SORPTION MECHANISM OF MERCURIC IONS BY KERATEINE GELS OF THIOL-TYPE

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Sorption behavior of Hg²⁺ by the kerateine gel of thiol-type, which was prepared from wool keratin, was investigated. The behavior was discussed in relation to (i) the amphoteric nature, (ii) the chemical modification of side-chain polar groups, and (iii) the heat denaturation of kerateine gels. It was found that the apparent isoelectric point of the kerateine gel was closely related to the pH at which the Hg²⁺ uptake showed the minimum. The Hg²⁺ uptakes by modified kerateine gels in acid media depended on their modification history. The results indicated that in acid media, the dominant binding modes of Hg²⁺ consist in chemosorption by the thiol as well as amino groups in the kerateine gel and in chelation of Hg²⁺ through these groups. On the other hand, in neutral and alkaline media, very high Hg²⁺ uptakes were observed for all types of kerateine gels independent of their modification history. Such high Hg²⁺ uptakes could be explained in terms of the cooperative interactions between electron negative atoms incorporated in the kerateine gel, namely, sulphur, nitrogen, and oxygen rather than the ionization effect of the acidic groups present. The swelling volume of the heat-denatured kerateine gels decreased considerably with increasing the treatment temperature, and a good relation was found between the rate of Hg²⁺ uptake and swelling volume of the gel: the rate of Hg²⁺ uptake decreased with decrease in the swelling volume.