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Distinct Effects of Humic Acid on Transport and Retention of TiO2 Rutile Nanoparticles in Saturated Sand Column
Citation:
Chen, G., X. Liu, AND C. Su. Distinct Effects of Humic Acid on Transport and Retention of TiO2 Rutile Nanoparticles in Saturated Sand Column. Presented at 86th ACS Colloid and Surface Science Symposium, June 10 - 13, 2012.
Impact/Purpose:
Presentation for the 86th ACS Colloid and Surface Science Symposium at the Johns Hopkins University in Baltimore, Maryland on June 10-13, 2012.
Description:
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.