Mechanistic study of selective adsorption of Hg2+ ion by porous alginate beads

Porous alginate beads prepared by template-assisted emulsion polymerization were investigated as selective adsorbents for Hg2+ ions from tri-metallic solutions containing the same molar concentration of Hg2+, Cd2+ and Pb2+. We observed experimentally that mechanisms of selective adsorption of Hg2+ ion by the porous alginate beads are complicated and highly dependent on solution pH. Interactions between metal cations and the porous adsorbent at different pH values were further characterized by IR and XPS analyses. Results indicated that metal uptake at low solution pH is regulated predominantly by cation exchange, making the adsorbent extremely selective to Hg2+. Nonetheless, metal binding by electrostatic attraction and complexation become important with the increase of solution pH. To validate the sorption mechanisms, density functional theory (DFT) was employed to study the binding properties of metal cations with the porous alginate adsorbent. Our computational results are completely consistent with our experimental findings, providing the theoretical basis and explanations for selective adsorption of Hg2+ by porous alginate beads.