Transport Unsaturated column Two-site kinetic attachment model a b s t r a c tThis study aims to explore the mechanisms governing the transport and retention kinetics of TiO 2 nanoparticle aggregates (NPAs) in flow-through columns of packed sand, particularly under unsaturated conditions. The study was carried out at different pHs (2.6, 7.1, and 9.6) and ionic strengths (ISs) (1.0, 10, and 50 mM). A two-site kinetic attachment model was used to describe transport behaviors of TiO 2 NPAs. At low ISs (i.e., 1.0 and 10 mM) and in neutral/alkaline conditions, high mobility of TiO 2 NPAs was observed in both saturated and unsaturated conditions. However, the retention of TiO 2 NPAs was substantially enhanced at the high IS (50 mM) and in extremely acidity condition (pH ¼ 2.6), because of increased aggregation and straining of TiO 2 NPAs during their transport course. The breakthrough curves (BTCs) of TiO 2 NPAs under unsaturated and saturated conditions almost overlapped, suggesting that decreasing the water saturation did not enhance the retention of TiO 2 NPAs in sand columns. This was probably due to the repulsive interactions existed between negatively charged airewater and TiO 2 NPAs systems that resulted in unfavorable attachment conditions. The two-site kinetic attachment model provided a good description for the BTCs of TiO 2 NPAs both in saturated and unsaturated conditions. The fitted parameters could successfully explain the transport behaviors of TiO 2 NPAs under various solution chemistries. ª
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