Date of Award

5-2009

Level of Access Assigned by Author

Campus-Only Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemistry

Advisor

Carl Tripp

Second Committee Member

Alice Bruce

Third Committee Member

Mitchell Bruce

Abstract

Meso-dimercaptosuccinic acid (DMSA) covalently attached to silica gel via amide bond linkages (DMSA-[silica]) was evaluated as a chelate for Hg(II), Cd(II), and Pb(II). When DMSA-[silica] was individually stirred with aqueous solutions containing stiochiometric amounts of the three metal ions at room temperature for 2 hrs all three metals are separately chelated by DMSA-[silica]; 95% of Hg(II) is chelated, 81% of Cd(II) and 74% of Pb(II). When equal molar concentrations of the three metals are allowed to react simultaneously with DMSA-[silica] for two hrs, Hg(II) is preferentially bound (99%) compared to Cd(II) (13%) or Pb(II) (0.4%). Attachment of DMSA to silica via an amide bond linkage reduces the number of carboxylate sites, thus increasing the thiol to carboxylic acid ratio over free DMSA. The enhancement in selectivity towards mercury with DMSA-[silica] compared to free DMSA in solution may be in part a consequence of creating a more thiophilic rich material and/or the close proximity of the thiolate binding sites. Cd(II) and Pb(II) have lower thiophilicity compared to Hg(II) and thus prefer chelation by carboxylate over thiolate moities. In order to gain additional insight into mercury selectivity with DMSA-[silica], we have investigated a series of ligands related to DMSA, these are: monomercaptosuccinic acid; MMSA, 2-mercapto-4-methyl-5-thiazoleacetic acid; MCT, ortho-thiosalicylic acid; o-TSA and para-thiosalicylic acid; p-TSA. The MMSA chelate is structurally similar to DMSA except that it has only one thiol group. The chelates o-TSA and p-TSA each have one thiol and one carboxylic acid group. MCT includes neutral S and N atoms in close proximity to the thiolate binding site. MCT, o-TSA and p-TSA resemble each other in having equal numbers of carboxylic acid and thiol groups and formation of amide bonds with the linker on silica is expected to eliminate the carboxylate binding sites, making thiolates the only binding sites for Hg(II), Cd(II) and Pb(II) metals ions in MCT-[silica], o-TSA-[silica] and p-TSA-[silica], MMSA has only a single thiolate binding site in contrast to the two in DMSA. Each of MMSA-[silica], MCT-[silica], o-TSA-[silica] and p-TSA-[silica], show a higher preference for Hg(II) over Cd(II) and Pb(II) compared to the same free chelates in solution, respectively. In addition, there are differences in the level of metal ion chelation for each functionalized silica. These differences in degree of metal chelation for each functionalized silica surface are explained by the difference in thiolate/carboxylate ratio upon attachment to the surface and on steric reasons based on the orientation of the thiol groups on the surface, When attached via amide bond formation, the thiolate site in o-TSA-[silica] will be facing towards the silica surface, while for p-TSA-[silica], the thiolate site is expected to be pointed outwards away from the silica surface. In comparing MMSA-[silica] to DMSA-[silica], the thiolate/carboxylate ratio decreases from 2/1 in DMSA-[silica] to 1/1 in MMSA-[silica] (assuming attachment via one amide bond in each case). This effect of increasing the ratio of thiolate to carboxylate upon attachment to the surface is believed to play a role in the selectivity enhancement towards Hg(II) over Cd(II) and Pb(II). The use of functionalized silicas for the detection of Hg(II) was also examined. Specifically, Hg(II)-catalyzed transformation of immobilized 3-aminorhodanine into the oxadiazole-ketene zwitterionic form on a silica coated chip was employed as an IR spectroscopic-based method for the detection of Hg(II) in aqueous environments. The immobilized 3-aminorhodanine substrate reacts in the presence of Hg(II) and IR spectral changes were detected in the presence of as low as as 10 nanomoles Hg(II). Similar changes in the IR signal were not observed with aqueous solutions containing 25 micromoles of Cd(II), Pb(II), Mg(II), Ca(II) and Na(I).

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