[Show abstract][Hide abstract] ABSTRACT: The performance of three electrochemical advanced oxidation processes, namely electro-oxidation with electrogenerated H2O2 (EO–H2O2), electro-Fenton (EF) and photoelectro-Fenton (PEF) for the treatment of aqueous solutions of the food azo dye Ponceau 4R in an undivided cell with a BDD anode and an air-diffusion cathode was compared in terms of colour, dye concentration and total organic carbon (TOC) removals. PEF treatments in ultrapure water with Na2SO4 were performed to assess the effect of current density, as well as supporting electrolyte and dye concentrations. At 100 mA cm−2, solutions of 130 mL of 254 mg L−1 of the dye in 0.05 M Na2SO4 became colourless and totally mineralized after 50 and 240 min, respectively, which can be explained by the synergistic action of BDD(OH) at the anode surface and homogeneous OH formed in the bulk from Fenton’s reaction promoted in the presence of Fe2+ catalyst. Furthermore, UVA photons induced the continuous Fe2+ regeneration and photolytic decomposition of refractory intermediate complexes. In that aqueous matrix, the cleavage of the dye molecules proceeded through several reaction routes to yield N-containing and non-N-containing derivatives with one or two aromatic rings, short-chain aliphatic carboxylic acids and inorganic ions. Oxalic and oxamic acids and sulfate ions were accumulated at different rates in EO–H2O2, EF and PEF. The three methods allowed the progressive decontamination of Ponceau 4R solutions in a real water matrix even without the addition of electrolyte, although complete TOC abatement after 360 min at 33.3 mA cm−2 was only ensured by the iron-catalyzed PEF process.
[Show abstract][Hide abstract] ABSTRACT: The degradation of 100 cm3 of 177 mg dm-3 of the triphenylmethane dye Malachite Green oxalate at pH 3.0 was studied by anodic oxidation with stainless steel cathode (AO-SS), AO with air-diffusion cathode (AO-H2O2), electro-Fenton (EF) and photoelectro-Fenton (PEF) with UVA light. The main oxidizing species were hydroxyl radicals formed from either water oxidation at the anode surface or in the bulk between added Fe2+ and H2O2 generated at the air-diffusion cathode. The use of a Pt anode led to slower decolorization and mineralization than BDD in all treatments because of the higher oxidation power of the latter. The decolorization was much faster for EF and PEF compared to AO-SS and AO-H2O2 due to the contribution of hydroxyl radicals in the bulk. PEF allowed the quickest color removal by the rapid Fe2+ regeneration from the photolysis of Fe(III) complexes with oxalate. The most powerful process was PEF with BDD, which yielded total decolorization in 6 min and 97% mineralization at 240 min operating at 100 mA cm-2, thanks to hydroxyl radicals formed at the anode surface and in the bulk along with the photolytic action of UVA radiation. The evolution of final carboxylic acids like maleic, fumaric, succinic, acetic, oxalic, formic and oxamic was followed by ion-exclusion HPLC. All these acids and their Fe(III) complexes were removed more slowly with Pt anode. The initial N atoms of the dye were pre-eminently accumulated as NH4+ ion, along with small amounts of NO3- ion.
[Show abstract][Hide abstract] ABSTRACT: The effect of various parameters on the performance of electrochemical advanced oxidation processes (EAOPs) like electro-Fenton (EF), photoelectro-Fenton (PEF) and solar PEF (SPEF) was assessed for the treatment of a sanitary landfill leachate previously subjected to biological and coagulation processes. The tested operational variables included: (i) anode material (boron-doped diamond (BDD) and Pt), (ii) initial total dissolved iron concentration (20-80mgL-1), (iii) pH (2.8-4.0), (iv) initial addition of 1:3 Fe(III)-to-oxalate molar ratio at various pH values (2.8-5.0), (v) temperature (15-40°C) and (vi) radiation source (UVA, UVA-Vis and UVC lamps and natural sunlight). The BDD anode showed high superiority over the Pt one for EF, PEF with UVA light (PEF-UVA) and SPEF processes, thereby advising an important role of the physisorbed hydroxyl radicals (OH) at the anode surface on landfill leachate oxidation even under the potent solar radiation. An initial total dissolved iron content of 60mgL-1 was chosen as the best dose for the PEF-UVA process with the BDD anode (PEF-BDD-UVA). While PEF-BDD-UVA without external addition of oxalic acid yielded the best results at pH 2.8, the initial addition of 1:3 Fe(III)-to-oxalate molar ratio allowed operating at pH 3.5 with even higher efficiency and at pH 4.0 with only slightly lower efficiency. Effluent temperatures from 20 to 40°C led to similar mineralization rates for the PEF-BDD-UVA technique. The use of UVA and UVC lamps and natural sunlight as radiation sources in PEF-BDD and SPEF-BDD systems led to similar mineralization profiles as a function of time. The UVA-Vis lamp induced lower effluent mineralization mainly for longer reaction times.
[Show abstract][Hide abstract] ABSTRACT: The occurrence of food color additives in waters is becoming a hot topic due to their potential health effects, especially on children. The treatment of Ponceau 4R solutions by electro-oxidation (EO) with or without H2O2 production, electro-Fenton (EF) and photoelectro-Fenton (PEF) using small undivided and divided batch cells with either Pt or BDD as the anode has been thoroughly studied. The electrolyses were performed in different electrolytes like Na2SO4, NaCl, NaNO3 and LiClO4 in order to elucidate the changes in reactivity. Depending on the anode, cathode, process and electrolyte, the azo dye could be degraded due to the single or combined action of: (i) direct cathodic reduction and/or anodic oxidation, (ii) •OH-mediated oxidation at the anode vicinity or in the solution bulk, and (iii) active chlorine-mediated oxidation. The presence of Cl− led to the fastest decolorization, whereas it became detrimental for total organic carbon abatement. The oxidation of Cl− to active chlorine (Cl2 and HClO) and oxychlorine anions (ClO3− and ClO4−) by direct charge transfer or by M(•OH) and •OH was investigated. Color removal was much slower in SO42−, ClO4− and NO3− media, although the latter was particularly beneficial in PEF and/or using BDD. Regarding the mineralization, PEF outperformed the other technologies. Similar trends were observed in SO42− and ClO4− media, being slightly favored in NO3−. In contrast, Cl− medium tended to be detrimental due to the formation of refractory chloroderivatives, the destruction of M(•OH) to form less oxidizing (oxy) chloro radicals, oxychlorine anions and active chlorine, and the reaction between HClO and H2O2. Experiments in divided cells demonstrated the very small contribution of cathodic reduction of the dye and its by-products. Linear sweep voltammetry revealed the direct oxidation of both, Ponceau 4R and Cl− on the anode surface.
[Show abstract][Hide abstract] ABSTRACT: A solution with 0.245 mM of the diazo dye Evans Blue and 0.50 mM Fe2+ as catalyst of pH 3.0 was comparatively degraded by electrochemical processes based on Fenton’s reaction chemistry like electro-Fenton (EF), photoelectro-Fenton (PEF) with a 6 W UVA light and solar photoelectro-Fenton (SPEF). Electrolytic trials were made in a 100 mL stirred tank reactor with a boron-doped diamond (BDD) anode and an air-diffusion cathode at constant current density. Organics were destroyed by OH produced at the anode surface from water oxidation and in the bulk from Fenton’s reaction between added Fe2+ and H2O2 generated at the cathode. Evans Blue decay obeyed a pseudo-first-order kinetics and was much faster than solution decolorization due to the formation of colored aromatic products. The mineralization rate rose in the sequence EF < PEF < SPEF. Almost total mineralization was rapidly achieved in SPEF at current density ⩾66.7 mA cm−2 because of the potent UV radiation from sunlight. Up to 19 aromatic intermediates and 16 hydroxylated derivatives including diazo, monoazo, biphenylic, benzenic, naphthalenic and phthalic acid compounds were detected by LC–MS. The SPEF process was performed in a 10 L flow plant with a Pt/air-diffusion cell coupled to a CPC photoreactor in order to confirm its viability at industrial scale. 88% mineralization with 42% current efficiency and 2.13 kWh kg−1 DOC energy consumption were obtained after 300 min of treatment at 55.4 mA cm−2. Nine short-linear carboxylic acids were identified as final products, oxalic, formic and oxamic acids being the most persistent. The photodecarboxylation of Fe(III)–carboxylate complexes explained the good oxidation ability of PEF and SPEF. The initial S of the diazo dye was transformed into SO42− ion, whereas its initial N was mineralized to NO3− ion but largely lost as N-volatile products.
[Show abstract][Hide abstract] ABSTRACT: The degradation of solutions with 0.260 mM of the diazo dye Congo Red at pH 3.0 has been studied by electrochemical advanced oxidation processes (EAOPs) like anodic oxidation with electrogenerated H2O2 (AO-H2O2), electro-Fenton (EF) and photoelectro-Fenton (PEF) with a 6 W UVA light. Experiments were made in a 100 mL stirred tank reactor with a boron-doped diamond (BDD) anode and an air-diffusion cathode at constant current density (j). In these systems, organics were mainly destroyed by OH formed at the anode surface from water oxidation and/or in the bulk from Fenton’s reaction between added Fe2+ and cathodically generated H2O2. The oxidation power of the EAOPs increased in the sequence AO- H2O2 < EF < PEF. Almost total mineralization was attained after 360 min of PEF at j ≥ 66.7 mA cm−2 due to the parallel photolytic action of UVA light. In all the EAOPs, increasing j enhanced the degradation process, but with a loss of mineralization current efficiency and higher energy consumption. Congo Red decay always obeyed a pseudo-first-order kinetics. The study of the Congo Red degradation in a 2.5 L solar flow plant with a Pt/air-diffusion cell confirmed the viability of the solar PEF (SPEF) treatment at industrial scale. Optimum conditions were found for 0.260 mM of Congo Red with 0.50 mM Fe2+ at 100 mA cm−2, yielding almost total mineralization in 240 min with about 49% current efficiency and 0.45 kWh (g DOC)−1 energy consumption. LC-MS analysis of treated solutions allowed the identification of 21 aromatic intermediates and 13 hydroxylated derivatives, including diazo, monoazo, biphenyl, benzene, naphthalene and phthalic acid compounds. Tartatic, tartronic, acetic, oxalic and oxamic acids were detected as final carboxylic acids in all the EAOPs. The fast photodecarboxylation of the Fe(III)-carboxylate complexes explained the higher oxidation ability of the photo-assisted methods of PEF and SPEF. The initial N of the dye was mainly lost as N-volatile products and mineralized to NO3− ion and in lesser extent to NH4+ ion, whereas its initial S was converted into SO42− ion.
[Show abstract][Hide abstract] ABSTRACT: Here, 2.5 L of solutions of the pharmaceutical ranitidine (RNTD) at pH 3.0 have been comparatively degraded by electro-Fenton (EF) and solar photoelectro-Fenton (SPEF) processes using a pre-pilot flow plant with a Pt/air-diffusion cell. RNTD was oxidized by hydroxyl radicals coming from water oxidation at the Pt anode and Fenton’s reaction between added Fe2+ and H2O2 generated at the air-diffusion cathode. In SPEF, the cell was coupled to a flat solar photoreactor to irradiate the solution with sunlight. The potent combined action of hydroxyl radicals and photolysis by sunlight explains the higher oxidation ability of SPEF compared to EF, attaining 80% mineralization as maximal. The optimum Fe2+ content as catalyst was 0.50 mM. The effect of current density and drug concentration on the degradation rate of RNTD, mineralization current efficiency and energy consumption for EF and SPEF was examined. The RNTD decay always followed a pseudo-first-order kinetics, having a greater oxidation rate for SPEF by the additional generation of hydroxyl radicals induced by photolysis of Fe(III) species. Malic, pyruvic, acetic, oxalic, oxamic and formic acids were detected as final carboxylic acids. The three latter acids were the main components of the final treated solution in EF because their Fe(III) complexes were not destroyed by generated hydroxyl radicals. The quick photolysis of such Fe(III)-carboxylate species by sunlight explains the greater oxidation power of SPEF. The initial S atom of RNTD was released as sulfate ion, whereas its initial N atoms were converted into ammonium ion along with a smaller proportion of nitrate ion.
[Show abstract][Hide abstract] ABSTRACT: Electrochemical technology has attracted increasing interest in recent years as an environment-friendly solution to many industrial problems and challenges. First, we give an overview on the fundamentals of electrochemical processes for the removal of pharmaceuticals from water, particularly electrochemical oxidation and Fenton-based processes, such as electro-Fenton, and ultraviolet (UV) and solar photoelectro-Fenton, that have improved performance. We also mention other less studied methods, although the main focus is on reactivity elucidation by chromatography with UV or conductivity detection, especially by mass spectrometry techniques (e.g., coupled to gas chromatography or liquid chromatography), in order to discuss the degradation pathways of pharmaceuticals on the basis of the reactive species electrogenerated in each technology. In some cases, simultaneous assessment of toxicity adds crucial information for the future integration of these technologies in water-treatment facilities where pharmaceuticals and their byproducts can occur.
[Show abstract][Hide abstract] ABSTRACT: As the environment preservation gradually becomes a matter of major social concern and more strict legislation is being imposed on effluent discharge, more effective processes are required to deal with non-readily biodegradable and toxic pollutants. Synthetic organic dyes in industrial effluents cannot be destroyed in conventional wastewater treatment and consequently, an urgent challenge is the development of new environmentally benign technologies able to mineralize completely these non-biodegradable compounds. This review aims to increase the knowledge on the electrochemical methods used at lab and pilot plant scale to decontaminate synthetic and real effluents containing dyes, considering the period from 2009 to 2013, as an update of our previous review up to 2008. Fundamentals and main applications of electrochemical advanced oxidation processes and the other electrochemical approaches are described. Typical methods such as electrocoagulation, electrochemical reduction, electrochemical oxidation and indirect electro-oxidation with active chlorine species are discussed. Recent advances on electrocatalysis related to the nature of anode material to generate strong heterogeneous OH as mediated oxidant of dyes in electrochemical oxidation are extensively examined. The fast destruction of dyestuffs mediated with electrogenerated active chlorine is analyzed. Electro-Fenton and photo-assisted electrochemical methods like photoelectrocatalysis and photoelectro-Fenton, which destroy dyes by heterogeneous OH and/or homogeneous OH produced in the solution bulk, are described. Current advantages of the exposition of effluents to sunlight in the emerging photo-assisted procedures of solar photoelectrocatalysis and solar photoelectro-Fenton are detailed. The characteristics of novel combined methods involving photocatalysis, adsorption, nanofiltration, microwaves and ultrasounds among others and the use of microbial fuel cells are finally discussed.
[Show abstract][Hide abstract] ABSTRACT: The synthesis of core-shell Pt(Cu) and Pt-Ru(Cu) electrocatalysts allows for a reduction in the amount of precious metal and, as was previously shown, a better CO oxidation performance can be achieved when compared to the nanoparticulated Pt and Pt-Ru ones. In this paper, the carbon black used as the support was previously submitted to electrochemical oxidation and characterized by XPS. The new catalysts thus prepared were characterized by HRTEM, FFT, EDX, and electrochemical techniques. Cu nanoparticles were generated by electrodeposition and were further transformed into Pt(Cu) and Pt-Ru(Cu) core-shell nanoparticles by successive galvanic exchange with Pt and spontaneous deposition of Ru species, the smallest ones being 3.3 nm in mean size. The onset potential for CO oxidation was as good as that obtained for the untreated carbon, with CO stripping peak potentials about 0.1 and 0.2 V more negative than those corresponding to Pt/C and Ru-decorated Pt/C, respectively. Carbon oxidation yielded an additional improvement in the catalyst performance, because the ECSA values for hydrogen adsorption/desorption were much higher than those obtained for the non-oxidized carbon. This suggested a higher accessibility of the Pt sites in spite of having the same nanoparticle structure and mean size.
[Show abstract][Hide abstract] ABSTRACT: The degradation of 10 dm3 of solutions of the heterocyclic antibiotic metronidazole in 0.10 mol dm−3 Na2SO4 of pH 3.0 has been comparatively studied by electro-Fenton (EF) and solar photoelectro-Fenton (SPEF). Experiments were performed in a solar flow plant equipped with a Pt/air-diffusion cell and coupled to a compound parabolic collector (CPC) photoreactor. A very weak mineralization was found for the EF process in the dark, indicating a large recalcitrance of heterocyclic compounds to be destroyed by hydroxyl radicals formed at the Pt anode from water oxidation and mainly in the bulk from Fenton's reaction between added Fe2+ and cathodically generated H2O2. The quick photolysis of intermediates by UV radiation from sunlight enhanced largely the mineralization process by SPEF. The effect of applied current density and Fe2+ and drug contents on the SPEF treatment was examined. The best process was found for 1.39 mmol dm−3 metronidazole with 0.50 mmol dm−3 Fe2+ at 55.4 mA cm−2 giving 53% mineralization, 36% mineralization current efficiency and 0.339 kWh (g DOC)−1 in 300 min. Metrodinazole was completely removed and its decay obeyed a pseudo-first-order kinetics. LC-MS analysis allowed identifying five heterocyclic products and twelve hydroxylated derivatives. Ion-exclusion HPLC analysis revealed that final oxalic and oxamic acids were practically removed at the end of electrolysis due to the efficient photolysis of their Fe(III) complexes by sunlight. The initial N of metronidazole gave NO3− ion as main inorganic ion. A large proportion of initial N remained in solution as unidentified N-products and their major part was lost as N-volatile species. Based on the detected products, a reaction sequence for metronidazole mineralization by SPEF is proposed.