title: X-ray absorption spectroscopic evidence for the formation of Pb(II) inner-sphere adsorption complexes and precipitates at the calcite-water interface.
abstract: Combined batch sorption and in situ X-ray absorption spectroscopy provide direct assessment of the mechanisms for Pb(II) sorption atthe calcite--water interface under low-temperature conditions. At low metal concentration, 1 microM 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 A 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 microM Pb, precipitation of hydrocerussite and cerussite secondary phases dominates Pb partitioning. At 5 and 10 microM 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.
authors.affiliation: Department of Geosciences, Center for Environmental Molecular Science, State University of New York at Stony Brook, Stony Brook, New York 11794, USA. arouff@ic.sunysb.edu
authors.forename: Ashaki A
authors.initials: AA
authors.lastname: Rouff
authors.forename: Evert J
authors.initials: EJ
authors.lastname: Elzinga
authors.forename: Richard J
authors.initials: RJ
authors.lastname: Reeder
authors.forename: Nicholas S
authors.initials: NS
authors.lastname: Fisher
mesh_terms: D000327:Adsorption; D002119:Calcium Carbonate / Q000737:chemistry*; D011232:Chemical Precipitation; D004785:Environmental Pollutants / Q000032:analysis*; D007854:Lead / Q000032:analysis* / Q000737:chemistry*; D013057:Spectrum Analysis / Q000379:methods; D013696:Temperature; D014867:Water / Q000737:chemistry
chemical_list: D004785:Environmental Pollutants; D014867:Water; D007854:Lead; D002119:Calcium Carbonate