Abstract
Although, the kinetic reactivity of a mineral surface is determined, in part, by the rates of exchange of surface-bound oxygens and protons with bulk solution, there are no elementary rate data for minerals. However, such kinetic measurements can be made on dissolved polynuclear clusters, and here we report lifetimes for protons bound to three oxygen sites on the AlO.sub.4Al.sub.12(OH).sub.24(H.sub.2O).sub.12.sup.7+ (Al.sub.13) molecule, which is a model for aluminum-hydroxide solids in water. Proton lifetimes were measured using.sup.1H NMR at pH[approximately equal to]5 in both aqueous and mixed solvents. The.sup.1H NMR peak for protons on bound waters (I*-H.sub.2O) lies near 8ppm in a 2.5:1 mixture of H.sub.2O/acetone-d.sub.6 and broadens over the temperature range -20 to -5[degrees]C. Extrapolated to 298K, the lifetime of a proton on a I*-H.sub.2O is I.sup.298 [approximately equal to]0.0002s, which is surprisingly close to the lifetime of an oxygen in the I*-H.sub.2O ([approximately equal to]0.0009s), but in the same general range as lifetimes for protons on fully protonated monomer ions of trivalent metals (e.g., Al(H.sub.2O).sub.6.sup.3+). The lifetime is reduced somewhat by acid addition, indicating that there is a contribution from the partly deprotonated Al.sub.13 molecule in addition to the fully protonated Al.sub.13 at self-buffered pH conditions. Proton lifetimes on the two distinct sets of hydroxyls bridging two Al(III) ([mu].sub.2-OH) differ substantially and are much shorter than the lifetime of an oxygen at these sites. The average lifetimes for hydroxyl protons were measured in a 2:1 mixture of H.sub.2O/dmso-d.sub.6 over the temperature range 3.7-95.2[degrees]C. The lifetime of a hydrogen on one of the [mu].sub.2-OH was also measured in D.sub.2O. The I.sup.298 values are [approximately equal to]0.013 and [approximately equal to]0.2s in the H.sub.2O/dmso-d.sub.6 solution and the I.sup.298 value for the [mu].sub.2-OH detectable in D.sub.2O is I.sup.298 [approximately equal to]0.013s. The.sup.1H NMR peak for the more reactive [mu].sub.2-OH broadens slightly with acid addition, indicating a contribution from an exchange pathway that involves a proton or hydronium ion. These data indicate that surface protons on minerals will equilibrate with near-surface waters on the diffusional time scale.