Fine particle attachment to quartz sand in the presence of multiple interacting dissolved components

Abstract

In natural aquatic systems water chemistry is complicated and fine particles encounter multiple water components simultaneously, yet the combined effects of some multiple components on the fate and transport of these particles have not been elucidated. In this study nTiO2 and illite colloid attachment to quartz sand was investigated in 1 mM NaCl and 0.5 mM CaCl2 background solutions using a range of phosphate concentrations (0 to 10 mg/L) at pH 5 and 9. The results obtained from the batch experiments indicated that without using phosphate, nTiO2 aggregation and attachment was strongly influenced by pH and Ca2+, both of which modified nTiO2 surface charges. nTiO2 attachment was high in CaCl2 solution at pH 9 due to attractive forces between nTiO2 and sand, as well as ripening. Furthermore, phosphate adsorption to nTiO2 was higher in CaCl2 solution at pH 9 than that at pH 5 due to attractive forces between nTiO2 and phosphate anions, and also potential surface precipitation of Ca-P minerals at pH 9. Phosphate adsorption to illite was low owing to strong repulsive forces between illite and phosphate. The effect of phosphate on nTiO2 and illite attachment to sand was influenced by pH and cation valency. A decreasing trend in nTiO2 attachment with phosphate addition was observed in NaCl solution at pH 5 and 9, and in CaCl2 solution at pH 5; however, in CaCl2 solution at pH 9, the surface charge of nTiO2 reversed from negative to positive and a substantial amount of nTiO2 attached to sand. Moreover, illite attachment to sand was much lower than that of nTiO2 under all the conditions tested in this study. These findings are important for understanding of the fate and transport of nTiO2 and illite colloids in natural aquatic systems where various anions and cations co-exist.