Facilitated transport of titanium dioxide nanoparticles by humic substances in saturated porous media under acidic conditions

The transport behavior of titanium dioxide nanoparticles (TiO₂ NPs, 30 nm in diameter) was studied in well-defined porous media composed of clean quartz sand over a range of solution chemistry under acidic conditions. Transport of TiO₂ NPs was dramatically enhanced by humic substances (HS) at acidic pH (4.0, 5.0 and 6.0), even at a low HS concentration of 0.5 mg L⁻¹. Facilitated transport of TiO₂ NPs was likely attributable to the increased stability of TiO₂ NPs and repulsive interaction between TiO₂ NPs and quartz sands due to the adsorbed HS. The mobility of TiO₂ NPs was also increased with increasing pH from 4.0 to 6.0. Although transport of TiO₂ NPs was insensitive to low ionic strength, it was significantly inhibited by high concentrations of NaCl and CaCl₂. In addition, calculated Derjaguin–Landau–Verwey–Overbeek (DLVO) interaction energy indicated that high energy barriers were responsible for the high mobility of TiO₂ NPs, while the secondary energy minimum could play an important role in the retention of TiO₂ NPs at 100 mmol L⁻¹ NaCl. Straining and gravitational settlement of larger TiO₂ NPs aggregates at 1 mg L⁻¹ HS, pH 5.0, and 2 mmol L⁻¹ CaCl₂ could be responsible for the significant retention even in the presence of high energy barriers. Moreover, more favorable interaction between approaching TiO₂ NPs and TiO₂ NPs that had been already deposited on the collector resulted in a ripening-shape breakthrough curve at 2 mmol L⁻¹ CaCl₂. Overall, a combination of mechanisms including DLVO-type force, straining, and physical filtration was involved in the retention of TiO₂ NPs over the range of solution chemistry examined in this study.