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

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.