Effects of oxidation of immobilized poly(L-cysteine) on trace metal chelation and preconcentration

The on-line reversible oxidation of poly(L-cysteine) immobilized on controlled-pore glass (PLC-CPG) was demonstrated using 1% hydrogen peroxide, aerated buffer solution, and Cu(II) as oxidizing agents and dithiothreitol (DTT) as a reducing agent. Cd breakthrough curves on the oxidized forms of PLC-CPG, used as an ion exchanger, revealed that hydrogen peroxide was an effective oxidizing agent for the sulfhydryl groups and eliminated all of the strong binding sites for Cd, while oxidation by aerated water removed only 33% of the total Cd capacity after an 18-h reaction. A Cu(II) breakthrough curve on a partially reduced form of PLC-CPG showed that strong Cu binding sites were present and that the Cu(II) oxidized PLC-CPG, most likely through the binding of Cu(II) to sulfhydryl binding sites, followed by their oxidation to disulfides with the production and likely binding of Cu(I). At least 10% of the bound Cu had to be removed using KCN, which suggested Cu(I) was strongly complexed. Oxidation of PLC-CPG using Cu(II) eliminated fewer Cd binding sites than hydrogen peroxide, which may, in addition to forming disulfides, oxidize lone sulfhydryl groups to sulfenites or sulfonates. A competitive binding experiment with Cd and Cu(II) suggested Cd was able to compete with Cu(II) for binding sites on PLC-CPG and that the complexed Cd may inhibit Cu(II) oxidation of those sites. PLC-CPG could be fully reduced in 10 min from its peroxide-oxidized form with pH 8.1 DTT, while reduction with pH 4.5 DTT was much slower and allowed for intermediate (partially reduced) oxidation states. Oxidized forms of PLC-CPG were used successfully for on-line Cd, Zn, and Pb chelation and preconcentration at pH 7.0 and influent now rates of 1.0 mL/min. Oxidized PLC-CPG also retained enough strong binding sites for Cd to permit its quantitative recovery from a 400-fold excess Cu(II) matrix and average recoveries of 93+% of the Cd in alkali and alkaline earth matrixes.