Evaluation of the Hg2+ binding potential of fulvic acids from Fluorescence Excitation-Emission Matrices.

ANDREA M. BERKOVIC; FERNANDO S. GARCÍA EINSCHLAG*; MÓNICA C. GONZALEZ; REINALDO PIS DIEZ; DANIEL O. MARTIRE*

Photochemical and Photobiological Sciences

ROYAL SOC CHEMISTRY

Lugar: CAMBRIDGE; Año: 2013 vol. 12 p. 384 - 392

1474-905X

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 nonfluorescent 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. 1. Introduction Dissolved organic matter (DOM) constitutes a complex mixture of organic macromolecules with a variety of building blocks and functional groups, and it is ubiquitous in water, soils and sediments. Humic substances (HS) are the major component of DOM in natural water, and account for up to 50?80% of DOM, in terms of dissolved organic carbon.1,2 According to its solubility, HS can be separated into fulvic acid (FA, soluble at all pH), humic acid (HA, soluble in alkaline media and insoluble at pH = 1), and humin (insoluble at all pH).3 DOM plays a key role in influencing the solubility, mobility, bioavailability and toxicity of trace metals in aquatic environments because of its strong ability to bind to metal ions.4?7 It has been reported that DOM plays a key role in the biogeochemical cycling of mercury in aquatic environments.6?8 Recent studies have demonstrated that fluorescence