Study of the complexation of Cu(II) by fulvic acids extracted from a sewage sludge and its compost

The complexation of Cu(II) by two samples of fulvic acids (FA) extracted from the raw sewage sludge (RsFA) of a waste water processing plant and from the composted sludge (CsFA) obtained upon aerobical digestion was studied at pH 6.0. Synchronous molecular fluorescence spectroscopy was used to monitor the association of FA with Cu(II) and a self-modelling mixture analysis method (SIMPLISMA) was used to preprocess the spectral data to calculate the number of components with different quenching profiles as function of the Cu(II) concentration and their spectra. The stability constants and a rough estimation of the binding site concentrations were obtained by a Stern-Volmer analysis, by a non-linear least-square method and by a linear procedure. The analysis of the SyF spectra allowed the identification of two binding sites for both samples. The logarithm of the conditional stability constant corresponding to the 1:1 complex formed between the stronger binding site and Cu(II) is about 4 for both samples.