Influence of natural organic matter on the aggregation and deposition of titanium dioxide nanoparticles

The aggregation kinetics of TiO2 nanoparticles was studied in the absence and presence of Suwanee River humic acid (SRHA) in either NaCl or CaCl2 electrolytes. The CCC[Ca2+]/CCC[Na+] ratios were found to yield a proportionality fraction of z−7.2 (in the absence of SRHA) and z−5.6 (in the presence of SRHA), near the theoretical prediction of z−6, where z is the cation's valence. SRHA drastically increased the stability of TiO2 nanoparticles under most conditions, due to the combined effect of increased electrostatic and steric repulsions. Deposition rates of TiO2 nanoparticles onto a silica surface were quantitatively measured using a quartz crystal microbalance with dissipation (QCM -D) over a broad range of solution (pH and ionic strength, IS) conditions, and the effects of the SRHA on particle deposition behavior were evaluated. In general, zeta potential can be used to predict the interaction energies between particles or particles and surfaces, and from there an inference can be made as to the potential for aggregation and deposition. The presence of SRHA significantly hinders TiO2 deposition onto silica surfaces via steric repulsion in addition to repulsive electrostatics even under high ionic strength, which has important implications for the mobility of these nanoparticles.