Oxidation of carboxylic acids in water at IrO2-Ta2O5 and Boron Doped Diamond anodes

The electrochemical oxidation of different carboxylic acids (namely, oxalic, formic and maleic) in water at boron doped diamond (BDD) and IrO2-Ta2O5 (DSA-O2) anodes was performed to study the influence of the operative parameters and of the nature of the acid on the performances of the process. Higher abatements were obtained at BDD with respect to DSA anodes for all the selected carboxylic compounds. The rate of abatement decreased in the order oxalic > formic >> maleic at iridium anodes while an opposite trend was observed at diamond anodes (formic maleic > oxalic), thus indicating that different oxidant agents are involved at these two electrodes. Also the effect of the temperature depends on both the nature of the acid and of the anode. Higher current efficiencies were obtained when most part of the process was under the kinetic control of the oxidation reaction, i.e., when low current densities and high flow rates were imposed. High concentrations of carboxylic acids enhanced the current efficiency at all kinetic regimes

Innovative chemical processes for the treatment of water polluted by recalcitrant organic substances

In the last years, many research groups have focused their attention on the innovative chemical processes adopted for the treatment of water effluents polluted by recalcitrant organic substances, i.e., substances resistant to biological treatment. The electrochemical oxidation is one of the most studied technologies because it presents high versatility and low cost, it is realized under mild conditions of pressure and temperature and generally it does not involve the use of toxic substances [1]. In this work, the comparative performance of different electrochemical approaches such as direct oxidation processes, oxidation by means of electrogenerated chlorine and electro-Fenton was investigated. The influence of numerous parameters, such as the nature of the electrodic material and of the organic pollutant, the pH, the flow dynamic regime, the current density, the pollutant concentration and the temperature, on the electrochemical incineration of some carboxylic acids and aliphatic chlorides, chosen as model organic compounds, was studied in detail. Two very different anodes were used: Ti/IrO2-Ta2O5, which presents a quite low oxygen overpotential, and boron-doped diamond (BDD), which certainly is one of the most promising materials for the electrochemical incineration [2,3]. Incineration of carboxylic acids was favored by high flow rate and low current density, i.e., when the oxidation process was mainly under kinetic control. Moreover, the process resulted to be favored by high initial concentrations of the organic substrate and low pH but it did not depend, under the adopted operative conditions, on the nature of the supporting electrolyte. The effect of sodium chloride on the electrochemical oxidation of oxalic acid was observed to depend on the nature of the anode as well as on the pH. The best results were achieved by using IrO2-Ta2O5 electrode, with addition of sodium chloride at acid pH. Furthermore, in the presence of high amounts of sodium chloride, a higher abatement of oxalic acid was obtained when high current densities and low flow rates were imposed. Comparing the performance of electro-Fenton process coupled with anodic oxidation at BDD anode and that of simple anodic oxidation, higher abatements of 1,2-dichloroethane and 1,1,2,2-tetrachloroethane were obtained in the first case thanks to the presence of Fe2+ in the solution. Higher applied currents led to a faster electrogeneration of H2O2 and regeneration of Fe2+, thus giving rise to a faster degradation of the starting compounds. The time course of the concentration of the main intermediates accumulated in the electrochemical cell during the treatments, particularly short-chain carboxylic acids and chlorinated ions, was also reported. [1] Jüttner K. et al., Electrochimica Acta, 45 (2000) 2575–2594. [2] Panizza M. and Cerisola G., Electrochimica Acta, 51 (2005) 191–199. [3] Martinez-Huitle C. A. and Ferro S., Chem. Soc. Rev., 35 (2006) 1324-1340

Loading carbonaceous materials with silver for the treatment of chloro-organic compounds in aqueous phase

Many electrochemical technologies, either based on novel concepts (such as microbial fuel cells), experimental setups (such as photoelectrochemical or solar photoelectro-Fenton reactors) or materials (mainly focused on the use of large O2-overpotential anodes like BDD) have been devised in recent years for water remediation. Special attention has been paid to highly toxic, biorefractory organic pollutants such as the chlorinated hydrocarbons, which conjugate toxicity with chemical stability, bioaccumulation and long-range diffusivity [1]. Electroreduction at silver cathodes becomes an interesting alternative to degrade chloro-organic compounds, but it may lead to the accumulation of reaction by-products, even upon coupling with electro-oxidation at BDD [2]. On the other hand, some Fenton-based processes have proven very effective for the destruction of organic matter due to the action of •OH formed when cathodically electrogenerated H2O2 reacts with added Fe2+ [3]. Based on this, we have envisaged a potential strategy for the enhanced removal of chloro-organic pollutants and their by-products: electro-Fenton process in the bulk upon H2O2 electrogeneration at a carbonaceous cathode, which can simultaneously act as the substrate for electroreduction at loaded Ag nanoparticles. To achieve this goal, a highly efficient material for H2O2 production, i.e., a gas diffusion electrode (GDE), has been chosen for Ag-loading experiments. Several authors have reported the preparation of Ag-loaded carbonaceous materials based on a simple electroless deposition (ELD) process from Ag+ solutions. Some of them have addressed the full preparation of GDEs with Ag catalysts [4,5]. Here, we report the use of a commercial GDE as a suitable substrate to obtain conveniently dispersed Ag nanoparticles. The effect of several ELD parameters (e.g., nature of reductant, mode of application and deposition time) on the surface morphology has been mainly studied by SEM-EDX. Bulk electrolyses in 50 mM Na2SO4 at various pH were subsequently performed with the best materials to assess their ability to electrogenerate H2O2. For comparison, carbon paper was used as an alternative substrate. An important objective of the research was to find the optimum conditions to load the substrate so as to keep the balance between covered and uncovered area, in order to favor both H2O2 production and pollutant electroreduction. The performances of these electrodes for the electrogeneration of H2O2 and the abatement of chloro-organic pollutants is currently being investigated. [1] S. Rondinini, A. Vertova, in Electrochemistry for the environment, 2010, pp. 279–306. [2] O. Scialdone, A. Galia, L. Gurreri, S. Randazzo, Electrochim. Acta 55 (2010) 701–708. [3] E. Brillas, I. Sirés, M.A. Oturan, Chem. Rev. 109 (2009) 6570–6631. [4] E. Gülzow, N. Wagner, M. Schulze, Fuel Cells 3 (2003) 67–72. [5] S. Rondinini, G. Aricci, Z. Krpetic, C. Locatelli, A. Minguzzi, F. Porta, A. Vertova, Fuel Cells 3 (2009) 253–263

Electrochemical incineration of organic pollutants: effect of the nature of the pollutants and of the temperature

The electrochemical oxidation of some organic pollutants, including three carboxylic acids (oxalic, formic and maleic), at boron doped diamond (BDD) and DSA anodes, using galvanostatic alimentation, was investigated in order to study the effect of the temperature and of the nature of the pollutants on the process. In incineration electrolyses, the performances of the process in terms of carboxylic acid conversion and current efficiency dramatically depend on the adopted operative conditions. The abatement of the organic pollutant depends on the nature of the carboxylic acid. Quite interestingly, the opposite effect of the nature of the substrate on the performances of the process was observed at BDD and DSA anodes

Economic Analysis of an Innovative Scheme for the Treatment of Produced Waters

During the crude oil extraction processes, for each barrel of oil turns out an equivalent of 3 barrels of wastewaters on average. These wastes are known as Produced Waters (PWs) and their dramatic impact on the environment has attracted the attention of researchers in order to find an economic and efficient method for their treatment. Dealing with PWs is not easy: the long exposure with oil increases their hydrocarbon fraction, while the contact with the underground wells increases their concentration in salts and minerals. The direct discharge of PWs into the sea is obviously not allowed by law and PWs are usually re-injected into the well. The present work deals with a novel and innovative treatment chain (including assisted reverse electrodialysis (ARED) as dilution step) able to reduce both the salinity and organic content of PWs. The innovative scheme includes an ultrafiltration unit as pre-treatment, upstream an ARED unit for the PW dilution. Once the salinity level has been reduced down to a value affordable for a bioremediation step, PWs are sent to a bio-reactor, where the organic compounds are digested. Finally, a reverse osmosis unit is used to recover water from the treated PWs and to recycle it as diluted stream in the ARED unit. A techno-economic model was purposely developed in the present work to assess the economic feasibility of the proposed scheme. Preliminary results suggest that the treatment costs are lower than 5 € m-3 PW and fully competitive with current PWs treatment technologies

Salinity Reduction of Real Produced Waters via Assisted Reverse Electrodialysis

Produced waters (PWs) are waste streams generated during the crude oil extraction processes. The management of these wastewaters is complicated by the large volumes extracted during the oil recovery operations: these depends on the life of the oil-well: typically, 3 barrels of PWs on average are produced for each barrel of oil extracted. After oil separation, PWs are usually re-injected into the well, but this approach is not always possible without a preliminary and suitable treatment. Bioremediation techniques might be a good option, but they fail due to the PWs high salinity, which inhibit bacteria growth and metabolism. Thus, reducing their salinity upstream a bioremediation unit is a matter of crucial importance. To this aim, Assisted Reverse electrodialysis (ARED) along with the use of a dilute stream typically available on site is here proposed as a novel solution. In ARED an additional voltage is applied in the same direction of the salinity gradient through the membranes in order to enhance the passage of ions from the PW to the diluted solution, thus significantly reducing the required membrane area. An experimental campaign was carried out in order to assess the process feasibility. A fixed volume of real PWs was fed to a laboratory scale ARED unit. Each experimental test lasted for three days to reduce the salinity down to about 20 g l-1, a value compatible with the biomass metabolism for a downstream bioremediation step. Two different types of commercial membranes were tested and relevant energy consumptions were calculated. The long-runs performed did not show a significant loss of efficiency due to fouling, thus suggesting that ARED might a suitable technology for a pre-dilution of produced water

Combining Membrane and Zero Brine Technologies in Waste Acid Treatment for a Circular Economy in the Hot‑Dip Galvanizing Industry: A Life Cycle Perspective

An innovative approach of combining membrane and zero brine technologies for a joint treatment of industrial liquid waste is investigated regarding its environmental impacts compared to the existing liquid waste treatment. The object of investigation is the generation of waste acid solution by a hot dip galvanizing plant in Sicily, Italy. The waste acid solution contains hydrochloric acid, iron and zinc, which makes it a hazardous waste according to EU classifcations. Environmental impacts are studied for two scenarios in the Tecnozinco hot-dip galvanizing plant in Sicily, Italy: (i) the current process of pickling with linear disposal of waste acid and (ii) the pickling combined with in-situ treatment of the waste acid using a combination of difusion dialysis (DD), membrane distillation (MD) and a precipitation reactor. Results are obtained via an attributional life cycle assessment (LCA) approach focusing on the water footprint profle of the process. The linear disposal path creates signifcant costs, environmental burdens and risks during the 1500 km transport of hazardous liquid waste. The combination of DD and MD, complemented with a zero-brine precipitation reactor, closes internal material loops, could save local water resources and reduces costs as well as environmental impacts. Reduction potentials of 70–80% regarding most LCA impact categories can be expected for the application of the novel technology combination supporting the galvanizing pre-treatment process under study. Therefore, the application of such technology on the way forward to a more circular economy is recommended from an environmental viewpoint, especially in process plants similar to the investigated one

Electrochemical processes for the treatment of chlorinated ethanes in water solutions

In the present work, the electrochemical treatment of water solutions containing chloro ethanes was performed by cathodic reduction, anodic oxidation and coupled processes with the aim of evaluate the effectiveness of these methodologies

TRATAMIENTO ELECTROQUIMICO DE CONTAMINANTES ORGANOCLORADOS ALIFATICOS. ELUCIDACION DE LOS CAMINOS DE REACCION.

Los hidrocarburos alifáticos clorados conjugan toxicidad con una alta estabilidad química, capacidad de bioacumulación y difusión de largo alcance. Los cloroetanos son particularmente ubicuos en la industria y en los productos domésticos, y su introducción en el medio ambiente puede entrañar riesgos para los seres vivos. Actualmente, la USEPA (US Environmental Protection Agency) está llevando a cabo el llamado “Endocrine Disruptor Screening Program (EDSP)”, en el cual tanto el 1,2-dicloroetano (DCA) como el 1,1,2,2-tetracloroetano (TCA) son considerados prioritarios debido a sus efectos potenciales sobre el sistema endocrino. El DCA se encuentra también en la lista de substancias prioritarias establecida recientemente por la Comisión Europea.1 Para evitar o, cuando menos, minimizar la entrada de cloroetanos en el medio acuático es necesaria la aplicación de tecnologías de tratamiento de aguas más eficaces que las tradicionalmente empleadas en las plantas de tratamiento de aguas residuales, como por ejemplo los procesos de oxidación avanzada (AOPs). De entre éstos, los AOPs electroquímicos (EAOPs) como la oxidación anódica (OA) y el proceso electro-Fenton (EF) vienen suscitando un gran interés para la degradación de compuestos orgánicos debido a sus excepcionales características técnicas y al uso reducido de compuestos químicos de elevado coste. Sin embargo, hasta la fecha, la electrorreducción con cátodos de plata ha sido el método electroquímico más utilizado para la destrucción de cloroetanos, observándose en general una conversión parcial de los contaminantes iniciales en lugar de su mineralización total. En este trabajo, se han tratado disoluciones acuosas ácidas de DCA y TCA mediante los procesos EF y OA. Las electrólisis se han realizado a corriente constante usando un ánodo de diamante dopado con boro (BDD) y un cátodo a difusión de aire (ADE) capaz de generar H2O2 in situ, el cual reacciona con el Fe2+ añadido para obtener •OH en el seno de la disolución a partir de la conocida reacción de Fenton. A los 420 min de tratamiento EF a 300 mA, se alcanzó la mineralización prácticamente total de disoluciones con 4 mM de DCA o TCA. Tratamientos comparativos en ausencia de Fe2+ (es decir, OA) o con un ánodo de menor poder oxidante como el Pt condujeron a una mineralización más pobre. Los resultados tan positivos obtenidos mediante el proceso EF con BDD se pueden atribuir a la acción sinérgica de los radicales oxidantes, es decir, BDD(•OH) en la superficie del ánodo y •OH en el seno de la disolución, además de la minimización de las limitaciones difusivas. El descenso de la concentración del contaminante inicial se ajusta perfectamente a una cinética de pseudo primer orden. Los subproductos acumulados en mayor concentración durante la degradación del DCA y el TCA son los ácidos cloroacético y dicloroacético, respectivamente. Así mismo, se han identificado los ácidos acético, oxálico y fórmico. Los caminos de reacción propuestos incluyen etapas de decloración oxidativa y reductiva (catódica). Por otra parte, se ha encontrado que el cloro se libera inicialmente como ion Cl, el cual es oxidado a ion ClO3 y, fundamentalmente, a ion ClO4, gracias a la acción de los radicales BDD(•OH) y •OH generados en gran proporción. En conclusión, se ha demostrado que el proceso EF con una celda BDD/ADE es una tecnología muy efectiva para la descontaminación de disoluciones acuosas de DCA y TCA, así como de mezclas de ambos, lo cual constituye un resultado importante de cara a proseguir con su tratamiento en matrices de agua real. Referencias 1. European Commission, Identification of Priority Hazardous Substances under the Water Framework Directive, Directorate-General Environment, 2000