Electrochemical Destruction of Dilute Cyanide by Copper‐Catalyzed Oxidation in a Flow‐Through Porous Electrode

Abstract

Cyanide ion is a pollutant that occurs in many industrial wastewater streams, particularly rinse waters from electroplating operations. 1 Based on aquatic toxicity tests, regulatory agencies have set point-source cyanide emission limits on the order of 1 ppm. Thus, it is necessary to treat wastewaters containing cyanide prior to discharge to the environment.Two common methods for destruction of cyanide are chemical oxidation, with hypochlorite for instance, or electrochemical oxidation. Neither is ideal for attaining the low levels required. Chemical methods may leave an excess of other objectionable chemicals in the water stream, and electrochemical methods are not efficient when the cyanide level drops below 100 ppm because of mass-transfer limitations and low current efficiency. This paper addresses these latter limitations in order to identify a more effective electrochemical treatment method.On many electrode surfaces, the anodic overpotential for cyanide oxidation is large and the current efficiency is low. The cyanide solutions must necessarily be kept alkaline to prevent liberation of hydrogen cyanide (pK ϭ 9.2). In the potential range where cyanide reacts, and in the absence of halides, the concurrent oxidation of hydroxide to oxygen is the major source of current inefficiency. Researchers have investigated a variety of electrodes seeking a good catalyst for cyanide oxidation. Metal oxides have been found to reduce the cyanide overpotential, but most are not stable in the solutions of interest. One exception may be lead dioxide. 2 Another appears to be copper oxide. [3][4][5] The problem of mass-transfer limitations with a very dilute reactant such as cyanide is best solved by use of an extended-surface electrode, such as a flow-through porous electrode. 6-9 Such electrodes not only promote high mass-transfer rates, but they also provide high surface areas within a compact volume.In this study a reticulated vitreous carbon porous anode was activated with copper to catalyze cyanide oxidation. Synthetic waste solutions were treated, both on a once-through basis and in a semibatch recycle reactor, in order to determine the efficacy of this approach.System Chemistry The solution chemistry of cyanide and its electrochemical oxidation are rather complicated. Depending on reaction conditions one can observe a variety of reaction products, but the reaction of primary interest for pollution control is the two-electron oxidation of cyanide to the much less toxic product cyanateThe competing anodic reaction that robs current efficiency in an excess of hydroxide isOn electrodes such as platinum reaction 1 exhibits a large overpotential such that reaction 2 can proceed preferentially. Reaction 1 can be catalyzed either homogeneously or heterogeneously by copper. Katagiri et al. [10][11][12] have discussed a homogeneous mechanism in which cuprous in a cyanide complex is oxidized anodically, and the resulting cupric complex undergoes an intramolecular redox reaction regenerating cuprous and producing cyanogen, which hy...

Electrocatalytic oxidation of cyanide on copper-doped cobalt oxide electrodes

“…This enhancement was assigned to an electrocatalytic effect caused by copper oxide films in-situ deposited on the anode [23][24][25][26][27][28]31,32]. This catalytic activity is in agreement with the reported promotion of cyanide decomposition by copper species when using other elimination methods [38,39].…”

“…In this sense, and because both (10) and (11) are competing reactions, the high activity of Co 3 O 4 electrodes for the OER in alkaline media The electrocatalytic effect of copper on CN − oxidation in alkaline media has been previously studied and different mechanisms have been reported depending on the conditions used (pH, cyanide and copper concentrations, cell design and operating conditions) [11,[23][24][25][26][27][28]31,32,62].…”

“…whereas C 0 refers to the initial cyanide concentration (500 ppm) and Q th refers to the theoretical charge necessary for the complete removal of cyanide by direct oxidation to cyanate according to the following reaction [23,27]:…”

“…Under highly alkaline conditions, cyanate can subsequently undergo electrochemical mineralization to carbonate and N 2 as final products (reaction (8)) [27,28,35]. By contrast, if the alkalinity is not high enough, highly toxic cyanogen can be derived from the electro-oxidation of cyanide [23,28], whereas cyanate can chemically decompose to other by-products [27]. Previous studies have demonstrated that an initial pH of around 13, like that used in present work, is high enough to ensure the preferred elimination pathway via reactions (1) and (8) [27,28,35].…”

“…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.…”