Interpreting the role of NO₃ ⁻, SO₄ ²⁻, 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. SO₄ ²⁻ 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.