Quantitative understanding of the impact of flooding durations on Cd variations in an acidic paddy soil during the flooding and drainage processes

The solubility and transformation of cadmium (Cd) are controlled by paddy soil pH, whereas paddy soil pH varies during flooding and drainage and is significantly controlled by flooding durations. However, there is still a lack of modeling approaches for simulating the impact of flooding durations on pH fluctuation and concomitant Cd effectiveness in Cd-polluted acidic paddy soils. Herein, laboratory findings combined with a process-based numerical modeling method were used to quantify the observed key geochemical processes of Fe/C/N/S and the accompanying Cd partitioning dynamics during flooding and drainage. During flooding stage, the number of protons consumed by Fe(III), NO₃⁻, and SO₄²⁻ reduction increased with flooding durations, resulting in an increase of pH, which enhanced Cd immobilization and reduced Cd potential risk. After entering drainage stage, the number of protons released from Fe(II), NH₄⁺, and S²⁻ oxidation increased with flooding time, leading to pH decrease, which increased Cd release. A process-based kinetic model fitting results showed that Fe(III) reduction and Fe(II) oxidation were key processes that increased and decreased pH, respectively, and that increasing flooding durations were not beneficial to Cd immobilization during flooding and drainage processes. The results of this study shed light on the impact of redox state of paddy soil on Cd dynamics under different flooding durations and provide a theoretical approach to quantify the contribution of key processes controlling changes in metal species, which can be used to simulate the dynamic behavior of heavy metals in paddy soil under different natural conditions by coupling other significant processes.