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Binding of Mercury(II) to Aquatic Humic Substances: Influence of pH and Source of Humic Substances

Markus Haitzer, * ,
George R. Aiken , and
Joseph N. Ryan

Department of Civil, Environmental, and Architectural Engineering, University of Colorado, Boulder, Colorado 80309, and U.S. Geological Survey, Water Resources Division, 3215 Marine Street, Boulder, Colorado 80303

Posted (abstracted/excerpted) with permission from Haitzer, M.; Aiken, G.R.; Ryan, J.N. Environ. Sci. Technol. 2003, 37, 2436-2441. Copyright 2003 American Chemical Society. Note: Entire paper is available from the Environmental Science and Technology Journal website (journal subscription is required)

Abstract | Figures | Tables | Literature Cited

Abstract

Conditional distribution coefficients (KDOM') for Hg(II) binding to seven dissolved organic matter (DOM) isolates were measured at environmentally relevant ratios of Hg(II) to DOM. The results show that KDOM' values for different types of samples (humic acids, fulvic acids, hydrophobic acids) isolated from diverse aquatic environments were all within 1 order of magnitude (1022.5±1.0 -1023.5±1.0 L kg-1), suggesting similar Hg(II) binding environments, presumably involving thiol groups, for the different isolates. KDOM' values decreased at low pHs (4) compared to values at pH 7, indicating proton competition for the strong Hg(II) binding sites. Chemical modeling of Hg(II)-DOM binding at different pH values was consistent with bidentate binding of Hg(II) by one thiol group (pKa = 10.3) and one other group (pKa = 6.3) in the DOM, which is in agreement with recent results on the structure of Hg(II)-DOM bonds obtained by extended X-ray absorption fine structure spectroscopy (EXAFS).


Figures

plot showing the atomic C/S ratio versus atomic C/N ratio for various dissolved organic matter isolates graph of conditional distribution coefficients measured for the binding of mercury (II) to various dissolved organic matter isolates
Figure 1. Plot showing the atomic C/S ratio versus atomic C/N ratio for the various DOM isolates. [larger image] Figure 2. Conditional distribution coefficients (log KDOM') measured for the binding of Hg(II) to various DOM isolates. Error bars represent standard deviations calculated from three replicates. [larger image]


graph of binding of mercury (II) to F1 hydrophobic organic acid as a function of solution pH
Figure 3. Binding of Hg(II) to F1 HPOA as a function of solution pH. Binding is given as conditional distribution coefficient log KDOM' (see eq 2) at I = 0.1 M. Circles are the experimental data points (± standard deviation). The solid line represents the model fit to the experimental data using parameters in Table 2. [larger image]


Tables

Table 1. Site Descriptions for Aquatic Organic Matter Isolates
sample site description
Suwannee River Humic (SRHA) and Fulvic Acid (SRFA) Black water river draining the Okeefenokee Swamp; Sampled at Fargo, GA; Vegetation types: Southern Flood plain Forest (Quercus, Nyassa, Taxodium); International Humic Substances Society Standard Humic and Fulvic Acids.
Ogeechee River Humic (OGHA) and Fulvic Acid (OGFA) Small river draining the piedmont in Eastern Georgia. Sampled at Grange, GA. Vegetation types: Oak-Hickory-Pine Forest (Quercus, Carya, Pinus).
Coal Creek Fulvic Acid (CCFA) Small mountain stream draining the Flattops Wilderness Area, CO. Vegetation type: Spruce-Fir Forest (Picea, Abies).
F1 Hydrophobic Acid (F1 HPOA) Eutrophied marshland located in Water Conservation Area 2A in the northern Everglades. Vegetation dominated by cattails (typha latifolia).
2BS Hydrophobic Acid (2BS HPOA) Relatively pristine marshland located in Water Conservation Area 2B in the northern Everglades. Vegetation dominated by saw grass.


Table 2. Chemical Characteristics of Isolated Samples
  ash-free elemental
composition (wt %)
       
sample C H O N S relative
% Sred
relative %
thiol+sulfide
MW
(daltons)
% aromatic C
SRHA 53.4 3.9 40.9 1.1 0.7 55 14 1399a 35.1
SRFA 54.2 3.9 38.0 0.7 0.4 48 12 1360b 22.9
OGHA 54.6 4.9 36.8 1.6 1.8 37 14 1906 40.8
OGFA 54.0 4.0 38.5 0.9 1.3 44 18 1021 24.7
CCFA 52.8 4.5 38.4 1.0 0.7 47 11 1180 28.0
F1 HPOA 52.2 4.6 39.9 1.5 1.7 60 22 103a 25.4
2BS HPOA 52.3 4.8 40.2 1.6 1.2 51 23 953a 21.3

a Ref 6. b Ref 23.


Table 3. Equilibrium Binding Constants Resulting from the Best Fit of the Model to the Experimental Data at Various pH Valuesa
pKa1
10.3
± 0.1
pKa2
6.3
± 0.1
log KHgsite
28.7
± 0.1
site concentration
5 x 10-9 mol mg-1

apKa1 and pKa2 are deprotonation constants for the two different functional groups of the bidentate Hg(II) binding site. log KHgsite is the equilibrium binding constant for the reaction of Hg2+ with the fully deprotonated binding site. Errors in binding constants were calculated as a variation in binding constant resulting in a 5% variation of the sum of fit errors. Note that errors reported here only relate to the fitting procedure. Overall errors for absolute binding constants are significantly higher (see ref 12).


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* Corresponding author phone: (303)492-0772; fax: (413)702-4196;
e-mail: mhaitzer@usgs.gov.
† University of Colorado.
‡ U.S. Geological Survey.

Related information:

SOFIA Project: Interactions of Mercury with Dissolved Organic Carbon in the Florida Everglades



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