Aggregation kinetics of inorganic colloids in eutrophic shallow lakes: Influence of cyanobacterial extracellular polymeric substances and electrolyte cations

The stability/aggregation propensity of inorganic colloids in eutrophic shallow lakes is of great essence in governing the water transparency and contaminant behavior. In this study, time-resolved dynamic light scattering was employed to investigate the aggregation kinetics of Al2O3 inorganic Colloids over a wide range of cyanobacterial extracellular polymeric substance (EPS) concentrations in the absence and presence of electrolyte cations. The results showed that EPS adsorption alone greatly decreased the hydrodynamic diameters of colloidal particles, whose stability behavior followed closely the predictions of the classical DLVO theory. Electrolyte cations, however, can induce the aggregation of colloidal particles, and divalent Ca2+ were found to be more efficient in destabilizing the colloids than monovalent Na+, as indicated by the considerably lower critical coagulation concentrations (2.5 mM for Ca2+ vs. 170 mM for Na+). Further addition of Ca2+, i.e., >2.5 mM, caused an extremely high aggregation degree and rate. High resolution transmission electron microscopy revealed that this enhanced aggregation should be attributed to the gel-like bridging between colloidal particles, which were verified to be the amorphous EPS-Ca2+ complexes. Field-emission scanning electron microscopy coupled with elemental mapping provided additional evidence that the bridging interaction of EPS with Ca2+ was the predominant mechanism for the aggregation enhancement. (C) 2016 Elsevier Ltd. All rights reserved.