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

摘要

Interactions of three heavy metal ions, Cu~(2+), Cd~(2+), and Pb~(2+), and, for comparison, Na~+ with electrodialytic clay fractions (less than 2 μm 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 x 10~(-4)/z mol L~(-1), where z is the cation valence, and a nitric acid solution of 3 x 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~+ < Cd~(2+) < Cu~(2+) < Pb~(2+). 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~+ > Cu~(2+) > Cd~(2+) > Pb~(2+) for all tested soils. A characteristic parameter of the REC-field strength curves, ΔREC_(200), REC 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 ΔREC_(200) were generally in the order: Na~+ < Cu~(2+) ≤ Cd~(2+) ≤ Pb~(2+).