Reactivity of Pb(II) at the Mn(III,IV) (oxyhydr)oxide-water interface

In this study, the reactivity of lead (Pb(II)) on naturally occurring Mn(III,IV) (oxyhydr)oxide minerals was evaluated using kinetic, thermodynamic, and spectroscopic investigations. Aqueous Pb(II) was more strongly adsorbed to birnessite (δ-MnO_1.7) than to manganite (γ-MnOOH) under all experimental conditions. The isoteric heat of Pb adsorption (ΔH_Γ) on birnessite was 94 kJ mol^-1 at a surface loading of 1.1 mmol g^-1, and decreased with increasing adsorption density. This indicated that adsorption was an endothermic process and that birnessite possessed heterogeneous sites of reactivity for Pb. X-ray absorption fine structure (XAFS) spectra revealed that Pb was adsorbed as inner-sphere complexes on both birnessite and manganite with no evidence to suggest oxidation as an operative sorption mechanism. Lead appeared to coordinate to vacancy sites in the birnessite layer structure with concurrent release of Mn to solution, which resulted in a greater number of second shell Mn scatterers in Pb-birnessite when compared to Pb-manganite samples. The difference in Pb coordination apparently explained the contrasting desorption behavior between the two Mn minerals. These results have significant implications for Pb partitioning in soil environments containing solid-phase Mn(III,IV) (oxyhydr)oxides.