Electro-oxidation of cyanide on active and non-active anodes: Designing the electrocatalytic response of cobalt spinels

[EN] The feasibility of the electrochemical technologies for wastewater treatment greatly relies on the design of efficient but inexpensive electrocatalysts. It is generally accepted that the so-called ¿non-active¿ anodes (like the boron-doped diamond (BDD) or SnO2-based anodes), producing highly oxidizing hydroxyl radicals, are the most promising candidates for pollutants abatement. In this work, the electrocatalytic performance of various cobalt oxides, pure and doped with Cu or Au, for CN¿ oxidation has been studied and compared with that of conventional graphite, BDD, SnO2-Sb and SnO2-Sb-Pt. The metal oxide electrodes were prepared by thermal decomposition of the salt precursors onto Ti. For the M-doped Co3O4 electrodes, the nominal M/Co ratios were Cu/ Co=0.07¿1.00; and Au/Co=0.05¿0.20. The electrodes were characterized by different techniques (XRD, SEM, EDX, XPS) and their electrocatalytic response was studied by cyclic voltammetry and galvanostatic electrolysis in a H-type cell in aqueous 0.1M NaOH. The obtained results show that the nature of the dopant plays a key role on the electrocatalytic behavior of cobalt spinels. Thus, while Cu catalyzes the CN¿ electro-oxidation, Au declines it. This is explained by the fact that, unlike Au (which segregates as Au-rich particles), Cu is effectively incorporated into the spinel structure by forming a solid solution (CuxCo3-xO4). In this solid solution, atomic scale Cu(spinel)-CN¿ specific interactions occur to catalyze the reaction, whereas in segregated Au particles the oxidation is hindered probably by a too-strong adsorption of cyanide and/or its inaccessibility to oxide active sites. Electrolysis runs have revealed that ¿active¿ over-saturated Cu-doped spinels (Cu/Co=1.00) exhibit higher current efficiencies than conventional graphite and ¿non-active¿ BDD and SnO2-based anodes. Hence, we hereby demonstrate that an inexpensive ¿active¿ electrocatalyst can show even higher efficiency than the most powerful BDD anode. These results highlight the significance of anode design in the application of the electrochemical technique for wastewater treatment.Financial support from the Spanish Ministerio de Economia y Competitividad and FEDER funds (MAT2016-76595-R, IJCI-2014-20012) is gratefully acknowledgedBerenguer, R.; Quijada, C.; La Rosa-Toro, A.; Morallón, E. (2019). Electro-oxidation of cyanide on active and non-active anodes: Designing the electrocatalytic response of cobalt spinels. Separation and Purification Technology. 208:42-50. https://doi.org/10.1016/j.seppur.2018.05.024S425020

A FTIR characterization of surface interactions of cyanide and coppercyanide with a platinum electrode in alkaline solution

In this paper, the nature of the species formed in the electrochemical interactions between a polycrystalline platinum surface and cyanide and copper-cyanide species in alkaline solution (pH 13) has been investigated by combining cyclic voltammetry with in situ FTIR spectroscopy. This study was performed because electrochemical reactors for the treatment of cyanide-containing industrial wastewaters have been shown to achieve viable destruction rates when copper ions are present in the alkaline solution through the formation of copper oxide films [1-3]. It has been found that the complexation of cyanide ions by copper species hinders the adsorption of the anion at the electrode, thus avoiding the characteristic electrode poisoning by CO that occurs in the absence of soluble copper species. Conversely, soluble copper-cyanide complexes can be electrooxidized at the platinum surface through the formation of cyanate species as intermediate, and of HCOOH and NO as end products. The addition of copper species to the electrolyte also facilitates this route for cyanide oxidation to occur at higher rates without poisoning the platinum electrodeFTIRvoltammetryplatinumcyanide oxidationcopper ionscatalysi

Sanitation of blackwater via sequential wetland and electrochemical treatment

The discharge of untreated septage is a major health hazard in countries that lack sewer systems and centralized sewage treatment. Small-scale, point-source treatment units are needed for water treatment and disinfection due to the distributed nature of this discharge, i.e., from single households or community toilets. In this study, a high-rate-wetland coupled with an electrochemical system was developed and demonstrated to treat septage at full scale. The full-scale wetland on average removed 79 +/- 2% chemical oxygen demand (COD), 30 +/- 5% total Kjeldahl nitrogen (TKN), 58 +/- 4% total ammoniacal nitrogen (TAN), and 78 +/- 4% orthophosphate. Pathogens such as coliforms were not fully removed after passage through the wetland. Therefore, the wetland effluent was subsequently treated with an electrochemical cell with a cation exchange membrane where the effluent first passed through the anodic chamber. This lead to in situ chlorine or other oxidant production under acidifying conditions. Upon a residence time of at least 6 h of this anodic effluent in a buffer tank, the fluid was sent through the cathodic chamber where pH neutralization occurred. Overall, the combined system removed 89 +/- 1% COD, 36 +/- 5% TKN, 70 +/- 2% TAN, and 87 +/- 2% ortho-phosphate. An average 5-log unit reduction in coliform was observed. The energy input for the integrated system was on average 16 +/- 3 kWh/m(3), and 11 kWh/m(3) under optimal conditions. Further research is required to optimize the system in terms of stability and energy consumption