Evaluation of the Hg2+ binding potential of fulvic acids from fluorescence excitation-emission matrices

The effect of Hg2+ on the fluorescence intensity of three fulvic acids (Pahokee Peat, Pony Lake and Suwannee River) was studied. The fluorescence intensity decreased in the presence of added Hg2+, while the fluorescence lifetimes were independent of the concentration of Hg2+ in solution. These results are indicative of ground-states association between the fulvic acids and Hg2+ with formation of stable non-fluorescent complexes (static quenching process). The analysis of the excitation-emission matrices with the Singular Value Decomposition (SVD) and Multivariate Curve Resolution - Alternating Least Squares (MCR-ALS) methods provided additional valuable information regarding the binding properties between Hg2+ ions and specific fluorescence components of the fulvic acids. The three fulvic acids were shown to contain the same three groups of fluorophores characterized by excitation/emission pairs in the following regions: (320-330 nm/425-450 nm), (370-375 nm/465-500 nm), (290-295 nm/370-395 nm). These pairs are almost not affected by the change of pH from 2.0 to 7.0. Ryan-Weber and modified Stern-Volmer methods were used to analyze the static fluorescence quenching of the individual components. Similar conditional stability constants of Hg2+ binding for the three components were found by both methods. The obtained log K values are in the range of 4.4 to 5.4.