Fluorescence characterization of dissolved organic matter in an urban river and its complexation with Hg(II)

Dissolved organic matter (DOM) in the Nanming River, an urban river in Guiyang City in SW China, and its complexation behavior with Hg(II) were investigated using fluorescence spectroscopy and the quenching titration technique. Three major fluorophores, two humic-like and one protein-like fluorescence, were observed in most of the DOM samples. Significant correlations were observed between the humic-like and protein-like fluorescence intensities, as well as, between them and other water quality parameters such as dissolved organic carbon, PO43-, chemical oxygen demand and NH4+ concentrations, suggesting that agricultural and municipal wastewaters may be the source for both protein-like and humic-like fluorescence materials in the river. The fluorescence quenching titration resulted in similar values for the conditional stability constants for Hg(II) complexes with the humic-like and protein-like fluorophores, likely due to the dominance of Hg binding with O-containing function groups at the high Hg(II) concentrations used in the titration. Effects of Cl−, Ca2+, Mg2+ and Cu2+ ions on the binding between Hg(II) and three different fluorophores were also studied. The fluorescence index from the Nanming River was further found to be controlled by pH and Hg(II), cautioning its use in discriminating the sources of DOM.