Influence of aggregation on nanoscale titanium dioxide (nTiO2) deposition to quartz sand

Although extensive research has been conducted to investigate nTiO2 aggregation and deposition, effects of aggregation on concurrent/subsequent deposition of nTiO2, which has important implications to the fate and transport of nTiO2 in groundwater, has received only limited attention. The objective of this study was to investigate how pH, dissolved organic matter (DOM), and valence of background solution cation influence aggregation and concurrent/subsequent deposition of nTiO2. Experiments were performed to examine nTiO2 aggregation and deposition onto quartz sand with co-present illite, kaolinite, and montmorillonite colloids under various geochemical conditions. Results showed that nTiO2 formed hetero-aggregates (i.e., nTiO2-clay aggregates) at low pH when nTiO2 and clay colloids carried opposite charges, and the hetero-aggregates may either deposit or remain suspended depending on their interactions with quartz sand and Fe/Al oxyhydroxide coatings. Deposition of nTiO2 and/or nTiO2-clay aggregates occurred as a result of electrostatic attraction, secondary minimum, and potentially Mg2+ bridging. Humic acid prevented nTiO2 aggregation and deposition under most conditions. In MgCl2 solutions, however, it facilitated deposition by adsorbing to nTiO2 and Fe/Al oxyhydroxides, thereby enabling Mg2+ bridging. This study demonstrated the important and complex roles of pH, DOM, cation valence, and clay colloids in controlling aggregation and subsequent deposition of nTiO2.