Structural charge and cesium retention in a chronosequence of tephritic soils

Mineral transformation during the course of soil development is probably coupled to time-dependent trends in surface-charge and selective adsorption behavior, but the relationships are not well documented. The density of permanent (structural) charge is subject to change as a result of accretion or depletion of 2:1 layer-type silicates. The objectives of this study were to measure changes in (i) Cs⁺ selectivity and (ii) structural charge as affected by pedogenic mineral transformations in an age sequence of Hawaiian soils. The Cs⁺ → Li⁺ exchange experiments were conducted on soils collected from six sites [basaltic parent material deposited 0.3, 20, 150, 400, 1400 and 4100 thousands of years (ky) ago]. Identical exchange experiments were performed with kaolinite, montmorillonite, and illite for comparison. Selectivity for Cs⁺ on soils and clays increased with adsorbed mole fraction of Cs⁺. Cesium-accessible structural charge of the surface soils increased initially with soil age from 20 mmol_c kg^-1 at the 0.3 ky site to 113 mmol_c kg^-1 at the 400-ky site. Increased weathering beyond 400 ky reduced structural charge to 21 mmol_c kg^-1 for the oldest site. Parallel results were observed for subsurface soils with maximum structural charge of 138 mmol_c kg^-1 measured for the 400 ky site. The magnitude of Cs retention in the soils is correlated with the presence of 2:1 layer-type silicates detected by x-ray diffraction (XRD) after removal of poorly crystalline constituents. The results indicate a modest accumulation of secondary 2:1 layer-type silicates (with larger accumulations of poorly crystalline clays), followed by their subsequent decline, during the course of soil weathering.