Simultaneous photocatalytic reduction of silver and oxidation of cyanide from dicyanoargentate solutions

The feasibility of heterogeneous photocatalysis for the treatment of dicyanoargentate complexes in solution using titanium dioxide was investigated. The best results were obtained in the presence of oxygen, as the simultaneous deposition of metallic silver on the catalyst and oxidation of released cyanide ions to cyanate species was achieved. It is proposed that O2 plays a dual role in the reaction: it supports the reduction of the metal acting as intermediate in the transfer of electrons and acts as reagent in the oxidation of released cyanide to cyanate species. The influence on the kinetics of the addition of methanol was studied. In anoxic conditions the rate of silver reduction was increased, what is attributed to the effectiveness of methanol as hole scavenger and its ability to form reducing radicals, whereas the oxidation of released CN− was inhibited. On the contrary, in aerobic medium the presence of the alcohol had a detrimental effect on the metal reduction but no cyanide accumulation was produced. The photocatalytic treatment of an industrial spent silver plating bath was carried out. In anoxic conditions, the recovery of silver upon deposition on the catalyst as Ag0 was achieved. As the large amount of organic matter in the solution inhibited the oxidation of cyanide ions, a two-step procedure is proposed for the overall treatment of those wastewaters.