Comparison between ion activity method and suspension Wien effect method in determining binding energy of divalent cations to soil particles

Purpose The binding energy of ions on soils is one of the most important parameters to quantify the sorption strength of ions on soils. The aim of this study was to determine the mean free binding energies of four divalent cations Ca 2+ , Cd 2+ , Cu 2+ , and Pb 2+ in suspensions of yellow-brown soil and black soils clay particles through ion activity and Wien effect methods, respectively, and to justify which method will be positive. Materials and methods The homoionic suspensions of yellow-brown soil (YB) and two black soils (BS I , BS II ), which were saturated with divalent cations Ca 2+ , Cd 2+ , Cu 2+ , and Pb 2+ , were allowed to stand for 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 measurement and another for the ion activity measurement, and the binding energies of divalent cations on yellow-brown soil and black soils that were determined by the two methods were calculated and compared. Results and discussion The mean Gibbs free binding energies values (ΔG bi ) of Ca 2+ , Cu 2+ , and Pb 2+ on YB and BS I determined by ion activity method are obviously greater than ones by Wien effect method. The ΔG bi values (I) of Cd 2+ , Cu 2+ , and Pb 2+ to subsamples of black soil II determined by ion activity method are also larger than ones (II) by Wien effect method, the values (I) of the three cations for top black soil II were in the order Pb (14 kJ mol −1 ) > Cu (9 kJ mol −1 ) > Cd (7 kJ mol −1 ), and the values for bottom black soil II and OM-free black soil II were in the order Pb (14–16 kJ mol −1 ) > Cu (11–13 kJ mol −1 ) ≈ Cd (11–13 kJ mol −1 ). The ratios (I/II) of ΔG bi are in the range of 1.3–2.4 except the suspensions of Cd-top black soil II in which I/II value is equal to about 1. Conclusions The mean Gibbs free binding energies between divalent cations and soil particles determined by ion activity method were larger than those determined by Wien effect method. The Wien effect method supported by the results acquired from atom absorption spectrum is the better method to determine the binding energies of divalent cations to soil particles because the activities of divalent cations in suspensions determined by ion activity method were underestimated.