Understanding the mechanism of metal adsorption on polysaccharides possesing suparamolecular structure and molecular design of separation materials

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Biosorption, the metal-ion adsorption process by biomass, represents a potentially cost-effective way of eliminating toxic heavy metals. As the ability of microorgamisms to remove metal ions in aqueous solution has been extensively studied, fungi have been recognized as a promising class of low-cost adsorbents for metal ions. Adsorption performance of lanthane, copper, and cobalt by Rhizopus oligosporus (Rh.oligo.) fermented in liquid media was investigated. Among the Rh.oligo. species, Rh.oligo. P12 demonstrated the highest metal-adsorption capacity of 3.2 mmol/g-mycelia, which was comparable to the conventional ion-exchange beads for wastewater treatments. From the adsorption isotherm, the Freundlich curve, and the Scatchard plot analysis, the several kinds of functional groups in Rh.oligo. P12 were found to contribute to the metal adsorption. The slope analysis of the pH dependence showed that the ion-exchange is not a dominant factor of the interactions between the mycelia and metals such as copper, lanthane and cobalt. These results indicated that the polysaccharides in cell wall were involved in the metal adsorption. We carried out the adsorption tests with various kinds of biomass containing polysaccharides in order to elucidate the relationship between the supramolecular structure of polysaccharides and the metal adsorption ability. For example, we have utilized the cellulose containing the plural hydroxyl groups as the antimony (III) adsorbent. Amount of antimony (III) adsorbed to the cellulose was 5.2mg/g at maximum. Two adjacent hydroxyl groups of two polyol moieties are in the glucose unit of cellulose, cellulose could capture antimony (III) with free hydroxyl groups, that is, non-hydrogen-bonding-forming hydroxyl groups.