Method for the differentiation of leaf litter extracts and study of their interaction with Cu(II) by molecular fluorescence

Six leaf litter extracts (LLE) (eucalyptus (Eucaliptus globulus), fern (Pte ridium aquilinium), oak (Quercus robur), chestnut (Castanea sativa), laurel (Laurus nobilis), and ulex (Ulex europoeus) canopies) were isolated follow ing an extraction procedure similar to that used for fulvic acids (FA) and were characterized by elemental analysis and UV-Vis, FT-IR, and synchronous molecular fluorescence (SyF) spectroscopies. Moreover, information about t heir interaction with the Cu(II) ion in aqueous solution (100 mg/L of LLE i n 0.1 M KNO3 at pH = 6) was obtained from the measurement of SyF spectra at increasing concentrations of Cu(II). These spectral sets were treated by a self-modeling mixture analysis method (SIMPLISMA) to obtain improved quenc hing profiles to be used in the estimation by the method of Ryan and Weber of the conditional stability constants (K-c), concentration of binding site s, and percentage of fluorescent binding sites accessible for complexation. For comparison purposes, two samples of FA extracted from two horizons of an oak forest soil (0-5 cm and 5-15 cm) were also studied. The spectroscopi c data obtained for LLE and FA were different. The results suggest that LLE are characterized by relatively high concentrations of individualized simp le molecules and include reactive structures (alkene and protein residues). The ratio of aliphatic/aromatic structures was higher in LLE than for FA. Both LLE and soil FA form stable complexes with Cu(II), but the logarithm o f the conditional stability constant of the 1:1 complexes is larger for LLE (about 5) than for the soil FA (about 4.5).