title: Speciation of Pb(II) sorbed by Burkholderia cepacia/goethite composites.
abstract: Bacterial-mineral composites are important in the retention of heavy metals such as Pb due to their large sorption capacity under a wide range of environmental conditions. However, the partitioning of heavy metals between components in such composites is not probed directly. Using Burkholderia cepacia biofilms coated with goethite (alpha-FeOOH) particles, the partitioning of Pb(II) between the biological and iron-(oxyhydr)oxide surfaces has been measured using an X-ray spectroscopic approach. EXAFS spectra were fit to quantitatively determine the fraction of Pb(II) associated with each component as a function of pH and [Pb]. At pH < 5.5, at least 50% of the total sorbed Pb(II) is associated with the biofilm component, whereas the total uptake within the composite is dominated by goethite (> 70% Pb/goethite) above pH 6. Direct comparison can be made between the amount of Pb(II) bound to each component in the composite vs separate binary systems (i.e., Pb/biofilm or Pb/goethite). At high pH, Pb(II) uptake on the biofilm is dramatically decreased due to competition with the goethite surface. In contrast, Pb uptake on goethite is significantly enhanced at low pH (2-fold increase at pH 5) compared to systems with no complexing ligands. The mode of Pb(II)-binding to the goethite component changes from low to high [Pb]. Structural fitting of the EXAFS spectra collected from 10(-5.6) to 10(-3.6) M [Pb]eq at pH 6 shows that the Pb-goethite surface complexes at low [Pb] are dominated by inner-sphere bidentate, binuclear complexes bridging two adjacent singly coordinated surface oxygens, giving rise to Pb-Fe distances of approximately 3.9 A. At high [Pb], the dominant Pb(II) inner-sphere complexes on the goethite surface shift to bidentate edge-sharing complexes with Pb-Fe distances of approximately 3.3 A.
authors.affiliation: Surface & Aqueous Geochemistry Group, Department of Geological & Environmental Sciences, Stanford University, Stanford, California 94035-2115, USA. atempleton@ucsd.edu
authors.forename: Alexis S
authors.initials: AS
authors.lastname: Templeton
authors.forename: Alfred M
authors.initials: AM
authors.lastname: Spormann
authors.forename: Gordon E
authors.initials: GE
authors.lastname: Brown
mesh_terms: D000327:Adsorption; D016956:Burkholderia cepacia / Q000254:growth & development*; D004785:Environmental Pollutants; D006863:Hydrogen-Ion Concentration; D058085:Iron Compounds / Q000737:chemistry*; D007854:Lead / Q000737:chemistry*; D008854:Microscopy, Electron; D008903:Minerals; D009566:Nitrates / Q000737:chemistry*; D013057:Spectrum Analysis; D014965:X-Rays
chemical_list: D004785:Environmental Pollutants; D058085:Iron Compounds; D008903:Minerals; D009566:Nitrates; C094886:goethite; D007854:Lead; C017461:lead nitrate