Characterization of electrostatic binding sites of extracellular polymers by linear programming analysis of titration data

Abstract

Electrostatic binding sites of extracellular polymeric substances (EPS) were characterized from titration data using linear programming analysis. Test results for three synthetic solutions of given solutes comprising amino, carboxyl, and phenolic groups indicated that this method was able to identify the electrostatic binding sites. For the six sites with pKa between 3 and 10, the estimated pKa deviated 0.11 ± 0.09 from the theoretical values, and the estimated concentrations deviated 3.0% ± 0.9% from the actual concentrations. Two EPS samples were then extracted from a hydrogen-producing sludge (HPS) and a sulfate-reducing biofilm (SRB). Analysis of charge excess data in titration from pH 3 to 11 indicated that the EPS of HPS comprised of five electrostatic binding sites with pKa ranging from 3 to 11. The pKa values of these binding sites and the possible corresponding functional groups were pKa 4.8 (carboxyl), pKa 6.0 (carboxyl/phosphoric), pKa 7.0 (phosphoric), pKa 9.8 (amine/phenolic), and pKa 11.0 (hydroxyl). EPS of the SRB comprised five of similar binding sites (with corresponding pKa values of 4.4, 6.0, 7.4, 9.4, and 11.0), plus one extra site at pKa 8.2, which was likely corresponding to the sulfhydryl group. The total electrostatic binding site concentration of EPS extracted from HPS were 10.88 mmol/g-EPS, of which the highest concentration was from the site of pKa 11.0. The corresponding values for the EPS extracted from SRB were 16.44 mmol/g-EPS and pKa 4.4. The total concentrations of electrostatic binding sites found in this study were 20- to 30-fold of those reported for bacterial cell surface, implying that EPS might be more crucial in biosorption of metals than bacterial cell surface in wastewater treatment and in bioremediation. © 2002 Wiley Periodicals, Inc.