The aggregation and charging of natural clay allophane: Critical coagulation ionic strength in the presence of multivalent counter-ions

Abstract

Allophane is an amorphous aluminosilicate clay mineral which is typically found in young volcanic ash soils. The aggregation-dispersion and charging behaviors of allophane may affect the usability and protection of the volcanic ash soils and surrounding environment. To understand the aggregation-dispersion and charging of natural allophane, dynamic and electrophoretic light scattering experiments were carried out at pH 5 as a function of potassium salt concentration and counter-ion (Cl - , SO 4 2- , Fe(CN) 6 3- , and Fe(CN) 6 4- ) valence. The temporal increase of particle hydrodynamic diameters and electrophoretic mobilities (EPM) were measured, and the stability ratio, critical coagulation concentration (CCC), critical coagulation ionic strength (CCIS), zeta potential, and surface charge density were further obtained. The experimental results of stability ratio and EPM of allophane particles were affected by the counter-ion valence, and the CCCs strongly depended on the counter-ion valence. The behavior was well described by the well-known empirical Schulze-Hardy rule. The allophane particles showed a low surface charge density in the presence of multivalent counter-ions. Thus, Derjaguin-Landau and Verwey-Overbeek (DLVO) theory with the Debye-Huckel (DH) approximation was applied to consider the CCIS. The experimental relationship between CCIS and surface charge density was well explained by DLVO theory by taking account of particle size. We conclude that, in the presence of multivalent counter-ions, the DLVO theory with DH approximation has a good applicability to the aggregation-dispersion of natural clay allophane particles. • Counter-ion valence affects the aggregation and charging of allophane particles. • Tetravalent counter-ion induces charge reversal and re-stabilization of allophane. • Critical coagulation ionic strength vs. surface chare follows DLVO theory. • DLVO theory has a good applicability to the aggregation and dispersion of allophane.