SURFACE-CHARGE PROPERTIES OF KAOLINITE

The surface charge components of 2 Georgia kaolinites of differing deg rees of crystallinity (KGa-1 and KGa-2) were determined using procedur es based on charge balance concepts. Permanent structural charge densi ty (sigma(0)) was determined by measuring the surface excess of Cs, wh ich is highly selective to permanent charge sites. The values of sigma (0) determined were -6.3 +/- 0.1 and -13.6 +/- 0.5 mmol kg-L for kaoli nites KGa-1 and KGa-2, respectively. The net proton surface charge den sity (sigma(0)) was determined as a function of pH by potentiometric t itration in 0.01 mol dm(-3) LiCl. Correction from apparent to absolute values of sigma(H) was made by accounting for Al release during disso lution, background ion adsorption and charge balance. Lithium and Cl a dsorption accounted for the remainder of the surface charge components . Changes in surface charge properties with time were measured after m ixing times of 1, 3 and 15 h, the latter representing ''equilibrium''. Time-dependent behavior is believed to be caused by mineral dissoluti on followed by readsorption or precipitation of Al on the mineral surf ace. Both the point of zero net charge (p.z.n.c.) and the point of zer o net proton charge (p.z.n.p.c.) changed with mixing time, generally i ncreasing. The ''equilibrium'' p.z.n.c. values were approximately 3.6 for KGa-1 and 3.5 for KGa-2, whereas the corresponding p.z.n.p.c. valu es 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 most other values reported for specimen kaolinite.