The reaction of aluminium with silicic acid in acidic solution: an important mechanism in controlling the biological availability of aluminium?

The reaction of aluminium (Al) with monomeric silicic acid (Si(OH)₄) to form an hydroxyaluminosilicate (HAS) has been well documented over the past 40 or so years. The formation of an aluminium hydroxide template, upon which Si(OH)₄ will condense in competition with Al, was demonstrated to be a prerequisite to HAS formation. This initial reaction results in the formation of a slowly aggregating HAS, with a Si:Al ratio of 0.5, in which silicon tetrahedra are bonded to Al octahedra through three SiOAl linkages. We have called this HAS_A. In solutions in which the concentration of Si(OH)₄≥Al HAS_A acts as a template for the incorporation of further silicon tetrahedra to give a rapidly precipitating HAS (that we have called HAS_B), with a Si:Al ratio of 1.0, in which up to 50% of the constituent Al has adopted tetrahedral geometry. There are, at present, no reliable constants to describe either the formation or the solubility of these HAS. They are extremely insoluble and are likely to play an important role in the control of the release of Al from the edaphic to the aquatic environment. They may also have an important role in Al homeostasis in biota though the evidence to support this is more tentative.