Wien effect of Cd/Zn on soil clay fraction and their interaction

BACKGROUND The coexistence of Cd2+ and Zn2+ ions in nature has a significant influence on their environmental behaviors in soils and bioavailability for plants. While many studies have been done on the mutual toxicity of Cd2+ and Zn2+, few studies can be found in the literature focused on the interaction of Cd2+ and Zn2+ on soil clay fractions especially in terms of energy relationship. RESULTS The binding energies of Cd2+ on boggy soil (Histosols) particles and Zn2+ on yellow brown soil (Haplic Luvisols) particles were the highest, while those of Cd2+ and Zn2+ on paddy soil (Inceptisols) particles were the lowest. These results indicated that Cd2+ and Zn2+ have a strong capacity to adsorb in the solid phase at the soil-water interface of boggy soil and yellow brown soil, respectively. However, both Cd2+ and Zn2+ adsorbed on paddy soil particles easily release into the solution of the soil suspension. Unlike the binding energy, the higher adsorption energies of ions in boggy and yellow brown soils showed a weak binding force of ions in boggy soil and yellow brown soil. A 1:1 ratio of Cd2+ to Zn2+ promotes the mutual inhibition of their retentions. Cd2+ and Zn2+ have high mobility and bioavailability in paddy soil and yellow drab soil (Ustalfs), whereas they have high potential mobility and bioavailability in boggy soil and yellow brown soil. CONCLUSION In the combined system, Zn2+ had preferential adsorption than Cd2+ on soil clay fractions. Boggy soil and yellow brown soil have a low environmental risk with lower mobility and bioavailability of Cd2+ and Zn2+ while paddy soil and yellow drab soil present a substantial environmental risk. In the combined system, Cd2+ and Zn2+ restrain each other, resulting in the weaker binding force between ions and soil particles at a 1:1 ratio of Cd2+-Zn2+.