Sources and speciation of aluminium and silicon in natural waters

The aluminosilicate minerals of igneous and metamorphic rocks are mostly unstable in earth-surface weathering conditions. In the tropics and subtropics, they are transformed to stable end-products (crystalline clay minerals, oxides and hydroxides) that largely conserve aluminium and iron. In noncalcareous soils in temperature and boreal climates, aluminium can be markedly mobile, and is precipitated as metastable products that include hydrous aluminosilicates, hydroxyaluminium polymers in or on 2:1 layer silicates, and complexes with soil organic matter. The aluminosilicate precipitates formed at pH less than 5.5 have structures related to imogolite, a unidimensional crystal in the form of a tube of 2.3 nm outer diameter. These metastable precipitates, both organic and inorganic, are readily remobilized on further acidification, and can release aluminium into streams if the solutions are not neutralized in the subsoil. Three classes of soluble aluminium species in natural waters have been distinguished by their rate of reaction with complexing reagents, and their rate of adsorption on cation-exchange columns. These are: (a) unreactive, acid-soluble, Al, (b) labile monomeric Al, and (c) non-liable monomeric Al. Group (b) includes simple inorganic species (e.g. Al3+, AlOH2+, AlF2+), and group (c) is thought to include organic complexes. In contrast, silicon occurs dominantly as Si(OH)4 monomers in natural water. Its metastable precipitates include hydrous aluminosilicates and biogenic opal.