New diamond coatings for a safer electrolytic disinfection.

In this work, a new coating of boron-doped diamond ultra-nanocrystalline (U-NBDD), tailored to prevent massive formation of perchlorates during disinfection, is evaluated as electrode for the reclaiming of treated secondary wastewater by the electrochemically assisted disinfection process. Results obtained are compared to those obtained by using a standard electrode (STD) that was evaluated as a standard in previous research showing outstanding performance for this application. First tests were carried out to evaluate the chlorine speciation obtained after the electrolysis of synthetic chloride solutions at two different ranges of current densities. Concentrations of hypochlorite obtained using the U-NBDD anode at 25 mA cm-2 were 1.5-fold higher, outperforming STD anode; however, at 300 mA cm-2, an overturn on the behavior of anodes occurs where the amount of hypochlorite produced on STD anode was 1.5-fold higher. Importantly, at low current density the formation of chlorates and perchlorates is null using U-NBDD. Then, the disinfection of the real effluent of the secondary clarifier of a municipal wastewater treatment facility is assessed, where inactivation of Escherichia coli is achieved at low charge applied per volume electrolyzed (0.08 A h L-1) at 25 mA cm-2 using the U-NBDD. These findings demonstrate the appropriateness of the strategy followed in this work to obtain safer electro-disinfection technologies for the reclaiming of treated wastewater.

Highly Efficient Electrochemical Production of Hydrogen Peroxide Using the GDE Technology.

This work examines the role of oxygen supply in the improvement of the hydrogen peroxide (H2O2) electrochemical production efficiency and the generation of high H2O2 concentrations in electrochemical processes operated in a discontinuous mode. To conduct this study, a highly efficient Printex L6 carbon-based gas diffusion electrode (GDE) as a cathode was employed for the electrogeneration of H2O2 in a flow-by reactor and evaluated the effects of lowering the operation temperature (to increase solubility) and increasing the air supply in the system on H2O2 electrogeneration. The results obtained in this study show that unlike what is expected in flow-through reactors, the efficiency in the H2O2 production is not affected by the solubility of oxygen when GDE is employed in the electrochemical process (using the flow-by reactor); i.e., the efficiency of H2O2 production is not significantly dependent on O2 solubility, temperature, and pressure. The application of the proposed PL6C-based GDE led to the generation of accumulated H2O2 of over 3 g L-1 at a high current density. It should be noted, however, that the application of the electrocatalyst at lower current densities resulted in higher energy efficiency in terms of H2O2 production. Precisely, a specific production of H2O2 as high as 131 g kWh-1 was obtained at 25 mA cm-2; the energy efficiency (in terms of H2O2 production) values obtained in this study based on the application of the proposed GDE in a flow-by reactor at low current densities were found to be within the range of values recorded for H2O2 production techniques that employ flow-through reactors.

Biodegradability improvement and toxicity reduction of soil washing effluents polluted with atrazine by means of electrochemical pre-treatment: Influence of the anode material.

This work focuses on the partial anodic electro-oxidation of atrazine-polluted soil washing effluents (SWE) in order to reduce its toxicity and to improve its biodegradability. Concretely it has been evaluated the influence of the anodic material used. It is hypothesized that such partial oxidation step could be considered as a pre-treatment for a subsequent biological treatment. At first, atrazine was extracted from a polluted soil by means of a surfactant-aided soil-washing process. Then, four different anodic materials were studied in partial electro-oxidation pre-treatment batch experiments at different electric charges applied: Boron Doped Diamond (BDD), Carbon Felt (CF), and Mixed Metal Oxides Anodes with Iridium and Ruthenium. Atrazine, TOC, surfactant and sulphate species concentrations, as well as changes in toxicity and biodegradability, were monitored during electrochemical experiments, showing important differences in their evolution during the treatment. It was observed that BDD was the most powerful anodic material to completely degrade atrazine. The other materials achieve an atrazine degradation rate about 75%. Regarding mineralization of the organics in SWE, BDD overtakes clearly the rest of anodes tested. CF obtains good atrazine removal but low mineralization results. All the anodes tested slightly reduced the ecotoxicity of the water effluents. About the biodegradability, only the effluent obtained after the pre-treatment with BDD presented a high biodegradability. In this sense, it must be highlighted the mineralization obtained during the BDD pre-treatment was very strong. These results globally indicate that it is necessary to find a compromise between reaching efficient atrazine removal and biodegradability improvement, while also simultaneously avoiding strong mineralization. Additional efforts should be made to find the most adequate working conditions.