Quantitative Insights into Phosphate-Enhanced Lead Immobilization on Goethite

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 K_sp value of -82.02. The adjustment of log K_sp 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.