Crosslinked N-(2-pyridylmethyl)chitosan, N-(2-thienylmethyl)chitosan, and N-[3-(methylthio)propyl]chitosan were newly synthesized. The amino group of the active adsorption site of chitosan was protected by Schiff’s base formation prior to being crosslinked by 2-(chloromethyl)oxirane. The final chitosan derivatives were obtained by reducing the imine moiety of the Schiff’s bases with sodium borohydride. The adsorption behavior of metal ions on the chitosan derivatives was examined from a solution containing hydrochloric acid using a batchwise method. These chitosan derivatives exhibited high selectivities and excellent loading capacities for gold(III), palladium(II), and platinum(IV) over base metals in hydrochloric acid. In order to elucidate the adsorption species of palladium(II) on the chitosan derivatives, the effect of the chloride ion concentration on the distribution ratio in the adsorption of palladium(II) was examined. The adsorption capacities of these chitosan derivatives for palladium(II) were found to be about 2.5—3 times greater than that of the cross-linked original chitosan. The desorption of palladium(II) was performed to an extent of 70% by a single batchwise treatment with aqueous thiourea solution.

Crosslinked N-(2-pyridylmethyl)chitosan was newly synthesized by crosslinking N-(2-pyridylmethylidene)chitosan of the intermediate compound before reducing it so that the adsorptive activity is not lowered by the crosslinking. The pyridylmethylchitosan selectively adsorbed palladium(II), platinum(IV), and mercury(II) over base metals from hydrochloric acid.

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2-Thienylchitosan and 3-(methylthio)propylchitosan were newly synthesized by crosslinking the corresponding Schiff base derivative before reducing it so that the adsorptive activity was not lowered by the crosslinking. These chitosan derivatives selectively adsorbed gold(III), palladium(II) and platinum(IV) over base metals from hydrochloric acid.

N-(2-Hydroxybenzyl)chitosan (=HR) of an insoluble chitosan derivative was synthesized by reducing N (2-hydroxybenzylidene)chitosan obtained from chitosan and 2-hydroxybenzaldehyde.We examined its adsorption selectivities for various metal ions in terms of the pH dependence of the distribution ratio of metal ions at 303 K. The chelating ability of N-(2-hydroxybenzyl)chitosan for various metal ions was measured by a batchwise operation from 1 mol dm-3 of ammonium nitrate solution. The separations of gallium and iron(III) from zinc, and nickel from cobalt were possible because of the high selectivity of the present derivative. Since N-(2-hydroxybenzyl)chitosan showed the high adsorption abilities for silver(I) and Copper(II), the adsorption equilibria of both silver(I) and copper(II) were investigated in order to elucidate the adsorbed species by examining the concentration dependencies of hydrogen ion and ammonium nitrate. Silver(I) was adsorbed according to this adsorption reaction, accompanied by an ammonia molecule: AgNH3++ HR RAgNH3 + H+ while, copper(II) was adsorbed according to the following reactions:Cue++2HR ;=± i i +2H+ and CuNH32++2HR R2CuNH3 +2Ht