Preparation of Thiocyanate-Retaining Tannin Gel for Adsorption Recovery of Palladium from Strong Hydrochloric Acid Solutions

Abstract

A tannin gel (TG) synthesized from condensed tannin molecules has a remarkable ability to adsorb various metal ions in aqueous solutions. In the present study, we have introduced thiocyanate ions (SCN – ) into the TG by adsorption process in acidic thiocyanate solutions to improve its adsorbability for palladium ion, Pd(II). To determine the optimum conditions for preparing the SCN – -retaining tannin gel (SCN-TG), we have investigated the introduction mechanism of SCN – into the TG at 298 K and HCl concentrations less than or equal to 1 M, where the decomposition of SCN – can be negligible. The adsorption amount of SCN – onto the TG increased with increasing HCl concentration, indicating that SCN – is adsorbed onto the TG through the electrostatic attraction between anionic SCN – and the protonated hydroxy groups in the TG at higher HCl concentrations. Using the resultant SCN-TG and the TG, we have conducted the Pd(II) adsorption experiment at molar-level concentrations of HCl and found that the SCN-TG can adsorb Pd(II) efficiently, whereas the TG adsorbs almost no Pd(II). Besides, the adsorbability of the SCN-TG for Pd(II) decreased only slightly with increasing HCl concentration up to at least 5 M.