Solar photoelectro-Fenton Research Articles

Vermiculite as heterogeneous catalyst in electrochemical Fenton-based processes: Application to the oxidation of Ponceau SS dye

Modified sodium vermiculite, an iron-rich clay mineral, has been used in novel heterogeneous electrochemical Fenton-based treatments, so-called electro-Fenton (EF)-vermiculite, UVA photoelectro-Fenton (PEF)-vermiculite and solar photoelectro-Fenton (SPEF)-vermiculite. Tests were made with 130 mL of 0.150 mM Ponceau SS diazo dye in 0.050 M Na2SO4 at pH 3.0, in the presence of 1.0 g L−1 catalyst microparticles. The electrolyses were performed in an undivided cell with a boron-doped diamond anode (BDD) and air-diffusion cathode for H2O2 production, at 33.3 mA cm−2. Decolorization and mineralization were upgraded in the sequence: EF-vermiculite < PEF-vermiculite < SPEF-vermiculite. The removal of organics occurred by the combined action of OH oxidant formed at the BDD surface and homogeneous and heterogeneous Fenton's reactions, along with the photolysis caused by UVA light or sunlight. The homogeneous Fenton's reaction resulted from iron ions leaching, but the heterogeneous mechanism was prevalent. Comparative treatments by anodic oxidation in the presence of H2O2 and homogeneous EF were less effective than EF-vermiculite. The diazo dye absorbance decays agreed with a pseudo-first-order kinetics. SPEF-vermiculite was the most powerful process, yielding total decolorization and 84.1% mineralization after 300 and 360 min, respectively. The influence of catalyst concentration, current density and diazo dye content on PEF-vermiculite performance was examined. Oxalic, oxamic, malic, tartronic and acetic acids were detected as final short-linear carboxylic acids.

Electrosynthesis of Hydrogen Peroxide in a Novel Flow-through Electrochemical Reactor with in Situ generated Oxygen

Most of electrochemical advanced oxidation processes (EAOP´s) such as the electroFenton (EF), photo electroFenton (PEF), solar photo electroFenton (SPEF) and electro-peroxone (E-peroxone) processes use hydrogen peroxide (H2O2) as its core reactant due to its possibility to react with some catalysts (Fe2+, O3) yielding high oxidant hydroxyl radicals to be used in the degradation of persistent organic pollutants [1]. Carbonaceous materials like carbon felt, carbon cloth and reticulated vitreous carbon (RVC) are the most used because of its capability to accumulate H2O2 [2,3]. The use of these materials is often in gas diffusion electrodes which are based in the injection of air from an external source to an air chamber located behind the electrode allowing the dispersion of the gas inside the electrode promoting the oxygen reduction [3]. This work deals with the in situ electrosynthesis of H2O2 in a flow through reactor using RVC as cathode material and Ti|Ir-Ta oxides expanded mesh as anode to improve the oxygen evolution reaction with subsequent reduction of O2 on the RVC to yield H2O2. The electrochemical cell consists of a flow channel where the electrodes are placed along its length, allowing the electrolyte to flow through them forcing the transport of oxygen bubbles inside the porous carbon material for its reduction to H2O2. [1] Sirés I., Brillas., E. Remediation of water pollution caused by pharmaceutical residues sed on electrochemical separation and degradation technologies: A review. Environmental International, 40 (2018) 212-229. [2] Coria G., Pérez T., Sirés I., Nava J.L., Mass transport studies during dissolved oxygen reduction to hydrogen peroxide in a filter-press electrolyzer using graphite felt, reticulated vitreous carbon and boron-doped diamond as cathodes. Journal of Electroanalytical Chemistry, 757 (2015) 225-229. [3] Pérez T., Coria G., Sirés I., Nava J.L., Uribe A.R., Electrosynthesis of hydrogen peroxide in a filter-press flow cell using graphite felt as air-diffusion cathode. Journal of Electroanalytical Chemistry, 812 (2018) 54-58.

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.

Treatment of industrial textile wastewater by the solar photoelectro-Fenton process: Influence of solar radiation and applied current

This work addresses the removal of Acid Blue 29 (AB29) textile dye by a solar photo-electro-Fenton (SPEF) process. Ten liter AB29 solutions containing 100 mg L−1 total organic carbon (TOC), 0.5 mM Fe2+ and 0.05 M Na2SO4 at a pH of 3.0 were electrolyzed by applying current densities of 25 and 50 mA cm−2 in a pilot plant built in our laboratory. The pilot plant was composed of an electrochemical filter press cell with a dimensionally stable anode (DSA-type) and an air-diffusion cathode coupled to a solar photo-reactor.AB29 was completely mineralized by SPEF at the end of electrolysis, while the solution achieved decolorization in short periods of time on sunny days. Reaction intermediates, such as aromatic compounds, carboxylic acids and inorganic ions, were found during the oxidation process. Solar irradiation favors the mineralization of the dye in cost and efficiency compared to other electrochemical methods. Moreover, the treatment of an industrial textile effluent containing AB29 by SPEF produced mineralization greater than 90% on sunny days.

Effect of anodic materials on solar photoelectro-Fenton process using a diazo dye as a model contaminant

Boron-doped diamond (BDD), Pt and Ti/RuO2 anodes were chosen to treat 118 mg L−1 Acid Blue 29 (AB29) solution in 0.050 M Na2SO4 at pH 3.0 by electrochemical oxidation with electrogenerated H2O2 (EO-H2O2), electro-Fenton (EF) and solar photoelectro-Fenton (SPEF) processes. Pseudo-first order decolourisation decays were obtained with the following order of enhancement: EO-H2O2< EF < SPEF. In EO-H2O2 are present the electrogenerated heterogeneous OH radicals on the surface of the anode, while in EF and SPEF, there is the additional influence of homogeneous OH radicals generated in the bulk by Fenton's reaction. In SPEF, the solar radiation enhances catalyst regeneration and OH generation, as well as the photolysis of Fe3+ complexes. Regarding the anodic materials, its effect was less significant in SPEF due to the predominance of homogeneous reactions. In this process, the influence of current density is more meaningful over time than as a function of charge. At the end of 15 min, applying 33.3 mA cm−2, almost complete degradation of AB29 was achieved, regardless of the anode used; 285 min after, complete mineralisation of the organic matter was reached. The energy consumption attained with BDD and Pt was quite similar, ≈ 0.11 kWh gCOD−1, whereas the Ti/RuO2 was 0.09 kWh gCOD−1. Furthermore, the evolution of short-chain carboxylic acids was followed over time and the concentration of nitrogen species was monitored.This study highlights the potential of low cost anode materials to be applied in SPEF in order to achieve a promising and competitive alternative to wastewater treatments.

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

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

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.

Optimization of electrocatalytic H2O2 production at pilot plant scale for solar-assisted water treatment

This manuscript summarizes the successful start-up and operation of a hybrid eco-engineered water treatment system, at pilot scale. The pilot unit, with 100 L capacity, has been devised for the efficient electrocatalytic production of H2O2 at an air-diffusion cathode, triggering the formation of OH from Fenton’s reaction with added Fe2+ catalyst. These radicals, in combination with those formed at a powerful boron-doped diamond (BDD) anode in an undivided cell, are used to degrade a mixture of model pesticides. The capability of the plant to produce H2O2 on site was initially optimized using an experimental design based on central composite design (CCD) coupled with response surface methodology (RSM). This aimed to evaluate the effect of key process parameters like current density (j) and solution pH. The influence of electrolyte concentration as well as liquid and air flow rates on H2O2 electrogeneration and current efficiency at optimized j and pH was also assessed. The best operation conditions resulted in H2O2 mass production rate of 64.9 mg min−1, 89.3% of current efficiency and 0.4 kWh m-3 of energy consumption at short electrolysis time. Performance tests at optimum conditions were carried out with 75 L of a mixture of pesticides (pyrimethanil and methomyl) as a first step towards the elimination of organic contaminants by solar photoelectro-Fenton (SPEF) process. The combined action of homogeneous (OH) and heterogeneous (BDD(OH)) catalysis along with photocatalysis (UV photons collected at a solar CPC photoreactor) allowed the removal of more than 50% of both pesticides in 5 min, confirming the fast regeneration of Fe2+ catalyst through cathodic reduction and photo-Fenton reaction.

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.

Electrochemical Combustion of the Antibiotic Levofloxacin By Electro-Peroxone Process

Over the last years pharmaceuticals have been receiving a lot of attention as emergent pollutants worldwide. These compounds can reach the environment through many vias, i.e. excretion in urine and faeces of humans and livestock, household disposal or hospital wastes and their concentration in water can vary between ng L-1 to mg L-1 [1]. Despite their low concentration these persistent organics can have adverse effects on the environment, for this reason its elimination is a big concern. Different processes have been tried to remove these compounds from water among which are the electrochemical advanced oxidation processes (EAOP´s) like electroFenton (EF), photo electroFenton (PEF), solar photo electroFenton (SPEF) and electro-peroxone (E-peroxone) [1]. E-peroxone is a relatively new process that employs two reactants, the electrogenerated hydrogen peroxide (via O2 reduction in acidic media) and ozone to produce homogeneous hydroxyl radicals [2]. The E-peroxone process has been studied for the removal of several pharmaceuticals showing great potential in their elimination from water [3]. This work deals with the removal of the antibiotic Levofloxacin (LVN) from synthetic wastewater using the E-peroxone process. The electrolyzes were performed in a laboratory flow plant containing 1.5 dm3 of solution having 50 mg dm-3 of LVN and 0.05 M NaSO4 as background electrolyte at pH 3; a Ti-Ir-Sn-Sb oxides was used as anode and a graphite felt on top of carbon cloth was used as gas diffusion cathode [4], where compressed air was fed at 0.36 psi of pressure. The trials were conducted at a constant cathodic potential (Ecat ) of -0.3 V vs SHE at a volumetric rate (Q) of 1.5 dm3 min-1. The results showed a chemical oxygen demand (COD) decay of 70 % at 6 hours of electrolysis. The influence of current density, O3 feed and initial concentration of LVN on the rate of degradation of LVN and mineralization current efficiency was also analyzed. Finally, the by-products of LVN were followed by chromatographic techniques to propose a reaction pathway. [1] Sirés I., Brillas., E. Remediation of water pollution caused by pharmaceutical residues sed on electrochemical separation and degradation technologies: A review. Environmental International, 40 (2018) 212-229. [2] Li X., Wang Y., Yuan S., Li Z., Wang B., Huang J., Deng S., Yu G. Degradation of the anti-inflammatory drug ibuprofen by electro-peroxone process. Water Research, 63 (2014) 81-93. [3] Wang H., Zhan J., Yao W., Wang B., Deng S., Huang J., Yu G., Wang Y. Comparison of pharmaceutical abatement in various water matrices by conventional ozonation, peroxone (O3/H2O2), and an electro-peroxone process. Water Research, 130 (2018) 127-138. [4] Pérez T., Coria G., Sirés I., Nava J.L., Uribe A.R. Electrosynthesis of hydrogen peroxide in a filter-press flow cell using graphite felt as air diffusion cathode. Journal of Electroanalytical Chemistry, 812 (2018) 54-58.

Air diffusion electrodes based on synthetized mesoporous carbon for application in amoxicillin degradation by electro-Fenton and solar photo electro-Fenton

The aim of this work was to develop an air diffusion mesoporous carbon electrode (ADE-MC) as cathode for the in-situ electrogeneration of hydrogen peroxide and its application on electro-Fenton (EF) and solar photo electro-Fenton (SPEF) processes. Three different mesoporous carbon materials were obtained by organic-organic self-assembly synthesis (soft-template route) by changing concentrations of resorcinol-formaldehyde polymer as carbon precursor and Pluronic F-127™ surfactant as matrix-forming agent. The materials were characterized by X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), linear voltammetry (LV) and linear rotating disk electrode voltammetry (LV-RDE). MC materials showed pore diameter between 3.6 and 4.6 nm. The efficiency of ADE-MC was explored as a function of H2O2 produced at current densities (50, 100 and 150 mA cm−2). The highest production of H2O2 was 10.85 mmol L−1 by applying 150 mA cm−2. Finally, the ADE-MC were used for the mineralization of amoxicillin (50 mg L−1), allowing complete degradation and mineralization percentages of 55% and 85% with EF and SPEF processes, respectively. MC electrodes produce H2O2 in enough amounts that provide a viable option to degradation and mineralization of others organic pollutants in aqueous media by EF and SPEF processes.

Abatement of the antibiotic levofloxacin in a solar photoelectro-Fenton flow plant: Modeling the dissolved organic carbon concentration-time relationship

The degradation of solutions of the antibiotic levofloxacin (LVN) in sulfate medium at pH 3.0 has been investigated at pre-pilot scale by solar photoelectro-Fenton (SPEF) process. The flow plant included an FM01-LC filter-press cell equipped with a Ti|Pt anode and a three-dimensional-like air-diffusion cathode, connected to a compound parabolic collector as photoreactor and a continuous stirred tank under recirculation batch mode. The effect of volumetric flow rate on H2O2 electrogeneration from O2 reduction was assessed. Then, the influence of initial LVN concentration and Fe2+ concentration as catalyst on dissolved organic carbon (DOC) removal was thoroughly investigated. LVN was gradually mineralized by SPEF process, with faster DOC abatement at 0.50 mM Fe2+, yielding 100% after 360 min at applied cathodic potential of −0.30 V|SHE. The high mineralization current efficiency (MCE) and low specific energy consumption (ECDOC) revealed the extraordinary role of homogeneous hydroxyl radicals and natural UV light, which allowed the degradation of the antibiotic and its by-products with MCE values greater than 100%. Five cyclic by-products, N,N-diethylformamide and three short-chain linear carboxylic acids were detected by GC-MS and HPLC analyses. A parametric model to simulate the DOC decay versus electrolysis time was implemented for the SPEF pre-pilot flow plant, showing good agreement with experimental data.

Electrochemical advanced oxidation processes as decentralized water treatment technologies to remediate domestic washing machine effluents.

Water scarcity is one of the major concerns worldwide. In order to secure this appreciated natural resource, management and development of water treatment technologies are mandatory. One feasible alternative is the consideration of water recycling/reuse at the household scale. Here, the treatment of actual washing machine effluent by electrochemical advanced oxidation processes was considered. Electrochemical oxidation and electro-Fenton technologies can be applied as decentralized small-scale water treatment devices. Therefore, efficient decolorization and total organic abatement have been followed. The results demonstrate the promising performance of solar photoelectro-Fenton process, where complete color and organic removal was attained after 240min of treatment under optimum conditions by applying a current density of 66.6mAcm-2. Thus, electrochemical technologies emerge as promising water-sustainable approaches.

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.

Sulfamethoxazole mineralization by solar photo electro-Fenton process in a pilot plant

The sulfamethoxazole (SMX) mineralization through the solar photo electro-Fenton (SPEF) process in a pilot plant containing 20L of 50mgL−1 SMX solution was investigated. The process was carried out at pH 3 under different operational conditions varying temperature, volumetric flow and applied current density. Box-Behnken experimental design and response surface methodology (RSM) were used to optimize the experimental variables of the SPEF process. Under the optimal conditions, the SPEF was compared with electro-Fenton (EF) and anodic oxidation (AO- H2O2) processes. Under optimum design variables of 47mAcm−2, 45°C and 571Lh−1, SPEF was more efficient yielding 90% of TOC removal after 360min of reaction with a maximum of 83% of mineralization current efficiency (MCE). The main by-products detected on the final solution treated by SPEF were oxalic, maleic, oxamic and fumaric acids, while NH4+ and NO3− were the main inorganic nitrogen ions released during the process.

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.

Solar photoelectro-Fenton treatment of a mixture of parabens spiked into secondary treated wastewater effluent at low input current

Aqueous mixtures of methyl, ethyl and propyl paraben (MeP, EtP and PrP) prepared in real urban wastewater with low conductivity were treated by solar photoelectro-Fenton (SPEF) process at low input current (j=10mAcm−2) using a pre-pilot plant with an electrochemical reactor equipped with an air-diffusion cathode to electrogenerate H2O2 and a boron-doped diamond (BDD) or RuO2-based anode. Comparative trials in simulated water matrices with or without Cl− in the absence of natural organic matter (NOM) always led to a slower decay of parabens concentration and total organic carbon (TOC). This was mainly due to the superior regeneration of Fe2+ from photoreduction of Fe(III) complexes formed with NOM in real wastewater compared to that from Fe(OH)2+. In all matrices, a catalyst concentration as low as 0.20mM Fe2+ was enough to ensure the production of OH in the bulk from Fenton’s reaction. SPEF with BDD yielded a complete removal of parabens in 180min and 66% mineralization at 240min. This gave rise to the greatest mineralization current efficiencies reported so far, up to 1000%, with a low energy consumption of 84kWh (kg TOC)−1. The synergy between homogeneous and heterogeneous catalysis, which allowed the efficient dosage of OH and M(OH) at low j, with simultaneous action of high UV power from sunlight justified such a good performance. Analogous apparent rate constants were determined for MeP, EtP and PrP. Slower decays were found with RuO2-based anode due to its lower oxidation power. As a result, the MCE was 425% as maximum, but a lower energy consumption of 52kWh (kg TOC)−1 was needed. Since the role of active chlorine was of minor importance, the formation of toxic, refractory chloroderivatives was minimized. All by-products were transformed into malic, formic and oxalic acids prior to total mineralization.

Electro-Fenton, UVA Photoelectro-Fenton and Solar Photoelectro-Fenton Treatments of Organics in Waters Using a Boron-Doped Diamond Anode: A Review

The characteristics and applications of electrochemical advanced oxidation processes (EAOPs) like electro-Fenton (EF), UVA photoelectro-Fenton (PEF) and solar PEF (SPEF) with a boron-doped diamond (BDD) anode are reviewed. Their oxidative properties are based on the attack of •OH formed at the anode surface and in the medium from Fenton’s reaction between cathodically generated H2O2 and added Fe2+. The illumination with UVA light in PEF and sunlight in SPEF enhances the degradation process due to the photolysis of complexes of Fe(III) with generated carboxylic acids. Examples on the removal of industrial chemicals, pesticides, dyes and pharmaceuticals by these EAOPs using bench-scaled stirred tank reactors and pre-pilot plants are described. The influence of experimental variables on the mineralization and energetic parameters is detailed. The decay kinetics of pollutants and the evolution of intermediates are discussed. The SPEF process shows the best performance, being the most potent EAOP tested.

Solar photoelectro-Fenton flow plant modeling for the degradation of the antibiotic erythromycin in sulfate medium

A solar photoelectro-Fenton (SPEF) plant containing a filter-press FM01-LC flow reactor in series with a compound parabolic collector (CPC) as photoreactor, operating in batch recirculation mode, was simulated using a parametric model. The degradation of 10dm3 of solutions of the heterocyclic antibiotic erythromycin (ERY) in 0.050moldm−3 Na2SO4 at pH 3.0 was used for validation. The filter-press reactor contained a platinized titanium plate anode and a graphite-felt cathode that produced H2O2 from the reduction of dissolved oxygen (0.24mmoldm−3). Trials were performed under potentiostatic and galvanostatic conditions with predominance of H2O2 production, minimizing H2 evolution reaction. The effect of initial catalyst (Fe2+) concentration, current density (j), initial antibiotic concentration as dissolved organic carbon (DOC) and volumetric flow rate on the ERY mineralization was studied. Good agreement between simulations and experimental DOC decays was obtained. Mineralization current efficiencies and specific energy consumptions were also determined. The best performance under galvanostatic conditions was found for 0.225mmoldm−3 ERY (100mgdm−3 DOC), 0.50mmoldm−3 Fe2+, volumetric flow rate of 3.0dm3min−1 and jcath=-0.16mAcm−2, reaching 69% mineralization with current efficiency of 75% and specific energy consumption of 0.059kWh (g DOC)−1. Six organic by-products were identified by gas chromatography-mass spectrometry, whereas final short-chain carboxylic acids like formic and oxalic acid were detected by ion-exclusion high-performance liquid chromatography. The initial N atom of ERY was predominantly converted into NO3− ion, although NH4+ ion was formed as well.

Catalytic activity of MOF(2Fe/Co)/carbon aerogel for improving H2O2 and [rad] OH generation in solar photo–electro–Fenton process

A novel bifunctional MOF(2Fe/Co)/CA cathode with a high surface possessing area, and good electrocatalytic and photocatalytic activity, was proposed for the solar photo–electro–Fenton (SPEF) process. The optimal nFe: nCo ratio was 2:1 for successfully introducing MOF(2Fe/Co) on the surface of CA, which is due to the specific interaction affinity of FeOOH between MOF(2Fe/Co) and CA. Continuous on–site generation of H2O2 through an oxygen reduction reaction (ORR) was achieved over the CA cathode. The electron transfer number (n) of the ORR with CA was 2.5 in the selected potential range. Moreover, the addition of MOF can behave as the active sites for enhancing the ORR activity and hence can increase the yield of the H2O2. Simultaneously, the photoinduced electron (ecb−) of MOF(2Fe/Co) efficiently in–situ decomposed H2O2 to form hydroxyl radicals (OH). The observation indicated that the ecb− is an efficient and new Fenton–like catalysts that is pH–independent. Efficient photo–electro–catalytic performance was exhibited when applied to degrade Rhodamine B (RhB) and dimethyl phthalate (DMP). Almost 100% RhB removal and 85% DMP removal were reached in 45min and 120min, respectively. The MOF(Fe/Co)/CA presented very low iron and cobalt leaching (<0.5ppm) even in an acidic media, and it retained an efficient degradation efficiency in the wide pH range of 3–9.