Influence of extracellular polymeric substances on the aggregation kinetics of TiO₂ nanoparticles

The early stage of aggregation of titanium oxide (TiO₂) nanoparticles was investigated in the presence of extracellular polymeric substance (EPS) constituents and common monovalent and divalent electrolytes through time-resolved dynamic light scattering (DLS). The hydrodynamic diameter was measured and the subsequent aggregation kinetics and attachment efficiencies were calculated across a range of 1-500 mM NaCl and 0.05-40 mM CaCl₂ solutions. TiO₂ particles were significantly aggregated in the tested range of monovalent and divalent electrolyte concentrations. The aggregation behavior of TiO₂ particles in electrolyte solutions was in excellent agreement with the predictions based on Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. Divalent electrolytes were more efficient in destabilizing TiO₂ particles, as indicated by the considerably lower critical coagulation concentrations (CCC) (1.3 mM CaCl₂ vs 11 mM NaCl). The addition of EPS to the NaCl and low concentration CaCl₂ (0.05-10 mM) solutions resulted in a dramatic decrease in the aggregation rate and an increase in the CCC values. For solutions of 11 mM NaCl (the CCC values of TiO₂ in the absence of EPS) and above, the resulting attachment efficiency was less than one, suggesting that the adsorbed EPS on the TiO₂ nanoparticles led to steric repulsion, which effectively stabilized the nanoparticle suspension. At high CaCl₂ concentrations (10-40 mM), however, the presence of EPS increased the aggregation rate. This is attributed to the aggregation of the dissolved extracellular polymeric macromolecules via intermolecular bridging, which in turn linked the TiO₂ nanoparticles and aggregates together, resulting in enhanced aggregate growth. These results have important implications for assessing the fate and transport of TiO₂ nanomaterials released in aquatic environments.