Characterization of H+ and Cd2+ Binding Properties of Bacterial Exopolymers

The proton and Cd binding capacities of microbially produced exopolysaccharides, EPS, were quantified by the determination of complex stability constants and the concentration of complexing sites using H+ or Cd2+ selective electrodes in dynamic titrations. The influence of ionic strength, pH and the Cd to EPS ratio was determined over large concentration ranges. The applicability of the non-ideal competitive adsorption isotherm combined with a Donnan electrostatics approach was tested with respect to the EPS. Proton and cadmium binding data were compared with literature data examining other ubiquitous environmental ligands including humic substances, alginate, bacteria, etc. Subsequent modelling of Cd speciation in aquatic (fresh and marine waters) and soil systems suggested that the exopolysaccharides would play non-negligible role under most conditions. The quantitative information provided here therefore represents an important advance in our understanding on Cd transport, bioavailabil