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

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