Role of pH and ionic strength in the aggregation of TiO nanoparticles in the presence of extracellular polymeric substances from Bacillus subtilis.

Increased use of commercial titanium dioxide nanoparticles (TiO NPs) in consumer products most likely leads to their additional environmental release. Aggregation and disaggregation processes are expected to play an important role in the fate and transport of TiO NPs in natural aquatic ecosystems. Therefore, in this work, we have studied the colloidal stability of TiO NPs in the presence of extracellular polymeric substances (EPS) from Bacillus subtilis and the adsorption behavior of EPS on TiO NPs in aqueous solutions at different pH values and ionic strengths (IS). The adsorption and aggregation processes were found to depend on the solution chemistry. The mass fraction of EPS on TiO NPs decreased with increased pH and NaCl concentrations, which was verified by Fourier transform infrared spectroscopy. The presence of EPS can substantially influence the colloidal stability of TiO NPs. In deionized water, the aggregation of NPs was induced by the addition of EPS only when the pH was below the TiO NP point of zero charge (≈6). When the pH was equal to pH, the TiO NPs would rapidly form large aggregates, but the adsorption of EPS leads to partial fragmentation via electrostatic repulsion and steric hindrance. When the pH was greater than pH, the aggregation rate was minimally affected by the increased EPS concentration. In NaCl solution, the aggregation rate of TiO NPs obviously increased with increased NaCl concentration. The critical coagulation concentration (CCC) of TiO NPs is 13.9 mM in the absence of EPS and increases to 155.6, 213.7 and 316.4 mM in the presence of 1, 5 and 10 mg/L EPS in NaCl solution, respectively, which indicates that the steric hindrance occurs after the addition of EPS. This study suggests that environmental conditions and EPS concentration greatly modify the colloidal stability of TiO nanoparticles.