Contribution of extracellular polymeric substances fractions to the adsorption of silver nanoparticles by activated sludge

Abstract

The extracellular polymeric substances (EPS) from activated sludge played significant roles in the removal of nanoparticles from wastewater. A series of batch experiments were carried out to determine the adsorption mechanism of three nano-Ag by activated sludge, as well as the contributions of EPS fractions including dissolved EPS (DEPS), loosely bound EPS (LB-EPS) and tightly bound EPS (TB-EPS). The results demonstrated that the adsorption of nano-Ag by sludge biomass agreed with pseudo-second-order kinetic reaction model and Freundlich isotherm model. About 26.0-41.2% of nano-Ag was trapped by the bound EPS (BEPS) matrix of activated sludge (especially LB-EPS) and 42.5-52.6% of them was adsorbed onto the inner cells after the adsorption. Moreover, the interaction energy contributions of EPS fractions followed the order of EDE > 0 > ETB > ELB, suggesting DEPS in wastewater went against the removal of nano-Ag due to steric repulsion while LB-EPS and TB-EPS were positive to nano-Ag adsorption by modifying biomass surface and providing extensive binding sites. Besides, EPS fractions played significant roles in the adsorption of nano-Ag with low initial concentrations but had limited effect at high concentrations. Overall, this study investigated the effect of EPS fractions on the adsorption behaviors of nano-Ag by activated sludge biomass, which is meaningful to understand the removal mechanism of nanoparticles in sewage and the potential role of EPS fractions.