Solubility of trace metals in two contaminated paddy soils exposed to alternating flooding and drainage

- Dynamics in Eh, pH, and DOM greatly affect the solubility of trace metals.
- Metal sulfides largely control trace metal solubility under anaerobic conditions.
- Speciation modeling for anaerobic soil environments should include sulfide minerals.
- Stoichiometry of copper sulfides can affect solubility of other trace metals.

Uptake of trace metals by crops is determined by the solubility of trace metals. In paddy soils, flooding and drainage influence redox chemistry and consequently trace metal solubility and thus uptake by rice plants. Current knowledge on how the dynamics in redox chemistry affect the solubility of trace metals in contaminated paddy soils is still limited. The objectives of our study were to investigate (i) the effects of flooding and drainage on trace metal solubility in paddy soils and (ii) to what extent a multi-surface modeling approach can predict trace metal solubility under changing redox conditions. We performed a column experiment with two contaminated paddy soils with similar soil properties but contrasting pH. During two successive flooding and drainage cycles, dynamics in Eh, pH and dissolved organic matter concentrations greatly affected trace metal solubility for both soils. Multi-surface model predictions indicate that under aerobic conditions, the higher pH of the alkaline soil leads to a stronger complexation of trace metals by reactive surfaces of the soil and, consequently, to lower dissolved concentrations than in the acidic soil. Under anaerobic conditions, predictions shows that sulfide precipitates control trace metal solubility in both soils, but still the higher pH of the alkaline soil leads to lower trace metal concentrations in soil solution at equilibrium. Furthermore, model calculations showed that stoichiometry and solubility of copper sulfide minerals can substantially affect solubility of other trace metals especially when trace element concentrations exceed soil sulfate concentrations. This stoichiometry and solubility should be considered when predicting the solubility of trace metals under anaerobic conditions.