Interface mechanism of Cd bioavailability by porewater prediction in paddy field system

Abstract

Cadmium (Cd) contamination poses a significant threat to human health. Predicting the risk of Cd in rice grains is challenging due to the heterogeneity and complexity of bioavailable Cd in paddy soils. We proposed that porewater during the grain-filling period can effectively predict Cd bioavailability in rice (R² > 0.5, p < 0.05). The prediction mechanism was elucidated through soil-porewater interface characterization analysis and DFT calculations. Key factors determining Cd bioavailability included Cd²⁺ and SO₄²⁻ concentration, pH, and ORP of porewater, with pH showing the highest correlation. As porewater pH increased from 5 to 9, typical mineral surfaces gradually deprotonated and formed complexation bonds {SOCd⁺} instead of {SOH} or {SOH₂⁺}. Additionally, the complexation energy between montmorillonite and Cd was 1.57–1.78 eV higher than that between goethite and Cd, while the protonation activation energy barrier on montmorillonite was 1.33–1.52 eV lower than on goethite. Therefore, {H⁺} concentration in porewater had the potential to quantify {SOCd⁺} content and binding capacity, aiding Cd bioavailability prediction. This study outlines the mechanism of predicting heavy metal health risks in rice grains through porewater and offers a potential regulation approach for agricultural product safety.