Environmental Behavior of Heavy Metal Chemical Speciation and Migration Regulated by Dissolved Organic Matter in Soil

Heavy metal soil pollution is still a serious environmental issue as the risk assessment relies not only on total metal concentration but also on chemical speciation and mobility. DOM impacts the metal transport via complexation, competitive adsorption, and colloid-facilitation; however, its effect varies depending on molecular structure and concentrations. The column experiments of soil for Cd, Cu, and Pb were carried out with the application of control, low- and high-concentration straw-derived DOM, and low- and high-concentration humic-rich DOM. The BCR sequential extraction, ICP-MS, UV-Vis spectroscopy, EEM-PARAFAC, and XGBoost-SHAP techniques were combined to associate DOM properties with the speciation and mobility of metals. Simulation results which fit the experimental setting demonstrated that high straw-derived DOM increased the Cd acid-soluble fraction up to 61.3% and cumulative migration up to 31.4%; however, for high humic-rich DOM, the parameters equal 39.5% and 11.6%. The R² of XGBoost models was higher than that of Random Forest and SVR models and equal to 0.93, 0.90, and 0.88 for Cd, Cu, and Pb migration, correspondingly. According to SHAP analysis, acid-soluble fraction and DOC played the dominant role for the migration of Cd; however, humic-like fluorescence and SUVA254 had a greater impact on Cu and Pb.