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

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, MeHCF 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.