Rates and mechanisms of oxygen exchanges between sites in the AlO4Al12(OH)24(H2O)127+(aq) complex and water:: Implications for mineral surface chemistry

Rate coefficients for steady exchange of oxygens between sites in the AlO4Al12(OH)(24)(H2O)(12)(7+)(aq) (Al-13) complex and bulk solution were determined over the temperature range of 9-19 degrees C and 4.6 < pH < 5.4 using O-17-NMR. The molecule has 12 equivalent water molecules (eta(1)-OH2 sites), two structurally distinct hydroxyl bridges (mu(2)-OH sites), and four coordinated oxo groups (mu(4)-O sites). The rate coefficients for exchange of the 12 coordinated water molecules on the complex at 298 K are: k(ex)(298) = 1100 (+/- 300) s(-1), Delta H-double dagger = 53 (+/- 12) kJ mol(-1) kJ mol(-1), and Delta S-double dagger = -7 (+/- 25) J mol(-1) K-1 and fall within the range measured for dissolved aluminum monomers. The two hydroxyl bridges in the Al-13 complex differ in their positions relative to the mu(4)-O sites but have identical nearest neighbors (two aluminum, one hydrogen) and virtually identical bond lengths to aluminums. The rates of isotopic exchange, however, differ considerably and are independent of pH to within the experimental uncertainties. Rate coefficients for exchange of the less-reactive hydroxyl are k(ex)(298) = 1.6 (+/- 0.1) (.) 10(-5) s(-1), with Delta H-double dagger = 104 (+/- 20) kJ mol(-1) and Delta S-double dagger = 5 (+/- 4) J mol(-1) K-1. These rate parameters compare well with those for uncatalyzed dissociation of small hydroxy-bridged metal complexes, such as dimers. The rate parameters for the more reactive hydroxyl: k(ex)(298) = 1.6 (+/- 0.4) (.) 10(-2) s(-1), Delta H-double dagger = 202 (+/- 23) kJ mol(-1) and Delta S-double dagger = 403 (+/- 43) J mol(-1) K-1, are inconsistent with previous work and suggest that an unidentified reaction may be involved in a precursor step. No steady exchange of the mu(4)-O site was observed over several hundreds of hours of reaction. We postulate a mechanism for dissolution of the molecule that requires protonation of the mu(4)-O site and hydration of the central AlO4 unit as the rate-controlling step. Furthermore, the measured rates of exchange of eta(1)-OH2 sites also allow us to constrain solvolysis at structurally similar aluminum (hydr)oxide mineral surfaces. Copyright (C) 2000 Elsevier Science Ltd.