HR: 0830h
AN: B51C-0972    [PDF]
TI: Determination of Mercury Complexation in Seawater by Competitive Ligand Equilibration Using Thiosalicylic Acid
AU: * Han, S
EM: shan@tamug.tamu.edu
AF: Laboratory for Oceanographic and Environmental Research, Department of Oceanography, Texas A\&M University, 5007 Avenue U, Galveston, TX 77551 United States
AU: Gill, G A
EM: gillg@tamug.edu
AF: Laboratory for Oceanographic and Environmental Research, Department of Oceanography, Texas A\&M University, 5007 Avenue U, Galveston, TX 77551 United States
AB: The measurement of quantitative information, such as the concentration and binding strengths, of organic ligands which complex mercury in natural water is essential for understanding the aquatic biogeochemistry of mercury. Organic thiols have been suggested as a binding site for mercury complexation in natural waters based on its high binding constant and experimental observations with soil humic matter. To date, the direct characterization of the complexation between aquatic organic matter and mercury is limited. This work describes a competitive ligand equilibration method, using thiosalicylic acid as a competing ligand, to determine the complexation of mercury by organic ligands in natural water. Two classes of ligands were observed, one hydrophilic and one hydrophobic. The hydrophilic organic ligands had concentrations of 10 - 60 pM and conditional stability constants of 10$^{26.4 - 27.9}$ in coastal and estuarine samples at pH 9.7. Hydrophobic organic ligands with concentrations between 0.5 and 8.0 nM and stability constants of 10$^{21.7 - 24.1}$ were detected in the same samples at pH 7.0 - 7.5. These latter ligands were determined using the natural chloride ion present as a competing ligand. Both results agree with the extractability of natural mercury complexes. The hydrophilic ligand dominates at low pM mercury levels, while the hydrophobic ligand becomes dominant at nM mercury concentrations. The accuracy of the method was tested using a model ligand, diethyldithiocarbamate, yielding a relative error of 6 %. Speciation calculations using Galveston Bay estuarine water show that $>$99.9 % of the dissolved mercury is organically complexed at salinities $<$5 ppt. Competition with chloride reduces organic complexation at higher salinities. A linear relationship exists between log K­_ and the log [L] of the mercury complexing organic ligands, indicating a continuity of binding characteristics over a broad salinity range in natural water.
DE: 4805 Biogeochemical cycles (1615)
DE: 4807 Chemical speciation and complexation
DE: 4835 Inorganic marine chemistry
DE: 4875 Trace elements
SC: Biogeosciences [B]
MN: 2003 Fall Meeting