Abstract
BackgroundThe 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.ResultsThe 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.ConclusionIn 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+.
Highlights
The coexistence of Cd2+ and Zn2+ ions in nature has a significant influence on their environmental behaviors in soils and bioavailability for plants
The combined amounts of Cd2+ and Zn2+ on the clay-sized particles ranged from 122.1 to 173.3 mmol kg−1 and conformed to the cation exchange capacity (CEC) values determined for the whole soil samples
Most of EC0 values of the suspensions containing clay-sized particles extracted from the paddy soil (Inceptisols) and yellow drab soils (Ustalfs) were the highest measured ones (Table 3)
Summary
The coexistence of Cd2+ and Zn2+ ions in nature has a significant influence on their environmental behaviors in soils and bioavailability for plants. In China, about 13% of soils included in the national assessment of soil contamination hold excessive amounts of inorganic pollutants [1] Most of these contaminants are metals and metalloids that can be found in fertilizers [2] or in several classes of hazardous residues that are improperly disposed of in the soils [3]. Such elements can pose a threat to human health because of their accumulation throughout the food chain [4,5,6,7]. Even the direct assessment of binding strengths between adsorbates and soil particles does not account for the competitive effects that take place
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