A bio-mimetic cadmium adsorbent: design, synthesis, and characterization

A cadmium-binding adsorbent has been designed based on insights gained from the study of cadmium-binding molecules isolated from living organisms. Cadmium-chelating thiolate groups-common in proteins that bind cadmium-have been covalently attached to a commercially available ion-exchange resin. The new adsorbent exhibits a favorable affinity (2 x 10(-10)M), selectivity (25-fold greater affinity for Cd(2+) than Zn(2+)), and capacity (1.4 mmol Cd(2+)/g dry resin) for cadmium ion. Adsorbed Cd(2+) may be released with 20mM pyro-phosphate at pH 2. The adsorbent also recovers Cd(2+) in the presence of NH(4)CI and KCN.The apparent adsorption rate at pH 7.0 (2M(-1) min(-1)) increases 30-fold as pH is increased to 11. The rate dependence on pH may be due to the inhibition of adsorbent-metal association by intramolecular hydrogen bonding at neutral pH.