Evolving factor analysis of synchronous fluorescence spectra of humic substances in the presence of Cu(II)

Synchronous fluorescence spectra of soil and marine fulvic acids and of a commercial humic acid collected by varying Cu(II) concentration at constant pH (quenching experiments) were analyzed by evolving factor analysis (EFA). The number of components in each data set were related to different binding sites on the molecules of the corresponding humic substances. Two binding sites were found for the marine fulvic acid (mfua) and humic acid (hua) samples and two or three binding sites, depending on the pH, for the soil fulvic acid (sfua) sample. Rules were developed for the EFA procedure to obtain, in a self-modeling base, spectra and fluorescence intensity profiles for the detected binding sites. From the calculated fluorescence intensity profiles, and using a 1 : 1 stoichiometry for the Cu(II)/binding site complex formation, the conditional stability constants (K) and complexing capacities for the detected stronger binding sites were calculated. The following values for log K (standard deviation) were obtained: mfua (pH = 6), log K1 = 5.2 (0.3); sfua (pH = 6), log K1 = 5.57 (0.06) and log K2 = 4.92 (0.04); and hua (pH = 7), log K1 = 5.38 (0.07).