The formation of metastable aluminosilicates in the Al–Si–H2O system: Results from solution chemistry and solid-state NMR spectroscopy

Abstract

We present the results of a series of experiments designed to probe the interactions between Al and the amorphous silica surface as a function of thermodynamic driving forces. The results from 27 Al single pulse magic angle spinning (SP/MAS) and 27 Al{ 1 H} rotational echo double resonance (REDOR) allow us to identify the reaction products and constrain their structure. In all cases, despite low Al and Si concentrations we observe the formation of metastable aluminosilicates. Results from low temperature experiments indicate that despite thermodynamic driving forces for the formation of gibbsite we observe the precipitation of separate octahedrally coordinated Al (Al [6] ) and tetrahedrally coordinated Al (Al [4] ) silicate phases. At higher temperatures the Al [4] silicate phase dominates the speciation. Structural models derived from the NMR data are also proposed, and the results are discussed as they relate to previous work on Al/Si cycling. Published by Elsevier Ltd.

The effects of initial acetate concentration on CO2–brine-anorthite interactions under geologic CO2 sequestration conditions

“…57,58 Moreover, the potential presence of amorphous silica may further consume dissolved Al by adsorption, surface enhanced precipitation, or/and bulk precipitation of Al-containing phase(s), 51 even when the concentrations of Al and Si are low. 59 Based on the Al:Ca ratios in Fig. 4, less Al-containing precipitation formed when [NaCl] ¼ 1.0 M (Exps.…”

Biogenic and pedogenic controls on Si distributions and cycling in grasslands of the Santa Cruz soil chronosequence, California

“…HAS phases, produced under both laboratory and natural conditions, range from highly soluble colloids to relative insoluble amorphous phases with stiochiometries generally ranging from Al:Si ratios of 0.5 to 1.0 (Wada and Kudo, 1975;Barron et al, 1982;Douchet et al, 2001;Dobrzynski, 2007;Houston et al, 2008;Mason et al, 2011). Naturally occuring HAS phases, with Si-Al ratios close to 1:1 have been described as defect or kaolinitic allophanes which serve as precursors to crystalline kaolinite (Parfitt, 2009).…”

Analysis of trivalent cation complexation to functionalized mesoporous silica using solid-state NMR spectroscopy

“…This result agrees with the literature where Al was found to bind to the silanol groups on the silica surface in bidentate tetrahedral complexes. 24,25 As noted by Houston, 25 the width of this peak is indicative of a motion-constrained species which supports a bidentate complex to the silica surface. The Al here is binding to the deprotonated silanol groups on the silica surface forming an inner sphere complex.…”

“…44 Additionally, the concentration and pH used for these studies limits Al-hydroxide formation. 24 Based on this spectrum and the total amount of sorbed Al, a 1 : 1 metal ligand coordination for the octahedral Al is likely with the remaining space in the coordination sphere occupied by water molecules.…”

“…Despite the relatively large number of publications studying metal uptake on functionalized silica, NMR has not been used to probe these metals directly. NMR has been used to probe metals in organometallic crystals and in mineral environments of these same metals, [24][25][26][27][28][29][30] but the work presented here is the first to examine the binding mechanism of the metal to functionalized mesoporous silica using NMR of the metal nuclei.…”