SURFACE-CHARGE CHARACTERISTICS OF KAOLINITIC TROPICAL SOILS

Surface charge properties of four representative kaolinitic soils from the humid tropics (Brazil) were investigated by a methodology involvi ng independent measurements of net total permanent and variable charge components. Permanent structural charge was determined by Cs+ adsorpt ion, whereas variable charge was quantified by simultaneous proton tit ration and background electrolyte (LiCl) adsorption measurements. Data were obtained for homoionic soils suspended in LiCl solutions of ioni c strength 1-10 mmol L-1 and pH value 2-6. Corrections were made in th e titration data for proton consumption resulting from dissolution and aqueous-phase complexation reactions. Conjunctive use of proton titra tion and electrolyte adsorption data yielded independent assessments o f proton surface charge densities and points of zero charge. The surfa ce charge data were tested successfully for consistency with the law o f surface charge balance. Three of the soils exhibited similar surface charge behavior, with no pronounced effect of differences in either o rganic C or Fe and aluminum oxide content. One soil containing signifi cant manganese oxides showed points of zero charge well below those of the other three soils. The point of zero net charge (p.z.n.c.) for th e soils was less-than-or-equal-to 4, lower than values reported for sp ecimen kaolinite. The point of zero net proton charge (p.z.n.p.c.) inc reased with decreasing ionic strength. In all cases, the presence of s mall quantities of structural charge in 2:1 clay minerals had a signif icant effect on surface charge properties; e.g., for all soils, p.z.n. c. < p.z.n.p.c. These characteristics of surface charge were shown to be consistent with the behavior of a mixture of kaolinite, organic mat ter, and a small quantity of 2:1 clay minerals. In conformity with the law of surface charge balance, ionic strength effects were found to b e removed by plotting net adsorbed ion charge against net proton surfa ce charge density.