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 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.

Carbon sequestration in paddy soils: Contribution and mechanisms of mineral-associated SOC formation.

In this work, comparative study of paddy and upland soils were carried out to unravel mechanisms of enhanced soil organic carbon (SOC) sequestration in paddy soils using fractionation methods, 13C NMR and Nano-SIMS analysis, as well as organic layer thickness calculations (Core-Shell model). The results showed that although there is a strong increase in particulate SOC in paddy soils compared to that in the upland soils, the increase in mineral-associated SOC is more important, explaining 60-75% of SOC increase in the paddy soils. In the wet and dry alternate cycles of paddy soil, iron (hydr)oxides adsorb relatively small and soluble organic molecules (fulvic acid-like), promote catalytic oxidation and polymerization, thus accelerating formation of larger organic molecules. Upon reductive iron dissolution, these molecules are released and incorporated into existing less soluble organic compounds (humic acid or humin-like), which are coagulated and associated with clay minerals, becoming part of the mineral-associated SOC. The functioning of this "iron wheel" process stimulates accumulation of relatively young SOC into mineral-associated organic carbon pool, and reduces the difference in chemical structure between oxides-bound and clay-bound SOC. Further, the faster turnover of oxides and soil aggregates in paddy soil also facilities interaction between SOC and minerals. The formation of mineral-associated SOC may delay degradation of organic matter during both wet and dry period in the paddy field, therefore enhancing carbon sequestration in paddy soils.