Studies toward the development of an electronically switchable ion exchange system
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Early studies toward the development of a chemical-free means of reclaiming metals from an ion exchange system were begun by exploring the option of controlling metal-ligand interactions electrochemically. Poly-L-cysteine (PLC), a soft acid cationic metal chelator, was explored as an option for an electroactive ligand for use as an electronically switchable ion exchange system. Initial studies were conducted by immobilizing PLC onto a glassy carbon electrode creating a modified electrode. The modified electrode was characterized using electrochemical techniques, atomic spectroscopy and molecular spectroscopy. Initial studies were conducted with poly-L-cysteine immobilized onto controlled pore glass and packed in a microcolumn for low pressure liquid chromatography experiments. Flow parameters, as well as the ability of poly-L-cysteine to act as a preconcentration agent, were established. Electrochemical methods, specifically cyclic voltammetry, were used to confirm polymer attachment and to estimate that near monolayer coverage on the glassy carbon electrode surface existed. Further studies examined the kinetics of electron transfer through the use of large amplitude potential steps to demonstrate consistency with other types of modified electrodes. The pKa of the polymer was determined to be between 7.5 and 8 by using surface charge density as a function of pH. The poly-L-cysteine modified electrode, as well as an electrode modified with cysteine monomers, was evaluated for its metal binding capabilities. Using Cd2+ as a target metal, optimal binding conditions were established. Binding experiments were repeated with Co2+, Cu2+, Ni2+ and Pb2+, as well as competitive binding studies comprised of binding in the presence of all five metals. Quantitative release of bound metals was achieved by repeatedly pulsing the potential from open circuit to a number of anodic potentials. Raman microscopy was used to probe the redox states of PLC, confirm the complexation of Cd2+ to the thiol groups of PLC and to establish an average formal reduction potential for PLC of -0.3 V vs. SHE. The possibility of multiple formal potentials was explored through the use of mathematical models based on the Nernst equation. The models suggest that many formal potentials do exist although the exact nature of the distribution could not be determined. As a plausible electronically switchable ion exchange option for heavy metal oxyanions, pyrrole was electropolymerized and investigated as a ligand for toxic chromate remediation. Polypyrrole, which bears a structural similarity to poly-L- histidine, a previously studied chromate chelator, was shown to have potential to both remediate and speciate Cr(VI) from Cr3+.