Direct detection of aluminosilicate species in aqueous solution by silicon-29 and aluminum-27 NMR spectroscopy

Silicon-29 NMR spectra of solutions containing 1 mol kg-1 SiO2 and 0.07 mol kg-1 Al in 2.4 mol kg-1 aqueous NaOH at 5 degree C show five bands which are attributable to dissolved species containing Si-O-Al links. The major aluminosilicate species in solution appear to be Al-substituted derivatives of the silicate dimer, cyclic trimer, cyclic tetramer, and acyclic trimer. 29Si spectra suggest and 27Al NMR spectra confirm that little or no free Al(OH)4- is present and that a major fraction of the bound Al is connected to more than one SiO4 unit. The time constant for Al exchange is on the order of 60-120 ms for the most of these aluminosilicate species. In solutions containing only 0.01 mol kg-1 of each of Si and Al in 0.03 mol kg-1 NaOH, 27Al NMR spectroscopy shows that free Al(OH)4- predominates but some aluminosilicate species are present. The behavior of Al in alkaline silicate solutions reflects Loewenstein's rule. 18 refs., 3 figs., 1 tab