Decomposition of Thiocyanate in Wastewater by Electron Beam Radiolysis

Abstract-Wastewater from cyanidation leaching of gold from ore, coal coking, and electroplating processes contains high concentration of thiocyanate, an anionic toxin of particular concern to aquatic environments. This study investigated potential use of electron beam radiolysis of thiocyanate in aqueous solution. Results showed that in 500 mg/L thiocyanate solution with pH of 7 and 12, as irradiation dose increased from 60 to 550 KGy, the conversion percentage that sulfur element in thiocyanate into sulfate was 47.1 - 84.5% and 26.9 - 67.7%; carbon element into carbonate were 1 - 11% and 5 - 31%, conversion into α-amnio acid were 56 - 57% and 58 - 68%, and conversion into urea were 2 - 8.3% and 2 - 8.4%, with a total carbon mass balances of 61 - 71 % and 75-97% respectively. The highest decomposition efficiencies were 98.61% and 99.46% for the solution of pH of 7 and 12 respectively, at a dose of 550 kGy and an initial concentration of 500 mg/L. The thiocyanate decomposition mechanism is postulated to be a series of radical reactions. The reaction steps were that thiocyanate firstly formed cyanide ion by removal of sulfur atom, and then the cyanide ion reacts with radicals such as -OH, -H and e-aq to form intermediate such as OCN^-, H₄C₂N₂, and HCNO.