BIODEGRADATION OF FREE CYANIDE, THIOCYANATE AND METAL COMPLEXED CYANIDES IN SOLUTIONS WITH DIFFERENT COMPOSITIONS

The feasibility of using biological processes for the detoxification o f cyanide solutions was established in the 1980's. However, most of th e proposed schemes of treatment have been developed for relatively low concentrations. The present work reports on a biological sludge accli matisation procedure and the process performance when treating cyanide solutions with different compositions, including the elimination of a carbon source in the influent. The experiments were carried out in co ntinuous bench scale stirred reactors (5 litres). The following parame ters were monitored for evaluation purposes: chemical oxygen demand (C OD), free cyanide, thiocyanate, copper, iron and zinc concentrations. The acclimatisation time to obtain a specific biological sludge was (r educed from 12 to) 6 months. Microbial activity was found to be the ma in mechanism of free cyanide and thiocyanate degradations, copper and iron removal. An organic carbon source was necessary in the influent t o the system to improve process performance. It was possible to treat efficiently solutions containing 31 mg l(-1) free cyanide, 34 mg l(-1) thiocyanate, 7.2 mg l(-1) copper, 7.4 mg l(-1) iron, and 8.5 mg l(-1) zinc and the removal rates were respectively 47 mg l(-1) day(-1), 50 mg l(-1) day(-1), 10 mg l(-1) day, 11 mg l(-1) day(-1), and 13 mg l(-1 ) day(-1) under the conditions tested. A solid residue was produced co ntaining 42% in metal content (dry weight), making attractive the recy cling of those metals.