Experimental Design and Analysis of the Fate of Nanoparticulate Titanium Dioxide in Aquatic Environment

The increase in the production and use of engineered nanomaterials has been considered to result in potential environmental risks and health issues. Of the commonly used nanomaterials, TiO2 has raised intensive concern due to its wide-spread application in food, drugs, cosmetics, catalysis, ultraviolet blocker, and sorbents for water treatment, etc. It is believed that TiO2 nanomaterials possess distinct transport, transfer and toxicity in the aquatic environment. This research applies the design of experiments methodology to investigate the fate of engineered TiO2 nanoparticles with various functional groups in the simulated aquatic environment. Multiple linear regression models were used to analyze the experimental fate data.;The engineered TiO2 NPs with -CH3, -SH, -OH, -COOH and -SO3H functional groups were prepared by the surface silanization, and characterized by X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR). The surface charge, aggregation and surface chemistry of engineered TiO2 NPs were investigated by dynamic (electrophoretic) light scattering.;Results obtained from the multiple linear regression model show that the pH, the ionic strength and the cation type affect the surface charge, aggregation, and adsorption capability in individual and associated forms. Moreover, the surface functional group on TiO2 NPs surface dominates the fate in the simulated aquatic environment. Especially, the zeta potential of TiO2 NPs decreased with the increase of pH value in low salt concentration, whereas SO3H-, COOH-, OH- and SH- TiO2 NPs slightly increased when pH increased. The zeta potential of TiO2 NPs also increased with the increasing salt concentration at the whole pH range and COOH-TiO2 NPs are more sensitive to the salt concentration in the zeta potential. Furthermore, the zeta potential of TiO2 NPs in solutions containing CaCl2 is higher than that in NaCl. The hydrodynamic size was little affected by the pH value or had a little decrease with the increasing pH in the low salt concentration while it significantly increased with the increasing pH value at the high salt concentration, except NH 2-TiO2 NPs. The addition of CaCl2 significantly increased the size. The NH2- and COOH-TiO2 NPs showed high sensitivities to the salt concentration in comparison with other engineered TiO2 NPs. Addition of CaCl2 leaded to the larger hydrodynamic size of TiO2 NPs compared to the addition of NaCl. The adsorption efficiency humic acid on TiO2 NPs decreased with the increase of pH value whereas increased with the increase of salt concentration regardless of the surface functional group. More adsorption of humic acid onto TiO 2 NPs in the CaCl2 solution was obtained than that in the NaCl solution. In addition, significant negative correlation between the hydrodynamic size and absolute value of zeta potential and positive correlation between the adsorption efficiency and the zeta potential value were obtained in this research.