The reaction of aluminium with silicic acid in acidic solution: an important mechanism in controlling the biological availability of aluminium? Christopher Exley a, *, Celine Schneider a , Fre ´de ´ric J. Doucet b a Birchall Centre for Inorganic Chemistry and Materials Science, School of Chemistry and Physics, Keele University, Staffordshire ST5 5BG, UK b Department of Chemistry, The Radiochemical Centre of Excellence, The University of Manchester, Oxford Road, Manchester M13 9PL, UK Received 30 November 2001; accepted 4 March 2002 Abstract 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. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Aluminium; Silicic acid; Hydroxyaluminosilicate 1. Preface The primary objective of this review was to critically evaluate scientific literature that has reported the interaction of aluminium (Al) with monomeric silicic acid (Si(OH) 4 ) in acidic solution. (A comprehensive and authoritative review of all silicate complexes of Al has recently been published [1].) A secondary objective was to update the significance of this chemistry to the biological availability of Al. A particular bone of contention in this field is the definition of what constitutes Si(OH) 4 . We have attempted to reduce the ambiguity that surrounds this definition by only review- ing research in which every attempt had been made to ensure that the only reactive form of silicon in experi- mental solutions was the neutral monomer and was not charged or polymeric forms of this weak acid. Our interpretation of ‘every attempt’ has excluded any research in which: (i) the concentration of Si(OH) 4 in either stock or experimental solutions exceeded 2.00 mmol l 1 (Si(OH) 4 will autocondense at concentrations Contents Abstract ......................................................................... 127 1. Preface ....................................................................... 127 2. Historical perspective ............................................................... 128 3. The mechanism of formation of HAS ..................................................... 130 4. HAS and the biological availability of Al ................................................... 134 Acknowledgements ................................................................... 135 References ........................................................................ 135 * Corresponding author. Tel.:  /44-1782-584080; fax:  /44-1782- 712378. E-mail address: cha38@keele.ac.uk (C. Exley). Coordination Chemistry Reviews 228 (2002) 127  /135 www.elsevier.com/locate/ccr 0010-8545/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII:S0010-8545(02)00077-2