Mechanisms of cyanide attenuation in soils
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Three types of tests, stirred reactor, tall column, and large tank, were run to identify the mechanisms of cyanide attenuation in soils. Ten pure minerals that comprise the major part of soils were tested. Minerals with high content of alumina are found to attenuate cyanide in an appreciable amount through adsorption. Calcareous and silicous minerals do not tend to react with cyanide. Six major mechanisms of cyanide attenuation in soils were identified. They are adsorption/ion exchange, complexation, oxidation, precipitation, volatilization, and transformation to nitrate. Oxidation is found to be a catalytic process with metal cyanide complexes, [N i( CN)4]2 - and [Cu(CN)4]3 -, being the catalysts. Precipitation of metal cyanide complexes occurs usually at low pH, 3-9 for Zn(CN)2(s) and 2 -8 for Cd(CN)2(s). Iron existing as hematite in soils does not react with cyanide. The attenuation of cyanide in pure minerals is mainly an adsorption process with a first-order rate. pH is a key factor f o r volatilization, adsorption/ion exchange and precipitation processes, all of which were enhanced at low pH values. Zinc, either existing in the soils originally or added to the fresh cyanide solutions was completely precipitated in the soils. Nickel, and copper in the effluent were found to be complexed with CN-. A mass balance on CN- showed that 33.6% cyanide originally added was attenuated by transformation to nitrate, 11.7% by adsorption , 7.3% by precipitation, 4.2% by oxidation, 5-13% by volatilization , and 4% by complexing. About 2 6% cyanide was attenuated through unidentified mechanisms.
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