Distinct Effects of Humic Acid on Transport and Retention of TiO₂ Rutile Nanoparticles in Saturated Sand Columns

The distinct effects of humic acid (HA, 0–10 mg L^–1) on the transport of titanium dioxide (rutile) nanoparticles (nTiO₂) through saturated sand columns were observed under conditions of environmental relevance (ionic strength 3–200 mM NaCl, pH 5.7 and 9.0). Specifically, the transport of nTiO₂ was dramatically enhanced in the presence of HA at pH 5.7, even at a low HA concentration of 1 mg L^–1. The mobility of nTiO₂ was further increased with greater concentrations of HA. In contrast, this enhancement of the nTiO₂ transportability due to the presence of HA was limited at pH 9.0 because of the negligible adsorption of HA onto nTiO₂, regardless of the concentrations of HA examined in this study. The distinct effects can be explained by the adsorption behaviors of HA to nTiO₂ and sand surfaces and the resulting interactions between nTiO₂ and sand surfaces under different conditions, which resulted in a large variation of the nTiO₂ transport and deposition behaviors at various conditions. In addition, theoretical interaction energy calculations and additional elution experiments indicate that the secondary energy minimum played an important role in controlling the nTiO₂ transport and deposition in porous media observed in this study. Moreover, the interaction energy calculations suggest that at pH 5.7, HA affected nTiO₂ transport by increasing the negative surface charge of nTiO₂ at low HA adsorption densities; whereas, combinations of increased electrostatic and steric interactions due to the presence of HA were the main mechanisms of enhanced transportability of nTiO₂ at high HA adsorption densities. Overall, results from this study suggest that natural organic matter and solution pH are likely key factors that govern the stability and mobility of nTiO₂ in the natural aquatic environment