Outer-sphere adsorption of Pb(II)EDTA on goethite

Abstract

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 2-. Since substantial uptake of PbEDTA (II) 2-occurred in the samples, we infer that Pb(II)EDTA 2-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 inner-and 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.

Inorganic Ligand Effects on Pb(II) Sorption to Goethite (α-FeOOH)

“…Macroscopically, innersphere sorption is suggested by the fact that uptake is unaffected by changes in ionic strength (12,13) and occurs well below the pH pzc , where the mineral surface is positively charged. Direct, in situ EXAFS analyses have confirmed the dominance of innersphere sorption for Pb on goethite (α-FeOOH) as well as on hematite (α-Fe 2 O 3 ) and poorly crystalline hydrous ferric oxide (1,(14)(15)(16)(17), although outer-sphere binding has recently been reported for Pb sorbed to goethite in the presence of EDTA, based on EXAFS and ATR-FTIR results (18).…”

Structure and Bonding of Cu(II)–Glutamate Complexes at the γ-Al2O3–Water Interface

“…In addition, hydrogen bonding through the nonbonded oxygen atoms of the ␣-carboxylate groups is observed in metal-amino acid salts (25), and may also contribute to the stability of this complex. This type of hydrogen bonding has been implicated as the dominant contribution to the stability of Pb-EDTA complexes at Fe-and Al-(hydr)oxide surfaces (45). The proposed outer-sphere Type B interaction has additional implications for surface complexation models.…”

ATR-FTIR spectroscopic characterization of coexisting carbonate surface complexes on hematite

“…The observation of a symmetry-broken interfacial CO 3 2Ϫ species distinct from that in the diffuse layer further implies the existence of nonelectrostatic stabilizing forces. Hydrogen bonding mechanisms have often been invoked to account for the stability of outer-sphere complexes that occur in the absence of electrostatic driving forces (Bargar et al, 1999a;Axe and Persson, 2001;Peak et al, 2003), and are likely to be present between surface sites, water, and the putative complex. In addition, it is plausible that a net increase in the entropy of the system may occur upon adsorption of the outer-sphere complex (entropic driving force).…”