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.