PROTON SURFACE-CHARGE DENSITY IN SOILS WITH STRUCTURAL AND PH-DEPENDENT CHARGE

Nearly all soils contain both variable- and permanent-charge surface sites. The objective of this research was to quantify the variable-charge properties of three soils that contain both permanent structural charge (primarily on external siloxane surfaces of mica and collapsed vermiculite) and variable (pH-dependent) charge. When the three soils were suspended in 0.02 or 0.05 mol kg-1 NaCl solutions at eight equilibrium pH values between 4 and 8, the net ion adsorption was approximately two times greater at pH 8 than at pH 4 and decreased with increasing ionic strength. The point of zero net proton charge (PZNPC), which was calculated from proton titration data, net NaCl adsorption data, and previously measured values of the structural charge density, ranged from 4 to 5. When the soils were suspended in 0.05 mol kg-1 CsCl solutions at equilibrium pH values between 4 and 9, the PZNPC was st least 1 pH unit lower than for NaCl suspensions. Cesium adsorption was greater than Na adsorption at low pH and was less pH dependent; the net proton surface-charge density also was less pH dependent in CsCl suspensions. The three soils contain < 10 g kg-1 organic C, yet the proton-titration, ion-adsorption, and PZNPC data suggest that organic matter is extremely important in the surface chemistry of the three soils.