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...