Abstract
BACKGROUND: The coexistence of Cd(2+) and Zn(2+) 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 Cd(2+) and Zn(2+), few studies can be found in the literature focused on the interaction of Cd(2+) and Zn(2+) on soil clay fractions especially in terms of energy relationship. RESULTS: The binding energies of Cd(2+) on boggy soil (Histosols) particles and Zn(2+) on yellow brown soil (Haplic Luvisols) particles were the highest, while those of Cd(2+) and Zn(2+) on paddy soil (Inceptisols) particles were the lowest. These results indicated that Cd(2+) and Zn(2+) 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 Cd(2+) and Zn(2+) 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 Cd(2+) to Zn(2+) promotes the mutual inhibition of their retentions. Cd(2+) and Zn(2+) 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, Zn(2+) had preferential adsorption than Cd(2+) on soil clay fractions. Boggy soil and yellow brown soil have a low environmental risk with lower mobility and bioavailability of Cd(2+) and Zn(2+) while paddy soil and yellow drab soil present a substantial environmental risk. In the combined system, Cd(2+) and Zn(2+) restrain each other, resulting in the weaker binding force between ions and soil particles at a 1:1 ratio of Cd(2+)-Zn(2+).