Browsing by Author "Davis, T. A."
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- Cd(II) speciation in alginate gelsPublication . Davis, T. A.; Kalis, E. J. J.; Pinheiro, J. P.; Town, R. M.; Van Leeuwen, H. P.Polysaccharides, such as those occurring in cell walls and biofilms, play an important role in metal speciation in natural aqueous systems. This work describes the speciation of Cd(II) in alginate gels chosen as a model system for biogels. The gels are formed by bridging calcium ions at junction zones present along adjacent homopolymeric guluronic acid chain sequences. The free Cd2+ concentration in the gel phase is measured by a novel in situ microelectrode voltammetric technique that monitors the electroactive probe cation Cd2+ by its reduction at a Au-amalgam microelectrode. In situ voltammetric measurement, coupled with total Ca(II) and Cd(II) determinations, as well as potentiometric titration, permits the full reconstruction of the charging environment and the cation binding for the gel phase. Three independent combinations of measuring and modeling the charged gel layer thereby permit accurate prediction of the Donnan potential, ΨD, and the Donnan enrichment coefficient, ΠD. At an ionic strength of 10 mM, Donnan potentials in the gel ranged from approximately −10 to −20 mV, corresponding to an enhancement in the level of free Cd2+ ions in the gel phase relative to the bulk solution by a factor of approximately 3. In contrast, the total level of Cd(II) was found to be enhanced by a factor of approximately 60, resulting predominantly from the specific binding of the Cd by the uronic acids of the alginate gel. These results emphasize that large differences in Cd(II) speciation can arise due to the combination of specific and electrostatic modes of binding. The results of this speciation analysis, for charged biological gels, have important consequences for mechanistic interpretation of metal biouptake processes involved in complex media.
- Measurement of Donnan potentials in gels by in situ microelectrode voltammetryPublication . Davis, T. A.; Yezek, L. P.; Pinheiro, J. P.; Van Leeuwen, H. P.This work describes the electrochemical methodology for the determination of the Donnan potential from diffusion-limited steady-state voltammograms of acrylamide gels. The technique is based upon the measurement of gel–sol systems that have reached Donnan equilibrium and contain Cd2+ as a probe ion. Au-amalgam microelectrodes are used to measure the Cd concentration in the gel phase relative to the solution phase, thus permitting comparison of the Cd voltammograms obtained in both phases. This approach yields two independent measures of the Donnan potential resulting from (i) the potential shift relative to the reference electrode, and (ii) the enhancement of the Cd2+ wave. Two suites of acrylamide gels containing 0.2% and 0.5% Na-acrylate were studied as a function of ionic strength by varying [NaNO3] and maintaining a constant concentration of the electroactive probe ion, [Cd2+] = 1 · 10 5 mol/L in the equilibrating solutions. Independent model predictions of the Donnan potential as a function of ionic strength that consider the effects of differential swelling on the charge density, the influence of a mixed electrolyte on the potential developed in the gel at the limit of low ionic strength and the effects of incomplete dissociation of the carboxylic functional groups were in agreement with the Donnan potentials independently measured by the twofold steady-state voltammetric approach.
- Stability of lead(II) complexes of alginate oligomersPublication . Davis, T. A.; Pinheiro, J. P.; Grasdalen, H.; Smidsrød, O.; Van Leeuwen, H. P.The current work reports on the Pb(II) complexes formed with oligomeric uronic acids (carboxylated saccharide residues) found polymerized in the cell walls and envelopes of algae and bacteria alike. The application of partial acid hydrolysis, sizeexclusion chromatography (SEC), 1H NMR, and scanned deposition stripping chronopotentiometry (SSCP) has permitted the determination of stability constants for Pb(II) with both mannuronic (M) and guluronic (G) acid oligomers ranging from the dimer to the pentamer. The determined logarithm of the stability constants range between 4.11 ( 0.05 and 5.00 ( 0.04 mol-1 ·dm3 for the eight oligomers studied (pH 6; I ) 0.1 mol ·dm-3). Additional experiments under the same experimental conditions employing galacturonic and glucuronic acid oligomers yielded slightly lower values (2.19 ( 0.10 to 4.02 ( 0.07 mol-1 ·dm3) that were expected based on their structure, whereby the monomers which were not included in the alginate oligomer series (unavailable by SEC), yielded the lowest stability constants. Thisworkdemonstrates the applicability of the SSCP technique for the determination of stability constants for metal–ligand complexes in which the ligands display relatively low molecular mass. Previous studies on heavy metal interaction with the matrix polysaccharide alginate have largely been restricted to the whole polymer that forms a gel upon binding to network bridging ions such as calcium. The results will be discussed in this context with the emphasis being placed on the relevance of these findings to processes occurring at the biointerface and results from the relevant literature.