X-ray Absorption Spectroscopic Evidence for the Formation of Pb(II) Inner-Sphere Adsorption Complexes and Precipitates at the Calcite−Water Interface

Combined batch sorption and in situ X-ray absorption spectroscopy provide direct assessment of the mechanisms for Pb(II) sorption at the calcite−water interface under low-temperature conditions. At low metal concentration, 1 μM initial Pb, X-ray absorption fine structure data indicate the formation of Pb mononuclear inner-sphere complexes at the surface. A first-shell Pb−O bond length of 2.34 Å is consistent with nearest oxygen neighbors in 3- or 4-fold coordination with a distorted trigonal pyramidal or square pyramidal geometry with a stereochemically active electron lone pair. For high initial Pb concentrations, 20 and 60 μM Pb, precipitation of hydrocerussite and cerussite secondary phases dominates Pb partitioning. At 5 and 10 μM initial Pb, the sorption mechanism is dual in nature with persistence of the mononuclear adsorption complex combined with precipitation of a cerussite phase occurring prior to saturation of theoretically available surface sites. The formation of inner-sphere complexes implies strong metal interactions with the surfacethe mechanistic reason for the affinity of Pb for calcite as observed in macroscopic studies. The geometry of the adsorbed complex can influence Pb coprecipitation, as a change to octahedral coordination is required for incorporation into calcite. The results provide the basis for predictions of Pb sequestration by calcite in natural systems.