Association of dissolved aluminum with silica: Connecting molecular structure to surface reactivity using NMR

摘要

We studied uptake mechanisms for dissolved Al on amorphous silica by combining bulk-solution chemistry experiments with solid-state Nuclear Magnetic Resonance techniques (Al-27 magic-angle spinning (MAS) NMR, Al-27{H-1} cross-polarization (CP) MAS NMR and 29Si{H-1} CP-MAS NMR). We find that reaction of Al (1 mM) with amorphous silica consists of at least three reaction pathways; (1) adsorption of Al to surface silanol sites, (2) surface-enhanced precipitation of an aluminum hydroxide, and (3) bulk precipitation of an aluminosilicate phase. From the NMR speciation and water chemistry data, we calculate that 0.20 (+/- 0.04) tetrahedral Al atoms nm(-2) sorb to the silica surface. Once the surface has sorbed roughly half of the total dissolved Al (similar to 8% site coverage), aluminum hydroxides and aluminosilicates precipitate from solution. These precipitation reactions are dependent upon solution pH and total dissolved silica concentration. We find that the Si:Al stoichiometry of the aluminosilicate precipitate is roughly 1:1 and suggest a chemical formula of NaAlSiO4 in which Na+ acts as the charge compensating cation. For the adsorption of Al, we propose a surface-controlled reaction mechanism where Al sorbs as an inner-sphere coordination complex at the silica surface. Analogous to the hydrolysis of Al(OH2)(6)(3+), we suggest that rapid deprotonation by surface hydroxyls followed by dehydration of ligated waters results in four-coordinate (> SiOH)(2)Al(OH)(2) sites at the surface of amorphous silica. (C) 2008 Elsevier Ltd. All rights reserved.