This article reviewed author's studies on the selective adsorption and separation of valuable metals based on ion exchange or chelate formation using chitin and chitosan. In order to develop the new adsorbents and processes for recovery and separation of precious metals, valuable or harmful metal ion, the authors have synthesized new ion exchangers and chelating resins using chitin and chitosan. Fist of all, some kinds of chitosan derivative were synthesized by chemically modifying chitosan. To protect the amino groups of chitosan from the attack of cross-linking reagents, the authors have developed a new synthesis process via the Schiff's base formation as an amino protective group. New chitosan derivatives exhibited the high selectivity for gold (III), palladium (II), and platinum (IV) over base metals in hydrochloric acid. In addition, the chitosan derivative containing pyridyl groups exhibited the high selectivity for copper (II) over iron (III) in 1 M aqueous ammonium nitrate solution. The adsorption capacities of these chitosan derivatives for palladium (II) were found to be about 2-4 times greater than those of the cross-linked original chitosan and commercial chelating resins.
Highly-porous chitosan microspheres(OWOC) were prepared by coexisting the O/W/O emulsion containing chitosan and the W/O emulsion containing high concentrations of NaCl through the difference in their osmotic pressures. In addition, cross-linking and introduction of functional moieties to OWOC were simultaneously performed using the activated OWOC with chloromethyloxirane and bifunctional reagents for recovery of metals. Thus, the 1,2-dithioethane-containing OWOC chitosan derivative (EDTSC) was synthesized to examine the selectivity for precious metals. EDTSC exhibited the high selectivity for precious metals from hydrochloric acid. The adsorption rate of palladium (II) on EDTSC was measured to elucidate its adsorption mechanism. In the adsorption of palladium (II) on EDTSC, it was found that the intraparticle-diffusion process is not the rate-determining step, but the chemical reaction of PdCl₃⁻ with EDTSC is the rate-determining step owing to through-pores in EDTSC. EDTSC can be expected as an adsorbent for a perfusion chromatography.

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The investigation looked at the separation of divalent metal ions in an aqueous solution. The adsorption materials for the metal ions were synthesized with β-diketones retained on the octadecyl-group bonded to silica gel (C18). The β-diketones such as TTA and BFA were used as reagents for separation of the metal ions. The reaction ability of these materials (TTA-, BFA-C18) was evaluated by solid-liquid distribution behavior. It was found that these metal ions were adsorbed on these materials at pH 2-6 by an ion-exchange reaction, and their selectivity series was Cu²⁺>Ni²⁺>Co²⁺≥Zn²⁺>Mn²⁺. The mutual separation ability was found to be better than that of reported inorganic ion-exchangers such as dihydrogen tetratitanate hydrate fibers. BFA-C18 had an especially high selectivity for Cu²⁺.
Then, the adsorption behavior of Cu²⁺, Cd²⁺ and Pb²⁺ in aqueous solution was examined by same method. The reaction behavior of same tendency to divalent transition metal ions was obtained. BFA-C18 has better mutual selectivity for Pb²⁺-Cd²⁺ than that of the TTA-C18. In contrast, TTA-C18 shows good separation ability between Cu²⁺-Pb²⁺. The synthesized materials have high mutual separation ability of the metal ions.

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Among fission products in spent fuels, extremely high-grade rare metals are contained. In nuclear rare metals, Mo and Tc nuclides are very useful in the field of metallic materials, chemistry, electric industry and medical treatment. Lately, much attention has been focused on the selective separation and recovery of nuclear rare metals in the high-level liquid wastes generated from the reprocessing of spent fuels. The selective separation, recovery and effective utilization of nuclear rare metals result in a conversion of wastes to valuable resources and the accomplishment of rationalization of treatment and disposal of high-level radioactive wastes. As for the separation of nuclear rare metals, ion-exchange methods are more promising for the compact column process compared to solvent extraction techniques. This special issue deals with the following items (1) Generation and characteristics of nuclear rare metals such as Mo and Tc oxoanions, (2) Separation properties of promising ion-exchangers, and (3) Advanced nuclide separation system including the recovery processes of oxoanions.

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