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

In this study, the reactivity of lead (Pb(ll)) on naturally occurring Mn(II I,IV) (oxyhydr)oxide minerals was evaluated using kinetic, thermodynamic, a nd spectroscopic investigations. Aqueous Pb(ll) was more strongly adsorbed to birnessite (delta -MnO1.7) than to manganite (gamma -MnOOH) under all ex perimental conditions. The isoteric heat of Pb adsorption (DeltaH(Gamma)) o n 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 adsorptio n was an endothermic process and that birnessite possessed heterogeneous si tes of reactivity for Pb. X-ray absorption fine structure (XAFS) spectra re vealed that fb was adsorbed as inner-sphere complexes on both birnessite an d manganite with no evidence to suggest oxidation as an operative sorption mechanism. Lead appeared to coordinate to vacancy sites in the birnessite l ayer structure with concurrent release of Mn to solution, which resulted in a greater number of second shell Mn scatterers in Pb-birnessite when compa red to Pb-manganite samples. The difference in Pb coordination apparently e xplained the contrasting desorption behavior between the two Mn minerals. T hese results have significant implications for Pb partitioning in soil envi ronments containing solid-phase Mfi(lll,IV) (oxyhydr)oxides.