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publications > paper > binding of mercury(II) to dissolved organic matter

Binding of Mercury(II) to Dissolved Organic Matter: The Role of the Mercury-to-DOM Concentration Ratio

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

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

Posted (abstracted/excerpted) with permission from Haitzer, M.; Aiken, G.R.; Ryan, J.N. Environ. Sci. Technol. 2002, 36, 3564-3570. Copyright 2002 American Chemical Society. Note: Entire paper is available from the Environmental Science and Technology Journal website (journal subscription is required)

Abstract | Figures | Tables | Bibliography

Abstract

The binding of Hg(II) to dissolved organic matter (DOM; hydrophobic acids isolated from the Florida Everglades by XAD-8 resin) was measured at a wide range of Hg-to-DOM concentration ratios using an equilibrium dialysis ligand exchange method. Conditional distribution coefficients (K_DOM') determined by this method were strongly affected by the Hg/DOM concentration ratio. At Hg/DOM ratios below approximately 1 µg of Hg/mg of DOM, we observed very strong interactions (K_DOM' = 10^23.2±1.0 L kg^-1 at pH = 7.0 and I = 0.1), indicative of mercury-thiol bonds. Hg/DOM ratios above approximately 10 µg of Hg/mg of DOM, as used in most studies that have determined Hg-DOM binding constants, gave much lower K_DOM' values (10^10.7±1.0 L kg^-1 at pH = 4.9-5.6 and I = 0.1), consistent with Hg binding mainly to oxygen functional groups. These results suggest that the binding of Hg to DOM under natural conditions (very low Hg/DOM ratios) is controlled by a small fraction of DOM molecules containing a reactive thiol functional group. Therefore, Hg/DOM distribution coefficients used for modeling the biogeochemical behavior of Hg in natural systems need to be determined at low Hg/DOM ratios.

Figures


Figure 1. Partitioning of Hg (1.0 µg L^-1) spiked in the outside compartment in the absence (A) and presence (B) of DOM in the inside compartment. (C) Partitioning of DOM spiked in the inside compartment. Conditions for all experiments: RCBT 3500 membrane, pH 7.0, ionic strength 0.1 M. [larger image]	Figure 2. Distribution ratios between HgDOM and HgL in a hypothetical EDLE experiment, where a single type of Hg-binding site is assumed for DOM, the concentration of Hg is smaller than the concentration of the Hg binding site, and the auxiliary ligand L forms 1:1 complexes with Hg. Bubble size and numbers represent Hg distribution ratios (Q = [HgDOM]/[HgL]). [larger image]

Figure 3. Distribution ratios between HgDOM and HgEDTA in a real EDLE experiment using order of magnitude concentration steps for [EDTA] and total Hg. Bubble size and numbers represent Hg distribution ratios (Q = [HgDOM]/[HgEDTA]). Nearly "ideal" behavior (see Figure 2) is only observed at [Hg]_total < 0.1 µg L^-1. At [Hg]_total > 1 µg L^-1, the HgDOM/HgEDTA distribution ratios are lower than expected under "ideal" conditions, suggesting a limitation of the amount of Hg that can be bound by DOM under these experimental conditions. [larger image] Figure 4. Graphical representation of the concept suggesting that the experimental distribution of Hg at concentrations above approximately 1 µg L^-1 was limited by the binding of Hg to a small number of strong binding sites on the DOM ([DOM] = 0.5 mg L^-1). All of the Hg that was not bound by the strong sites was bound by EDTA, which outcompeted weak Hg binding sites on the DOM. The nature of strong Hg binding sites on the DOM is oversimplified in this plot by assuming a certain concentration of one type of strong Hg binding site, whereas real DOM is a mixture of different types of strong binding sites. [larger image]

Figure 5. Distribution ratios between HgDOM and Hg(OH)₂ in an EDLE experiment using hydroxide as the auxiliary ligand. Bubble size and numbers represent Hg distribution ratios (Q = [HgDOM]/[Hg(OH)₂]). Competitive binding of Hg to DOM and OH^- under these conditions suggests interactions of Hg with weak DOM sites that are present at high concentrations. [larger image] Figure 6. Relationship between Hg/DOM concentration ratio (measured in the inside compartment of EDLE experiments) and the conditional distribution coefficient K_DOM' calculated according to eq 3. Experimental conditions: (DOM)_inside = 0.5 mg L^-1, (DOM)_outside = 0.04 mg L^-1, I = 0.1 M, pH = 7.0 (EDTA experiments, < 10 µg of Hg/mg of DOM), pH = 5.6 (OH^- experiment, 10-100 µg of Hg/mg of DOM), pH = 4.9 (OH^- experiment, 100-1000 µg of Hg/mg of DOM). For exact numbers, see Table 1 in the Supporting Information. [larger image]

Tables

Table 1. Selected Characteristics of the F1HPoA Isolate (after (40))
number-average molecular wt (Da)	C^a(%)	total S^a(%)	reduced S^b(% of total S)	carboxyl content^a(meq/g)
1030	52.2	1.73	60.0	5.45

^a Analyzed on dried samples and reported here on an ash-free basis. ^b The relative content of reduced sulfur was measured by sulfur K-edge X-ray absorption near edge structure (XANES) spectroscopy (27).

Table 2. Suitability of Different Auxiliary Ligands for EDLE Experiments^a

low Hg: DOM high Hg/DOM
hydroxide

bromide iodide cysteine EDTA

strong Hg binding + ++ ++ 0+ -

Hg recovery 0 - + + +

no DOM-Hg-L ? ? ? + +^b

suitable for pH 6-8 -^c 0 + + +

reliable log K (Hg-L)^d 0 0 - ++ +

^a (+) = good, (0) = fair, (-) = bad, (?) = not known. ^b Chelate formation with DOM is regarded more likely than formation of DOM-Hg-OH. ^c HgBr_n is only competitive with HgDOM at low pH, where H⁺ competition decreases Hg-DOM binding but does not affect Hg-Br binding. ^d Based on number of literature values, agreement between data, and IUPAC recommendation.

Table 3. Summary of Binding Constants Used for the Calculation of K_DOM'

reaction log ^a ionic strength,
temperature^b ref

Hg²⁺ + EDTA^4- = HgEDTA^2- 21.8^c 0.1 M, 20 °C (41)

HgEDTA^2- + H⁺ = HgEDTAH^- 3.1^c 0.1 M, 20 °C (41)

EDTA^4- + H⁺ = EDTAH^3- 10.2^d 0.1 M, 20 °C (41)

EDTA^4- + 2H⁺ = EDTAH₂^2- 16.4^d 0.1 M, 20 °C (41)

EDTA^4- + 3H⁺ = EDTAH₃^- 19.1^d 0.1 M, 20 °C (41)

EDTA^4- + 4H⁺ = EDTAH₄ 21.1^d 0.1 M, 20 °C (41)

Hg²⁺ + OH^-= HgOH⁺ 10.2 0.1 M, 25 °C (42)

Hg²⁺ + 2OH^-= Hg(OH)₂ 21.2 0.1 M, 25 °C (42)

^a Overall binding constants. ^b Corrections for the experimental temperature of 22 °C using the van't Hoff equation are not significant. ^c IUPAC tentative recommendation (19). ^d IUPAC recommendation (19).

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* Corresponding author phone: 303 541 3009; fax: 303 447 2505; e-mail: mhaitzer@usgs.gov; address: U.S. Geological Survey, 3215 Marine Street, Boulder, CO 80303.
** U.S. Geological Survey.
*** University of Colorado.

Related information:

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

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