Anodic Oxidation With Electrogenerated H2O2 Research Articles

Greywater treatment by anodic oxidation, photoelectro-Fenton and solar photoelectro-Fenton processes: Influence of relevant parameters and toxicity evolution

In this study, the applicability of factorial design to the treatment of greywater (GW) containing dodecyl-benzene sulfonic acid (LAS) by electrochemical advanced oxidation processes (EAOPs) is demonstrated. At bench scale, anodic oxidation with electrogenerated H2O2 (AO-H2O2) and photoelectro Fenton (PEF) processes were studied following a 23 factorial design with central point insertion, using a first-order mathematical polynomial. In the former process, the combination of a boron-doped diamond (BDD) anode with a carbon-PTFE air-diffusion cathode, both of 3 cm2, yielded a 76% degradation of LAS at 40 mg L−1 along with 52% TOC removal under optimized conditions. The PEF process with 5 mg L−1 Fe2+ at current density of 77.5 mA cm−2 allowed attaining a 63% of LAS degradation and 78% of TOC abatement. The best conditions found for PEF according to the factorial design, in terms of Fe2+ concentration and current density, were applied for the treatment of 10 L of raw GW by solar PEF (SPEF) using a compound parabolic collector (CPC) as solar reactor and a filter-press electrochemical cell. A 70% of LAS removal and a 55% of GW mineralization were attained after 240 min of treatment. Artemia salina toxicity tests were performed with effluents resulting from the different methods under optimum conditions, and the SPEF process was proven to be the most effective and promising EAOP for the reduction of GW toxicity.

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.

Electrochemical oxidation of Naphthol Blue Black with different supporting electrolytes using a BDD /carbon felt cell

The electrochemical oxidation of Naphthol Blue Black (NBB) solution by means of anodic oxidation with electro-generated H2O2 (AO-H2O2) and Electro-Fenton (EF) was studied, using boron doped diamond (BDD)/ carbon felt (CF) cell. The experiments were carried out in NaCl and Na2SO4 as supporting electrolytes with initial concentration of 0.1 mM of NBB. The studied parameters were pH, applied current, concentration of Fenton catalyst, concentration of supporting electrolytes, and Cl-/SO42- mixture. The degradation of NBB was almost total when NaCl was used compared to Na2SO4, thanks to the electro-generated active chlorine (HClO/ClO-). The higher degradation is found with EF compared to AO-H2O2 process, the kinetic of degradation of NBB always follows a pseudo first-order reaction. The optimum conditions for the mineralization of NBB (i.e., 0.1 mM NBB, 50 mM Na2SO4 at pH 3.0, 0.1 mM Fe2+, and a current of 300 mA) were determined. These conditions yielded a total color removal in less than 10 min and 98% of total organic carbon (TOC) removal at 120 min electrolysis time. The biochemical oxygen demand/ Chemical oxygen demand (BOD/COD) ratio was decreased from 0.5 to 0.3, during the same timescales. Whereas, the mineralization current efficiency (MCE%) dropped from 21.5% to 0.05% in the electrolysis time range from 15-120 min suggesting the concomitant parasitic reactions. The evolution of nitrite NO2-, nitrate NO3-, ammonium NH4+, and sulfate SO42- concentrations were also followed as the end-products during the electrolysis.

A comprehensive study on the electrochemical advanced oxidation of antihypertensive captopril in different cells and aqueous matrices

Anodic oxidation with electrogenerated H2O2 (AO-H2O2) was applied to treat 130 mL of antihypertensive drug captopril in sulfate medium, urban wastewater and synthetic urine using a BDD, Pt or IrO2 anode and an air-diffusion cathode. Oxidants were OH formed during O2 evolution and active chlorine formed via anodic oxidation of Cl−. Drug removal decreased as: BDD > Pt > IrO2. The effect of pH and current density was examined. Further, 2.5 L of drug solutions in the same matrices with Fe2+ at pH 3.0 were treated by solar photoelectro-Fenton (SPEF) using a solar pre-pilot flow plant with a Pt/air-diffusion cell and a planar photoreactor. In sulfate medium, SPEF outperformed AO-H2O2 and electro-Fenton because of the efficient Fe3+ photoreduction. Low mineralization was achieved by the small generation of photoactive Fe(III)-carboxylate complexes. In urban wastewater, captopril was more rapidly removed due to active chlorine. Two heterocyclic derivatives and four aliphatic acids were detected.

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.

Influence of electrolysis conditions on the treatment of herbicide bentazon using artificial UVA radiation and sunlight. Identification of oxidation products

The main objective of this work is to demonstrate the viability of solar photoelectro-Fenton (SPEF) process to degrade pesticides in urban wastewater matrix, selecting the herbicide bentazon as a model molecule. In order to provide a correct assessment of the role of the different oxidants and catalysts involved, bentazon was comparatively treated by anodic oxidation with electrogenerated H2O2 (AO-H2O2), electro-Fenton (EF) and UVA-assisted EF (i.e., PEF) processes as well, either in sulfate or chloride media. Trials were made in a stirred tank reactor with an air-diffusion cathode and a boron-doped diamond (BDD), RuO2-based or Pt anode. In chlorinated matrices, the herbicide disappeared more rapidly using a RuO2-based anode because of the generated active chlorine. The best mineralization performance was always obtained using BDD due to its higher oxidation power, which allowed the complete destruction of refractory chloroderivatives. A concentration of 0.50 mM Fe2+ was found optimal to catalyze Fenton's reaction, largely enhancing the mineralization process under the action of OH. Among photo-assisted treatments, sunlight was proven superior to a UVA lamp to promote the photolysis of intermediates, owing to its greater UV irradiance and contribution of visible photons, although PEF also allowed achieving a large mineralization. In all cases, bentazon decay obeyed a pseudo-first-order kinetics. SPEF treatment in urban wastewater using BDD at only 16.6 mA cm−2 yielded 63.2% mineralization. A thorough, original reaction pathway for bentazon degradation is proposed, including seven non-chlorinated aromatics, sixteen chloroaromatics and two chloroaliphatics identified by GC-MS, most of them not previously reported in literature. Ion-exclusion HPLC allowed the detection of seven short-chain linear carboxylic acids.

Ensuring the overall combustion of herbicide metribuzin by electrochemical advanced oxidation processes. Study of operation variables, kinetics and degradation routes

This article reports the electrochemical degradation of the herbicide metribuzin (MTZ) in sulfate medium by advanced oxidation processes like anodic oxidation with electrogenerated H2O2 (AO-H2O2), electro-Fenton (EF) and UVA photoelectro-Fenton (PEF). A boron-doped diamond (BDD) anode was combined with an air-diffusion cathode with ability to produce H2O2. Unprecedented overall combustion was feasible by all methods at a constant current density (j) ≥100.0 mA cm−2. The total organic carbon (TOC) removal achieved by AO-H2O2 was independent from pH within the range 3.0–9.0, whereas the oscillatory dependence of the pseudo-first-order MTZ decay rate constant with this variable was ascribed to adsorption on the BDD surface. In EF and PEF at pH 3.0, 0.50 mM Fe2+ was determined as optimum catalyst content and the MTZ removal showed two consecutive pseudo-first-order kinetic stages. These were related to the fast reaction of the target molecule with OH formed from Fenton’s reaction, followed by a slower attack of physisorbed BDD(OH) onto Fe(III)-MTZ complexes. The effect of j and MTZ content on decay kinetics and TOC removal was examined. PEF was the best treatment due to the decomposition of photoactive intermediates by UVA radiation, yielding total mineralization of a 0.523 mM herbicide solution after 420 min of electrolysis at 100.0 mA cm−2. A thorough reaction pathway for MTZ degradation is proposed from the sixteen heteroaromatic by-products and three aliphatic molecules identified by GC–MS and LC-MS/MS. Oxalic and oxamic acids were detected as final carboxylic acids by ion-exclusion HPLC.

Total mineralization of mixtures of Tartrazine, Ponceau SS and Direct Blue 71 azo dyes by solar photoelectro-Fenton in pre-pilot plant

Mixtures of monoazo Tartrazine, diazo Ponceau SS and triazo Direct Blue 71 dyes with 105 mg L−1 of total organic carbon (TOC) in 0.050 M Na2SO4 at pH 3.0 have been treated by solar photoelectro-Fenton (SPEF). Experiments were carried out in a 2.5 L pre-pilot plant with a Pt/air-diffusion cell coupled to a solar planar photoreactor. Comparative trials were made by anodic oxidation with electrogenerated H2O2 (AO-H2O2) and electro-Fenton (EF) to better understand the role of oxidizing agents. AO-H2O2 gave poor degradation due to the low oxidation ability of OH formed at the Pt anode and H2O2 produced at the cathode. Similar color removal was achieved in EF and SPEF because the main oxidant was OH formed in the bulk from Fenton's reaction. EF yielded partial mineralization by formation of molecules with high stability against OH. In contrast, these by-products were rapidly photolyzed under sunlight irradiation in SPEF, which was the most powerful treatment. Up to 8 linear final carboxylic acids were detected, along with the release of sulfate and ammonium ions. The effect of Fe2+ and azo dye concentrations, and current density over the SPEF performance was assessed. Total mineralization of azo dyes mixtures occurred when operating up to 105 mg L−1 TOC with 0.50 mM Fe2+ at 100 mA cm−2.

Chemical and electrochemical advanced oxidation processes as a polishing step for textile wastewater treatment: A study regarding the discharge into the environment and the reuse in the textile industry

Abstract This study was focused on the application of various chemical and electrochemical advanced oxidation processes (AOPs/EAOPs) as a polishing step for the remediation of a textile wastewater, concerning its discharge into the environment and its reuse in the textile industry. The textile wastewater was previously subjected to biological treatment, clarification and filtration. The following processes were compared: hydrogen peroxide (H2O2) photolysis with ultraviolet C (UVC) radiation (H2O2/UVC), anodic oxidation (AO), AO with electrogenerated H2O2 (AO-H2O2) and AO-H2O2 with UVC radiation (AO-H2O2/UVC). Furthermore, the influence of H2O2 concentration on the H2O2/UVC process and the influence of current density on the AO-H2O2/UVC process were evaluated. The efficiency of the AOPs/EAOPs was assessed in terms of decolorization and mineralization. The decolorization ability of the various processes up to 25–30 min of reaction could be arranged in the following sequence: AO > AO-H2O2/UVC ≈ H2O2/UVC > AO-H2O2. For treatment times above 25–30 min, the AO-H2O2/UVC and H2O2/UVC processes provided higher decolorization than the AO process. Complete color removals were found, contrasting with a maximum mineralization of 39%, thereby indicating the presence of recalcitrant non-colored organic compounds. The German color discharge standards were achieved after 5–9 min of reaction for all processes, whereas the Brazilian color discharge limit was reached after 10 min of reaction for the AO process, 20 min for the AO-H2O2/UVC and H2O2/UVC processes, and 35 min for the AO-H2O2 process. The feasibility of using the recycled textile wastewater in textile processing was proved for scouring, bleaching and dyeing processes, taking into account hydrophilicity, color difference and whiteness index. The treatment times required for reuse purposes applying the H2O2/UVC process were equal or higher than the ones necessary for discharge purposes: 45 min for scouring, 20 min for bleaching, and 30 min for dyeing.

Electrochemical Treatment of 4-Hydroxyphenylacetic Acid in Olive Oil Mill Wastewater

INTRODUCTION The decontamination of olive oil mill wastewater (OMWW) is of great interest in Mediterranean countries as the main producers of olive oil worldwide. The large variety of recalcitrant substances contained in OMWW and the difficulty to remove them or recycle the large volumes of wastewater encourage the development of new technologies for decontamination, like electrochemical advanced oxidation processes (EAOPs) that present high performance for recalcitrant compounds [1,2]. 4-Hydroxyphenylacetic acid (4-HPA) is a main component of this kind of wastewater [3]. This communication presents the effective removal of 4-HPA from an OMWW matrix by EAOPs including anodic oxidation with electrogenerated H2O2 (AO-H2O2), electro-Fenton (EF) and UVA photoelectro-Fenton (PEF). EXPERIMENTAL Analytical grade 4-HPA was purchased from Sigma. The OMWW was provided by a local company of olive oil extraction, which was characterized by analyzing its metal content, BOD, COD, TOC, conductance, TS, TDS and organic components before use. As a first step, the OMWW was filtered with a 45 µm cloth. As a second step, several mixtures with 100 mg L-1 TOC of 4-HPA in OMWW were prepared. The OMWW/water ratio was varied from 20/80 to 100/0 v/v. Experiments were also carried out using 100 mg L-1 TOC of 4-HPA in 50 mM of Na2SO4 to clarify the matrix effect. AO-H2O2, EF and PEF experiments were performed with 100 mL of each solution at pH 3.0 and 25 ºC using a stirred tank reactor with a 3 cm2 boron-doped diamond (BDD) anode and a 3 cm2 carbon-polytetrafluoroethylene air-diffusion cathode by applying 16.7 mA cm-2 for 540 min. In the two latter methods, 0.50 mM Fe2+ was added as catalyst for Fenton’s reaction. PEF trials were run with a 6W UVA light. RESULTS The characteristics of the real OMWW were: 505.35 mg L-1 of TOC, 1.51 mS of conductance, 1600 mg L-1 of BOD, 5536 mg L-1 of TS and 82 mgL-1 of TDS. For each method, a gradual decay in TOC was found as the OMWW ratio in the sample increased. For example, a final TOC removal of 89% was achieved by AO-H2O2 in 50 mM Na2SO4, whereas it was reduced by 75% in a 60/40 OMWW/water mixture and 60% in pure OMWW. The best results were obtained by PEF, with final TOC decays of 98%, 88% and 87% in a the above media due to the combined action of ●OH produced at the BDD surface from water oxidation, the ●OH formed during Fenton’s reaction and the extra decomposition of Fe(III)-carboxylate complexes by UVA light. The kinetic assays also showed a drop in 4-HPA decay with raising OMWW content. Thus, total disappearance of the substrate was attained in 360 min by AO-H2O2 in 50 mM Na2SO4 and in 500 min for the other solutions. For the best process (i.e., PEF), these times diminished to 60 and 80 min, respectively. CONCLUSIONS 4-HPA can be effectively degraded in an OMWW matrix using the EAOPs studied, especially PEF. This process was the most efficient one, reaching an almost total mineralization, to a larger extent in pure Na2SO4. The decay and mineralization of 4-HPA rose in the order AO-H2O2 < EF < PEF, regardless of the matrix used. The oxidation power of ●OH formed from water oxidation at the BDD surface in AO-H2O2 was strongly enhanced by the additional ●OH in the bulk from Fenton’s reaction, being the degradation improved under the potent action of UVA irradiation in PEF. For all the EAOPs studied, the presence of OMWW decelerated TOC removal and 4-HPA decay. This can be related to the parallel reactions of ●OH with the organic components present in OMWW that act as oxidant scavengers. ACKNOWLEDGEMENTS We are grateful for economical support from project CTQ2016-78616-R (MINECO, FEDER, EU) and for the scholarship given to N.E Flores from SENESCYT (Ecuador). BIBLIOGRAPHY [1] A. Thiam, E. Brillas, F. Centellas, P.L. Cabot, I. Sirés, Electrochemical reactivity of Ponceau 4R (food additive E124) in different electrolytes and batch cells, Electrochim. Acta 173 (2015) 523. [2] A. Thiam, E. Brillas, J.A. Garrido, R.M. Rodríguez, I. Sirés, Routes for the electrochemical degradation of the artificial food azo-colour Ponceau 4R by advanced oxidation processes, Appl. Catal. B: Environ. 180 (2016) 227. [3] C. Belaid, M. Khadraoui, S. Mseddi, M. Kallel, B. Elleuch, J.F. Fauvarque, Electrochemical treatment of olive mill wastewater: Treatment extent and effluent phenolic compounds monitoring using some uncommon analytical tools, J. Environ. Sci. 25 (2013) 220.

Abatement of the fluorinated antidepressant fluoxetine (Prozac) and its reaction by-products by electrochemical advanced methods

The degradation of the fluorinated antidepressant fluoxetine, as hydrochloride, was comparatively studied in sulfate medium at pH 3.0 by anodic oxidation with electrogenerated H2O2 (AO-H2O2), electro-Fenton (EF) and photoelectro-Fenton (PEF). Experiments were performed with 100mL solutions in an undivided tank reactor equipped with a Pt, RuO2-based or boron-doped diamond (BDD) anode and an air-diffusion cathode for continuous H2O2 production. The BDD anode showed higher mineralization rate due to the great production of physisorbed BDD(OH), which has large reactivity to oxidize the drug and intermediates. The degradation rate was enhanced by EF with 0.50mM Fe2+ due to the additional production of OH in the bulk from Fenton’s reaction. The degradation was even faster using PEF owing to the additional photolytic action of UVA radiation. The most effective process was PEF with a BDD anode achieving 94% mineralization at 300min. The fluoxetine decay followed a pseudo-first-order kinetics, being quicker in the order:AO-H2O2<EF<PEF. The effect of the current density and drug concentration on the mineralization rate and fluoxetine decay was clarified. Oxidation of fluoxetine by hydroxyl radicals yielded four aromatic by-products, as found by GC–MS. Additionally, a chloroaromatic compound was identified as a result of the reaction of active chlorine, which was formed in situ from the oxidation of chloride ion at the BDD anode. Four short-chain linear carboxylic acids, being oxalic and formic acid more abundant, were identified. In PEF, fluorine atoms of fluoxetine were completely released as fluoride ion, whereas the initial nitrogen was converted into nitrate ion in all cases. A reaction pathway for fluoxetine mineralization by the electrochemical advanced methods is finally proposed.

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.

Tertiary treatment of a municipal wastewater toward pharmaceuticals removal by chemical and electrochemical advanced oxidation processes

This study focuses on the degradation of pharmaceuticals from a municipal wastewater after secondary treatment by applying various advanced oxidation processes (AOPs) and electrochemical AOPs (EAOPs) like UVC, H2O2/UVC, anodic oxidation (AO), AO with electrogenerated H2O2 (AO-H2O2), AO-H2O2/UVC and photoelectro-Fenton (PEF) using either UVC radiation (PEF-UVC) or UVA radiation (PEF-UVA). The municipal wastewater after secondary treatment was spiked with 5.0 mg L−1 of trimethoprim (TMP) antibiotic. The efficiency of processes to remove TMP followed the order UVC < AO-H2O2 < PEF-UVA << AO ≈ PEF-UVC < AO-H2O2/UVC < PEF-UVA (pH = 2.8) < H2O2/UVC ≈ PEF-UVC (pH = 2.8), using neutral pH, except when identified. While the UVC radiation alone led to a very low TMP removal, the H2O2/UVC process promoted a very high TMP degradation due to the production of hydroxyl radicals (OH) by H2O2 cleavage. In the AO-H2O2/UVC process, the electrogeneration of H2O2 can avoid the risks associated with the transportation, storage and manipulation of this oxidant and, furthermore, OH at the anode surface are also formed. Nevertheless, low contents of H2O2 were detected mainly at the beginning of the reaction, leading to a lower initial reaction rate when compared with the H2O2/UVC system. In the PEF-UVC, the addition of iron at neutral pH led to the visible formation of insoluble iron oxides that can filter the light. At pH 2.8, the iron remained dissolved, thereby promoting the Fenton's reaction and increasing the organics removal. The UVA-driven processes showed limited efficiency when compared with those using UVC light. For all processes with H2O2 electrogeneration, the active chlorine species can be scavenged by the H2O2, diminishing the efficiency of the processes. This can explain the lower efficiency of AO-H2O2 when compared with AO. Moreover, the degradation of the MWWTP effluent spiked with 18 pharmaceuticals in μg L−1 during AO process was assessed as well as the influence of the following operational variables on the process efficiency: (i) H2O2 concentration on H2O2/UVC, (ii) current density on AO, AO-H2O2, AO-H2O2/UVC, PEF-UVC and PEF-UVA, and (iii) pH on PEF-UVA.

Removal of 4-hydroxyphenylacetic acid from aqueous medium by electrochemical oxidation with a BDD anode: Mineralization, kinetics and oxidation products

The degradation of 100mL of solutions containing 4-hydroxyphenylacetic acid in 0.050M Na2SO4 at pH3.0 has been performed by anodic oxidation with electrogenerated H2O2 (AO-H2O2) using a stirred tank reactor equipped with a boron-doped diamond (BDD) anode and an air-diffusion cathode. An almost total mineralization with 95.5% total organic carbon (TOC) removal was achieved for a 1.03mM substrate solution at 100mAcm−2. The effect of current density between 16.7 and 100mAcm−2 and 4-hydroxyphenylacetic acid content between 0.21 and 2.06mM was examined. Greater current efficiency with lower specific energy consumption and smaller mineralization was found at low current density and high substrate concentration. The TOC abatement as well as the 4-hydroxyphenylacetic acid concentration decay obeyed a pseudo-first-order kinetics. The oxidation role of hydroxyl radical formed from water discharge at the BDD anode is explained on the basis of its electrogeneration rate and competitive wasting reactions. 4-Hydroxybenzenemethanol and its derivative 4-hydroxybenzenealdehyde were identified as primary aromatic by-products by gas chromatography-mass spectrometry. Ion-exclusion HPLC allowed the detection of low amounts of the persistent oxalic acid during the AO-H2O2 process. The remaining TOC in final electrolyzed solutions is related to the presence of a large proportion of unidentified by-products that are even more recalcitrant than common short-chain aliphatic carboxylic acids.

Application of anodic oxidation, electro-Fenton and UVA photoelectro-Fenton to decolorize and mineralize acidic solutions of Reactive Yellow 160 azo dye

The degradation of 100cm3 of a solution with 0.167mmoldm−3 Reactive Yellow 160 (RY160) azo dye in sulfate medium at pH 3.0 has been comparatively studied by anodic oxidation with electrogenerated H2O2 (AO-H2O2), electro-Fenton (EF) and UVA photoelectro-Fenton (PEF). Trials were carried out with a stirred tank reactor equipped with a boron-doped diamond (BDD) anode and an air-diffusion cathode for H2O2 production, upon addition of 0.50mmoldm−3 Fe2+ as catalyst in EF and PEF. The solution was slowly decolorized by AO-H2O2 because of the low rate of reaction of the azo dye and its colored products with hydroxyl radicals generated at the BDD anode from water oxidation. The color loss was enhanced in EF by the larger oxidation ability of hydroxyl radicals produced in the bulk from Fenton’s reaction between added Fe2+ and generated H2O2, whereas the solution was more rapidly decolorized by PEF owing to the additional generation of hydroxyl radicals from the photolysis of Fe(III)-hydroxy complexes by UVA light. The relative mineralization ability of the processes also increased in the sequence AO-H2O2<EF<PEF. The PEF method was the most powerful due to the synergistic oxidation by hydroxyl radicals and UVA irradiation, yielding 94% mineralization after 360min at 100mAcm−2. The influence of current density and RY160 concentration on the performance of all processes was assessed. Final carboxylic acids like maleic, fumaric, tartronic, acetic, oxalic, oxamic and formic were quantified by ion-exclusion HPLC. All these acids were totally removed by PEF, but the formation of small amounts of other highly recalcitrant products impeded the total mineralization. Chloride, sulfate, ammonium and, to a smaller extent, nitrate ions were released to the solution from the heteroatoms of the azo dye in all cases.

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.

Anodic oxidation, electro-Fenton and photoelectro-Fenton degradations of pyridinium- and imidazolium-based ionic liquids in waters using a BDD/air-diffusion cell

The degradation of two representative ionic liquids, 1-butyl-4-methylpyridinium chloride and 1-ethyl-3-methylimidazolium chloride, has been studied in 100cm3 of aqueous sulfate medium of pH 3.0 by electrochemical advanced oxidation processes (EAOPs) like anodic oxidation with electrogenerated H2O2 (AO-H2O2), electro-Fenton (EF) and photoelectro-Fenton (PEF). The electrolytic cell contained a boron-doped diamond (BDD) anode and an air diffusion cathode. The main oxidants were hydroxyl radicals formed at the anode surface from water discharge and in the bulk from Fenton’s reaction between generated H2O2 and added Fe2+. The solutions of both compounds underwent an almost total mineralization with 97% and 94% of dissolved organic abatement by the most powerful PEF at high current density, being similar and slightly lower for AO-H2O2 and EF, respectively. At low current density, the pyridinium-based ionic liquid showed similar mineralization rate by AO-H2O2 and PEF, whereas the imidazolium-based ionic liquid was more rapidly degraded in the sequence: AO-H2O2<EF<PEF, where the positive effect of UVA irradiation in PEF was more apparent. The current efficiency decreased gradually with increasing current density in all EAOPs and its value was always superior for the latter compound. The NO3− and NH4+ concentrations produced in each process were quantified by ion chromatography. This technique confirmed a drastic oxidation of Cl− contained in the parent products at the BDD anode to be partially converted into ClO3− and, in much smaller proportion, into ClO4−. Primary heteroaromatic products were identified by LC-MS and final carboxylic acids were detected by ion-exclusion HPLC. Plausible reaction sequences for the mineralization of the cation of each ionic liquid including all detected products are proposed.

Salicylic acid degradation by advanced oxidation processes. Coupling of solar photoelectro-Fenton and solar heterogeneous photocatalysis

A 3.0 L solar flow plant with a Pt/air-diffusion (anode/cathode) cell, a solar photoreactor and a photocatalytic photoreactor filled with TiO2-coated glass spheres has been utilized to couple solar photoelectro-Fenton (SPEF) and solar heterogeneous photocatalysis (SPC) for treating a 165mgL−1 salicylic acid solution of pH 3.0. Organics were destroyed by OH radicals formed on the TiO2 photocatalyst and at the Pt anode during water oxidation and in the bulk from Fenton’s reaction between added Fe2+ and cathodically generated H2O2, along with the photolytic action of sunlight. Poor salicylic acid removal and mineralization were attained using SPC, anodic oxidation with electrogenerated H2O2 (AO-H2O2) and coupled AO-H2O2-SPC. The electro-Fenton process accelerated the substrate decay, but with low mineralization by the formation of byproducts that are hardly destroyed by OH. The mineralization was strongly increased by SPEF due to the photolysis of products by sunlight, being enhanced by coupled SPEF-SPC due to the additional oxidation by OH at the TiO2 surface. The effect of current density on the performance of both processes was examined. The most potent SPEF-SPC process at 150mAcm−2 yielded 87% mineralization and 13% current efficiency after consuming 6.0AhL−1. Maleic, fumaric and oxalic acids detected as final carboxylic acids were completely removed by SPEF and SPEF-SPC.

Treatment of single and mixed pesticide formulations by solar photoelectro-Fenton using a flow plant

Solutions of single real formulations of herbicides tebuthiuron and ametryn have been degraded by anodic oxidation with electrogenerated H2O2 (AO-H2O2), electro-Fenton (EF) and solar photoelectro-Fenton (SPEF). The assays were made in a flow plant of 2.5L equipped with a boron-doped diamond (BDD)/air-diffusion cell and a solar planar photoreactor. Main oxidants were hydroxyl radicals formed from water discharge at the BDD anode and in the bulk from Fenton’s reaction between added Fe2+ and cathodically generated H2O2. Partial mineralization was achieved for all the treatments, with the relative oxidation ability growing in the sequence: AO-H2O2<EF<SPEF. The superiority of SPEF over EF was related to the contribution of photo-oxidation of intermediates and/or their Fe(III) complexes upon sunlight irradiation. No short-linear carboxylic acids were identified, whereas heteroatoms were mainly released as NO3− and SO42− ions. For herbicide contents ⩽0.18mM in EF and SPEF, a very rapid pseudo-first-order decay kinetics was related to the oxidation of the Fe(II) complexes of each herbicide at short time, whereupon a much slower pseudo-first-order kinetics associated with the decay of their Fe(III) complexes was observed. Ametryn disappeared more quickly than tebuthiuron, but its solutions were more poorly mineralized. The effect of j and substrate concentration on the performance of SPEF was examined. Mixtures of both formulations were efficiently treated by SPEF, showing similar but slower decays than single solutions. Four primary heteroaromatic products were detected by GC–MS upon prolonged electrolysis of an equimolar mixture. Our results indicate that SPEF is viable for the treatment of water contaminated with tebuthiuron and ametryn herbicides, either single or mixed.

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

Acidic solutions of trans-cinnamic acid at pH3.0 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.