This article reports the synthesis of TiO2 nanotube arrays (TiO2 NTs), grown by Ti anodization, and their use as photoanode in a hybrid photelectrocatalytic (PEC)/photoelectro-Fenton (PEF) treatment of Indigo Carmine solutions in sulfate medium at pH 3.0. The anode was combined with an air-diffusion cathode that ensured continuous H2O2 electrogeneration. Comparative trials by electrochemical oxidation with electrogenerated H2O2 (EO-H2O2), electro-Fenton (EF) and PEF with Pt anode were made. The photoanode was stable operating up to 3 mA cm -2 with irradiation from a 36-W UV LED lamp, showing photoelectroactivity from an anodic potential (Ean) of +0.20 V, as determined by cyclic voltammetry. At 3 mA cm -2 , color removal by EO-H2O2 with Pt and PEC with TiO2 NTs was very slow, being much faster in EF, PEF and PEC/PEF due to main role of • OH formed from Fenton's reaction upon addition of Fe 2+ . The absorbance and dye concentration decays agreed with a pseudo-first-order kinetics, yielding a slightly lower rate constant for decolorization because of the formation of colored products.The mineralization ability increased as: EO-H2O2 << EF << PEF < PEC/PEF. The holes photogenerated at the TiO2 NTs surface had higher oxidation ability than • OH formed at the Pt surface from water discharge. In PEC/PEF, a slower mineralization was found at 2 mA cm -2 , although the final mineralization percentage was similar to that attained at 3 mA cm -2 . Both, SO4 2− and NH4 + ions were released during the treatments, along with isatin-5-sulfonic and formic acids as main products.

Groundwater is one of the main freshwater resources on the Earth, but its contamination by NO3 − and pesticides jeopardizes its suitability for consumption. In this work, the simultaneous electro-oxidation of insecticide imidacloprid (IMC) and electroreduction of NO3 − in softened groundwater containing a large amount of Cl − has been addressed. The assays were carried out in a stirred undivided tank reactor containing either a boron-doped diamond (BDD) or IrO2 anode, and Fe cathode, which showed greater electrocatalytic activity than stainless steel to reduce NO3 − . Comparative assays in simulated water mimicking the anionic composition of groundwater were made to assess the influence of natural organic matter (NOM) on the decontamination process. The BDD/Fe cell had much greater performance than the IrO2/Fe one, although the former produced larger amounts of ClO3 − and ClO4 − . In all cases, the NO3 − , Cl − and IMC decays agreed with a (pseudo)-first-order kinetics. In the BDD/Fe cell, total NO3 − removal was reached at j ≥ 10 mA cm -2 in softened groundwater, at similar rate in the presence and absence of IMC, but it was decelerated using the simulated matrix. The N-products formed upon NO3 − electroreduction contributed to IMC degradation, but its decay was inhibited by NOM because of the partial consumption of oxidants like hydroxyl radical and active chlorine.Operating at 5 mA cm -2 for 240 min, total removal of the insecticide and 61.5% total organic carbon (TOC) decay were achieved, also attaining a low NO3 − content that was suitable for humans. Eight heteroaromatic products were identified, allowing the proposal of a reaction sequence for IMC degradation in groundwater.

This article reports the high performance of a solid polymer electrolyte cell, equipped with a Nafion® N117 membrane packed between a Nb/boron-doped diamond (Nb/BDD) mesh anode and a Ti/RuO 2 mesh cathode, to degrade the insecticide imidacloprid spiked at 1.2-59.2 mg L À 1 into low conductivity groundwater by electrochemical oxidation. The natural water matrix was first softened using valorized industrial waste in the form of zeolite as reactive sorbent. Total removal of the insecticide, always obeying pseudo-first-order kinetics, and maximum mineralization degrees of 70 %-87 % were achieved, with energy consumption of 26.4 � 1.6 kWh m À 3 . Active chlorine in the bulk and * OH at the BDD surface were the main oxidants. Comparative studies using simulated water with analogous anions content revealed that the natural organic matter interfered in the groundwater treatment. Trials carried out in ultrapure water showed the primary conversion of the initial N and Cl atoms of imidacloprid to NO 3 À and Cl À ions, being the latter anion eventually transformed into ClO 3 À and ClO 4 À ions. 6-Chloro-nicotinonitrile, 6-chloro-pyridine-3-carbaldehyde, and tartaric acid were identified as oxidation products.[a] R.

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Nickel-cobalt electrocatalysts with atomic ratios of 2 : 1 and 1 : 2 were synthesized on nickel foam (NF) substrates by cathodic electrodeposition, further evaluating the performance of the pristine and thermally-treated materials as anodes for glycerol oxidation in alkaline medium. The electrodes were characterized by cyclic and linear sweep voltammetry at alkaline pH, showing an indirect oxidation of glycerol mediated by the metal oxyhydroxides. Under the selected conditions, a favourable potential window of 0.2 V upon comparison of water and glycerol oxidation was found. In addition, the increase in nickel content and the thermal treatment enhanced the anode polarization. After galvanostatic electrolysis at 10 mA cm À 2 , the products were analysed by HPLC, formate ion being the primary product, with a faradaic efficiency (FE) higher than 70 % in most cases. Both the FE to formate and the glycerol conversion were substantially enhanced using the thermally-treated anodes, whereas the effect of the Ni/Co ratio on these two parameters did not follow a clear trend.

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One of the main challenges of electrochemical Fenton-based processes is the treatment of organic pollutants at near-neutral pH. As a potential approach to this problem, this work addresses the use of a low content of soluble chelated metal catalyst, formed between Fe(III) and ethylenediamine-N,N'-disuccinic (EDDS) acid (1:1), to degrade the herbicide triclopyr in 0.050 M Na2SO4 solutions at pH 7.0 by photoelectro-Fenton with UVA light or sunlight (PEF and SPEF, respectively). Comparison with electro-Fenton treatments revealed the crucial role of the photo-Fenton-like reaction, since this promoted the production of soluble Fe(II) that enhanced the pesticide removal. Hydroxyl radicals formed at the anode surface and in the bulk were the main oxidants. A boron-doped diamond (BDD) anode yielded a greater mineralization than an IrO2-based one, at the expense of reduced cost-effectiveness. The effect of catalyst concentration and current density on the performance of PEF with BDD was examined. The PEF trials in 0.25 mM Na2SO4 + 0.35 mM NaCl medium showed a large influence of generated active chlorine as oxidant, being IrO2 more suitable than RuO2 and BDD.In SPEF with BDD, the higher light intensity from solar photons accelerated the removal of the catalyst and triclopyr, with small effect on mineralization. A plausible route for the herbicide degradation by Fe(III)-EDDS-catalyzed PEF and SPEF is finally proposed based on detected byproducts: three heteroaromatic and four linear N-aliphatic compounds, formamide and tartronic and oxamic acids.