27Al NMR Study of the Hydrolysis and Condensation of Organically Complexed Aluminum

Abstract

Aluminum is the most abundant metal of the Earth’s crust, of which it represents approximately 8%, ranking after oxygen and silicon. It exists mainly as oxides. In terrestrial environments, aluminum commonly exists as secondary (authigenic) hydroxide or aluminosilicate minerals, mainly clays. These minerals are highly insoluble at neutral pH. However, aluminum occurs in detectable amounts in natural waters, due to leaching of the soil minerals in acidic conditions. Soil acidity may have a natural origin, such as an acidic (silicic) mother rock, melted snow, dissolved carbonic acid, or biologically generated organic acids. During the past two decades, it has been demonstrated that one of the major origins of increased aluminum mobilization and transport in forested soils is introduction of strong acid through atmospheric sulfur and nitrogen deposition. It has also been shown that aqueous aluminum is the biogeochemical link between atmospheric pollution and damage caused to tree roots and aquatic organisms such as plankton, crustaceans, insects, and fish. Biological studies have shown that the different aluminum species exhibit various toxicities: the most toxic are the monomeric and the polynuclear species; complexation with organic acids results in low toxicity. The significance of aluminum to human health has long been regarded as negligible. There is a possible link between high-level aluminum contamination by renal dialysis or hemodialysis, and neurodegenerative health disorders such as Parkinson’s or Alzheimer’s diseases, but the part played by aluminum is not clear. However, since aluminum salts are used on an industrial level as coagulants and flocculants in water treatment, the aluminum concentration and speciation in drinking water deserve careful monitoring. Because of the specific toxicity of the aluminum species, there has been considerable concern in the past two decades over the speciation of aqueous aluminum present in soils and aquatic systems. To this end, several techniques have been developed in order to partition the aluminum species. The most common among them are chromatographic separation and categorization methods such as timed ferron reaction, and computational methods derived from thermodynamic equilibrium constants. However, significant discrepancies between the results have been noticed, and attributed to the dramatic interference of organic and inorganic anions in the Al fractionation.