Interactions of Silicate Ions with Zinc(II) and Aluminum(III) in Alkaline Aqueous Solution

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Abstract
We present 29Si, 27Al, and 67Zn NMR evidence to show that silicate ions in alkaline solution form complexes with zinc(II) (present as zincate, Zn(OH)3- or Zn(OH)42-) and, concomitantly, with aluminate (Al(OH)4-). Zincate reacts with monomeric silicate at pH 14−15 to form [(HO)O2Si−O−Zn(OH)3]4- and with dimeric silicate to produce [HO−SiO2−O−SiO2−O−Zn(OH)3]6-. The exchange of Si between these free and Zn-bound sites is immeasurably fast on the 29Si NMR time scale. The cyclic silicate trimer reacts relatively slowly and incompletely with zincate to form [(HO)3Zn{(SiO3)3}]7-. The concentration of the cyclic trimer becomes further depleted because zincate scavenges the silicate monomer and dimer, with which the cyclic trimer is in equilibrium on the time scale of sample preparation. Identification of these zincate−silicate complexes is supported by quantum chemical theoretical calculations. Aluminate and zincate, when present together, compete roughly equally for a deficiency of silicate to form [(HO)3ZnOSiO2OH]4- and [(HO)3AlOSiO2OH]3- which exchange 29Si at a fast but measurable rate.

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