Silicate complexes of aluminum(III) in aqueous systems

Aluminum(III) forms complexes with aqueous silicic acid in neutral or mildly acidic media, and these species are important in the protection of plant and animal life against aluminum toxicity. In highly alkaline media, surprisingly high concentrations of various aluminosilicate solute species can be achieved at least temporarily, the lifetime of the homogeneous solution depending on the pH, the nature of the cation(s) present, and the Al and Si concentrations. The longevity of aluminosilicate complexes in solution may have technological consequences - for example, in causing silica carryover in the Bayer process for aluminum production. The kinetics of silicate exchange on small, acyclic aluminosilicate solute species in alkaline solutions are much faster than on either the corresponding all-silicate ions or cyclic aluminosilicate species. The kinetic lability of small aluminosilicate species is attributable in part to the ability of Al-III to expand its coordination number easily from four to six in acyclic structures, but also to the availability of -OH groups for condensation reactions on aluminate centers, even at high pH where equivalent to Si-OH functions become deprotonated. One implication is that, contrary to the traditional picture of zeolite formation from structured 'secondary building units' pre-existing in solution, cyclic and cage-like aluminosilicate solute species are not directly involved in the crystallization of solid aluminosilicates such as zeolites from aqueous solutions but simply serve as reservoirs for small, active, acyclic species responsible for crystal growth. (C) 2001 Elsevier Science B.V. All rights reserved.