Study of the interaction of Al(III) with a soil fulvic acid in the acid pH range by self-modeling mixture analysis of synchronous fluorescence spectral data

The association of Al(III) with a soil fulvic acid (fua) was monitored in the pH range of 2.5-5.0 by synchronous fluorescence (SyF) spectroscopy. Sets of spectra of 100 mg/l fua solutions obtained at increasing Al(III) concentration (up to about 0.7 mM) at each pH (2.5, 3.0, 3.5, 4.0, 4.5 and 5.0) were preprocessed by the self-modeling mixture analysis method SIMPLISMA, with inverted second derivative spectra in the pure variable selection step, to determine the number and to calculate the spectra and concentration profiles of the components. This procedure detected two major components at all pH values: (i) one shows an increase of fluorescence which corresponds to a fluorescent complex formed between Al(III) and one of the binding sites of the fua; (ii) the other shows a decrease of fluorescence probably due to the decrease of the free concentration of a fluorescent binding site that complexes Al(III) forming a complex that does not show fluorescence. Considering the component that shows increase of fluorescence, conditional stability constants [log(K) with standard deviation in parenthesis] were estimated: pH=2.5, 4.5(3); pH=3.0, 4.6(3); pH=3.5, 4.8(2); pH=4.0, 5.2(1); pH=4.5, 6.1(2); and pH=5.0, 6.2(3). These results show that the stability of the complexes between fua and Al(III) decreases markedly for pH<4.5. This behavior may have an important influence on the changes of the environmental speciation of Al(III) upon acidification.