Improved Adsorption Behaviors of Amine-Modified Tannin Gel for Palladium and Platinum Ions in Acidic Chloride Solutions

Abstract

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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“…Then, a stronger bond with soft acidic precious metal ions, such as Pd(II) and Pt(IV), is expected if an amine-treatment is performed in tannin gels. Several works clearly demonstrated [41,54,59,60] the significant enhancement in the adsorbability of both precious metals. The adsorbed amounts of Pt(IV) were however found to be much smaller than the values obtained for Pd(II).…”

“…2 presents schematically several ways to obtain an amine-modified TBA. The simplest procedure involves an ammonia treatment of the tannin resin (after polymerization with formaldehyde) [14,60]. Kim et al [59] optimized ammonia treatment procedure, in terms of ammonia concentration, time and temperature, in order to get a maximum amount of amino groups introduced into tanningels.…”

“…3% of nitrogen. Evidences, based on the decrease in oxygen content and based in 13 CNMR (Carbon-13 nuclear magnetic resonance) spectra, suggested the replacement of hydroxyl groups of the tannin gel by amino groups during the ammonia treatment [59,60]. Slightly more complex procedures, involving an intermediate chloromethylation step, were performed in order to anchor bisthiourea (BTU) [41], N-aminoguanidine (AG) [54], tetraethylenepentamine (TEPA) [40], dimethylamine (DMA) [56], diethylamine (DEA) and triethylamine (TEA) ligands [45].…”

“…The difference is not so significant if we take the maximum adsorbed amounts in mg g À1 , but it is clearly visible if we consider the molar amounts adsorbed. The amine-treated tannin gels can then selectively adsorb Pd(II) from a Pd(II)/Pt(IV) mixture, although being also effective for Pt(IV) uptake in single metal solution [60].…”

“…The adsorption behavior of Pd(II) and Pt(IV) in modified/ unmodified tannin resins was found to be influenced by pH and total chloride ion concentration [60,96], since the speciation of both elements, chloropalladium(II) and chloroplatinum(IV) complexes, is dependent on these two parameters [60]. Higher chloride concentrations in aqueous solutions were found to decrease the adsorbed amounts of Pd(II) [60,96] and Pt(IV), which means that complexes with fewer chloride ligands are more favorable for being adsorbed or/and have lower stability in solution.…”