Adsorption mechanism of palladium by redox within condensed-tannin gel.

Condensed-tannin gel particles with polyhydroxyphenyl groups were synthesized as the adsorbent for the new recovery system of palladium (Pd), which was simple and generated little secondary waste in comparison with the conventional recovery processes. The properties of condensed-tannin gel particles for the Pd adsorption were examined in PdCl2 aqueous solution and it was found that Pd(II) was adsorbed onto the tannin gel particles as a reduced metallic Pd through redox reaction mechanism: chloropalladium(II) species were reduced to Pd(0), while hydroxyl groups of condensed-tannin gel were oxidized during the adsorption. Additionally, it was observed that Pd(II) species containing fewer Cl, such as PdCl2(H2O)2 and PdCl(H2O)3+, were more favorable for the adsorption than PdCl3(H2O)- and PdCl4(2-). By utilizing such characteristics of tannin gel particles, it is expected that they can be applied to recover Pd efficiently and simply with low cost.