Speciation of aluminum in soil environments: Application of NMR technique

Although aluminum (Al) is abundant in soil environments, it is not an essential element and it is toxic to most organisms. Since the toxicity of Al depends on their chemical forms, the importance of Al speciation has been recognized worldwide. Difficulties in Al speciation are caused by the complex coordination chemistry of Al for the hydrolysis and formation of polynuclear species with a variable degree of solubility in aqueous solution. Nondestructive analyses, such as nuclear magnetic resonance (NMR) spectroscopy, could supply primarily im-portant information on the chemical forms of Al and may enable to evaluate the results ob-tained by other methodologies. In the present report, NMR spectral characteristics of environmentally important Al-containing components, such as hydoxyaluminum ions, Alinorganic complexes, Al-organic complexes, and primary and secondary minerals, are summarized for the nuclei of ²⁷Al and ²⁹Si determined by solution NMR and solid-state magic angle spinning (MAS) NMR. Applications of NMR techniques to soil science, including speciation of phytotoxic Al in soil environments and whole soil NMR studies, are described.