Electro-Fenton, UVA photoelectro-Fenton and solar photoelectro-Fenton degradation of the drug ibuprofen in acid aqueous medium using platinum and boron-doped diamond anodes

The degradation of a 41 mg dm⁻³ ibuprofen (2-(4-isobutylphenyl)propionic acid) solution of pH 3.0 has been comparatively studied by electrochemical advanced oxidation processes (EAOPs) like electro-Fenton, UVA photoelectro-Fenton and solar photoelectro-Fenton at constant current density. Experiments were performed in a one-compartment cell with a Pt or boron-doped diamond (BDD) anode and an O₂-diffusion cathode. Heterogeneous hydroxyl radical (OH) is generated at the anode surface from water oxidation, while homogeneous OH is formed from Fenton's reaction between Fe²⁺ and H₂O₂ generated at the cathode, being its production strongly enhanced from photo-Fenton reaction induced by sunlight. Higher mineralization is attained in all methods using BDD instead Pt, because the former produces greater quantity of OH enhancing the oxidation of pollutants. The mineralization rate increases under UVA and solar irradiation by the rapid photodecomposition of complexes of Fe(III) with acidic intermediates. The most potent method is solar photoelectro-Fenton with BDD giving 92% mineralization due to the formation of a small proportion of highly persistent final by-products. The effect of Fe²⁺ content, pH and current density on photoelectro-Fenton degradation has been studied. The ibuprofen decay always follows a pseudo-first-order kinetics and its destruction rate is limited by current density and UV intensity. Aromatics such as 1-(1-hydroxyethyl)-4-isobutylbenzene, 4-isobutylacetophenone, 4-isobutylphenol and 4-ethylbenzaldehyde, and carboxylic acids such as pyruvic, acetic, formic and oxalic have been identified as oxidation by-products. Oxalic acid is the ultimate by-product and the fast photodecarboxylation of its complexes with Fe(III) under UVA or solar irradiation explains the higher oxidation power of photoelectro-Fenton methods in comparison to electro-Fenton procedures.     

Degradation of the beta-blocker propranolol by electrochemical advanced oxidation processes based on Fenton's reaction chemistry using a boron-doped diamond anode

The electro-Fenton (EF) and photoelectro-Fenton (PEF) degradation of solutions of the beta-blocker propranolol hydrochloride with 0.5 mmol dm⁻³ Fe²⁺ at pH 3.0 has been studied using a single cell with a boron-doped diamond (BDD) anode and an air diffusion cathode (ADE) for H₂O₂ electrogeneration and a combined cell containing the above BDD/ADE pair coupled in parallel to a Pt/carbon felt (CF) cell. This naphthalene derivative can be mineralized by both methods with a BDD anode. Almost overall mineralization is attained for the PEF treatments, more rapidly with the combined system due to the generation of higher amounts of hydroxyl radical from Fenton's reaction by the continuous Fe²⁺ regeneration at the CF cathode, accelerating the oxidation of organics to Fe(III)-carboxylate complexes that are more quickly photolyzed by UVA light. The homologous EF processes are less potent giving partial mineralization. The effect of current density, pH and Fe²⁺ and drug concentrations on the oxidation power of PEF process in combined cell is examined. Propranolol decay follows a pseudo first-order reaction in most cases. Aromatic intermediates such as 1-naphthol and phthalic acid and generated carboxylic acids such as maleic, formic, oxalic and oxamic are detected and quantified by high-performance liquid chromatography. The chloride ions present in the starting solution are slowly oxidized at the BDD anode. In PEF treatments, all initial N of propranolol is completely transformed into inorganic ions, with predominance of NH₄⁺ over NO₃⁻ ion.     

Electrochemical destruction of chlorophenoxy herbicides by anodic oxidation and electro-Fenton using a boron-doped diamond electrode

The degradation of herbicides 4-chlorophenoxyacetic acid (4-CPA), 4-chloro-2-methylphenoxyacetic acid (MCPA), 2,4-dichlorophenoxyacetic acid (2,4-D) and 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) in aqueous medium of pH 3.0 has been comparatively studied by anodic oxidation and electro-Fenton using a boron-doped diamond (BDD) anode. All solutions are totally mineralized by electro-Fenton, even at low current, being the process more efficient with 1 mM Fe²⁺ as catalyst. This is due to the production of large amounts of oxidant hydroxyl radical (OH) on the BDD surface by water oxidation and from Fenton’s reaction between added Fe²⁺ and H₂O₂ electrogenerated at the O₂-diffusion cathode. The herbicide solutions are also completely depolluted by anodic oxidation. Although a quicker degradation is found at the first stages of electro-Fenton, similar times are required for achieving overall mineralization in both methods. The decay kinetics of all herbicides always follows a pseudo first-order reaction. Reversed-phase chromatography allows detecting 4-chlorophenol, 4-chloro-o-cresol, 2,4-dichlorophenol and 2,4,5-trichlorophenol as primary aromatic intermediates of 4-CPA, MCPA, 2,4-D and 2,4,5-T, respectively. Dechlorination of these products gives Cl⁻, which is slowly oxidized on BDD. Ion-exclusion chromatography reveals the presence of persistent oxalic acid in electro-Fenton by formation of Fe³⁺-oxalato complexes, which are slowly destroyed by OH adsorbed on BDD. In anodic oxidation, oxalic acid is mineralized practically at the same rate as generated.     

Mineralization of an azo dye Acid Red 14 by photoelectro-Fenton process using an activated carbon fiber cathode

The mineralization of an azo dye Acid Red 14 (AR14) by the photoelectro-Fenton (PEF) process was studied in an undivided electrochemical reactor with a RuO₂/Ti anode and an activated carbon fiber (ACF) cathode able to electrochemically generate H₂O₂. Anodic oxidation and UV irradiation of AR14 were also examined as comparative experiments. Results indicate that the electro-Fenton process yielded about 60–70% mineralization of AR14, while the photoelectro-Fenton could mineralize AR14 more effectively (more than 94% total organic carbon (TOC) removal) even at low current densities assisted with UV irradiation after 6 h electrolysis. The mineralization current efficiency (MCE) of the PEF process increased with the increasing AR14 concentrations. In addition, the initial solution pH ranging from 1.49 to 6.72 had little influence on the TOC removal probably due to the formation of organic carboxylic acids which balanced the pH increase caused by the cathodic generation of hydrogen gas. The ACF cathode showed a long-term stability during multiple experimental runs for degradation of AR14, indicating its good potential for practical application in treating refractory organic pollutants in aqueous solutions.     

Effect of UV Radiation on the Removal of Carboxylic Acids from Water by H2O2 and O3 in the Presence of Metallic Ions

The effect of UV radiation on the removal of formic, oxalic and maleic acids from water by metallic ion (Fe²⁺ or Cu²⁺)/H₂O₂ and metallic ion/O₃ was studied and compared. The results showed that metallic ion/O₃/UV has higher efficiency than metallic ion/H₂O₂/UV for oxalic acid removal. UV radiation significantly increases the efficiency of metallic ion/H₂O₂ for formic and maleic acids removal while its effect on the efficiency of metallic ion/O₃ for formic acid removal is minor_. However, at pH 2, O₃ alone showed higher efficiency than metallic ion/H₂O₂/UV for formic acid removal. Contrary to the relative efficiency of metallic ions in the previous systems, Cu²⁺ exhibited higher rate than Fe²⁺ for the removal of the degradation products of maleic acid by O₃. UV radiation exhibited a minor effect on the efficiency of Cu²⁺/O₃, while it exhibited a large effect on the efficiency of Fe²⁺/O₃ for the removal of the degradation products of maleic acid.     

Mineralization of clofibric acid by electrochemical advanced oxidation processes using a boron-doped diamond anode and Fe and UVA light as catalysts

This work shows that aqueous solutions of clofibric acid (2-(4-chlorophenoxy)-2-methylpropionic acid), the bioactive metabolite of various lipid-regulating drugs, up to saturation at pH 3.0 are efficiently and completely degraded by electrochemical advanced oxidation processes such as electro-Fenton and photoelectro-Fenton with Fe²⁺ and UVA light as catalysts using an undivided electrolytic cell with a boron-doped diamond (BDD) anode and an O₂-diffusion cathode able to electrogenerate H₂O₂. This is feasible in these environmentally friendly methods by the production of oxidant hydroxyl radical at the BDD surface from water oxidation and in the medium from Fenton's reaction between Fe²⁺ and electrogenerated H₂O₂. The degradation process is accelerated in photoelectro-Fenton by additional photolysis of Fe³⁺ complexes under UVA irradiation. Comparative treatments by anodic oxidation with electrogenerated H₂O₂, but without Fe²⁺, yield much slower decontamination. Chloride ion is released and totally oxidized to chlorine at the BDD surface in all treatments. The decay kinetics of clofibric acid always follows a pseudo-first-order reaction. 4-Chlorophenol, 4-chlorocatechol, hydroquinone, p-benzoquinone and 2-hydroxyisobutyric, tartronic, maleic, fumaric, formic and oxalic acids, are detected as intermediates. The ultimate product is oxalic acid, which is slowly but progressively oxidized on BDD in anodic oxidation. In electro-Fenton this acid forms Fe³⁺–oxalato complexes that can also be totally destroyed at the BDD anode, whereas in photoelectro-Fenton the mineralization rate of these complexes is enhanced by its parallel photodecarboxylation with UVA light.     

Electro-Fenton degradation of antimicrobials triclosan and triclocarban

The antimicrobials triclosan (2,4,4′-trichloro-2′-hydroxydiphenyl ether) and triclocarban (N-(4-chlorophenyl)-N′-(3,4-dichlorophenyl)urea) have been degraded by four electro-Fenton systems using undivided electrolytic cells with a Pt or boron-doped diamond (BDD) anode and a carbon felt or O₂ diffusion cathode. The main oxidant is hydroxyl radical (OH) produced both on the anode surface from water oxidation and in the medium by Fenton's reaction, which takes place between electrogenerated H₂O₂ and Fe²⁺ coming from cathodic reduction of O₂ and Fe³⁺, respectively. Triclosan from saturated aqueous solutions of pH 3.0 is completely removed in all cells, decreasing its decay rate in the order: Pt/carbon felt > BDD/carbon felt > Pt/O₂ diffusion > BDD/O₂ diffusion, in agreement with their OH generation ability from Fenton's reaction. Glyoxylic, maleic and oxalic acids are identified as aliphatic intermediates. Complexes between oxalic acid and iron ions persist largely in solution, although Fe²⁺-oxalato complexes are mineralized by OH in the medium and Fe³⁺-oxalato complexes are destroyed by OH on BDD. Analogous treatments of more concentrated triclosan solutions using a 20:80 (v/v) acetonitrile/water mixture as solvent evidence the role of hydroxyl radicals along the degradation. In this hydroorganic medium hydroxylated derivatives such as 2,4-dichlorophenol, 4-chlorocatechol, chlorohydroquinone and chloro-p-benzoquinone, and carboxylic acids such as maleic, oxalic, formic and acetic acids are detected as products. Complete destruction of iron-oxalato complexes and released Cl⁻ ion involves some oxidizing species coming from parallel acetonitrile oxidation. The same electro-Fenton systems also yield the overall removal of triclocarban in acetonitrile/water mixtures, giving rise to urea, hydroquinone, chlorohydroquinone, 1-chloro-4-nitrobenzene and 1,2-dichloro-4-nitrobenzene as primary intermediates.     

Electrochemical degradation of 2,4,5-trichlorophenoxyacetic acid in aqueous medium by peroxi-coagulation. Effect of pH and UV light

Acidic aqueous solutions with 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) concentrations up to 270 ppm in the pH range 2.0-6.0 at 35 °C can be rapidly degraded by peroxi-coagulation using an Fe anode and an O₂-diffusion cathode. 2,4,5-T and its products are then efficiently oxidized with OH radicals produced from Fenton's reaction between Fe²⁺ and H₂O₂ generated by the electrodes. Under pH regulation at low currents, more than 90% of organics are destroyed at pH 3.0, although its optimum pH is 4.0. Higher degradations are reached without pH regulation. Solutions with 2,4,5-T concentration ≤200 ppm are more rapidly depolluted under UV irradiation, because of the production of more OH from photo-Fenton reaction. Coagulation of products with the Fe(OH)₃ precipitate formed predominates when pH is regulated to 2.0 and 3.0 in the absence of UV light, whereas parallel mineralization is favored without pH regulation and under UV irradiation. The 2,4,5-T decay follows a pseudo first-order reaction, with the same rate constant in the presence and absence of UV light. 2,4,5-trichlorophenol, 2,5-dichlorohydroquinone, 4,6-dichlororesorcinol and 2,5-dihydroxy-p-benzoquinone have been identified and quantified as aromatic intermediates by reverse-phase chromatography. Chloride ions are released from the oxidation of these chlorinated products. The evolution of generated carboxylic acids, such as glycolic, glyoxylic, formic, malic, maleic, fumaric and oxalic, has been followed by ion-exclusion chromatography. The great stability of oxalic acid and its complexes with Fe³⁺ at pH regulated to 3.0 can explain that concentrated solutions can not be completely degraded.     

Degradation of disperse azo dyes from waters by solar photoelectro-Fenton

Solutions of the azo dyes Disperse Red 1 (DR1) and Disperse Yellow 3 (DY3), commonly used in the Chilean textile industry, in 0.1 mol dm⁻³ Na₂SO₄ and 0.5 mmol dm⁻³ Fe²⁺ of pH 3.0 were comparatively degraded by electro-Fenton (EF) and solar photoelectro-Fenton (SPEF) using a 2.5 dm³ recirculation flow plant containing a BDD/air-diffusion cell coupled with a solar photoreactor. Organics were oxidized in EF with hydroxyl radicals formed at the anode surface from water oxidation and in the bulk from Fenton's reaction between electrogenerated H₂O₂ and added Fe²⁺. The oxidizing power of SPEF was enhanced by the additional production of hydroxyl radicals from the photolysis of Fe(III) hydrated species and the photodecomposition of Fe(III) complexes with intermediates by UV light of solar irradiation. Total decolorization, complete dye removal and almost overall mineralization for both dye solutions were only achieved using the most potent SPEF process, yielding higher current efficiencies and lower energy consumptions than EF. Final carboxylic acids like pyruvic, acetic, oxalic and oxamic were detected during the SPEF treatments. NO₃⁻ ion was released as inorganic ion. The use of a solution pH of 2.0–3.0 at 50 mA cm⁻² was found preferable for SPEF. Synthetic textile dyeing solutions containing the dyes were treated under these conditions yielding lower decolorization rate, slower dye removal and smaller mineralization degree than only using 0.1 mol dm⁻³ Na₂SO₄ due to the parallel oxidation of organic dyeing components. However, lower energy consumptions were obtained by the destruction of more amounts of total organic carbon, indicating that SPEF is a useful and viable method for the remediation of textile industrial wastewaters with high contents of disperse azo dyes.     

Oxidation pathways of malachite green by Fe-catalyzed electro-Fenton process

A very detailed scheme for the Fe³⁺-catalyzed electro-Fenton mineralization of malachite green as a model triarylmethane dye is presented. Bulk electrolyses of 250-mL aqueous solutions of 0.5 mM malachite green with 0.2 mM Fe³⁺ as catalyst have been carried out at room temperature and pH 3.0 under constant current in an undivided cell equipped with a graphite-felt cathode and a Pt anode to assess the performance of the electro-Fenton system. In situ electrogeneration of Fe²⁺ and H₂O₂ from quick cathodic reduction of Fe³⁺ and dissolved O₂ (from bubbled compressed air), respectively, allows the formation of the very oxidizing species hydroxyl radical (OH) in the medium from Fenton's reaction. A pseudo-first-order decay kinetics with an apparent rate constant of k_1,MG = 0.244 min⁻¹ was obtained for total destruction of malachite green by action of OH at 200 mA, requiring 22 min for total decoloration of the solution. In the same experimental conditions, overall mineralization was reached at 540 min. Up to 15 aromatic and short-chain carboxylic acid intermediates were identified along the treatment. The evolution of current efficiency was calculated from the chemical oxygen demand (COD) removal. Based on the time course of most of the by-products and the identification of inorganic ions released, some plausible mineralization pathways are proposed and thoroughly discussed. It has been found that the electro-Fenton degradation of malachite green proceeds via parallel pathways, all of them involving primary splitting of the triaryl structure initiated by attack of OH on the central carbon, thus yielding two different N-dimethylated benzophenones. Successive cleavage of the aromatic intermediates generates oxalic acid as the ultimate short-chain carboxylic acid, whereas N-demethylation of some of them produces formic acid as well. Oxalic acid and its Fe²⁺ complexes, as well as formic acid, can be slowly but totally mineralized by OH.     

Decomposition of Carboxylic Acids in Water by O3, O3/H2O2, and O3/Catalyst

This paper studies the decomposition of formic, oxalic and maleic acids by O₃, O₃/catalyst, and O₃/H₂O₂. The catalytic effect of Co²⁺, Ni²⁺, Cu²⁺, Mn²⁺, Zn²⁺, Cr³⁺, and Fe²⁺ ions is investigated. The results showed that—Co²⁺ and Mn²⁺ have the highest catalytic activity for the decomposition of oxalic acid while the catalytic effect of the studied ions is insignificant on the rate of decomposition of formic acid. Maleic acid decomposes by ozone into formic acid and glyoxylic acid, which subsequently oxidizes to oxalic acid. Though the studied ions have no effect on the decomposition of maleic acid, they have a significant effect on the produced oxalic and glyoxylic acids. In the presence of Cu²⁺ ions glyoxylic acid is mainly transformed into formic acid and traces of oxalic acid. In such case, a complete decomposition of maleic acid and its degradation products is achieved within 45 min. The results also show that combining H₂O₂ with O₃ results in an increase in the rate of decomposition of oxalic acid. However, O₃/H₂O₂ system is less active than O₃/Co²⁺ or O₃/Mn²⁺.     

Solar photoassisted anodic oxidation of carboxylic acids in presence of Fe using a boron-doped diamond electrode

This paper reports a comparative study on the anodic oxidation of 2.5 l of 50 mg l⁻¹ TOC of formic, oxalic, acetic, pyruvic or maleic acid in 0.1 M Na₂SO₄ solutions of pH 3.0 with and without 1.0 mM Fe³⁺ as catalyst in the dark or under solar irradiation. Experiments have been performed with a batch recirculation flow plant containing a one-compartment filter-press electrolytic reactor equipped with a 20 cm² boron-doped diamond (BDD) anode and a 20 cm² stainless steel cathode, and coupled to a solar photoreactor. This system gradually accumulates H₂O₂ from dimerization of hydroxyl radical (OH) formed at the anode surface from water oxidation. Carboxylic acids in direct anodic oxidation are mainly oxidized by direct charge transfer and/or OH produced on BDD, while their Fe(III) complexes formed in presence of Fe³⁺ can also react with OH produced from Fenton reaction between regenerated Fe²⁺ with electrosynthesized H₂O₂ and/or photo-Fenton reaction. Fast photolysis of Fe(III)-oxalate and Fe(III)-pyruvate complexes under the action of sunlight also takes place. Chemical and photochemical trials of the same solutions have been made to better clarify the role of the different catalysts. Solar photoassisted anodic oxidation in presence of Fe³⁺ strongly accelerates the removal of all carboxylic acids in comparison with direct anodic oxidation, except for acetic acid that is removed at similar rate in both cases. This novel electrochemical advanced oxidation process allows more rapid mineralization of formic, oxalic and maleic acids, without any significant effect on the conversion of acetic acid into CO₂. The synergistic action of Fe³⁺ and sunlight in anodic oxidation can then be useful for wastewater remediation when oxalic and formic acids are formed as ultimate carboxylic acids of organic pollutants, but its performance is expected to strongly decay in the case of generation of persistent acetic acid during the degradation process.     

Reaction sequence for the mineralization of the short-chain carboxylic acids usually formed upon cleavage of aromatics during electrochemical Fenton treatment

Electrochemical Fenton treatment of aromatic pollutants in aqueous medium always leads to the formation of short-chain carboxylic acids, which account for the slower degradation rate at the final stages of the process. In order to gain further insight into the fate of such compounds, bulk electrolyses of 200 ml aqueous solutions of eleven C₁-C₄ carboxylics, namely formic, glyoxylic, oxalic, acetic, glycolic, pyruvic, malonic, maleic, fumaric, succinic and malic acid have been carried out by electro-Fenton process with 0.1 mM Fe²⁺ as catalyst, at room temperature and pH 3.0, applying a constant current and using an open and undivided cell equipped with a carbon-felt cathode and a Pt anode. In situ cathodic electrogeneration of Fenton's reagent leads to the formation of the very oxidizing species hydroxyl radical (OH) in the medium, allowing the degradation and total mineralization of all carboxylics studied. Various goals have been accomplished: (a) identification of the degradation intermediates for each carboxylic acid and study of their time course, (b) discussion and proposal of the reaction mechanisms under the action of OH/O₂, (c) analysis of the decay kinetics and determination of the absolute rate constants, which agree well with those available in literature for processes involving OH, (d) verification of the great oxidation ability of the process to degrade mixtures containing all the carboxylics, upon variation of some experimental parameters such as current, concentration and cathode dimensions and, finally, (e) elucidation of a detailed reaction sequence for their mineralization, indicating the plausible pre-eminent pathways.     

Comparative study of chemical and electrochemical Fenton treatment of organic pollutants in wastewater

The technological and economic aspects of using the Fenton process to treat industrial wastewater containing morpholyne and diethylethanolamine, as well as sodium salts of naphthalene sulfonic acid and of ethylenediaminetetraacetic acid based on data obtained in pilot tests are discussed. Chemical Fenton technology was tested using commercial 30–35% solutions of H₂O₂ and iron (II) salts, which was followed by the additional electrochemical destruction of organic pollutants in an undivided reactor with catalytic stable anodes (CSA) and 1 g L⁻¹ NaCl as a supporting electrolyte and a source of active chlorine. An alternative electrochemical method involving the electrogeneration of hydrogen peroxide in polluted water at the gas -diffusion cathode was studied both with the addition of ferrous salt to the electrolyte prior to electrolysis (in-cell electro-Fenton) as well as with the post-electrolysis addition of Fe²⁺ in another reactor (ex-cell electro-Fenton). The accumulation of hydrogen peroxide in concentrations sufficient for the mineralization of organic pollutants was achieved in both cases with near 100% current efficiency. In comparison with wastewater treatment processes which use a purchased hydrogen peroxide reagent, the Fenton-like processes achieved an economic savings of as much as 64.5% in running costs due to the on-site electrochemical generation of H₂O₂. Preparative electrolysis in the membrane reactor showed higher current efficiencies and lower specific energy consumptions for H₂O₂ electrogeneration in comparison with the results of tests carried out in an undivided cell.     

Photoelectro-Fenton combined with photocatalytic process for degradation of an azo dye using supported TiO2 nanoparticles and carbon nanotube cathode: Neural network modeling

In this work, treatment of an azo dye solution containing C.I. Basic Red 46 (BR46) by photoelectro-Fenton (PEF) combined with photocatalytic process was studied. Carbon nanotube-polytetrafluoroethylene (CNT-PTFE) electrode was used as cathode. The investigated photocatalyst was TiO₂ nanoparticles (Degussa P25) having 80% anatase and 20% rutile, specific surface area (BET) 50 m²/g, and particle size 21 nm immobilized on glass plates. A comparison of electro-Fenton (EF), UV/TiO₂, PEF and PEF/TiO₂ processes for decolorization of BR46 solution was performed. Results showed that color removal follows the decreasing order: PEF/TiO₂ > PEF > EF > UV/TiO₂. The influence of the basic operational parameters such as initial pH of the solution, initial dye concentration, the size of anode, applied current, kind of ultraviolet (UV) light and initial Fe³⁺ concentration on the degradation efficiency of BR46 was studied. The mineralization of the dye was investigated by total organic carbon (TOC) measurements that showed 98.8% mineralization of 20 mg/l dye at 6 h using PEF/TiO₂ process. An artificial neural network (ANN) model was developed to predict the decolorization of BR46 solution. The findings indicated that artificial neural network provided reasonable predictive performance (R² = 0.986).     

Highly Efficient Photoelectrocatalytic Decomposition of Biomass Compounds Using a Nanoporous Semiconductor Photoanode and an O2-Reducing Cathode with Quantum Efficiency Over 100

UV light-activated highly efficient photoelectrocatalytic decomposition of biorelated compounds and biowastes was successfully achieved by a Biophotochemical Cell (BPCC) comprising a nanoporous semiconductor film photoanode and an O₂-reducing cathode with high internal quantum efficiency over 100. Major decomposition reaction was dark auto-oxidation of the activated substrate radicals (R^?) by the bulk O₂ into CO₂ or/and to N₂. Photocatalytic decomposition of aqueous solutions of urea, ammonia, pig urine, and oxalic acid was investigated. Combination cells were fabricated comprising a photoanode/cathode unit for activation of organic compounds and a large volume chamber for autooxidation of activated substrates.     

Electro-Fenton degradation of azo dye using polypyrrole/anthraquinonedisulphonate composite film modified graphite cathode in acidic aqueous solutions

Amaranth azo dye has been degraded by electro-Fenton method using an undivided cell containing the polypyrrole (PPy)/anthraquinonedisulphonate (AQDS) composite film modified graphite cathode and Pt anode. In acidic media, the PPy/AQDS composite film exhibits the characteristic of gas diffusion cathode and is highly efficient for hydrogen peroxide electrogeneration with high generation rate and current efficiency. This new electro-Fenton system can degrade amaranth azo dye efficiently in various acidic solutions. The amaranth decay and total organic carbon (TOC) removal were determined as a function of pH, cathode potential, Fe²⁺ and doping AQDS concentrations. Total dye decay and 80.3% mineralization were achieved at the optimum conditions (pH 3.0, E_cath = −0.65 V vs. SCE, 2.0 mM Fe²⁺ concentration). The electrochemical stability and electrocatalytic activity of the composite film after use in electro-Fenton process were also investigated using cyclic voltammetry (CV) and Fourier transfer infrared (FTIR) spectroscopy technologies.     

A dual-chamber microbial fuel cell with conductive film-modified anode and cathode and its application for the neutral electro-Fenton process

This study reports on the modification of the anode and the cathode in a dual-chamber microbial fuel cell (MFC) with a polypyrrole (PPy)/anthraquinone-2,6-disulfonate (AQDS) conductive film to boost its performance and the application of the MFC to drive neutral electron-Fenton reactions occurring in the cathode chamber. The MFC equipped with the conductive film-coated anode and cathode delivered the maximum power density of 823 mW cm⁻² that was one order of magnitude larger than that obtained in the MFC with the unmodified electrodes. This was resulted from the enhanced activities of microbial metabolism in the anode and oxygen reduction in the cathode owing to the decoration of both electrodes with the PPy/AQDS composite. The MFC with the modified electrodes resulted in the largest rate of H₂O₂ generation in the cathode chamber by the two-electron reduction of O₂. The increase in the concentration of H₂O₂ was beneficial for the enhancement in the amount of hydroxyl radicals produced by the reaction of H₂O₂ with Fe²⁺, thus allowing an increased oxidative ability of the electro-Fenton process towards the decolorization and mineralization of an azo dye (i.e., Orange II) at pH 7.0.     

Modeling dye degradation kinetic using dark- and photo-Fenton type processes

Dark- and photo-Fenton type processes, Fe²⁺/H₂O₂, Fe³⁺/H₂O₂, Fe⁰/H₂O₂, UV/Fe²⁺/H₂O₂, UV/Fe³⁺/H₂O₂ and UV/Fe⁰/H₂O₂, were applied for the treatment of model colored wastewater containing two reactive dyes, C.I. Reactive Blue 49 and C.I. Reactive Blue 137, and degradation kinetics were compared. Dye degradation was monitored by the means of UV/VIS, adsorbable organic halides (AOX) and total organic carbon (TOC) analysis, thus determining decolorization and dechlorination of triazine structure, as well as mineralization of model colored wastewater. Both dark- and photo-Fenton type processes were proven to be very efficient for color removal; ≥98% was achieved in all cases. Significant improvements in the mineralization of studied dyes were achieved by the assistance of UV light, as it was expected. It was demonstrated that the degradation kinetic of applied dyes depended on the presence of UV light, as well as type of iron catalyst and dye structure. On bases of the obtained experimental results, the mathematical models were developed describing dye degradation kinetics in all studied systems. Since UV light was used in order to enhance the efficiency of dark-Fenton type processes, mathematical model describing dye degradation by UV photolysis providing the values of quantum yields for each of the dye was developed and incorporated in model for photo-Fenton type processes. A sensitivity analysis for the evaluation of importance of each reaction used in mathematical models was also performed.     

Degradation of the fluoroquinolone enrofloxacin by electrochemical advanced oxidation processes based on hydrogen peroxide electrogeneration

Solutions of the veterinary fluoroquinolone antibiotic enrofloxacin in 0.05 M Na₂SO₄ of pH 3.0 have been comparatively degraded by electrochemical advanced oxidation processes such as anodic oxidation with electrogenerated H₂O₂ (AO-H₂O₂), electro-Fenton (EF), photoelectro-Fenton (PEF) and solar photoelectro-Fenton (SPEF) at constant current density. The study has been performed using an undivided stirred tank reactor of 100 ml and a batch recirculation flow plant of 2.5 l with an undivided filter-press cell coupled to a solar photoreactor, both equipped with a Pt or boron-doped diamond (BDD) anode and a carbon-polytetrafluoroethylene gas diffusion cathode to generate H₂O₂ from O₂ reduction. In EF, PEF and SPEF, hydroxyl radical (OH) is formed from Fenton's reaction between added catalytic Fe²⁺ and generated H₂O₂. Almost total decontamination of enrofloxacin solutions is achieved in the stirred tank reactor by SPEF with BDD. The use of the batch recirculation flow plant showed that this process is the most efficient and can be viable for industrial application, becoming more economic and yielding higher mineralization degree with raising antibiotic content. This is feasible because organics are quickly oxidized with OH formed from Fenton's reaction and at BDD from water oxidation, combined with the fast photolysis of complexes of Fe(III) with generated carboxylic acids under solar irradiation. The lower intensity of UVA irradiation used in PEF with BDD causes a slower degradation. EF with BDD is less efficient since OH cannot destroy the most persistent Fe(III)-oxalate and Fe(III)-oxamate complexes. AO-H₂O₂ with BDD yields the poorest mineralization because pollutants are only removed with OH generated at BDD. All procedures are less potent using Pt as anode due to the lower production of OH at its surface. Enrofloxacin decay always follows a pseudo first-order reaction. Its primary aromatic by-products and short intermediates including polyols, ketones, carboxylic acids and N-derivatives are detected by GC-MS and chromatographic techniques. The evolution of F⁻, NO₃⁻ and NH₄⁺ ions released to the medium during each process is also determined.