Abstract
Tannin gel (TG) with polyhydroxyphenyl groups was synthesized as an adsorbent for the hydrometallurgical recovery process of noble metals. It was further modified to enhance its adsorption performance by impregnating TG with SCN–, which produces SCN–-retaining tannin gel (SCN-TG). It was found that the adsorption mechanism of noble metal ions onto TG is a redox and Au(III) can be adsorbed selectively from the mixture with Pd(II) and Pt(IV) by difference in redox potential. SCN-TG adsorbs Pd(II) selectively in a highly acidic condition due to the great difference in kinetics and stability of metalligand complexation between Pd(II) and Pt(IV). After selective adsorption of Pd(II) under dark condition, the photosensitive Pt(IV)-SCN complexation can be employed to enhance the adsorbability of Pt(IV) on SCN-TG through light-irradiation. Consequently, it is suggested that the effective combination of the two adsorbents, TG and SCN-TG, according to metal species can make it feasible that each noble metal ion (Ag(I), Au(III), Pd(II) and Pt(IV)) is recovered selectively in sequence under acidic condition with high efficiency: Ag(I) in nitric acid media, Au(III) by redox potential higher than that of PGMs, Pd(II) by labile kinetics in complexation, and finally Pt(IV) recovery by light irradiation.
