The flux of oxygen from the basal surface of gibbsite (alpha-Al(OH)(3)) at equilibrium

摘要

Experiments were conducted on gibbsite to determine whether oxygen-isotope exchange rates at hydroxyl bridges (mu(2)-OH) on the basal sheet exhibit similar reactivity trends as in large aluminum polyoxocations, for which high-quality kinetic data exist. We followed the exchange of 180 from the mineral surface to solution by using a high-surface-area solid that had been enriched to tens of percent in O-18. To establish this high enrichment, we initially react the solid hydrothermally with highly enriched (H2O)-O-18 in order to tag all oxygens near the mineral surface, and then back exchange the most reactive oxygens with isotopically normal water. This enrichment procedure isolates O-18 into the least-reactive sites, which are presumably mu(2)-OH on the basal surface. By analogy with aqueous aluminum complexes, including large multimers, the eta-OH2 sites exchange within fractions of a second and should be isotopically normal using this procedure.When suspended in isotopically normal electrolyte solutions, we find that the rates of release of O-18 from the mineral fall close to the rates of dissolution. The lack of steady isotopic exchange of mu(2)-OH on gibbsite surfaces contrasts with the aluminum polyoxocations, where the mu(2)-OH exchange many hundreds of times with bulk water molecules before the molecule dissociates. Additional experiments were conducted in solutions at near-neutral pH to determine the flux of oxygens at conditions near thermodynamic equilibrium. As in more acidic solutions, rates are close to values expected from dissolution of the mineral and there is no evidence for steady exchange of hydroxyl bridges with water molecules in the bulk solution. Copyright (C) 2004 Elsevier Ltd