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Mercury(II) Sorption to Two Florida Everglades Peats: Evidence for Strong and Weak Binding and Competition by Dissolved Organic Matter Released from the Peat

R. Todd Drexel, Markus Haitzer,, Joseph N. Ryan,*, George R. Aiken, and Kathryn L. Nagy§

Department of Civil, Environmental, and Architectural Engineering, University of Colorado, Boulder, Colorado 80309-0428, U.S. Geological Survey, Water Resources Division, 3215 Marine Street, Boulder, Colorado 80303, and Department of Geological Sciences, University of Colorado, Boulder, Colorado 80309

Posted (abstracted/excerpted) with permission from Drexel, R.T.; Haitzer, M.; Ryan, J.N.; Aiken, G.R., Nagy, K.L. Environ. Sci. Technol., 2002, 36, 4058-4064. © 2002 American Chemical Society. Note: Entire paper is available from the Environmental Science & Technology Journal website (journal subscription is required)

Abstract | Figures | Tables | Literature Cited

Abstract

The binding of mercury(II) to two peats from Florida Everglades sites with different rates of mercury methylation was measured at pH 6.0 and 0.01 M ionic strength. The mercury(II) sorption isotherms, measured over a total mercury(II) range of 10-7.4 to 10-3.7 M, showed the competition for mercury(II) between the peat and dissolved organic matter released from the peat and the existence of strong and weak binding sites for mercury(II). Binding was portrayed by a model accounting for strong and weak sites on both the peat and the released DOM. The conditional binding constants (for which the ligand concentration was set as the concentration of reduced sulfur in the organic matter as measured by X-ray absorption near-edge structure spectroscopy) determined for the strong sites on the two peats were similar ( Kpeat,s = 1021.8±0.1 and 1022.0±0.1 M-1), but less than those determined for the DOM strong sites ( Kdom,s = 1022.8±0.1 and 1023.2±0.1 M-1), resulting in mercury(II) binding by the DOM at low mercury(II) concentrations. The magnitude of the strong site binding constant is indicative of mercury(II) interaction with organic thiol functional groups. The conditional binding constants determined for the weak peat sites ( Kpeat,w = 1011.5±0.1 and 1011.8±0.1 M-1) and weak DOM sites ( Kdom,w = 108.7±3.0 and 107.3±4.5 M-1) were indicative of mercury(II) interaction with carboxyl and phenol functional groups.

Figures

map of the Florida Everglades showing the sampling sites used for this study and other studies and the Water Conservation Area boundaries
FIGURE 1. (left) Map of the Florida Everglades showing the sampling sites used for this study (F1 and 2BS) and other studies and the Water Conservation Area (WCA) boundaries. [larger image]

graphs showing divalent mercury sorption to F1 and 2BS peat fractions graphs showing divalent mercury distribution coefficients as a function of aqueous divalent mercury concentration for peats from sites F1 and 2BS
FIGURE 2. (left) Mercury(II) sorption to F1 and 2BS peat fractions (75-104 µm) at pH 6.0, ionic strength 0.01 M NaNO3, and 40 mg L-1 peat. Fits of double site model using parameters in Tables 4 and 5 are shown as lines for each peat. [larger image] FIGURE 3. (right) Mercury(II) distribution coefficients as a function of aqueous mercury(II) concentration for peats from sites F1 and 2BS. Single site and double site models were used to fit these data with the parameters in Table 4 to produce the conditional binding constants in Table 5. [larger image]

FIGURE 4. (right) Dissolved organic carbon released from F1 and 2BS peat fractions during mercury(II) sorption experiments, pH 6.0, ionic strength 0.01 M NaNO3, and 40 mg L-1 peat. [larger image] graph showing dissolved organic carbon released from F1 and 2BS peat fractions during divalent mercury sorption experiments

Tables

TABLE 1. Water Quality Characteristics of the F1 and 2BS Sites in the Florida Everglades in July 1997 (21)
parameter units site F1 site 2BS
pH   7.3 7.4
specific conductance µS cm-1 1100 440
Mg2+ mM 0.96 0.32
Ca2+ mM 1.6 0.94
Cl- mM 4.0 1.1
SO42- µM 470 69
H2Stotal µM 0.22 0.063
O2 µM 10 100
Hgtotal pM 13 21
dissolved organic carbon mg C L-1 38 17
UVAa cm-1 1.37 0.41
SUVAa L (mg C)-1 cm-1 0.036 0.024
CH3Hgb mol kg-1 1.4 x 10-9 1.3 x 10-8
a Ultraviolet light (254 nm) absorption (UVA) and specific ultraviolet light absorption (SUVA). b Total methylmercury sediment concentrations reported by Gilmour et al. (2).


TABLE 2. Composition of the F1 and 2BS Peat Samples (75-104 µm Size Fraction)
property units site F1 site 2BS detection
limit
C wt %a 50.9 51.9 0.05
H wt % 4.9 5.5 0.05
O wt % 38.9 37.3 0.1
N wt % 3.58 4.05 0.01
S wt % 1.79 1.27 0.01
ash wt %b 5.4 18.0 0.1
aliphatic I (0-62 ppm) C%c 30.0 39.4  
aliphatic II (62-90 ppm) C% 18.9 18.2  
acetal (90-110 ppm) C% 9.0 7.4  
aromatic (110-160 ppm) C% 26.8 20.2  
carboxyl (160-190 ppm) C% 12.4 11.2  
ketone (190-230 ppm) C% 3.0 3.5  
carboxyl meq g-1 5.1 6.4  
phenol meq g-1 0.9 1.1  
total Hg nmol g-1 0.70 ± 0.05 0.90 ± 0.05  
a Elemental compositions reported as percentage of organic mass (corrected for ash). b Ash fraction reported as percentage of total mass of peat. c 13C NMR functional group composition reported as percentage of total carbon.


TABLE 3. Major Ion and Dissolved Organic Matter Concentrations Released from the F1 and 2BS Peat Samples (75-104 µm Size Fraction) into the Experimental Solution (40 mg L-1 Peat, 0.01 M NaNO3, pH 6.0)
property units site F1 site 2BS detection
limit
Na+ M 3 ± 1 x 10-5 2 ± 1 x 10-5 5 x 10-7
K+ M 1 ± 0.4 x 10-5 5 ± 3 x 10-6 2 x 10-7
NH4+ M bdl 2 ± 0.2 x 10-6 9 x 10-7
Mg2+ M bdl 4 ± 0.4 x 10-6 5 x 10-7
Ca2+ M 2 ± 0.2 x 10-5 8 ± 4 x 10-6 3 x 10-7
Hg(II) M bdl bdl 1 x 10-12
Cl- M 1 ± 0.6 x 10-5 6 ± 3 x 10-6 8 x 10-7
NO3- M bdl 1 ± 0.3 x 10-6 2 x 10-7
SO42- M 8 ± 2 x 10-7 6 ± 3 x 10-7 3 x 10-7
dissolved
organic
carbon
mg C L-1 0.9 ± 0.2 1.2 ± 0.3 0.2
UVAa cm-1 0.020 ± 0.004 0.020 ± 0.004  
SUVAa L (mg C)-1 cm-1 0.024 ± 0.008 0.020 ± 0.008  
a Ultraviolet light (254 nm) absorption (UVA) and specific ultraviolet light absorption (SUVA).


TABLE 4. Mercury(II) Complexation with Peat and Dissolved Organic Matter (DOM) in Single Site and Double Site Models and with Hydroxide and Chloride
    fixed parameters  
model reactions site F1 site 2BS fitted
parameters
single site Hg2+ + peat- = Hg-peat+ [peat]tot = 2.4 x 10-4 M [peat]tot = 3.0 x 10-4 M Kpeat
Hg2+ + dom- = Hg-dom+ [dom]tot = 8.3 x 10-6 M [dom]tot = 1.1 x 10-5 M Kdom
double sitea Hg2+ + peats- = Hg-peats+ [peats] = 6.4 x 10-6 M [peats] = 4.8 x 10-6 M Kpeat,s
Hg2+ + peatw- = Hg-peatw+ [peatw] = 2.4 x 10-4 M [peatw] = 3.0 x 10-4 M Kpeat,w
Hg2+ + doms- = Hg-doms+ [doms] = 2.83 x 10-7 M [doms] = 2.78 x 10-7 M Kdom,s
Hg2+ + domw- = Hg-domw+ [domw] = 8.3 x 10-6 M [domw] = 1.1 x 10-5 M Kdom,w
both modelsb, c H-peat = H+ + peat- log Ka,peat = -4.25    
H-dom = H+ + dom- log Ka,dom = -4.25    
Hg2++ OH- = HgOH+ log beta = 10.42    
Hg2++ 2 OH- = Hg(OH)20 log beta = 21.5    
Hg2++ 3 OH- = Hg(OH)3- log beta = 20.6    
Hg2++ Cl- = HgCl+ log beta = 7.1    
Hg2++ 2 Cl- = HgCl2 log beta = 13.73    
Hg2++ 3 Cl- = HgCl3- log beta = 14.7    
Hg2++ 4 Cl- = HgCl42- log beta = 15.4    
Hg2++ Cl- + OH- = HgOHCl log beta = 18.0
   
a Subscripts "s" and "w" refer to strong and weak sites, respectively. b Equilibrium constants for hydroxide and chloride complexation reactions for I = 0 corrected for I = 0.01 by Davies equation (29). c For all equilibrium calculations, pH = 6.0 and [Cl-] = 1.0 x 10-5 M.


TABLE 5. Conditional Equilibrium Binding Constants Resulting from the Best Fits of the Models to the Mercury(II) Isotherm Data
    conditional binding constantsa
model parameters
fit
site F1 site 2BS
single site Kpeat 1012.1±0.1 1011.6±0.1
Kdom 1012.8±0.2 109.0±2.6
double site Kpeat,s 1022.0±0.1 1021.8±0.1
(KRSHg+) (1026.0) (1025.8)
Kpeat,w 1011.8±0.1 1011.5±0.1
Kdom,s 1023.2±0.1 1022.8±0.1
(KRSHg+) (1027.2) (1026.8)
Kdom,w 107.3±4.5 108.7±3.0
a Conditional binding constants for reactions in Table 4 with strong sites as reduced sulfur concentration measured by XANES spectroscopy for Kpeat,s and Kdom,s; with organic thiols (RSH = RS- + H+, pKa = 10; RS- + Hg2+ = RSHg+) for KRSHg+ (shown in parentheses without errors). Errors in conditional binding constants reported as a variation in the binding constant resulting in a 5% variation in the fitting criterion.

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* Corresponding author phone (303)492-0772; fax: (801)327-7112; e-mail: joseph.ryan@colorado.edu.
Department of Civil, Environment & Architectural Engineering, University of Colorado.
U.S. Geological Survey.
§ Department of Geological Sciences, University of Colorado.

Related information:

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




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