X-ray Absorption Spectroscopic Evidence for the Formation of Pb(II) Inner-Sphere Adsorption Complexes and Precipitates at the Calcite−Water Interface
journal contributionposted on 15.03.2004, 00:00 by Ashaki A. Rouff, Evert J. Elzinga, Richard J. Reeder, Nicholas S. Fisher
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 surfacethe 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.