(217b) Deposition of TiO2 Nanoparticles On Surfaces in Parallel Plate Chamber: Role of Natural Organic Matter

Titanium dioxide (TiO₂) is one of the most widely used nanomaterials in industry; however, TiO₂ nanomaterials have shown to be toxic to both eukaryotic and prokaryotic cells. This presents a real concern and suggests a substantial need for fate, transport and toxicity studies of this widely used nanomaterial. Natural organic matter (NOM) is ubiquitous in aquatic environments and can significantly affect the fate and transport particles. Though transport of TiO₂ nanoparticles have been studied, the fundamental mechanisms involved in interactions of TiO₂ nanoparticles with NOM and its affect on deposition of nanoparticles on surfaces are not fully understood. In this study, a microscope-based technique, the parallel plate (PP) flow cell, was utilized to visualize deposition of TiO₂ nanoparticles on glass surfaces in the presence of NOM in both mono and divalent salts and at two pH values (5 and 7). A fluorescent microscope was used for capturing images of deposited TiO₂ nanoparticles labeled with fluorescein isothiocyanate for visualization. Suwanee River Humic Acid Standard II (SRHA) was used as model NOM at a concentration of 1 mg/L TOC. Preliminary results have shown that SRHA significantly affected the surface properties of nanoparticles and subsequent aggregation and deposition on surfaces. At pH 5, TiO₂ nanoparticles showed positive zeta potential without SRHA under all conditions tested; however, SRHA leads to the reversal of surface charge. At pH 7, TiO₂ nanoparticles were negatively charged, which was enhanced with SRHA. Aggregation of nanoparticles decreased with addition of SRHA under all conditions. Ion valence also was found to significantly impact the stability of TiO₂ nanoparticles with SRHA, with greater aggregation in the presence of Ca²⁺ versus K⁺ ions. Deposition results in the PP system also demonstrated sensitivity to NOM presence, pH and ion valence. SRHA led to a substantial increase in particle stability, and the sensitivity of nanoparticle deposition in the presence of SRHA was notably greater at pH 5 than at pH 7. Decrease of nanoparticle deposition rates was more pronounced for monovalent ions than divalent ions, which indicates that ion valence has significant effects on the interactions of NOM with TiO₂ nanoparticles. The results of the extensive characterization and deposition studies will be presented, along with a description of the fundamental mechanisms involved in the interactions of NOM with TiO₂ nanoparticles.