Abstract: Nowadays, the increasing pollution of natural water effluents with herbicides, such as atrazine (ATZ, 2-chloro-4-ethylamino-6-isopropylamino-s-triazine), is an emerging problem that has not received the sufficient attention. This work presents a study on ATZ degradation under an electrochemical advanced oxidation process (EAOP), such as anodic oxidation (AO). The degradation of 175 mL of 10 and 40 mg L-1 ATZ solutions was studied using Pt or BDD as anode. The assays were made with a stirred tank cell, using a supporting electrolyte of 0.050 mM of Na2SO4 at pH 3.0 by applying 0.18, 0.27 and 0.37 A cm-2. The degradation rate increased by increasing current density, regardless of the anode employed. Greater amounts of ATZ were removed at higher organic load. The pesticide decay always obeyed a pseudo-first-order kinetics. A high degradation efficiency of 97%-99% was obtained by the more powerful AO-BDD process at 0.37 A cm-2. High performance liquid chromatography (HPLC) was used to follow the evolution of major oxidation products by AO-BDD, such as desethyl atrazine, desethyl desisopropyl atrazine and cyanuric acid.

Resumen: En la actualidad, la creciente contaminación de los efluentes de aguas naturales con herbicidas como la atrazina (ATZ, 2-cloro-4-etilamino-6-isopropilamino-s-triazina) es un problema emergente que no ha recibido atención suficiente. Este trabajo presenta un estudio sobre la degradación ATZ bajo un proceso electroquímico de oxidación avanzada (EAOP) como la oxidación anódica (AO). La degradación de soluciones de 10 y 40 mgL-1 de ATZ en 175 mL se estudió usando Pt o BDD como ánodo. Los ensayos se realizaron con una celda de tanque agitada, usando un electrolito de soporte de Na2SO4 0,050 mM a pH 3,0 aplicando 0,18, 0,27 y 0,37 A cm-2. La tasa de degradación aumentó con el aumento de la densidad de corriente, independientemente del ánodo empleado. Se eliminaron cantidades mayores de ATZ a mayor carga orgánica. La descomposición del pesticida siempre obedeció a una cinética de pseudo primer orden. Se obtuvo una alta eficiencia de degradación de 97% -99% mediante el proceso más potente AO-BDD a 0,37 A cm-2. Se usó cromatografía líquida de alta resolución (HPLC) para seguir la evolución de los principales productos de oxidación mediante AO-BDD tal como desetil atrazina, desetil desisopropil atrazina y ácido cianúrico.

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.

Este artículo revisa las características y aplicaciones de procesos electroquímicos avanzados de oxidación (PEAOs) como el electro-Fenton (EF), el fotoelectro-Fenton UVA (FEF) y el FEF solar (FEFS) con un ánodo de diamante dopado con boro (DDB). Sus propiedades oxidativas se basan en el ataque del radical •OH formado en la superficie del ánodo y en el medio a partir de la reacción de Fenton entre el H2O2 generado catódicamente y el Fe2+ añadido. La iluminación con luz UVA en PEF y luz solar en FEFS mejora el proceso degradativo debido a la fotólisis de complejos de Fe(III) con ácidos carboxílicos generados. Se describen ejemplos sobre la eliminación de productos químicos industriales, pesticidas, colorantes y fármacos por estos PEAOs usando reactores de tanque agitado a pequeña escala y plantas pre-piloto. También se detalla la influencia de variables experimentales sobre los parámetros de mineralización y energéticos y se discute el descenso cinético de los contaminantes y la evolución de sus intermedios. El proceso de FEFS resulta ser el más eficiente, siendo el más potente PEAO ensayado.

Este artigo apresenta uma revisão sobre os emergentes processos eletroquímicos de oxidação avançada (PEOAs) tais como UV fotoeletro-Fenton (FEF) e fotoeletro-Fenton solar (SPEF) em que a irradiação do efluente com a luz UV e solar, respectivamente, causa um efeito sinérgico sobre o processo de degradação dos poluentes orgânicos pela formação de mais •OH e/ou da fotólise dos complexos de Fe3+ com a geração de ácidos carboxílicos. Fundamentos destes PEOAs são explicados para clarificar seu desempenho na remoção de produtos químicos industriais, pesticidas, corantes e fármacos. Exemplos com reatores de mistura perfeita em escala de bancada equipados com ânodos de Pt ou boro dopado de diamante (BDD) e cátodos de difusão à gás, feltro de carbono ou fibra de carvão ativado são discutidos. Tratamentos com uma planta piloto de recirculação acoplada a um fotoreator solar são também explicados. Um método de combinação envolvendo FEF e fotocatálise é descrito. A influência dos parâmetros experimentais sobre a taxa de mineralização, eficiência de corrente da mineralização e custo energético dos métodos FEF e FEFS é detalhada. A cinética de degradação dos contaminantes e a evolução de seus intermediários e íons inorgânicos finais são discutidas. O processo FEFS se mostrou mais eficiente e menos caro que outros PEOAs como a oxidação anódica com eletrogeração de H2O2, eletro-Fenton e FEF.

This paper presents a review on emerging electrochemical advanced oxidation processes (EAOPs) such as UV photoelectro-Fenton (PEF) and solar photoelectro-Fenton (SPEF) in which the irradiation of the effluent with UV light and sunlight, respectively, causes a synergistic effect on the degradation process of organic pollutants by the formation of more •OH and/or the photolysis of complexes of Fe(III) with generated carboxylic acids. Fundamentals of these EAOPs are explained to clarify their performance in the removal of industrial chemicals, pesticides, dyes and pharmaceuticals. Examples with bench-scaled stirred tank reactors equipped with Pt or boron-doped diamond (BDD) anodes and gas-diffusion, carbon-felt or activated carbon fiber cathodes are discussed. Treatments with a recirculation pre-pilot plant coupled to a solar photoreactor are also examined. A combined method involving PEF and photocatalysis is described. The influence of experimental parameters on the mineralization rate, mineralization current efficiency and energy cost of the PEF and SPEF methods is detailed. The decay kinetics of contaminants and the evolution of their intermediates and final inorganic ions are discussed. The SPEF process resulted more efficient and less expensive than other EAOPs like anodic oxidation with electrogenerated H2O2, electro-Fenton and PEF.

Actualmente se están desarrollando métodos químicos (AOPs) y electroquímicos (EAOPs) de oxidación avanzada que son capaces de destruir contaminantes orgánicos en aguas. Estas técnicas están basadas en la producción del radical hidroxilo (•OH) como oxidante. En los AOPs el radical •OH se puede obtener a partir de la reacción de Fenton entre el Fe2+ y el H2O2 adicionados al medio, de la fotorreducción de especies de Fe3+ o bien de la reacción entre el ozono y el Fe2+. En los EAOPs dicho radical se forma a partir de la oxidación del agua en la superficie de un ánodo de elevado sobrepotencial de O2 y/o de la reacción de Fenton entre el Fe2+ añadido al medio y el H2O2 electrogenerado en el cátodo por la reducción bielectrónica del O2. El presente trabajo describe la mineralización de algunos fármacos aromáticos como el paracetamol, el ibuprofeno, el ácido clofíbrico y el ácido salicílico usando AOPs basados en la ozonización catalizada por Fe2+, Cu2+ y/o luz UVA y EAOPs como la oxidación anódica, el electro-Fenton y el fotoelectro-Fenton, que son métodos electroquímicos compatibles con el medio ambiente. Para los últimos procesos, se ha estudiado la desaparición de los fármacos con el tiempo y la evolución de los intermedios aromáticos y de los ácidos carboxílicos generados. La oxidación anódica sólo es efectiva cuando se utiliza un ánodo de diamante dopado con boro (BDD), mientras que el proceso fotoelectro-Fenton con Fe2+, Cu2+ y luz UVA es el método más potente para destruir totalmente los fármacos. El uso conjunto de los catalizadores Fe2+, Cu2+ y luz UVA en la ozonización catalizada también conduce a la descontaminación total de las disoluciones de fármacos.

At present chemical (AOPs) and electrochemical (EAOPs) advanced oxidation processes with ability to destroy organic pollutants in waters are being developed. These methods are based on the production of hydroxyl radical (•OH) as oxidant. In AOPs •OH radical can be obtained from Fenton’s reaction between Fe2+ and H2O2 added to the medium, photoreduction of Fe3+ species or reaction between ozone and Fe2+. In EAOPs this radical is formed from water oxidation on the surface of a high O2-overvoltage anode and/or Fenton’s reaction between Fe2+ added to the medium and H2O2 electrogenerated at the cathode by the two-electron reduction of oxygen. The present work reports the mineralization of several aromatic drugs such as paracetamol, ibuprofen, clofibric acid and salicylic acid by AOPs based on ozonation catalyzed by Fe2+, Cu2+ and/or UVA light and EAOPs like anodic oxidation, electro-Fenton and photoelectro-Fenton, which are environmentally friendly electrochemical methods. For the latter processes, the drug decay with time and the evolution of aromatic intermediates and generated carboxylic acids are studied. Anodic oxidation is only effective when a boron-doped diamond (BDD) anode is used, whereas the photoelectro-Fenton process with Fe2+, Cu2+ and UVA light is the most potent method to completely destroy the drugs. The combined use of catalysts Fe2+, Cu2+ and UVA light in catalyzed ozonation also leads to overall decontamination of drug solutions.

Conventional processes for water treatment are inefficient for the remediation of wastewaters containing toxic and biorecalcitrant organic pollutants. A large number of advanced oxidation processes (AOPs) have been successfully applied to degrade pollutants present in waters. These methods are based on the generation of a very powerful oxidizing agent such as hydroxyl radical (·OH) in solution, able to destroy organics up to their mineralization. In recent years new AOPs based on the electrochemical technology are being developed. Electrochemical advanced oxidation processes (EAOPs) are environmentally friendly emerging methods for the decontamination of wastewaters contaminated with toxic and persistent herbicides, pesticides, chlorophenols, nitrophenols, polychlorinated biphenyls, pharmaceuticals, etc. This paper reports the fundamentals, main characteristics and recent developments of EAOPs such as anodic oxidation and electro-Fenton alone and coupled with other physicochemical processes. These techniques utilize electrolytic systems such as three-electrode divided and two-electrode undivided cells with different cathodes as working electrodes (carbon-felt or O2-diffusion cathode) and auxiliary electrodes (Pt, PbO2, boron-doped diamond (BDD) or iron anode). The effect of several experimental parameters that largely influence the degradation rate of organic pollutants is discussed. Chromatographic analyses and total organic carbon (TOC) and chemical oxygen demand (COD) measurements show a quick disappearance of initial pollutants and their aromatic and aliphatic reaction products in all cases. The great capacity of oxidation and/or mineralization of all these EAOPs to decontaminate acidic aqueous solutions of common herbicides and pesticides is described.