The effect of monovalent anion species on the aggregation and charging of allophane clay nanoparticles

Abstract

Allophane is a kind of clay mineral which has amorphous hollow spherical structure. To clarify the dispersion, aggregation and charging behaviour of allophane in the presence of various monovalent anions, F−, Cl−, Br−, I−, BrO3−, IO3−, and SCN−, the stability ratio and electrophoretic mobility of allophane were investigated. The stability ratio was obtained from the temporal change in the average hydrodynamic diameter of allophane measured by dynamic light scattering. The charging behaviour was evaluated from the measurement of electrophoretic mobility (EPM) of allophane. These experiments were performed as a function of electrolyte concentration at pH 5, where the net charge of allophane is positive. The experimental results demonstrated that the stability ratio decreased with increasing the salt concentration and finally it became unity independent of the salt concentration. Consequently, we observed slow aggregation regime, fast aggregation regime, and critical coagulation concentration (CCC). Therefore, the stability ratio follow the classical theory of Derjaguin, Landau, Verwey, Overbeek (DLVO) at least qualitatively. However, the CCC of allophane showed difference for each anion species; the CCC followed the order F− < IO3− < Cl− < SCN− < BrO3− < Br− < I−. We also confirmed the difference in EPM among anion species; the reduction of EPM magnitude was significant for well-hydrated ions. We presume that the CCC and EPM of allophane depend on the affinity of anions to the allophane surface characterized by the degree of hydration of each ion. That is, well-hydrated anions can adsorb to the hydrophilic surface of allophane and effectively decrease the positive charge of allophane. Besides this, the fluoride ion induced much lower CCC and charge reversal. These results are due to the strong affinity of fluoride ion for the allophane surface. Meanwhile the dependence of CCC on the zeta potential followed the DLVO prediction. Although the zeta potential of allophane is affected by anion species, the CCC behaviors can be understood by the DLVO theory through the evaluation of zeta potential of allophane. This means that the measurement of stability ratio and electrophoretic mobility accompanied with the DLVO theory are valuable tools for the understanding of the aggregation-dispersion of charge-stabilized natural clays.