In situ growth of metal hexacyanoferrate crystals on alginate beads for effective removal of radioactive cesium (137Cs) from seawater

Abstract

Metal hexacyanoferrates (Me-HCFs) are highly effective materials for removing radioactive Cs (137Cs) in seawater. For their practical application, granulation of Me-HCFs is necessary. However, conventional granulation techniques using a polymer as a binding material adversely affect the adsorption performance of Me-HCFs by blocking their adsorption sites. In the present work, to minimize the interference of the binding material, Me-HCF crystals were synthesized on an alginate bead surface in situ for the treatment of 137Cs+-contaminated seawater. Three different Me-HCFs immobilized on alginate beads (Fe-HCF, Cu-HCF, and Co-HCF) were prepared via a simple and rapid method involving a reaction between ionophorically crosslinked alginate beads (Fe3+, Cu2+, and Co2+ ions) and HCF solution. Among them, Fe-HCF crystals were successfully grown on the alginate bead surface; the other crystals were hardly synthesized despite a longer reaction time. The Fe-HCF crystals composed 15.0 % (w/w) of the beads and were located on the surface of the beads rather than being embedded in the alginate matrix. Because the Fe-HCF crystals were located at the surface of the alginate beads, Cs+ adsorption onto the Fe-HCF crystals showed fast reaction kinetics (achieving equilibrium in ∼ 3 h) and a high Cs+ adsorption capacity (∼33.5 mg/g). Also, a feasibility test was carried out for the removal of radioactive 137Cs+ in real seawater medium. Finally, a continuous Cs+ removal test was conducted in a packed column of the Fe-HCF beads. It was able to treat 65 bed volume of 50 mg/L of Cs+ solution with a flow rate of 1 mL/min.