Apparent differences in binding site distributions and aluminum(III) complexation for three molecular weight fractions of a coniferous soil fulvic acid

Abstract

A soil fulvic acid isolated from a northern coniferous forest (NCFA) was fractionated into three different molecular sizes ranging from less than 500 to 10 000 daltons by ultrafiltration and the fractions were studied by synchronous scan fluorescence spectroscopy (SSFS). The SSFS gives three distinct emission peaks (I, II and III) for these fractions of NCFA, which are attributed to different fluorophores. The lower-wavelength peaks (315 and 370 nm) are attributed to fluorophores that consist of a single aromatic ring with carboxyl and/or hydroxyl groups attached to it, whereas the longest-wavelength peak (470 nm) is associated with a fluorophore containing three or four condensed aromatic rings with substituted groups. The ratio of the intensity of peak III to II increases from the lower to higher molecular weight fraction which indicates a higher content of condensed aromatic rings in the high molecular weight fraction. This study also included aluminum(III) binding experiments with the different molecular weight fractions of NCFA. Results showed that the high molecular weight fraction has a stronger affinity for aluminum(III) than the low molecular weight fraction.