Multisite metal binding to fulvic acid determined using multiresponse fluorescence Original Research Article

Metal binding to Suwannee River fulvic acid (SRFA) is determined using multiresponse fluorescence. The fluorescence surface of SRFA is described by five average fluorescent components, determined by spectral resolution using SIMPLISMA [W. Windig, J. Guilment, Anal. Chem. 63 (1991) 1425]. Fluorescence at excitation and emission wavelengths corresponding to these components was measured during titration with Cu(II). The resultant multiresponse data were fit to a five-site speciation model for Cu–SRFA interactions. The excitation/emission wavelengths identified for the fives sites are: 320/425, 370/450, 420/500, 480/540 and 500/550 nm. Logarithms of the conditional stability constants assuming one to one complex stoichiometry for the five sites at pH 6.02 and ionic strength of 0.1 M (KClO4) are: 4.95 (0.23), 4.98 (0.23), 5.32 (0.21), 5.59 (0.24), and 5.62 (0.22). The respective concentrations (μmol/mg of SRFA) are 0.83 (0.44), 2.38 (0.56), 2.76 (0.29), 0.74 (0.09), 0.037 (0.005), where the bracket values indicate ±95% confidence intervals. Free copper predicted using these parameters agrees within 0.1 of log unit of predicted free copper concentration from Cabaniss and Shuman’s empirical equation [S.E. Cabaniss, M.S. Shuman, Geochim. Cosmochim. Acta 52 (1988) 185]. This agreement gives additional credence to the multiresponse multisite fluorescent method used in this paper. These values are compared to existing values for Al and H binding by SRFA, for the same five sites, also determined using multi-response fluorescence. The binding of Al, Cu and H are best represented as bidentate/diprotic sites. The conditional stability constants for Al and Cu are consistent with salicylic acid-like binding.