Potentiometric and 19F 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 [(beta)-Al(OH)3(s)] and boehmite [(gamma)-AlOOH(s)]-19F-NMR spectroscopy and aqueous surface chemistry," Geochim. Cosmochim. Acta 63, 3513–3524). Fluoride substitutes for oxygen at three different sites in the GaAl12 molecule, but at dramatically different rates....