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.