Removal of cadmium and cyanide from aqueous solutions through electrodialysis

Abstract

A descarga de efluentes contendo metais pesados e cianeto pelas indústrias de galvanoplastia é uma das maiores fontes de poluição dos recursos hídricos. O emprego da técnica de eletrodiálise para o tratamento de um efluente sintético contendo aproximadamente 0,0089 mol L -1 de cádmio e 0,081 mol L -1 de cianeto foi estudado utilizando uma célula de eletrodiálise de cinco compartimentos. Os resultados demonstram que a remoção de cádmio e cianeto depende da densidade de corrente aplicada e é limitada pela precipitação de cádmio sobre a membrana catiônica.The discharge of galvanic industry wastewaters containing heavy metals and cyanide is one of the largest sources of water pollution. The use of the electrodialysis technique for the treatment of a synthetic wastewater containing approximately 0.0089 mol L -1 cadmium and 0.081 mol L -1 cyanide was studied using a five-compartment electrodialysis cell. The results demonstrate that the removal of cadmium and cyanide depends on the applied current density and it is limited by the precipitation of cadmium on the cation-exchange membrane in the diluate central cell compartment. Keywords: cadmium, cyanide, electrodialysis, wastewater IntroductionThe release of industrial wastewaters containing cadmium and cyanide to the environment is strictly controlled due to the toxic nature of these substances. To reduce the environmental impacts it is necessary to remove these substances from wastewater before its discharge in the environment. 1 One of the largest cadmium and cyanide sources, which contributes largely to water pollution, are the electroplating industries that use cadmium cyanide baths. The main problems associated to this process, from the environmental point of view, are the deposition baths and rinsewaters discharges. The alkaline-chlorination process for cyanide destruction with further cadmium hydroxide precipitation is a common method employed for the treatment of the wastewaters characterized by the presence of these substances. However, chlorination of cyanides can result in highly toxic intermediates (cyanogen chlorides) and other toxic organo-chlorines that can be formed. These compounds together with residual chlorine create additional environmental problems. Metal precipitation can generate a large amount of cadmium hydroxide sludge classified as dangerous waste by the Brazilian Regulation -NBR10004 2 and its appropriate disposition constitutes a serious environmental and economical problem for the involved industries. [3][4][5] The development of more advanced technologies for wastewater treatment has increased in last years and new techniques have been suggested as promising possibilities, not only for the removal but also for the recovery of substances present in the wastewater. 6-9 Among these new techniques, electrodialysis can be mentioned. [10][11][12][13] Electrodialysis is a membrane separation process in which ionic species in solution are transported through ionexchange membranes, under the influence of an electric field from one solution to an...

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