Photochemical reduction of gold(III) on semiconductor dispersions of TiO 2 in the presence of CN − ions: disposal of CN − by treatment with hydrogen peroxide

Abstract

The reduction and recovery of gold from samples containing CN − ions have been investigated employing TiO 2 semiconductor dispersions irradiated with either UV light (λ ⩾ 210 nm) or AM1 simulated sunlight (λ ⩾ 310 nm). The photoreduction is very efficient at low pH; disposal of cyanide, which acts as a buffer to a pH of about 9.3, was imperative. The degradation of CN − was investigated employing two peroxides, H 2O 2 and S 2O 8 2−, in the presence and absence of light. Treatment of cyanide solutions with H 2O 2 and UV light irradiation leads to efficient conversion of CN − to NH 3 and CO 2 (the main products detected). The H 2O 2 decomposition of CN − also occurs in the dark, but to a lesser extent. The kinetics of the conversion process(es) have been studied; under irradiation, k obs is about ten times the rate of the dark process. A mechanism involving OH · radicals is proposed for the cyanide decomposition by UV light. Cyanate ion, OCN −, is the intermediate in both the dark and the illuminated pathways. Recovery of gold on TiO 2 powder, following cyanide oxidation and removal of unreacted H 2O 2, is favored in acidic aqueous media. The implications of our findings towards practical disposal of cyanide and recovery of gold from actual indusrial waste samples are discussed.