Facet-mediated interaction between humic acid and TiO2 nanoparticles: implications for aggregation and stability kinetics in aquatic environments

Abstract

Interfacial interactions between TiO₂ nanocrystals and the surrounding environmental media play a critical role in dictating their environmental behaviors. Here two specific-faceted TiO₂ crystals, {101} facet and {001} facet, were adopted to investigate facet-mediated aggregation kinetics in aquatic environments. In pristine electrolyte solution (NaCl and CaCl₂), {001} TiO₂ exhibited higher stability than {101} TiO₂ due to the abundant –OH groups on the {001} facet surface. The presence of Suwannee river humic acid (SRHA) significantly improved the stability of TiO₂ nanocrystals in a facet dependent manner, in which {101} TiO₂ exhibited a more remarkable improvement in stability than {001} TiO₂. At a high SRHA concentration (10 mg L⁻¹), both faceted TiO₂ displayed comparable stability. The adsorption experiments and attenuated total reflectance Fourier transformation infrared spectroscopy demonstrated that {101} TiO₂ induced more pronounced adsorption of SRHA molecules than {001} TiO₂, probably driven by the efficient bidentate coordination of –COO– groups in SRHA on the {101} facet. This facet-affected colloidal stability behavior would deepen our understanding on nanomaterial transport and exposure in natural waters.