Potentiometric and F nuclear magnetic resonance spectroscopic study of fluoride substitution in the GaAl12 polyoxocation: implications for aluminum (hydr)oxide mineral surfaces

Fluoride replacement of oxygens in the GaO₄Al₁₂(OH)₂₄(H₂O)127+(aq) molecule [GaAl₁₂] was studied via ¹⁹F nuclear magnetic resonance (NMR) at 4 < pH < 5 and 278 K in order to elucidate similar reactions at the surfaces of clays. Peaks are identified in the ¹⁹F-NMR spectra that correspond to both terminal and bridging fluorides on the GaAl₁₂ molecule, with relative peak positions similar to those previously identified in fluoridated aluminum (hydr)oxide mineral surfaces (Nordin, J. P., Sullivan, D. J., Phillips, B. L., and Casey, W. H. [1999], “Mechanisms for fluoride-promoted dissolution of bayerite [β-Al(OH)₃(s)] and boehmite [γ-AlOOH(s)]-¹⁹F-NMR spectroscopy and aqueous surface chemistry,” Geochim. Cosmochim. Acta63, 3513-3524). Fluoride substitutes for oxygen at three different sites in the GaAl₁₂ molecule, but at dramatically different rates.The kinetics of fluoride substitution follow a rate law that includes parallel and reversible transfer of fluoride from nonbridging sites to the two bridging sites. The essential features of the rate law are as follows: (1) fluoride replaces bound water molecules (η-OH₂) within minutes at 278 K at rates that are quantitatively similar to fluoride uptake by Al(H₂O)63+(aq) to form AlF²⁺(aq) at similar conditions; (2) fluoride substitutes onto the two topologically distinct μ₂-OH sites at different rates, as was previously observed for oxygen exchange, but here, the reaction is complete in hours to days at 278 K. Most importantly, rates of fluoride substitution onto μ₂-OH sites are 10² times more rapid than the corresponding rates of oxygen exchange with bulk waters, indicating that fluoride considerably labilizes the molecule, as is also observed at the surfaces of minerals. The largest cause of this labilization is the reduced molecular charge on the GaAl₁₂ upon replacement of bound waters by fluoride, which for mineral surfaces corresponds to a reduction in surface charge density.