Interpreting the role of NO3-, SO42-, and extracellular polymeric substances on aggregation kinetics of CeO2 nanoparticles: Measurement and modeling

The early stage of aggregation of cerium oxide nanoparticles (CeO2 NPs) in anion solutions was inspected in the absence and presence of extracellular polymeric substance (EPS) with a help of time-resolved dynamic light scattering (DLS). The aggregation kinetics and attachment efficiencies were calculated according to measured hydrodynamic diameter across a range of 1-500 mM NaNO3 and 0.01-100. mM Na2SO4. The aggregation of CeO2 NPs in both NaNO3 and Na2SO4 solution conformed with the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. In NaNO3 solution, the critical coagulation concentrations (CCC) of CeO2 NPs was calculated to be about 47 mM; in Na2SO4 solution, CeO2 NPs showed a re-stabilization process and thus there was no CCC value. SO42- had intenser effects on CeO2 NPs aggregation than NO3- might because of the distinction between their polarization, consisting in Hofmeister series. The presence of bound EPS (B-EPS), tightly bound EPS (TB-EPS) and loosely bound EPS (LB-EPS) in NaNO3 solutions all lead to significant decrease in CeO2 NPs aggregation. Steric repulsive force produced by absorbed EPS on CeO2 NPs might take main responsibility in stabilizing CeO2 NPs. Besides, Extended Derjaguin-Landau-Verwey-Overbeek (EDLVO) model successfully predicted the energy barrier between CeO2 NPs with B-EPS, TB-EPS and LB-EPS as a function of NaNO3 concentration. Furthermore, the difference in impeding the CeO2 NPs aggregation with B-EPS, TB-EPS and LB-EPS may be caused by the divergence in molecular weight and component mass fraction especially protein content. These results might subserve the assessment on the fate and transport behaviors of CeO2 NPs released in wastewater treatment plants.