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SOIL CHEMISTRY

Negative Wien Effect Measurements for Exploring Polarization Processes of Cations Interacting with Negatively Charged Soil Particles

^a State Key Lab. of Soil and Sustainable Agric., Institute of Soil Science, Chinese Academy of Sciences, P.O. Box 821, Nanjing 210008, China
^b College of Resource and Environ.Science, Nanjing Agricultural University, Nanjing 210095, China
^c Institute of Soil, Water, and Environ. Sci., Agric. Research Organization, The Volcani Center, Bet Dagan 50250, Israel

^* Corresponding authors (vwsfried{at}volcani.agri.gov.il).

The Wien effect, that is, the dependence of the electrical conductivity of dilute suspensions of soil particles on electrical field strength, was explored in the intermediate field-strengths range of 0.9 x 10⁶ to 5 x 10⁶ V m^–1, at higher resolution than in previous studies. This enabled the detection of a local minimum in the electrical conductivity–field strength relationship in which the declining phase is termed the negative Wien effect. Suspensions of clay-size soil particles of three soil types, with various mono-, di-, and trivalent cations, were tested. A negative Wien effect was observed with homoionic soil particles saturated with divalent cations, with electrodialyzed soil particles, and with suspensions of black soil particles that contained organic matter. Two quantifiers of the declining and increasing slopes: (i) polarizability (counter ion polarization and re-adsorption); (ii) ion-stripping intensity—on the respective sides of the local minimum were used for characterizing the interaction between the soil particles and the counter ions. The higher mean ion-stripping intensities found for the Na⁺ suspensions—twice those for the K⁺ suspensions—reflect the easier stripping off of the Na cations and the stronger adsorption of the K cations. The mean ion-stripping intensities of all divalent cations for the three soils were lower than those determined for the monovalent ones, which reflects the tighter binding of the divalent cations. The Ca²⁺ and Zn²⁺ ions were stripped off most easily from the surfaces of soil particles, and tighter binding was found for Pb²⁺ to brown soil and black soil, and for Cd²⁺ to yellow-brown soil. In general, lower polarizabilities were correlated with higher ion-stripping intensities (e.g., for Ca²⁺ and Zn²⁺). However, the exceptions (e.g., the polarizabilities of Cu²⁺ and Pb²⁺) shed more light on the interaction between the soil particles and the cations. The proposed method of characterization is superior to other methods for characterizing soil particle–counter ion interactions because it characterizes adsorption directly and not via exchange measurement, and it is less laborious than, for example, exchange isotherm measurements.

Abbreviations: DL, double layer • EC, electrical conductivity • i.d.m., induced dipole moment • OM, organic matter