Electrochemical treatment of diluted cyanide aqueous wastes

Abstract

The electrochemical oxidation of diluted cyanide aqueous wastes has been studied in a single compartment electrochemical flow cell. It has been determined that the anode material influences greatly the process's performance. Boron doped diamond and PbO 2 anodes can oxidize these wastes in the presence of both sulfate or chloride anions. On the contrary, dimensional stable anodes cannot oxidize cyanide in sulfate-containing wastewaters, and require the presence of chloride ions. The oxidation of cyanides leads to the formation of cyanate in a first step, and later to the formation of carbon dioxide and nitrogen. There is a net consumption of hydroxyl ions during the process. Energy consumptions in the range 20-70 kWh m −3 are required to decrease the initial pollutant load by 70-80%. Global current efficiencies in the range 3-8% are obtained. These low current efficiencies are justified by the low cyanide concentrations that the wastes used in this work contain.

Electrocatalytic oxidation of cyanide on copper-doped cobalt oxide electrodes

“…Accordingly, the obtained zero-order rate constants for cyanide electrooxidation on Cu x Co 3-x O 4 anodes cannot be directly compared with the kinetics of previously studied anode materials. Nonetheless, it can be inferred that the highest rate constants obtained in this work for the saturated and oversaturated spinels, of about 1x10 -3 mol m -3 s -1 , are in the order (�0(Cu x Co 3−x O 4) ~ �1(��ℎ ��) * ��−�=0) [26,30,33,34,36]. Furthermore, while it is well-known that the reaction rate generally increases with the applied current density [35], it is noteworthy to mention that these first-order rate constants (k 1 ) of other anodes in literature were obtained by using comparatively larger current densities (10 -50 mA/cm 2 ) [26,30,33,34,36].…”

“…Nonetheless, it can be inferred that the highest rate constants obtained in this work for the saturated and oversaturated spinels, of about 1x10 -3 mol m -3 s -1 , are in the order (�0(Cu x Co 3−x O 4) ~ �1(��ℎ ��) * ��−�=0) [26,30,33,34,36]. Furthermore, while it is well-known that the reaction rate generally increases with the applied current density [35], it is noteworthy to mention that these first-order rate constants (k 1 ) of other anodes in literature were obtained by using comparatively larger current densities (10 -50 mA/cm 2 ) [26,30,33,34,36]. Consequently, our results show that saturated and oversaturated Cu-Co spinel electrocatalysts enable complete cyanide conversion in an electrolytic process operated at low current density and high efficiency, at a fast and sustained (constant) rate, which is highly advantageous compared to previously reported first-order electrochemical oxidations, that exhibit typical rate decay as cyanide is consumed.…”

“…1). It is well-known that cyanate can be subsequently mineralized in strongly alkaline medium as follows [27,30]:…”

“…This is in agreement with previous studies, which showed that the kinetics of cyanate decomposition (Eq. 8) is considerably slower, so it begins when cyanide has mostly reacted [30,32,36]. As a result, cyanate oxidation (Eq.…”

“…4 Several electrodes including carbon materials [23][24][25][26], stainless steel [27], bare Pt and Ni [28,29], boron-doped diamond (BDD) [30], CuO [11], and Ti-supported metal and metal oxide systems (the so-called dimensionally stable anodes, DSA) like PbO 2 [30,31], Pt [28,32], RuO 2 -TiO 2 and TaO 2 -IrO 2 [33], SnO 2 -SbO x [34,35], Co 3 O 4 [36,37], etc. were previously studied for the direct and indirect electrochemical oxidation of cyanide.…”