A tannin gel (TG) synthesized from condensed tannin molecules has a remarkable ability to adsorb various metal ions in aqueous solutions. Previously, we found that the adsorbability of the TG for the palladium ion can be improved by the ammonia treatment ( Chem. Lett. 2007, 36, 1316). In the present study, we have performed solid-state 13C NMR measurements of the tannin gels before and after the ammonia treatment and found that some hydroxy groups in the TG are substituted by amino groups. Using this amine-modified tannin gel (ATG), we have investigated the adsorption behaviors of palladium and platinum ions, Pd(II) and Pt(IV), in acidic chloride solutions under various conditions. The adsorbability of the ATG for both precious metal ions was greatly improved compared to that of the original TG. Their adsorption amounts onto the ATG increased with increasing pH and temperature or decreasing chloride ion concentration. Under all conditions examined here, the adsorbability of the ATG for Pd(II) was much higher than that for Pt(IV), resulting in the success of the selective adsorption of Pd(II) from the solution mixture of Pd(II) and Pt(IV).

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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 À ) 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 À -retaining tannin gel (SCN-TG), we have investigated the introduction mechanism of SCN À into the TG at 298 K and HCl concentrations less than or equal to 1 M, where the decomposition of SCN À can be negligible. The adsorption amount of SCN À onto the TG increased with increasing HCl concentration, indicating that SCN À is adsorbed onto the TG through the electrostatic attraction between anionic SCN À 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.

A tannin gel (TG), synthesized from condensed tannin molecules, has the remarkable ability to adsorb various metal ions in aqueous solutions. In the present study, the adsorption behavior of rhodium ion [Rh(III)] in acidic chloride solutions was investigated using an aminemodified tannin gel (ATG) prepared by the ammonia treatment of a TG. The ATG was able to adsorb Rh(III) in strong hydrochloric acid solutions, whereas the TG failed to adsorb it. This is because the amino group in the ATG can form a stable complex with Rh(III) compared to the hydroxy group. With increasing chloride ion concentration, the adsorption amount of Rh(III) onto the ATG decreased somewhat, while the number of chloride ligands in the Rh(III) complex was expected to increase. This suggests that the Rh(III) complexes with fewer chloride ligands are more favorable for adsorption onto the ATG. Also, the Rh(III) adsorbability of the ATG decreased significantly with decreasing initial pH because of the protonation of the hydroxy and amino groups in the ATG. The results obtained here indicate that in the Rh(III) adsorption onto the ATG, the solution pH is more influential than the chloride ion concentration. V C 2012 Wiley Periodicals, Inc. J Appl Polym Sci 126: E34-E38, 2012

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 investigated the adsorption behaviors of palladium and platinum ions, Pd(II) and Pt(IV), in strong HCl solutions using the thiocyanate‐retaining tannin gel (SCN‐TG) prepared by adsorption of thiocyanate ion (SCN−) onto the TG in acidic thiocyanate solutions as reported previously (Ind Eng Chem Res 2011, 50, 12366). Although the adsorption rate of Pt(IV) is slow compared to that of Pd(II), the SCN‐TG was able to adsorb both Pd(II) and Pt(IV) in a 1 M HCl solution, where the TG can never adsorb them. Besides, we have succeeded in the selective adsorption of Pd(II) from the mixture solution of Pd(II) and Pt(IV) at a temperature somewhat lower than room temperature using the SCN‐TG. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012