Further insight into the mechanism of formation of hydroxyaluminosilicates

Abstract

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. HAS A which is formed when [Si(OH) 4] ⩽ [Al] and HAS B which is formed when [Si(OH) 4] ≫ [Al]. The formation of HAS B has been suggested to involve the further reaction of Si(OH) 4 with HAS A 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 HAS A 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 HAS B 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 HAS A which, in turn, is the precursor to the formation of HAS B. 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.