Strong Hg(II) Complexation in Municipal Wastewater Effluent and Surface Waters

The speciation of mercury(II) in the aquatic environment
is greatly affected by the presence of ligands capable of
forming extremely strong complexes with Hg(II). In this study,
a novel competitive ligand exchange (CLE) technique
was used to characterize Hg(II)-complexing ligands in
samples collected from three municipal wastewater treatment
plants, a eutrophic lake, a creek located downstream of
an abandoned mercury mine, and a model water containing
dissolved Suwannee River humic acid. These samples
contained 3.3−15.9 mg/L dissolved organic carbon and were
amended with 1.0−1.7 nM Hg(II) for CLE analysis. Results
indicated that all samples contained labile Hg(II)-complexing ligands with conditional stability constants
similar to those of reduced sulfur-containing ligands. Two
wastewater effluent samples also contained approximately
0.5 nM of ligands that formed extremely strong Hg(II)
complexes that did not dissociate in the presence of
competing ligands. The conditional stability constant of
these extremely strong or nonlabile complexes (i.e., cKHgL)
were estimated to be greater than 1030, for the reaction
Hg2+ + L‘ = HgL. The third wastewater sample and the
eutrophic lake sample contained lower concentrations, 0.07−0.09 nM, of nonlabile Hg(II)-complexing ligands. The
results suggested that these extremely strong Hg(II)-complexing ligands should account for most of the dissolved
Hg(II) species in municipal wastewater effluent and may
dominate Hg(II) speciation in effluent-receiving waters.