Research Article
INTRODUCTIONCadmium occurs together with zinc in the majority of zinc ores. Consequently, serious environmental pollutions by cadmium such as Itai-itai disease have been brought by unsuitable control of mining wastes of zinc ores and mining effluents [1] . Although the demand for cadmium has been decreased year by year due to the adverse effect caused by its toxicity [2] , cadmium compounds have been still employed for versatile applications such as pigments, batteries and solar panels. Therefore, the effective and selective removal of cadmium from spent materials has been strongly required.Adsorptive removal of toxic metals including cadmium using various adsorbents has been reported as review articles [3][4][5][6][7] and general articles [8,9] . Chitosan is a polysaccharide rich in primary amino groups produced by alkaline hydrolysis of shells of crustacean. It can be easily chemically modified via these amino groups. This modified chitosan has been investigated as adsorption gels for some metal ions as well as original chitosan [10][11][12][13][14][15][16][17][18] . Adsorptive removal of Cd(II) ion using gels of original unmodified chitosan has been also reported, in which primary amino groups were reported to be effective for adsorption of Cd(II) ion [19][20][21][22] . Muzzarelli et al. [23,24] reported the chemical modification of α-chitosan produced from krill (Euphausia superba) with functional groups of dithiocarbamate, which is abbreviated as DTC hereafter, and also briefly reported its adsorption behavior for some metal ions including Cd(II). In the previous paper [25] , we also prepared lyophilic chitosan containing DTC functional groups for a solvent extraction reagent and found its high selectivity to Cu(II) and Ni(II). The chitosan compounds chemically modified with DTC functional groups are abbreviated as DTC-chitosan, hereafter. In the present paper, the detailed adsorption behavior of DTCchitosan for Cd(II) was investigated from viewpoint of its separation from Zn(II) ion, in particular.According to the Pearson's HSAB theory [26] , Cd(II) is classified as a typical soft Lewis acid. Although nitrogen atoms of the primary amino group are classified as a slightly soft Lewis base exhibiting some affinity to some kinds of metal ions classified as soft Lewis acids, the affinity to Cd(II) ion is expected to be much more increased in case some functional groups containing sulfur atoms, classified as a strong soft Lewis base, such as the DTC group are immobilized onto polymer matrices of chitosan by means ABSTRACT Adsorption gels were prepared by means of chemical modification of β-chitosan by immobilizing functional groups of dithiocarbamate to investigate its adsorption behavior for cadmium(II) such as effect of cross-linking, adsorption rate, pH dependency, loading capacity by batch-wise experimental work and mutual adsorptive separation of cadmium(II) from zinc(II) using a packed column. Thus prepared adsorption gel exhibited high selectivity towards cadmium(II) over zinc(II) and h...
