Modeling the transport of TiO2 nanoparticle aggregates in saturated and unsaturated granular media: Effects of ionic strength and pH

Abstract

This study aims to explore the mechanisms governing the transport and retention kinetics of TiO2 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 TiO2 NPAs. At low ISs (i.e., 1.0 and 10 mM) and in neutral/alkaline conditions, high mobility of TiO2 NPAs was observed in both saturated and unsaturated conditions. However, the retention of TiO2 NPAs was substantially enhanced at the high IS (50 mM) and in extremely acidity condition (pH = 2.6), because of increased aggregation and straining of TiO2 NPAs during their transport course. The breakthrough curves (BTCs) of TiO2 NPAs under unsaturated and saturated conditions almost overlapped, suggesting that decreasing the water saturation did not enhance the retention of TiO2 NPAs in sand columns. This was probably due to the repulsive interactions existed between negatively charged air–water and TiO2 NPAs systems that resulted in unfavorable attachment conditions. The two-site kinetic attachment model provided a good description for the BTCs of TiO2 NPAs both in saturated and unsaturated conditions. The fitted parameters could successfully explain the transport behaviors of TiO2 NPAs under various solution chemistries.