Vanadium(V) adsorption onto goethite (alpha-FeOOH) at pH 1.5 to 12: A surface complexation model based on ab initio molecular geometries and EXAFS spectroscopy

We measured the adsorption of V(V) onto goethite (alpha-FeOOH) under oxic (P-O2 = 0.2 bar) atmospheric conditions. EXAFS spectra show that V(V) adsorbs by forming inner-sphere complexes as VO2(OH)(2) and VO3(OH). We predicted the relative energies and geometries of VO2(O, OH)(2)-FeOOH surface complexes using ab initio calculations of the geometries and energetics of analogue Fe-2(OH)(2)(H2O)(6)O2VO2(O, OH)(2) clusters. The bidentate corner-sharing complex is predicted to be substantially (57 kJ/mol) favoured energetically over the hypothetical edge-sharing bidentate complex. Fitting the EXAFS spectra using multiple scattering shows that only the bidentate corner-sharing complex is present with Fe-V and V-O distances in good agreement with those predicted. We find it important to include multiple scattering in the fits of our EXAFS data otherwise spurious V-Fe distances near 2.8 Angstrom result which may be incorrectly attributed to edge-sharing complexes. We find no evidence for monodentate complexes; this agrees with predicted high energies of such complexes. Having identified the Fe2O2V(OH)(2)(+) Fe2O2VO(OH)(O) surface complexes, we are able to fit the experimental vanadium(V) adsorption data to the reactions

2FeOH(2)(+) + VO2+ Fe2O2V(OH)(2)(+) + 2H(+)

and

2FeOH + HVO42- = Fe2O2VO(OH)(0) + 2OH(-)

also determined the first acid dissociation constant of the Fe2O2VO2H2+ surface complex. Fits of sorption edges to surface complexation models are ambiguous. This is one of the first studies to provide a surface complexation model of sorption edges that is consistent with both spectroscopic and quantum mechanical constraints. Copyright (C) 2004 Elsevier Ltd.