Wien Effect Measurements in Soil Colloidal Suspensions: A Novel Method for Characterizing the Interactions between Charged Particles and Counter Ions

Abstract

Adsorption is one of the most important chemical processes at the interface between soil particles and water. It determines the quantity of plant nutrients and pollutants which are retained on the surfaces of soil particles, and therefore, is one of the primary processes that affect transport of nutrients and contaminants in soils. The Wien effect, i.e., the dependence of the electrical conductivity of soil suspensions on electrical field strength, was proposed as the basis of a new method to characterize energy relationships between cations and soil particles. The results showed that the mean Gibbs free binding energies of the heavy metal ions with yellow-brown, black and brown soil particles decreased in the order of Pb2+>Zn2+>Cu2+>Cd2+, Pb2+>Cu2+>Zn2+>Cd2+ and Pb2+ >Cd2+>Cu2+>Zn2+, respectively, where the range of binding energies for yellow-brown soil (7.16∼8.54 kJ⋅mol−1) was less than that for black soil (9.05∼9.88 kJ⋅mol−1). The electrical field-dependent mean Gibbs free adsorption energies of these heavy metal ions for yellow-brown, black and brown soils descended in the order of Cu2+>Cd2+>Pb2+>Zn2+, Cu2+>Zn2+>Pb2+>Cd2+, and Cu2+>Pb2+>Cd2+>Zn2+, respectively. The mean Gibbs free adsorption energies of Cu2+, Zn2+, Cd2+ and Pb2+ at a field strength of 150 kV⋅cm−, for example, were in the range of 1.23 to 2.15 kJ⋅mol−1 for the three soils.