We measured the adsorption of V(V) onto goethite (α-FeOOH) under oxic (PO2 = 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 Fe2(OH)2(H2O)6O2VO2(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 Å 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+ and Fe2O2VO(OH)0 surface complexes, we are able to fit the experimental vanadium(V) adsorption data to the reactions
2 FeOH2+ + VO2+ = Fe2O2V(OH)2+ + 2 H+
and
2 FeOH + HVO42− = Fe2O2V(OH)0 + 2 OH−
We 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.
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