Outer Sphere Adsorption of Pb(II)EDTA on Goethite*

FTIR and EXAFS spectroscopic measurements were performed on Pb(II)EDTA adsorbed on goethite as functions of pH (4-6), Pb(II)EDTA concentration (0.11 μM 72 μM), and ionic strength (16 μM 0.5M). FTIR measurements show no evidence for carboxylate-Fe(III) bonding or protonation of EDTA at Pb:EDTA = 1:1. Both FTIR and EXAFS measurements suggest that EDTA acts as a hexadentate ligand, with all four of its carboxylate and both amine groups bonded to Pb(II). No evidence was observed for inner-sphere Pb(II)-goethite bonding at Pb:EDTA = 1:1. Hence, the adsorbed complexes should have composition Pb(II)EDTA. Since substantial uptake of PbEDTA(II) occurred in the samples, we infer that Pb(II)EDTA adsorbed as outer-sphere complexes and/or as complexes that lose part of their solvation shells and hydrogen bond directly to goethite surface sites. We propose the term “hydration-sphere” for the latter type of complexes because they should occupy space in the primary hydration spheres of goethite surface functional groups, and to distinguish this mode of sorption from common structural definitions of innerand outer-sphere complexes. The similarity of Pb(II) uptake isotherms to those of other divalent metal ions complexed by EDTA suggests that they too adsorb by these mechanisms. The lack of evidence for inner-sphere EDTA-Fe(III) bonding suggests that previously proposed metal-ligand promoted dissolution mechanisms should be modified, specifically to account for the presence of outer-sphere precursor species. Submitted to Geochimica et Cosmochimica Acta 1 Stanford Synchrotron Radiation Laboratory, PO Box 4349, Stanford, CA, 94309, [email protected]; 2 Dept. of Inorganic Chemistry, Umeå University, Umeå, S-901 87 Sweden, [email protected]; 3 Dept. of Geological and Environmental Sciences, Stanford University, Stanford, CA, 94305-2115, [email protected]. **Corresponding author. *Work supported by Department of Energy contract DE-AC-03-76SF00515 (SLAC)