Photocatalytic reduction - recovery of silver using polyoxometalates

Recovery of silver ions from aqueous solutions can be obtained through a homogeneous photocatalytic process that involves addition of a polyoxometalate (POM) (POM = PW12O403-, SiW12O404- or P2Mo18O626-) organic substrate, for instance propan-2-ol, and illumination with near-visible and UV-light. The process is effective for a wide range of Ag+ concentration, varying from about 3 to more than 1300 ppm. Prolonged irradiation leads to complete removal of silver up to non detected traces (<0.2 ppm). Silver removal follows thermodynamics, i.e. it depends on the redox potential difference of the photochemically reduced POM and Ag+ in the thermal reaction [POM(e(-)) + Ag+ --> POM + Ag]. Air oxygen has no effect on the rate of silver recovery. On the contrary, thiosulfate complexes Ag+, lowers the redox potential and hinders the reduction and precipitation of silver. Unlike TiO2 particulates, POM anions are not contaminated by the precipitated silver retaining their ability to remove large quantities of pure metal. (C) 2002 Elsevier Science B.V. All rights reserved.