INVESTIGATING PB(II) SORPTION PRODUCTS AT THE CALCITE-WATER INTERFACE: XAFS EVIDENCE FOR ADSORPTION AND PRECIPITATION

Pb(II) is a key contaminant which can be deleterious to humans even in minute concentrations. Due to the ubiquitous nature of calcite and its predominant occurrence in geological formations, it may prove to be an important sink for a number of such contaminants. Thus we have explored the interaction of Pb with calcite under low temperature conditions in an attempt to assess the mobility of the metal. This study couples batch sorption with in situ X-ray absorption spectroscopy to provide a direct assessment of the Pb(II) sorption mechanisms at the calcite-water interface. At a sorption density (G) of 1.0 x 10^-6 Moles per m² we observe 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 neighbors in 3-fold co-ordination with a distorted trigonal pyramidal geometry with a stereochemically active electron lone pair. At G=5.1 x 10^-6 and 1.1 x 10^-5 Moles per m² the sorption mechanism is dual in nature with persistence of the mononuclear adsorption complex combined with precipitation of a cerussite or hydrocerussite phase occuring prior to saturation of theoretically available Ca sites. At G=2.9 x 10^-5 and 9.8 x 10^-5 Moles per m², the system is dominated by the precipitation of lead carbonate phases. Overall, the formation of inner-sphere complexes implies strong metal interactions with the surface - an inference substantiated by the results of macroscopic studies. The geometry of the adsorbed complex can influence Pb co-precipitation as a change in co-ordination from trigonal to octahedral is required for incorporation into calcite. The results provide the basis for predictions of Pb sequestration by calcite in natural systems.