USGS: Speciation and Fractionation Modeling Studies -- Dissolved Organic Carbon (DOC)-Mercury Interaction
SPECIATION AND FRACTIONATION MODELING STUDIES -- DISSOLVED ORGANIC CARBON (DOC)-MERCURY INTERACTION
BY Michael M. Reddy, and George Aiken
U.S. GEOLOGICAL SURVEY
- U3 December 1995
We have used the computer ionic speciation model "WHAM" (Windermere Humic Aqueous Model) to characterize the mercury-organic species present in Everglades surface water. WHAM focuses on humic and fulvic acid-metal interactions. These calculations indicate that the major mercury species in solution change from uncharged chloro- and hydroxy-complexes to charged DOC-bound complexes in the presence of high DOC concentrations (that is greater than about 10 milligrams carbon per liter). The mercury ion plus hydroxide ion apparent (valid at the U3 temperature, ionic strength, and pH) binding constant to DOC is about 1023. Inorganic speciation calculated using WHAM has been compared to speciation determined with PHREEQC (pH-redox-equilibrium-equations) with good agreement. WHAM has also been satisfactorily tested using laboratory measurements of calcium ion binding to a soil fulvic acid.
Sulfide and sulfur containing ligands shift mercury speciation to mercury-sulfur and mercury-organosulfur complexes. Preliminary examination of mercury sulfide and mercury thiol binding constants suggests that sulfhydrl groups will preferentially bind mercury in competition with sulfide ion. Cinnabar is supersaturated in some Everglades surface and pore waters. However, Everglades DOC enhances mercury release from cinnabar.