Interactions of Heavy Metal Ions with Paddy Soils as Inferred from Wien Effect Measurements in Dilute Suspensions

Interactions of three heavy metal ions, Cu2+, Cd2+, and Pb2+, and, for comparison, Na+ with electrodialytic clay fractions (less than 2μm in diameter) of four paddy soils as well as a yellow-brown soil as a control soil were evaluated based on measurements of the Wien effect in dilute suspensions with a clay concentration of 10 g kg-1 in four nitrate solutions of 2×10-4/z mol L-1, where z is the cation valence, and a nitric acid solution of 3×10-5 mol L-1.Field strengths ranging from 15 to 230 kV cm-1 were applied for measuring the electrical conductivities (ECs) of the suspensions. The mean free binding energies between the various cations and all of the soils determined from exchange equilibrium increased in the order: Na+ < Cd2+ < Cu2+ < Pb2+. In general, the ECs of the suspensions in the sodium nitrate solution were smaller than those of the suspensions in the heavy metal solutions because of the lower electrophoretic mobility of sodium compared to the divalent cations. In terms of relative electrical conductivity-field strength relationships, relative electrical conductivity (REC) of suspensions containing various cations at field strengths larger than about 50 kV cm-1 were in the descending order: Na+ > Cu2+ > Cd2+ > Pb2+ for all tested soils. A characteristic parameter of the REC-field strength curves,△REG200, REG at a field strength of 200 kV cm-1 minus that at the local minimum of the concave segment of the REC-field strength curves, characterized the strength of adsorption of the cations stripped off by the applied strong electrical field, and for all soils the values of△REC200 were generally in the order: Na+ < Cu2+≤Cd2+≤Pb2+.

Wien Effect Characterization of Interactions Between Ions and Charged Sites on Clay Surfaces of Variable-Charge Soils

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,CaCl2,CdCl2 or LaCl3).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 μS cm-1 for NaCl,KCl,CaCl2,CdCl2,and LaCl3,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 μS cm-1 for NaCl,CaCl2,and CdCl2,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 μS cm-1 for NaCl,CaCl2,CdCl2,and LaCl3,respectively.