Francesc Centellas

Mineralization of the biocide chloroxylenol by electrochemical advanced oxidation processes

Electrochemical advanced oxidation processes (EAOPs) are environmentally friendly methods based on the destruction of organic pollutants in wastewaters with in situ electrogenerated hydroxyl radical. This species is formed in anodic oxidation (AO) from water oxidation at the anode and in indirect electro-oxidation methods like electro-Fenton (EF) and photoelectro-Fenton (PEF) also from reaction between catalytic Fe2+ and H2O2 continuously produced at the O2-diffusion cathode. The PEF method involves the irradiation of the treated solution with UVA light to enhance the photolysis of organics including Fe(III) complexes. In this work, the oxidation power of such EAOPs to decontaminate synthetic wastewaters of the biocide chloroxylenol (4-chloro-3,5-dimethylphenol) at pH 3.0 is comparatively examined with an undivided electrolytic cell containing a Pt or boron-doped diamond (BDD) anode and a stainless steel or O2-diffusion cathode. The initial chlorine is released as Cl(-) ion, which remains stable in the medium using Pt or is oxidized to Cl2 on BDD. The biocide solutions can be completely decontaminated using AO with a BDD anode, as well as PEF with a Pt or BDD anode. The PEF procedure with a BDD anode is the most powerful method leading to total mineralization in about 300 min, practically independent of current density. When current density rises, the degradation rate of processes increases, but they become less efficient due to the larger enhancement of waste reactions of oxidants. Chloroxylenol is much more rapidly removed in EF and PEF than in AO. 2,6-dimethylhydroquinone, 2,6-dimethyl-p-benzoquinone and 3,5-dimethyl-2-hydroxy-p-benzoquinone are identified as aromatic by-products, and maleic, malonic, pyruvic, acetic and oxalic acids are found as generated carboxylic acids. A general pathway for chloroxylenol mineralization by all EAOPs including the above by-products is proposed.

Mineralization of flumequine in acidic medium by electro-Fenton and photoelectro-Fenton processes

The mineralization of flumequine, an antimicrobial agent belonging to the first generation of synthetic fluoroquinolones which is detected in natural waters, has been studied by electrochemical advanced oxidation processes (EAOPs) like electro-Fenton (EF) and photoelectro-Fenton (PEF) with UVA light. The experiments were performed in a cell containing a boron-doped diamond (BDD) anode and an air-diffusion cathode to generate H(2)O(2) at constant current. The Fe(2+) ion added to the medium increased the solubility of the drug by the formation of a complex of intense orange colour and also reacted with electrogenerated H(2)O(2) to form hydroxyl radical from Fenton reaction. Oxidant hydroxyl radicals at the BDD surface were produced from water oxidation. A partial mineralization of flumequine in a solution near to saturation with optimum 2.0mM Fe(2+) at pH 3.0 was achieved by EF. The PEF process was more powerful, giving an almost total mineralization with 94-96% total organic carbon removal. Increasing current accelerated both treatments, but with decreasing mineralization current efficiency. Comparative treatments using a real wastewater matrix led to similar degradation degrees. The kinetics for flumequine decay always followed a pseudo-first-order reaction and its rate constant, similar for both EAOPs, raised with increasing current. Generated carboxylic acids like malonic, formic, oxalic and oxamic acids were quantified by ion-exclusion HPLC. Fe(III)-oxalate and Fe(III)-oxamate complexes were the most persistent by-products under EF conditions and their quicker photolysis by UVA light explains the higher oxidation power of PEF. The release of inorganic ions such as F(-), NO(3)(-) and in lesser extent NH(4)(+) was followed by ionic chromatography.

Electrochemical incineration of omeprazole in neutral aqueous medium using a platinum or boron-doped diamond anode: Degradation kinetics and oxidation products

The electrochemical incineration of omeprazole, a widely prescribed gastrointestinal drug which is detected in natural waters, has been studied in a phosphate buffer of pH 7.0 by anodic oxidation with electrogenerated H(2)O(2) (AO-H(2)O(2)) operating at constant current density (j). The experiments were carried out in a cell equipped with either a Pt or a boron-doped diamond (BDD) anode and an air-diffusion cathode to continuously produce H(2)O(2). In these systems, organics are mainly oxidized by hydroxyl radicals formed at the Pt or BDD surface from water oxidation. A partial total organic carbon (TOC) abatement close to 78% for omeprazole was achieved by AO-H(2)O(2) with a BDD anode after consumption of 18 Ah L(-1) at 100 mA cm(-2), whereas the alternative use of Pt did not allow mineralizing the drug. However, the drug was totally removed using both anodes, although it decayed more rapidly using BDD. In this latter system, increasing j accelerated the degradation process, but lowering the mineralization current efficiency. Greater drug content also enhanced the degradation rate with higher mineralization degree and current efficiency. The kinetics for omeprazole decay always followed a pseudo-first-order reaction and its rate constant increased with increasing j and with decreasing its concentration. Seven heteroaromatic intermediates and four hydroxylated derivatives were detected by LC-MS, while nine short-linear carboxylic acids were identified and quantified by ion-exclusion HPLC. These acids were largely accumulated using Pt and rapidly removed using BDD, thus explaining the partial mineralization of omeprazole achieved by AO-H(2)O(2) with the latter anode. The release of inorganic ions such as NO(3)(-), NH(4)(+) and SO(4)(2-) was followed by ionic chromatography. A plausible reaction sequence for omeprazole mineralization involving all intermediates detected is proposed.

Oxidation of enrofloxacin with conductive-diamond electrochemical oxidation, ozonation and Fenton oxidation. A comparison

The treatment of enrofloxacin synthetic wastewaters using conductive-diamond electrochemical oxidation (CDEO), ozonation and Fenton oxidation has been studied. Results show that the three technologies can reduce the organic content of enrofloxacin synthetic wastewaters but with different performances. CDEO was the most efficient technology in terms of mineralization but not on COD removal, which was more efficiently achieved by ozonation. This indicates that ozonation is efficient in the breakage of the complex molecules but not on the removal of final carboxylic acids. The high initial efficiency in terms of oxidant-use obtained by Fenton oxidation evidences that it is very efficient in the removal of the enrofloxacin, although it rapidly leads to the formation of refractory compounds to the treatment. This indicates the significance of other oxidation mechanisms (e.g. coagulation) that enhance the results obtained by the expected hydroxyl-mediated oxidation. Ammonium ions were the primary product species in CDEO and nitrate ions in ozonation, whereas Fenton effluents contained similar amounts of both nitrogen ionic species.

Degradation of pharmaceutical beta-blockers by electrochemical advanced oxidation processes using a flow plant with a solar compound parabolic collector.

The degradation of the beta-blockers atenolol, metoprolol tartrate and propranolol hydrochloride was studied by electro-Fenton (EF) and solar photoelectro-Fenton (SPEF). Solutions of 10 L of 100 mg L⁻¹ of total organic carbon of each drug in 0.1 M Na₂SO₄ with 0.5 mM Fe²⁺ of pH 3.0 were treated in a recirculation flow plant with an electrochemical reactor coupled with a solar compound parabolic collector. Single Pt/carbon felt (CF) and boron-doped diamond (BDD)/air-diffusion electrode (ADE) cells and combined Pt/ADE-Pt/CF and BDD/ADE-Pt/CF cells were used. SPEF treatments were more potent with the latter cell, yielding 95-97% mineralization with 100% of maximum current efficiency and energy consumptions of about 0.250 kWh g TOC⁻¹. However, the Pt/ADE-Pt/CF cell gave much lower energy consumptions of about 0.080 kWh g TOC⁻¹ with slightly lower mineralization of 88-93%, then being more useful for its possible application at industrial level. The EF method led to a poorer mineralization and was more potent using the combined cells by the additional production of hydroxyl radicals (•OH) from Fentons reaction from the fast Fe²⁺ regeneration at the CF cathode. Organics were also more rapidly destroyed at BDD than at Pt anode. The decay kinetics of beta-blockers always followed a pseudo first-order reaction, although in SPEF, it was accelerated by the additional production of •OH from the action of UV light of solar irradiation. Aromatic intermediates were also destroyed by hydroxyl radicals. Ultimate carboxylic acids like oxalic and oxamic remained in the treated solutions by EF, but their Fe(III) complexes were photolyzed by solar irradiation in SPEF, thus explaining its higher oxidation power. NO₃⁻ was the predominant inorganic ion lost in EF, whereas the SPEF process favored the production of NH₄⁺ ion and volatile N-derivatives.

Mineralization of sulfanilamide by electro-Fenton and solar photoelectro-Fenton in a pre-pilot plant with a Pt/air-diffusion cell

The mineralization of sulfanilamide solutions at pH 3.0 was comparatively studied by electro-Fenton (EF) and solar photoelectro-Fenton (SPEF) using a 2.5 L pre-pilot plant containing a Pt/air-diffusion cell coupled with a solar photoreactor. Organics were primordially oxidized by hydroxyl radical (OH) formed from Fentons reaction between H₂O₂ generated at the cathode and added Fe(2+) and/or under the action of sunlight. A mineralization up to 94% was achieved using SPEF, whereas EF yielded much poorer degradation. The effect of current density and Fe(2+) and drug concentrations on the degradation rate, mineralization current efficiency and energy cost per unit DOC mass of EF and/or SPEF was examined. The sulfanilamide decay always followed a pseudo first-order kinetics, being more rapid in SPEF due to the additional generation of OH induced by sunlight on Fe(III) species. Catechol, resorcinol, hydroquinone and p-benzoquinone were identified as aromatic intermediates. The final solutions treated by EF contained Fe(III) complexes of maleic, fumaric, oxamic and mainly oxalic acids, which are hardly destroyed by OH. The quick photolysis of Fe(III)-oxalate complexes by sunlight explains the higher oxidation ability of SPEF. The N content of sulfanilamide was mainly mineralized as NH₄⁺ ion and in much lesser extent as NO₃⁻ ion, whereas most of its initial S was converted into SO₄²⁻ ion.

Mineralization of metoprolol by electro-Fenton and photoelectro-Fenton processes.

Solutions of about 0.25 mM of the β-blocker metoprolol tartrate (100 mg L(-1) total organic carbon) with 0.5 mM Fe(2+) in the presence and absence of 0.1 mM Cu(2+) of pH 3.0 have been comparatively degraded under electro-Fenton (EF) and photoelectro-Fenton (PEF) conditions. The electrolyses were carried out with two systems: (i) a single cell with a boron-doped diamond (BDD) anode and an air-diffusion cathode (ADE) for H(2)O(2) electrogeneration and (ii) a combined cell with a BDD/ADE pair coupled with a Pt/carbon felt (CF) cell. Overall mineralization was reached in all PEF treatments using both systems due to the efficient production of hydroxyl radical ((•)OH) from Fentons reaction induced by UVA light and the quick photolysis of Fe(III) carboxylate complexes formed. In EF, the combined cell was much more potent than the single one by the larger (•)OH generation from the continuous Fe(2+) regeneration at the CF cathode, accelerating the oxidation of organics. However, almost total mineralization in EF was feasible using the combined cell in the presence of 0.1 mM Cu(2+), because of the parallel quick oxidation of Cu(II) carboxylate complexes by (•)OH. Metoprolol decay always followed a pseudo-first-order reaction. Aromatic products related to consecutive hydroxylation/oxidation reactions of metoprolol were detected by gas chromatography-mass spectrometry. The evolution of the aromatic 4-(2-methoxyethyl)phenol and generated carboxylic acids was followed by HPLC. The degradation rate and mineralization degree of metoprolol tartrate were limited by the removal of Fe(III) and Cu(II) complexes of ultimate carboxylic acids such as formic, oxalic, and oxamic. NH(4)(+) ion and to a lesser extent NO(3)(-) ion were released in all treatments, being quantified by ionic chromatography.

Electrochemical oxidation of high-purity and homogeneous Al–Mg alloys with low Mg contents

Abstract The susceptibility to the electrochemical corrosion of homogeneous Al–Mg alloys containing 0.33%, 0.43%, 0.77% and 0.88% (in weight) Mg in NaCl solutions has been studied by means of ocp measurements, cv , EIS, optical microscopy, SEM and EDX. The breakdown and repassivation potentials of these alloys were shifted in the negative direction when the Mg content was increased, indicating an increase in their susceptibility to localized corrosion in the same direction. The impedance measurements were interpreted according to suitable equivalent circuits. In the passive region, two capacitive semicircles were obtained, the first being related with the film capacitance and the second one with a diffusion process in the oxide film itself. The oxide film capacitances on the Al–Mg alloys were greater than those found on high purity Al, suggesting the formation of more defective and hydrated films on these alloys. Pit nucleation of Al–Mg alloys is discussed considering the formation of a defective oxide. In the pitting potential region, the Nyquist diagrams consisted of a capacitive semicircle followed by one or two inductive semicircles. The film capacitances were greater than those found for the passive oxide films because of the chloride-rich film formation in pitting conditions. In addition, the film capacitances increased with Mg content of the alloy because of the increase in the pitted surface area, confirmed by SEM, in the same direction.

Electrochemical study of aluminium corrosion in acid chloride solutions

Abstract In this work, oxidation and free corrosion of hydroxide-etched high-purity aluminium in deaerated HCI media at concentrations in the range 0.6–2.4 M using the potentiodynamic and galvanostatic transient techniques has been studied. Pitting potentials were determined from cyclic polarization curves. The stationary E corr values lay 200–100 mV more negative than the corresponding pitting potentials for the concentration range studied. The evolution of the electrode surface was examined by optical microscopy. For low current densities, the anodic galvanostatic curves show an initial region of constant potential with very low charge and a subsequent slow rise in potential up to quasistationary conditions. When the current density increases, the time corresponding to such an initial region decreases, showing the potential increase sooner and finally, for current densities about 0.3 mA cm −2 , an initial sharp maximum is found. The Tafel parameters have been determined in transient conditions from Mansfelds method. The anodic and cathodic Tafel for potentials ± 70 mV around E corr coincident within the experimental error, were 2R T /F. Free corrosion of Al in concentrated and deaerated HCl solutions is general and takes place in the passive mode through the formation and dissolution of a defective oxyhydroxide film, the total thickness being in the order of few monolayers. No pits develop in the latter conditions, in contrast with the electrode behaviour in aerated HCl solutions, in which pitting with intergranular fissuring attack is found. When a very low anodic current density is applied, aluminium dissolves into the medium through the oxyhydroxide formation and dissolution. The potential rise found at slightly higher current densities corresponds to an oxyhydroxide formation with chloride attack. At a certain value of the potential, pits are nucleated, which propagate in the quasistationary conditions of the galvanostatic curve. When the current applied increases, metal attack is more localized. The potential decrease found after the initial maximum for current about 0.3 mA cm −2 and higher, is ascribed to pit nucleation, the subsequent plateau corresponding to the propagation of pits.

Decolorization and mineralization of Allura Red AC azo dye by solar photoelectro-Fenton: Identification of intermediates.

The degradation of 2.5L of Allura Red AC solutions in sulfate medium containing 0.50mM Fe(2+) has been studied by solar photoelectro-Fenton (SPEF) using a flow plant equipped with a Pt/air-diffusion cell and a solar photoreactor. Comparative electro-Fenton treatment yielded rapid total decolorization but poor mineralization, since most products were slowly destroyed by OH formed from Fentons reaction between Fe(2+) and H2O2 generated at the air-diffusion cathode. In contrast, the potent action of UV radiation from sunlight in SPEF allowed the rapid photolysis of recalcitrant intermediates, thus giving rise to a quick mineralization. Sulfate and nitrate ions, along with a large proportion of volatile N-derivatives, were always released. The increase in current density and decrease in azo dye concentration accelerated the decolorization and mineralization in SPEF, although lower current efficiency and greater specific energy consumption were obtained. The most cost-effective SPEF treatment was found for 460 mg L(-1) azo dye in 0.05 M Na2SO4 at 50 mA cm(-2), which yielded 95% mineralization with 81% current efficiency and 8.50 kW h m(-3). No significant effect of sulfate concentration was found. Up to 16 aromatic intermediates and 11 short-chain carboxylic acids, including oxalic and oxamic as the most persistent ones, were detected by GC-MS and HPLC. The large oxidation ability of SPEF can be explained by the quick photolysis of Fe(III)-oxalate complexes and other undetected intermediates.