Acid-base properties of fulvic acids extracted from an untreated sewage sludge and from composted sludge

The acid-base properties of two samples of anthropogenic fulvic acids (FA) extracted from the untreated sewage sludge (rsFA) of a municipal wastewater treatment plant and from the compost (csFA) obtained in the aerobical digestion of this sludge were studied. Potentiometric titrimetry, infrared spectroscopy and synchronous molecular fluorescence (SyF) spectroscopy were used to obtain experimental information about the acid-base properties of the samples. Moreover, the SyF spectral data which were collected as function of the pH were analysed by a self-modelling curve resolution technique, evolving factor analysis with a gradient concentration window (EFA-GCW), to calculate the number of acid-base systems, their SyF spectra and the corresponding distribution diagrams, from which the pKa values were estimated. Two acid-base systems with pKa values about 6.2 and 8.9 were detected for rsFA and three systems with pKa values about 3.8, 6.6 and 9.4 for csFA. The amounts of acid groups directly titrated in water were (standard deviations in brackets) about 6.8(1) for rsFA and 6.73(3) for csFA. This information for rsFA and csFA was compared with similar information for natural soil FA samples extracted from a pinewood soil. Beyond different structural composition, this comparison showed that the anthropogenic samples have a higher amount of acid structures, but the structures of natural samples have stronger acid properties.