Interactions between uronic acids and chromium(III).
Laboratory ion-exchange experiments were performed to investigate the complexation behavior of Cr(III) with uronic acids, such as galacturonic, glucuronic, and alginic acid (main constituents of bacterial exopolymeric substances). The experimental data were analyzed with a chemical equilibrium model in FITEQL to determine the reaction stoichiometries and stability constants for the formation of Cr-ligand complexes. Analysis of ion-exchange data with a chemical model indicates that the accurate description of Cr(III) complexation with both glucuronic and galacturonic acids requires postulation of a mixture of 1:1/1:2 complexes between Cr(III) and ligands under the experimental conditions studied (e.g., pH 4), but that the Cr-alginic acid binding can be modeled based on a reaction stoichiometry of 1:1 Cr-alginic acid complexes. Because of the complex nature of alginic acid, a nonelectrostatic, discrete ligand approach was used to determine proton and Cr binding with the functional groups of alginic acid. In this approach, alginic acid was conceptualized as being composed of a suite of two monoprotic acids (HL1 and HL2) with arbitrarily assigned pKa values of two and four, respectively. The results indicate that Cr binding with uronic acids mainly occurs through carboxylic groups under acidic to slightly alkaline pH conditions (e.g., pH < 8). The overall results of the present study indicate that the formation of such Cr-ligand complexes may have a pronounced effect on Cr(III) transport, solubility and bioavailability in natural systems.