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 different molecular weight fractions (F1: less than 220 Da; F2:220-1000 Da; and F3: 1000-4000 Da) by gel filtration chromatography and the fractions were studied by synchronous fluorescence (SyF) spectroscopy. Analysis of the SyF spectra for the fulvic acid fractions showed that the fractions with molecules of larger sizes have a higher content of condensed aromatic compounds. The information about their interaction with UO₂(II) ions in an aqueous solution (100 mg 1^-1 of fulvic acid, in 0.1 M NaClO₄ at pH 3.5) was obtained from the measurement of SyF spectra at increasing concentrations of metal 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 analysis of the quenching profiles of the peaks, using a non-linear method, the concentration of binding sites (C_L), and the corresponding stability constants (log K) were calculated. The stability constants of the UO₂(II)-fulvate 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 fraction with molecules of larger size.