Measurement and modeling of mercury complexation by dissolved organic matter isolates from freshwater and effluents of a major wastewater treatment plant

Dissolved organic matter (DOM) samples were obtained from a low-density urbanized area located upstream of Paris (along the Marne River, France) and from the treated effluents at the Paris main wastewater treatment plant. These samples were then fractionated according to their hydrophobicity. DOM fractions consisted of nanomolar to sub-micromolar fresh organic substances with extremely strong Hg-complexing ligands. The conditional stability constants (i.e. KHgL′; pH ∼ 6.8, INaCl = 0.5 M, T = 25 °C) of the Hg-DOM complexes formed were greater than 1024 M−1, for the reaction Hg2+ + L = HgL (with L as ligand). For upstream of Paris, thermodynamic calculations indicated that the vast majority of Hg-DOM was associated with hydrophobic DOM. In contrast, in the Paris main wastewater treatment plant effluents, Hg-DOM was mainly bound to hydrophilic DOM. Simple dilution calculations highlighted that due to the large DOM loading of urban discharges, the hydrophilic urban DOM ligands may commonly dominate Hg-DOM speciation in the downstream Seine River, except under extreme dilution (i.e. high water episodes or floods).

► Treated WWTP effluents were a major source of fresh hydrophilic organic Hg-ligands.
► Their stability constants (pH ∼ 6.8, I = 0.5 M, T = 25 °C) were greater than 1024.
► They commonly dominated Hg-DOM speciation in the downstream Seine River.