Speciation of aluminum in soils and stream waters: The importance of organic matter

- Al(III) form mononuclear complexes with carboxylic groups in organic matter.
- Interactions with organic matter limit the hydrolysis and polymerization of Al.
- The stream waters contain substantial amounts of organic Al complexes.
- The organic soils contain both organic Al complexes and solid Al phases.
- Properties and concentration of organic matter and pH control the speciation of Al.

Aluminum (Al) is the most common metal in the Earth's crust, and exists mainly in the form of silicates, oxides and hydroxides, or in complexes with natural organic matter (NOM) or inorganic ligands. Since speciation is a key factor for understanding the environmental impact of Al, it is important to determine the chemical forms of Al that predominate in different natural media and under varying geochemical conditions. This study investigated how complexation with NOM influenced the speciation of Al(III) in different environmental samples (isolated aquatic NOM, organic soils and stream waters) using X-ray absorption spectroscopy (XAS) and Infrared (IR) spectroscopy. The combined spectroscopic results showed that Al(III) formed mononuclear complexes with carboxylic functional groups in NOM that were sufficiently stable to suppress the hydrolysis and polymerization of Al(III). These organic Al complexes were favored at slightly acidic pH values between pH 3 and 6 and at decreasing Al concentrations. The XAS results of the organic soils and the stream water samples indicated a variation in the speciation from a predominance of organically complexed Al in the stream waters to a mixture of Al-NOM complexes and precipitated Al phases (Al hydroxides and/or Al silicates) in the organic soils. Although the presented XAS results were limited by relatively low signal-to-noise caused by the low K-edge energy of Al, the combined spectroscopic results provided new and useful information about Al in different environmental samples and showed that NOM and pH are important parameters controlling the speciation of Al.