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Surface Charge Properties of Kaolinite

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Abstract

The surface charge components of two Georgia kaolinites of differing degree of crystallinity (KGa-1 and KGa-2) were measured using procedures based on charge balance concepts. Permanent structural charge density (σ0) was determined by measuring the surface excess of Cs, which is highly selective to permanent charge sites. The values of σ0 determined were −6.3 ± 0.1 and −13.6 ± 0.5 mmol kg−1 for kaolinites KGa-1 and KGa-2, respectively. The net proton surface charge density (σH) was determined as a function of pH by Potentiometric titration in 0.01 mol dm−3 LiCl. Correction from apparent to absolute values of σH was made by accounting for Al release during dissolution, background ion adsorption, and charge balance. Lithium and Cl adsorption accounted for the remainder of the surface charge components. Changes in surface charge properties with time were measured after mixing times of 1, 3, and 15 hours (the latter representing “equilibrium”). Time-dependent behavior is believed to be caused by mineral dissolution followed by readsorption or precipitation of Al on the mineral surface. Both the point of zero net charge (p.z.n.c.) and the point of zero net proton charge (p.z.n.p.c.) changed with mixing time, generally increasing. The “equilibrium” p.z.n.c. values were approximately 3.6 and 3.3 for KGa-1 and KGa-2, respectively, while the corresponding p.z.n.p.c. values were about 5.0 and 5.4. The p.z.n.c. results were in good agreement with previous studies, but the values of p.z.n.p.c. were higher than other values reported for specimen kaolinite.

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Acknowledgments

The research reported in this paper was supported in part by a faculty research grant from the College of Natural Resources, University of California at Berkeley, and in part by USDA-CSRS Regional Research Project W-82, “Pesticides and Other Toxic Organics in Soil and Their Potential for Ground and Surface Water Contamination.” The first author expresses gratitude for a predoctoral Fellowship from the Kearney Foundation of Soil Science. Thanks to Terri DeLuca for her assistance in preparation of the typescript.

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Correspondence to B. K. Schroth.

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Schroth, B.K., Sposito, G. Surface Charge Properties of Kaolinite. MRS Online Proceedings Library 432, 87–92 (1996). https://doi.org/10.1557/PROC-432-87

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