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

Although extensive research has been conducted to investigate nTiO aggregation and deposition, effects of aggregation on concurrent/subsequent deposition of nTiO, which has important implications to the fate and transport of nTiO 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 nTiO. Experiments were performed to examine nTiO aggregation and deposition onto quartz sand with co-present illite, kaolinite, and montmorillonite colloids under various geochemical conditions. Results showed that nTiO formed hetero-aggregates (i.e., nTiO-clay aggregates) at low pH when nTiO 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 nTiO and/or nTiO-clay aggregates occurred as a result of electrostatic attraction, secondary minimum, and potentially Mg bridging. Humic acid prevented nTiO aggregation and deposition under most conditions. In MgCl solutions, however, it facilitated deposition by adsorbing to nTiO and Fe/Al oxyhydroxides, thereby enabling Mg bridging. This study demonstrated the important and complex roles of pH, DOM, cation valence, and clay colloids in controlling aggregation and subsequent deposition of nTiO.