Fluorescence quenching of three molecular weight fractions of a soil fulvic acid by UO2(II)

A soil fulvic acid isolated from a Korean forest was divided into three dif ferent molecular weight fractions (F1: less than 220 Da; F2: 220-1000 Da; a nd F3: 1000-4000 Dal by gel filtration chromatography and the fractions wer e studied by synchronous fluorescence (SyF) spectroscopy. Analysis of the S yF spectra for the fulvic acid fractions showed that the fractions with mol ecules of larger sizes have a higher content of condensed aromatic compound s. The information about their interaction with UO2(II) ions in an aqueous solution (100 mg l(-1) of fulvic acid, in 0.1 M NaClO4 at pH 3.5) was obtai ned from the measurement of SyF spectra at increasing concentrations of met al ions. Self-modeling mixture analysis of the quenching spectra gives two distinct peak components having a maximum peak position of 386 (type I) and 498 nm (type II) for all the size-fractionated fulvic acids. From the anal ysis of the quenching profiles of the peaks, using a non-linear method, the concentration of binding sites (C-L), and the corresponding stability cons tants (log K) were calculated. The stability constants of the UO2(II)-fulva te complexes ranged from 4.10 to 5.33, and increased with higher molecular weight fractions, which indicates a stronger affinity for UO2(II) in the fr action with molecules of larger size. (C) 2001 Elsevier Science B.V. All ri ghts reserved.