3-), SO4(2-) and Cl(-) ions. Since Fenton's reaction was inhibited in this medium, only EO and EO-H2O2 processes were useful for mineralization, being the organics mainly degraded by HClO formed from Cl(-) oxidation. The EO process with a BDD/stainless steel cell was found to be the most powerful treatment for the urine solution, yielding 96% ciprofloxacin removal and 98% mineralization after 360 min of electrolysis at optimum values of pH 3.0 and current density of 66.6 mA cm(-2). The evolution of released inorganic ions was followed by ion chromatography.

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

In this study, a coupled photoelectrocatalytic/photoelectro-Fenton reactor was designed to enhance the degradation efficiency of organic pollutants and tested using the azo dye Orange G as a model compound. Pt-decorated TiO2 nanotubes were used as a photoanode with an air-diffusion polytetrafluoroethylene cathode for H2O2 generation. The sum of individual effects of coupling the photoelectrocatalytic and photoelectro-Fenton processes was evaluated as a function of the decolorization and mineralization of Orange G solutions. The dye solutions were only completely decolorized in more acidic conditions (pH 3.0). The mineralization of the Orange G solutions increased in the sequence photoelectrocatalytic<electro-Fenton<coupled photoelectrocatalytic/photoelectro-Fenton due to the gradual increase in the production of OH radicals. Total organic carbon reductions of 80% for photoelectrocatalysis, 87% for electro-Fenton and 97% for the coupled processes were obtained when using an applied electric charge per unit volume of electrolyzed solution of 200 mA h L(-1). The Orange G decays for all treatments followed pseudo-first-order kinetics, suggesting the attack of a constant concentration of OH radicals. Aromatics such as naphthalenic and benzenic compounds were formed as by-products and were identified using LC-MS/MS analysis. In addition, the generated aliphatic acids were identified using ion-exclusion high-performance liquid chromatography. The final by-products of oxalic and formic acid were identified as ultimate by-products and formed Fe(III) complexes that were rapidly mineralized to CO2 by UV-Vis irradiation. Then, according to the identified oxidation by-products, a plausible pathway was proposed for the degradation of Orange G dye by the coupled process.

Treatment of a mixture of food color additives (E122, E124 and E129) in different water matrices by UVA and solar photoelectro-Fenton

The degradation of 130 mL of mixtures of food azo dyes E122, E124 and E129 has been studied by electro-Fenton (EF) and UVA photoelectro-Fenton (PEF) using a stirred tank reactor with either a boron-doped diamond (BDD) or Pt anode and an air-diffusion cathode. The main oxidant was hydroxyl radical formed at the anode from water oxidation and in the bulk from Fenton's reaction between added Fe(2+) and H2O2 generated at the cathode. In sulfate medium, fast decolorization was found for all systems, but the almost total mineralization was more rapidly achieved by PEF with BDD. The performance with a real water matrix was slightly worse, although the removal of total organic load was still as high as 95%. The solar PEF (i.e., SPEF) treatment of dye mixtures using a 2.5 L flow plant with a BDD/air-diffusion cell coupled to a planar solar photoreactor is also reported. Fast decolorization and almost total mineralization was found in the presence of either sulfate, perchlorate, nitrate or a mixture of sulfate + chloride ions. In chloride medium, however, the formation of recalcitrant chloroderivatives decelerated the degradation process. Greater current efficiency and lower specific energy consumption were attained in sulfate medium at lower current density and higher azo dye content. A plausible reaction sequence based on 18 aromatic intermediates identified by GC-MS and 6 short-linear carboxylic acids detected by ion-exclusion HPLC has been proposed. The SPEF process promoted the photodegradation of Fe(III)-oxalate complexes and other undetected products. Sulfate and nitrate ions were always released to the medium.

Decolorization and mineralization of Allura Red AC aqueous solutions by electrochemical advanced oxidation processes

The decolorization and mineralization of solutions containing 230 mg L(-1) of the food azo dye Allura Red AC at pH 3.0 have been studied upon treatment by electrochemical oxidation with electrogenerated H2O2 (EO-H2O2), electro-Fenton (EF) and photoelectro-Fenton (PEF). Experiments were performed with a stirred tank reactor containing a boron-doped diamond (BDD) or Pt anode and an air-diffusion cathode to generate H2O2. The main oxidants were hydroxyl radicals formed at the anode surface from water oxidation and in the bulk from Fenton's reaction between H2O2 and added Fe(2+). The oxidation ability increased in the sequence EO-H2O2 < EF < PEF and faster degradation was always obtained using BDD. PEF process with BDD yielded almost total mineralization following similar trends in SO4(2-), ClO4(-) and NO3(-) media, whereas in Cl(-) medium, mineralization was inhibited by the formation of recalcitrant chloroderivatives. GC-MS analysis confirmed the cleavage of the −N=N− bond with formation of two main aromatics in SO4(2-) medium and three chloroaromatics in Cl(-) solutions. The effective oxidation of final oxalic and oxamic acids by BDD along with the photolysis of Fe(III)-oxalate species by UVA light accounted for the superiority of PEF with BDD. NH4(+), NO3(-) and SO4(2-) ions were released during the mineralization.