Insight into the roles of soluble, loosely bound and tightly bound extracellular polymeric substances produced by Enterobacter sp. in the Cd2+ and Pb2+ biosorption process: Characterization and mechanism

Extracellular polymeric substances (EPSs) are macromolecular polymers formed by metabolic secretion, and they have great potential for removing heavy metal (HM) ions from the aquatic phase. In this study, the contributions of soluble EPSs (S-EPSs), loosely bound EPSs (LB-EPSs) and tightly bound EPSs (TB-EPSs) secreted by Enterobacter sp. to Cd2+ and Pb2+ adsorption were analyzed. The results indicated that in a solution containing both Cd2+ and Pb2+, pH= 6.0 was best suited for the adsorption process, and adsorption equilibrium was reached in approximately 120 min. Moreover, the mechanism for adsorption of Cd2+ and Pb2+ by the different layers of EPSs involved spontaneous chemical processes. However, Cd2+ adsorption by the three layers of the EPSs was an exothermic process (∆H0 <0), but Pb2+ adsorption by the three layers of the EPSs was an endothermic process (∆H0 >0). The variations in zeta potentials indicated that ion exchange occurred during Cd2+ and Pb2+ adsorption. FT-IR, XPS and 3D-EEM analyses indicated that the functional groups of the EPSs involved in adsorption were mainly the CO, C-O and C-O-C groups of the polysaccharides; furthermore, fulvic acid-like substances, humic-like substances and tyrosine-like proteins played important roles in the adsorption of Cd2+ and Pb2+ by the different EPS layers.