Wien effect characterization of interactions between ions and charged sites on clay surfaces of variable-charge soils.

Abstract To investigate the interactions of oppositely charged sites on the surfaces of variable-charge soil particles with cations and anions, and to evaluate the mean Gibbs free binding and adsorption energies of various cations on particles of red soil and latosol, clay fractions smaller than 2 μm were separated from samples of the two variable-charge soils. Ferric oxides were removed from part of the clay fractions, which were then saturated with various chlorides (NaCl, KCl, CaCl 2 , CdCl 2 or LaCl 3 ). Electrical conductivities (EC) of dilute suspensions of the original and of the iron oxides-free clay fractions in deionized water were measured with the SHP-2 short high-voltage pulse apparatus, which enables measurement of the Wien effect at field strengths ( E ) from 14 to 250 kV cm −1 . The Wien effect (EC- E ) curves revealed EC increases of red soil suspensions between 14 and 200 kV cm −1 , of 8.3, 8.4, 12.1, 5.9, and 1.2 μScm −1 for NaCl, KCl, CaCl 2 , CdCl 2 , and LaCl 3 , respectively, reflecting the differing interactions with the various cations and chloride. The EC increments with the iron-free red soil suspensions were higher, being 29.7, 17.2, and 15.3 μScm −1 for NaCl, CaCl 2 , and CdCl 2 , respectively. In the natural latosol suspensions the EC increments were practically zero, whereas in the iron-free fractions there were significant EC increments of 10.3, 5.7, 5.0, and 1.6 μScm −1 for NaCl, CaCl 2 , CdCl 2 , and LaCl 3 , respectively.