Abstract
Photo‐oxidation of cyanide was studied in aqueous solution using a low‐pressure ultra‐violet (UV) lamp along with H~2~O~2~ as an oxidant. It was observed that by UV alone, cyanide degradation was slow but when H~2~O~2~ was used with UV, the degradation rate became faster and complete degradation occurred in 40 min. The rate of degradation increased as the lamp wattage was increased. It was also observed that cyanide oxidation is dependent on initial H~2~O~2~ concentration and the optimum dose of H~2~O~2~ was found to be 35.3 mmol dm^−3^. Photo‐oxidation reactions were carried out at alkaline pH values (10–11) as at acidic pH values, cyanide ions form highly toxic HCN gas which is volatile and difficult to oxidise. By the UV/H~2~O~2~ process, using a 25 W low‐pressure UV lamp and at alkaline pH of 10.5 with an H~2~O~2~ dose of 35.3 mmol dm^−3^, cyanide (100 mg dm^−3^) was completely degraded in 40 min when air was bubbled through the reactor, but when pure oxygen was bubbled the time reduced to 25 min. The cyanide degradation reaction pathway has been established. It was found that cyanide was first oxidised to cyanate and later the cyanate was oxidised to carbon dioxide and nitrogen. The kinetics of cyanide oxidation were found to be pseudo‐first order and the rate constant estimated to be 9.9 × 10^−2^min^−1^ at 40 °C. The power required for complete degradation of 1 kg of cyanide was found to be 167 kWh (kilowatt hour). Copyright © 2004 Society of Chemical Industry