Quantitative Insights into Phosphate-Enhanced Lead Immobilization on Goethite

Abstract

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.

This study provides comprehensive insights into mechanisms of lead immobilization on iron (hydr)oxides induced by phosphate at a molecular level.