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RESULTS AND DISCUSSIONS

Inorganic mercury speciation

The following species were included in the model calculations comparing WHAM and PHREEQC (the MINTEQ database used for mercury speciation calculation in PHREEQC was modified to match that of WHAM): HgOH+, Hg(OH)2, Hg(OH)3-, HgSO4, HgCl+, HgCl2, HgCl3- and HgCl4-2. WHAM and PHREEQC depict similar mercury species distributions as a function of solution pH (Figure 1 A). The predominant species in both model calculations is the uncharged dichloro complex below pH=7. Above pH=8 the predominant mercury containing ion pair is the dihydroxy species. Equal concentrations of each complex occur in the pH range 7 to 7.5.

The following species were included in the model calculations comparing WHAM and the unmodified (MINTEQ) PHREEQC: Hg0, Hg2+2, HgCl2, HgCl3-, HgClOH, HgCl4-2, Hg(OH)2, HgOH+, Hg+2, HgSO4 (Figure 1 B). Both the modified and the unmodified PHREEQC databases depict similar mercury species distributions as a function of solution pH. The results differ because the PHREEQC database contains a species not included in WHAM (i.e. HgClOH). This complex comprises a maximum of 20% of the total dissolved mercury concentration at pH=7 (Figure 1 B).

Mercury binding to dissolved organic carbon

In addition to the aforementioned inorganic species, the WHAM model also incorporates two mercury fulvic acid complexes: Hg2+-DOC and HgOH+DOC.

The dissociation of fulvic acid:

RAHZ=RAZ-1 + H+

is described in WHAM by an apparent (dependent on the degree of fulvic acid ionization and charge) ionization constant K(Z);

K(Z)=Kint(exp(2wZ))=[RAZ-1]{H+}/[RAHZ]

Where

Kint = the intrinsic ionization constant

W = the electrostatic interaction factor

Z = the charge on the fulvic acid molecule

[RAH]Z = solution concentration of fulvic acid of charge Z

[RAZ-1] = solution concentration of ionized fulvic acid of charge Z-1

{H+} = solution hydrogen ion activity

WHAM expresses metal binding to DOC as proton-metal exchange reactions:

RAHZ+MZ+ = RAMZ+z-1 + H+

for which an equilibrium expression dependent on the ionic charges Z and z, can be written in terms of an apparent constant as in the case of proton dissociation:

Kapp = KMHAexp(-2w(z-1)Z) = [RAM]{H+}/[RAH]{Mz+

where:

z = charge of the mercury containing species binding to DOC

{Mz+} = activity of solution mercury containing species

[RAM] = solution concentration of mercury containing species

K(MHA) = the intrinsic mercury species-DOC interaction coefficient

Values of the intrinsic mercury species-DOC interaction coefficient have been estimates from literature data.

Mercury species as a function of solution pH plots (Figure 2) show dissolved organic carbon concentrations above 10 milligrams per liter shift mercury species distribution so that there are equal amounts of the dichloro mercury complex and the hydroxy mercury-DOC complex at pH=5.8. At the measured DOC concentration (35.4 milligrams carbon per liter) and pH of the surface water sample collected at the WCA 2A site U3 in December 1995 the predominant dissolved mercury containing species was a mercury hydroxo-fulvic acid complex. The mercury ion plus hydroxide ion apparent (valid at the U3 temperature, ionic strength, and pH) binding constant to DOC is about 1023.

Sulfide ion (from sediment bacterial sulfate reduction) has been suggested as a key mediator of the mercury methylation process in the Everglades (C. Gilmore, personal communication, 1997). Sulfide-mercury ion pair formation reduces mercury bioavailability. Sulfide and sulfur containing ligands (in both simple organic compounds and incorporated into fulvic acids) shift mercury speciation to mercury-sulfur and mercury-organosulfur complexes. However, preliminary examination of mercury sulfide and mercury thiol binding constants suggests that for the average sulfur content (1% +/- 0.5%) of Everglades fulvic acid, fulvic acid sulfhydrl groups will preferentially bind mercury in competition with sulfide ion.

Cinnabar should form in Everglades waters containing sulfide ion (at concentrations above about 10-9 moles per liter). Calculations using PHREEQC for U3 suggest (depending on the redox potential) significant cinnabar supersaturation at very low DOC concentrations. Laboratory investigations show that Everglades DOC enhances mercury release from cinnabar. Laboratory studies in progress are also determining DOC-mercury binding constants initially at pH=6, total mercury concentration of about 10-8 moles per liter and DOC concentration of about 20 milligrams carbon per liter.

Analysis of DOC and sulfide competition for mercury binding is in progress. Everglades fulvic acid-mercury interactions are being characterized in cooperation with the Department of Civil Engineering, University of Colorado at Boulder.

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Last modified: Monday, 15-June-98 15:50:14 MST