Changes in Zero Point of Charge (ZPC), Specific Surface Area (SSA), and Cation Exchange Capacity (CEC) of kaolinite and montmorillonite, and strongly weathered soils caused by Fe and Al coatings

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Abstract
In order to analyze the effects of Al and Fe coatings on mineral grains, montmorillonite (Mt), kaolinite (Kt), and deferrated strongly weathered soils (T6B and T7B) were artificially coated with 2, 6, 10% of Fe or Al hydroxides. Changes in the values of the zero point of charge (ZPC), specific surface area (SSA), and cation exchange capacity (CEC) were examined for these samples. Furthermore, observations by X-ray diffraction (XRD) and scanning electron microsocpy (SEM) were made. The effect of sesquioxide coatings on the ZPC, SSA, and CEC values was found to be entirely dependent on the material coated and the type of coating material. In the case of Mt, Al coatings caused a decrease in the inter-lamellar spaces and CEC, and a shift of ZPC to a higher pH value, whereas Fe coatings caused no significant change except for the development of variable charges sufficient to induce a ZPC. In the case of Kt and the deferrated soil samples, Fe coatings brought about a shift of the ZPC to a higher pH value, an increase in the specific surface area (SSA) and a decrease in CEC, whereas Al coatings caused similar changes except for SSA. The shift of ZPC to a higher pH value was caused by the high value of the ZPC of Fe and Al hydroxides and/or blocking of negative charges by Al hydroxides. Decrease in CEC was caused by the addition of positive charges of hydroxides reflecting the higher ZPC value and, furthermore, through charge neutralization in the inter-lamellar spaces by the positive charges of Al hydroxides. The SSA was increased by the addition of particles of Fe hydroxides, but remained almost constant when Al hydroxides covered the whole mineral surfaces. In particular, Al precipitates occupied the inter-lamellar spaces of Mt and caused a decrease in SSA. Fe oxides accumulated in the strongly weathered soils were considered to increase the ZPC value and enlarge the SSA, but the increase in SSA did not always result in an increase in the CEC. In contrast, the CEC was reduced through the ZPC shift toward a higher pH value.