Surface charge properties of kaolinite from Thai soils

Abstract

Kaolinite is the major clay mineral in many Thai soils and the charge characteristics of soil kaolinites have not been properly evaluated. Soil samples from sub-soil horizons of fourteen Oxisols and Ultisols from Thailand were taken for this study. Clay fractions were separated from the soils, and free iron oxides and organic matter were removed to obtain soil kaolinites. Surface charge properties of soil kaolinites and a reference kaolinite (KGa-2) were evaluated by various procedures. Permanent negative charge (−σp) was determined by cesium adsorption method. Point of zero charge (PZC) was measured by ion adsorption, potentiometric titration and electrophoretic mobility methods. X-ray diffraction analysis showed that kaolinites isolated from the Thai soils contain traces to small amounts of quartz, anatase, gibbsite, hydroxy-Al interlayered vermiculite (HIV) and illite. The value of −σp of soil kaolinites varied between 34.9 and 145.3mmolc kg−1; excluding a few anomalous values for the kaolinites with impurities of 2:1 clay minerals, soil kaolinites have an average −σp of 52mmolc kg−1. The −σp on soil kaolinites was positively correlated with the specific surface area (SSA) of the samples. All kaolinites exhibited increasing negative charge and decreasing positive charge with increasing equilibrium solution pH confirming the presence of variable charge on the mineral surface. The PZNC values of soil kaolinites varies from 2.1–3.1, except for two samples where it could not determined as the CEC values were greater than the AEC values at all pH values. Potentiometric titration curves for all kaolinites exhibited variable net proton charge (ΔH−ΔOH or σH) with changing equilibrium solution pH from 2 to 10. The PZSE values were slightly higher than the PZNC values of the soil kaolinites, and varied between 2.3 and 2.9. The zeta potential of all kaolinites became more negative with increasing solution pH (2 to 6) but not low enough to reach a positive value for all but one soil kaolinites; hence the IEP of soil kaolinites could not be obtained. The PZC of kaolinite from Thai soils are at lower pH than the natural pH of these soils, thus soil kaolinites carry a net negative charge under natural conditions and they have some limited ability to adsorb cations. Changes in soil pH from soil acidification, liming or addition of any other amendments or practices can substantially change the cation- or anion-exchange capacity of soil kaolinites, which may influence the availability of nutrient elements in the soils.