Reaction of aquatic humic substances with aluminium: a Al-27 NMR study

Al-27 NMR spectroscopy showed that humic substances from swamp water can complex aluminium(III) ion. At the relatively low concentration of 0.002 M humic substances, the Al-27 NMR spectrum shows only one peak, with a chemical shift at 6.5 ppm. As the concentrations of the humic substances increased, the Al-27 NMR spectra showed three peaks with chemical shifts at 6.5, 12.4, and 16.5 ppm, respectively, indicating that there are at least three complexing species formed in the solutions of humic substance at the different concentrations. Humic substances from surface soil, pear and brown coal also formed complexes with aluminium(III). Investigation of the reaction of organic acids with aluminium(III) ion indicated that the chemical shifts of aluminium complexes observed in solutions of swamp humic substances are similar to those in oxalic acid solutions, suggesting that the three peaks represent aluminium complexes formed in the humic substances with one, two and three oxalic acid-like groups. Significant complexation of aluminium occurs at low pH, suggesting that humic substances play an important role in the transport of this metal in acidic environments.