Interaction of dissolved organic matter with Hg(II) along salinity gradient in Boston Lake

Mercury pollution has become one of the serious ecological and health problems in developing countries. In aquatic environments, widespread dissolved organic matter ( DOM ) affects the physical transfer, chemical transformation and bioavailability of heavy metal ions. In the arid inland lakes, water salinity has been increasing due to close hydrological system and much higher vaporization than precipitation, and thus there are usually salinity gradients in these inland lakes. The change of salinity may exert influences on complexation of DOM with heavy metals. However, response of the metal complexation of DOM to the salinity is still unclear. In this study, three-dimensional molecule fluorescence spectroscopy (3D-EEM) was used to examine the interaction of Hg(II) and DOM in sediment along a salinity gradient of the Bosten Lake, the biggest inland freshwater lake in China. Three fluorescence peaks were identified in the EEM fluorescence spectra of all DOM samples. All the three fluorescence peaks could be quenched by Hg(II), indicating the formation of stable nonluminous DOM -Hg complexes. The conditional stability constant and binding constant for DOM -Hg(II) were 3.82–4.34 and 3.76–5.54, respectively. Conditional stable constants and binding constants for the system of Hg(II) and protein-like substances were higher than those for the humic substances-Hg(II) system. The ligand ratio of fluorophore to Hg(II) correlated well with the salinity. The strong complexation of Hg(II) and DOM and its responses to salinity change may significantly influence transport, transformation and ecological risk of Hg in Bosten Lake.