Removal of silver in photographic processing waste by TiO2-based photocatalysis

Treatability data on actual waste show that titania-based photocatalysis can be used to remove silver ions from black and white photoprocessing waste. The silver ion is reduced to its metallic form producing particles comparable in size to the TiO2 catalyst particles. The mass of silver recovered approaches three times that of the titania. Thiosulfate (the predominant chemical in spent fixer) plays a complicated role in the process of silver ion reduction: (1) as a hole scavenger, it can increase the silver reduction rate; (2) as the chemical that stabilizes silver ion in solution, thiosulfate hinders photocatalysis when present at high concentration. Metallic silver can be separated from titania by the physical process of sonication. We also show that sunlight can be used directly to power the photo-electrochemical silver removal process.