Quantitative Insights into Phosphate-Enhanced Lead Immobilization on Goethite
Wanli Lian, Guanghui Yu, Jie Ma, Juan Xiong, Cuiyun Niu, Ran Zhang, Haijiao Xie, Liping Weng
Abstract
High Resolution Image Download MS PowerPoint Slide 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 (PO 4 ) induced Pb immobilization on metal (hydr)oxides. The results reveal that PO 4 mainly enhances bidentate-adsorbed Pb on goethite via electrostatic synergy at low PO 4 concentrations. At relatively low pH (below 5.5) and elevated PO 4 concentrations, the formation of the monodentate-O-sharing Pb-PO 4 ternary structure on goethite becomes important. Precipitation of hydropyromorphite (Pb 5 (PO 4 ) 3 OH) occurs at high pH and high concentrations of Pb and PO 4, 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-PO 4 precipitation enhanced by goethite. The CD-MUSIC model parameters for both the bidentate Pb complex and the monodentate-O-sharing Pb-PO 4 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.