Characterization of metal ion-exchange on modified surfaces of porous carbon Original Research Article

Two biologically based ligands, cysteine (Cys) and poly-l-cysteine (PLCys), were coupled with Carbopack-X, a commercially available porous carbon surface after acid activation, through simple cross-linking techniques. The resulting Cys-X and PLCys-X systems are effective for metal binding with capacities in the range of 1–25 μmol metal/g column, depending on the metal tested. Both columns demonstrate similar selectivity for the soft and moderately soft acid metals such as Cd2+, Cu+, and Pb2+, probably as a result of the soft basic character of the thiol ligand side chains, and exhibit minimal binding of the harder groups IA and IIA metals. Even though PLCys-X and Cys-X displayed similar strong and weak binding characteristics, the PLCys-X system contained stronger binding sites than those possessed by Cys-X. The strongest site displayed by Cys-X has a binding constant of 1×109, with the ability to compete for metal with moderately strong ligands such as ethylenediamine. However, the strongest PLCys-X site possessed a binding constant of about 1013, with the ability to compete for metals bound by very strong ligands such as EDTA. Both systems gave quantitative release of bound metals upon exposure to acid. Exposing the thiol-rich ligands to chemical reducing or oxidizing environments altered the metal binding characteristics of both columns, with at least a 50% decrease in metal binding on the oxidized columns due to thiol binding sites because of disulfide bond formation.