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A tannin-based novel adsorbent, named "BTU-PT gel", was prepared by immobilizing bisthiourea (BTU) ligand on persimmon tannin (PT) extract. The adsorption behaviors of precious metal ions along with other coexisting base metal ions onto BTU-PT gel were studied by batch and continuous column methods. The gel exhibited remarkable selectivity for precious metal ions such as Au(III), Pd(II), and Pt(IV) over base metal ions such as Cu(II), Fe(III), Ni(II), and Zn(II) in 1−5 mol dm −3 hydrochloric acid. The adsorption of precious metal ions on the present gel was found to obey the typical monolayer type of Langmuir model, and the maximum adsorption capacity of the adsorbent was evaluated as 5.18 mol kg −1 for Au(III), 1.80 mol kg −1 for Pd(II), and 0.67 mol kg −1 for Pt(IV). Combination of ion exchange, electrostatic interaction, and coordination through the thiocarbonyl group is the mechanism of adsorption of precious metals on BTU-PT gel. In the case of Au(III) adsorption, the adsorbed species was simultaneously reduced to elemental gold by abundant polyphenolic groups of the tannin matrix. Elution by using acidothiourea solution in continuous column experiment recovered the adsorbed precious metals almost quantitatively. The results of a potential reusability test of the gel for consecutive adsorption and elution cycles by continuous column experiment indicated that the gel was stable and regenerated with undiminished metal uptake capacity up to five cycles. The real time applicability of the adsorbent for the recovery of precious metals from real industrial liquor was evaluated from actual acidic leach liquor of printed circuit boards of spent mobile phones. The gel selectively adsorbed precious metal species but exhibited negligible affinity toward base metals present in the leach liquor. The BTU-PT gel is a potential adsorbent for selective recovery of precious metals from acidic leachate of spent mobile phones containing elevated concentrations of base metals.

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A fundamental investigation on the adsorption behavior of metal ions on chitosan from aqueous ammonium nitrate and hydrochloric acid solutions was conducted by batch-wise examining the effects by various factors: the pH, concentrations of ammonium nitrate and hydrochloric acid, and initial metal concentration on the amount of metals adsorbed. In order to prevent the dissolution loss of chitosan into an acidic aqueous solution, crosslinked copper(II)-complexed chitosan was prepared and its adsorption behavior was compared with that of the original chitosan. The amount of adsorption, not only of metal ions, but also of the hydrogen ion, was decreased by crosslinking in the adsorption from an ammonium nitrate solution. The decrease, however, was least for copper(II), compared with the other divalent metal ions examined: oxovanadium(II), zinc(II), nickel(II), and cobalt(II). It was considered that this result may be attributable to the “template” effect by the copper(II) ion, which was employed during crosslinking in order to protect the active adsorption site of chitosan from an attack by a crosslinking reagent. It was found that chitosan has an excellent loading capacity and a selectivity much greater than the commercial iminodiacetic acid type of chelating resin. A qualitative discussion is given concerning the excellent adsorption behavior. Contrary to the case of adsorption from an aqueous ammonium nitrate solution, no significant decreases in the distribution ratios were observed between the crosslinked copper(II)-complexed chitosan and the original chitosan regarding the adsorption of palladium(II) and platinum(IV) from hydrochloric acid.