Inorganic ligand effects on Pb(II) sorption to goethite (alpha-FeOOH) - I.Carbonate

Macroscopic measurements show that Pb(II) uptake on iron(hydr)oxides can be altered significantly by dissolved carbonate (enhanced up to 18% at pH 5 a nd decreased above pH similar to 6.5 in analyses at 1 atm CO2). This study elucidates the molecular-scale processes giving rise to these macroscopic e ffects by characterizing the structures of Pb(II) sorption complexes formed an goethite (alpha-FeOOH) in the presence of carbonate using in situ Pb L- III-EXAFS and ATR-FTIR spectroscopies. Bond valence and structural constrai nts are applied to develop mineral surface site-specific models for Pb sorp tion. Under all conditions studied (pH 5-7, Gamma(Pb) = 0.4-4 mu mol/m(2), and P-CO2 = 0-1 atm), Pb(II) forms predominantly inner-sphere edge-sharing (bidentate and/or tridentate) complexes with Fe(O,OH)(6) octahedra (RPb-Fe similar to 3.3 Angstrom). Corner-sharing complexes (RPb-Fe similar to 3.9 A ngstrom) are observed only in low pH (5) samples (P-CO2 0-1 atm). Consisten t with this pH sensitivity, site-specific analyses suggest that the relativ e abundance of corner-sharing sites reflects changes in the proton affinity of triply coordinated sites on the goethite (110) surface as suggested pre viously. FTIR results suggest the existence of ternary surface complexes in which carbonate groups bond to Pb as monodentate ligands. EXAFS data indic ate that these ternary complexes are bound to the surface through Pb, formi ng metal-bridged (Type A) complexes. Findings are summarized as structural models and corresponding mineral surface site-specific chemical reactions. (C) 2000 Academic Press.