APPARENT DIFFERENCES IN BINDING-SITE DISTRIBUTIONS AND ALUMINUM(III) COMPLEXATION FOR 3 MOLECULAR-WEIGHT FRACTIONS OF A CONIFEROUS SOIL FULVIC-ACID

A soil fulvic acid isolated from a northern coniferous forest (NCFA) w as fractionated into three different molecular sizes ranging from less than 500 to 10000 daltons by ultrafiltration and the fractions were s tudied by synchronous scan fluorescence spectroscopy (SSFS). The SSFS gives three distinct emission peaks (I, II and III) for these fraction s of NCFA, which are attributed to different fluorophores. The lower-w avelength peaks (315 and 370 nm) are attributed to fluorophores that c onsist of a single aromatic ring with carboxyl and/or hydroxyl groups attached to it, whereas the longest-wavelength peak (470 nm) is associ ated with a fluorophore containing three or four condensed aromatic ri ngs 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 mo lecular weight fraction. This study also included aluminum(III) bindin g experiments with the different molecular weight fractions of NCFA. R esults showed that the high molecular weight fraction has a stronger a ffinity for aluminum(III) than the low molecular weight fraction.