Biodegradation of free cyanide, thiocyanate and metal complexed cyanides in solutions with different compositions

The feasibility of using biological processes for the detoxification of cyanide solutions was established in the 1980's. However, most of the proposed schemes of treatment have been developed for relatively low concentrations. The present work reports on a biological sludge acclimatisation 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 continuous bench scale stirred reactors (5 litres). The following parameters were monitored for evaluation purposes: chemical oxygen demand (COD), free cyanide, thiocyanate, copper, iron and zinc concentrations. The acclimatisation time to obtain a specific biological sludge was (reduced from 12 to) 6 months. Microbial activity was found to be the main mechanism of free cyanide and thiocyanate degradations, copper and iron removal. An organic carbon source was necessary in the influent to 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 containing 42% in metal content (dry weight), making attractive the recycling of those metals.