Quantification of the interactions between Ca²⁺, Hg²⁺ and extracellular polymeric substances (EPS) of sludge

The interactions between metals (Ca(2+) and Hg(2+)) and extracellular polymeric substances (EPS) extracted from the aerobic and anaerobic sludge in wastewater treatment reactors were investigated using a combination of zeta potential measurement and 3-dimensional excitation-emission matrix (EEM) fluorescence spectroscopy with parallel factor (PARAFAC) analysis. Results show that Ca(2+) had no substantial effects on the EEM fluorescence spectra of the EPS, but their zeta potentials increased with the increasing Ca(2+) dosage. However, Hg(2+) had a significant effect on the EEM fluorescence spectra of the EPS, while their zeta potentials seemed not to be affected by the dose of Hg(2+). The interactions between Hg(2+) and EPS were elucidated using the fluorescence quenching with PARAFAC analysis, while the interactions between Ca(2+) and EPS were evaluated by the zeta potential technique. The binding constants for Hg(2+) and EPS were two orders of magnitude higher than those for Ca(2+) and EPS, suggesting that the binding mechanisms between Ca(2+) and EPS were different from those between Hg(2+) and EPS. The results might be useful for understanding the roles of EPS in bacterial self-protection against heavy metals and the aggregate formation mechanisms through ionic bridging interactions.