Further insight into the mechanism of formation of hydroxyaluminosilicates

Hydroxyaluminosilicates (HAS) are formed by the reaction of silicic acid (Si(OH)4) with adjacent hydroxyl groups on an aluminium hydroxide (Al(OH)3(s)) framework or template. They are important secondary mineral phases in the biogeochemical cycling of aluminium and are extremely insoluble. Two discrete forms of HAS have been identified. HASA which is formed when [Si(OH)4] ⩽ [Al] and HASB which is formed when [Si(OH)4] ≫ [Al]. The formation of HASB has been suggested to involve the further reaction of Si(OH)4 with HASA and it is this contention that, in the main, we have tested herein. Applying a number of analytical and structural tools we have demonstrated the critical importance of both absolute concentrations and relative ratio’s of Si(OH)4 and Al in solution in determining which form of HAS will be precipitated from solution. In addition, by collecting HAS both almost immediately upon their precipitation from solutions (ca 0.5 h) and after ageing in solutions for up to 336 h and analysing their stoichiometries and structural configurations we have shown that the formation of HASA was the first step in the formation of HAS including in those solutions in which [Si(OH)4] ≫ [Al] and ultimately (⩾72 h ageing) only HASB was identified. These are the first experimental results which support the long held belief that Al(OH)3(s) is a prerequisite to the formation of HASA which, in turn, is the precursor to the formation of HASB. The insight we have gained should enable a better understanding of the role of HAS both in their control of the biological availability of aluminium and in their potential future applications in materials science.

Graphical abstractWhen the kinetics of formation of hydroxyaluminosilicates (HAS) were investigated we obtained the first direct experimental evidence that the formation of HASA, an HAS with a Si:Al ratio of 0.5, was a prerequisite to the formation of HASB, an HAS with a Si:Al ratio of 1.0. We also found that the interconversions between Al(OH)3(s), HASA and HASB were critically dependent upon the concentration of silicic acid (Si(OH)4) with the rapid formation of HASA being favoured when [Si(OH)4] = [Al] and the rapid formation of HASB being favoured when [Si(OH)4] ≫ [Al].Figure optionsDownload full-size imageDownload as PowerPoint slide