Influence of Clay Particles on the Transport and Retention of Titanium Dioxide Nanoparticles in Quartz Sand

This study investigated the influence of two representative suspended clay particles, bentonite and kaolinite, on the transport of titanium dioxide nanoparticles (<i>n</i>TiO₂) in saturated quartz sand in both NaCl (1 and 10 mM ionic strength) and CaCl₂ solutions (0.1 and 1 mM ionic strength) at pH 7. The breakthrough curves of <i>n</i>TiO₂ with bentonite or kaolinite were higher than those without the presence of clay particles in NaCl solutions, indicating that both types of clay particles increased <i>n</i>TiO₂ transport in NaCl solutions. Moreover, the enhancement of <i>n</i>TiO₂ transport was more significant when bentonite was present in <i>n</i>TiO₂ suspensions relative to kaolinite. Similar to NaCl solutions, in CaCl₂ solutions, the breakthrough curves of <i>n</i>TiO₂ with bentonite were also higher than those without clay particles, while the breakthrough curves of <i>n</i>TiO₂ with kaolinite were lower than those without clay particles. Clearly, in CaCl₂ solutions, the presence of bentonite in suspensions increased <i>n</i>TiO₂ transport, whereas, kaolinite decreased <i>n</i>TiO₂ transport in quartz sand. The attachment of <i>n</i>TiO₂ onto clay particles (both bentonite and kaolinite) were observed under all experimental conditions. The increased transport of <i>n</i>TiO₂ in most experimental conditions (except for kaolinite in CaCl₂ solutions) was attributed mainly to the clay-facilitated <i>n</i>TiO₂ transport. The straining of larger <i>n</i>TiO₂-kaolinite clusters yet contributed to the decreased transport (enhanced retention) of <i>n</i>TiO₂ in divalent CaCl₂ solutions when kaolinite particles were copresent in suspensions