27Al 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 27 Al NMR
spectrum shows only one peak, with a chemical shift at 6.5 ppm. As the
concentrations of the humic substances increased, the 27
Al 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, peat 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.
