Despite extensive study, geochemical modeling often fails to accurately predict lead (Pb) immobilization in environmental samples. This study employs the Charge Distribution MUlti-SIte Complexation (CD-MUSIC) model, X-ray absorption fine structure (XAFS), and density functional theory (DFT) to investigate mechanisms of phosphate (PO4) induced Pb immobilization on metal (hydr)oxides. The results reveal that PO4 mainly enhances bidentate-adsorbed Pb on goethite via electrostatic synergy at low PO4 concentrations. At relatively low pH (below 5.5) and elevated PO4 concentrations, the formation of the monodentate-O-sharing Pb-PO4 ternary structure on goethite becomes important. Precipitation of hydropyromorphite (Pb5(PO4)3OH) occurs at high pH and high concentrations of Pb and PO4, with an optimized log Ksp value of −82.02. The adjustment of log Ksp compared to that in the bulk solution allows for quantification of the overall Pb-PO4 precipitation enhanced by goethite. The CD-MUSIC model parameters for both the bidentate Pb complex and the monodentate-O-sharing Pb-PO4 ternary complex were optimized. The modeling results and parameters are further validated and specified with XAFS analysis and DFT calculations. This study provides quantitative molecular-level insights into the contributions of electrostatic enhancement, ternary complexation, and precipitation to phosphate-induced Pb immobilization on oxides, which will be helpful in resolving controversies regarding Pb distribution in environmental samples.
Kaiyue Song,Chengjun Zhang,Jun Shan,Wenting Wang,Huiji Liu,Mengchang He
Simulating the adsorption behavior of Sb(V) on goethite is of great significance for predicting the mobility of Sb(V) in soil. However, there is still a lack of charge distribution and multisite surface complexation (CD-MUSIC) models that conform to the actual adsorption mechanism. Therefore, our research combined the previous EXAFS results with the SCMs and established two CD-MUSIC models (Model I, which introduced bidentate binuclear and bidentate mononuclear complexes, and Model II, which introduced bidentate mononuclear and monodentate mononuclear complexes). It is also one of the rare cases that bidentate edge- and corner-sharing adsorption complexes have been distinguished by introducing different amounts of H + . The results showed that Model I was more suitable for predicting the adsorption behavior of Sb(V) on the goethite surface than Model II. However, at low pH and high Sb(V) loading, the presence of the monodentate complex ≡FeOHSb(OH) 5 0.5- was possible due to the fast and slow two-step mechanism. Using the optimized CD-MUSIC model parameters, the contribution of surface species to the equilibrium adsorption under different conditions was determined. In the adsorption edge and adsorption isotherm simulation process, the edge-sharing bidentate mononuclear complex ≡(FeO) 2 H 2 Sb(OH) 4 was always the most important product, which was consistent with the results of extended X-ray absorption fine structure (EXAFS) analysis. The applicability of the model parameters was verified by predicting the competitive adsorption between PO 4 3− and Sb(V). The CD-MUSIC model established in the present study could be a useful tool for evaluating the equilibrium distribution behavior and environmental risk of Sb(V) in different types of soils.
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