Experimental and statistical analysis of surface charge, aggregation and adsorption behaviors of surface-functionalized titanium dioxide nanoparticles in aquatic system

Abstract

One hundred and fifty nanometers sized anatase titanium dioxide nanoparticles (TiO2 NPs) have been functionalized with the –CH3, –NH2, –SH, –OH, –COOH, and –SO3H terminal groups. Surface charge, aggregation, and adsorption behaviors of the functionalized NPs in aquatic phase have been investigated by a set of experiments following the full factorial design. The dependence of surface charge, suspension size, and surface adsorption upon the various factors (including surface chemistry of NPs, the pH value, and ionic strength of an aqueous solution) has been studied with the statistical methods such as multiple linear regressions and multiple comparison tests. The surface functional group on the TiO2 NPs affects the characteristics in the simulated aquatic environment. The correlations among the characteristics of NPs have also been investigated by obtaining Pearson’s correlation coefficient. The hydrodynamic size is negatively correlated with the absolute value of zeta potential, and positively correlated with the ionic strength. In the NaCl solution, the charge screening effect is responsible for the aggregation. In the CaCl2 solution, the charge screening effect is dominant mechanism for aggregation at a low salt concentration. In contrast, the interaction between Ca2+ ions and the specific functional group plays a significant role at a high salt concentration. The adsorption efficiency of humic acid decreases with an increase in the pH value, whereas increases with an increase in the ionic strength. The adsorption efficiency is positively correlated with the zeta potential. The statistical analysis methods and the results have implications in assessment of potential environmental risks posed by engineered nanoparticles.