Binding energies of monovalent anions with Fe/Al oxides based on ion activity and suspension Wien effect methods

Purpose Adsorption is one of the most important chemical processes at the interface between soil particles and water. It determines the quantities of plant nutrients and pollutants which can be retained on the surfaces of soil particles, and therefore, it 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, has recently been proposed as the basis of a novel method to characterize energy relationships between cations and soil particles, but little attention has been paid to the binding energies of anions with soil particles. The aim of this study was to examine the Wien effect of three anions F⁻, Cl⁻, and NO ₃ ⁻ in Fe/Al oxide suspensions and to investigate their binding energies with ion activity and Wien effect methods for comparison. Materials and methods The homoionic suspensions of hematite and γ-Al₂O₃, which were saturated with three sodium salts, NaF, NaCl, and NaNO₃, were allowed to stand for about 7 to 10 days and to achieve sufficient equilibration of ion reactions in the suspensions. The same sample was used to prepare two suspensions, one for the Wien effect measure and another for the ion activity measure, and the binding energies of anions on hematite and γ-Al₂O₃ with the two methods were calculated and compared. Results and discussion The mean free binding energies of F⁻ to Fe/Al oxides determined via the two methods possessed a good agreement, and the relative errors of the two methods were 3.88% and 8.33% for hematite and γ-Al₂O₃, respectively. The binding energies calculated from Cl⁻ ion activity (α) were less than or near those calculated from the weak-field electrical conductivity (EC₀) for the two oxides, and the relative errors of the two methods were 17.0% and 9.17% for hematite and γ-Al₂O₃, respectively. The binding energy of NO ₃ ⁻ to hematite calculated from α was much more than that calculated from EC₀, and the relative error was as high as 41.2%, which was due to the interference of H⁺ ions to nitrate ion-selective electrode (ISE) in the tested suspension of hematite. The binding energies of NO ₃ ⁻ on γ-Al₂O₃ calculated from α and EC₀ were almost the same, with relative error between the two methods being only 3.27%. Conclusions The mean Gibbs free binding energies between anions and minerals yielded by ion activity and Wien effect method were very identical. However, the