Hg
Development of Diffusive Gradient in Thin Film Probes for the Measurement of Hg/MeHg
Danny Reible, Tim Chess, YS Hong, Nate Johnson University of Texas at Austin
Outline Motivation- Mercury processes Porewater Sampling Techniques Background and Theory General DGT fabrication Laboratory Experiments
◦ Resin performance ◦ Measurement of Hg/MeHg
Future Work
Background
methyl mercury
Solid Phase
Aqueous phase
• Mercury vs. methyl mercury (MeHg) • Methylation is a byproduct of microbial activity sulfate reducing bacteria Inorganic Mercury
Methyl Mercury
Hg-NOM Hg(SH)20
HgS2HHgS22-
SO4
2
HS -
HgS0
Hg
2
OM
Hg2+
FeOOH FeS
uncharged, bioavailable mercury-sulfide
CH 3 Hg
Research Approach Aqueous speciation modeling -1
KS0 = 26.7 K-S2H = 31.5
-2
LogK
Hg2++2HS-=Hg(SH)20
37.7
Hg2++2HS-=HgS2H-+H+
31.5
Hg2++2HS-=HgS22-+2H+
23.2
Hg2++HS-=HgSH+
30.2
Hg2++HS-+H2O=HOHgSH0+H+
26.7
Hg2++RS-=HgRS-
0
Hg(SH)2 -3
-
HgS2H log ST
Equilibrium Reaction
-4 2-
HgS2 -5 +
HgSH -6
HgS
0
-7
22.4-23.8* -8 2
3
4
5
6
7
8
9
10
pH
• Equilibrium modeling for aqueous speciation based on sulfide and DOM complexes • Bioavailable complexes at ~20-100uM HS- neutral pH
11
Results
in-situ model application
model results & sensitivity analysis In-situ BGC observations
Methyl mercury model predictions MeHg [% Hg-tot]
Sulfide [mM],MeHg [%Hgtot]
2
4
6
2
4
-2
-2
-1
0
0
2
1
Depth [cm]
Depth [cm]
0
0
4 6 8
2 3 4 5 6
7.0
8.0
9.0
pH Total Total Measured Measured Sulfide Sulfide [mM] [mM] Obs % MeHg pH pH
7
Obs % MeHg Predicted MeHg Range (Model B')
Selected Characteristics South River Fine Bank Deposit Sample
Sediment
◦ Hg 9.7 ± 1 mg/kg ◦ AVS – 8.85 µmole/g ◦ OM – 9.2 ± 0.7
Porewater ◦ pH – 6.97 ◦ Hg – 106 ±7.9 ng/L aqueous equilibrium filtered
Goal
Primary Goal ◦ Insitu measurement of porewater concentrations Mercury 10-100 ng/L Methyl mercury 0.1-10 ng/L
Secondary Goal ◦ Couple with direct or indirect biogeochemical measures Sulfate reduction rate Sulfide concentrations
Porewater Sampling Techniques
Active sampling techniques ◦ Centrifugation and Filtration ◦ Displacement ◦ Direct water sampling (Henry’s sampler)
Passive sampling techniques ◦ Diffusive gradient in thin films (DGT) ◦ Advantages Minimal disturbance Suspension of particles Redox conditions Flexible Vertical Resolution
Diffusion Gel Thin Film Device
Resin – Chelex 100 ◦ Hg, MeHg – thiol (3mercaptylpropyl silica resin) ◦ Acrylamide gel base
Diffusion layer ◦ Agarose gel
Background and Theory
Davison & Zhang – Lancaster, UK Based on Fick’s 1st Law of Diffusion ◦ Measures flux, not an equilibrium device
Diffusion of metal = to that in pure water Cb
DBL
Resin Gel
Diffusive Gel
Concentration
Distance
Solution
Background and Theory
DGT theory also applicable to sediments ◦ Difference is solid phase influence
Ci
Resin Gel
Concentration
Diffusive Gel
Csoln
k1
Csolid
Sediment
Distance
Pseudo steady state achieved in ~ day deployments times
Background and Theory
Current Field Applications ◦ Bulk water, Sediment, and Soil ◦ Mn, Zn, Cu, Cd, Ni, Fe, Pb, Al, etc. (Chelex-100)
Developing Applications ◦ Hg & Me Hg ◦ Requires Different sorbing resin Strong and complete sorption of mercury species Complete extraction of mercury species during analysis
DGT Fabrication
Both diffusive and resin gel thickness controlled by glass plates/spacers Insert Picture
Probe Cover
0.45µm Filter Diffusive Layer Resin Gel Layer
Probe Base
Complete Probe
DGT Fabrication Probe Base Resin Layer Diffusive Layer 0.45 µm Filter Layer Probe Retaining Wall
Complete Sediment Probe
Laboratory Experiments (3-M)
Sorption and Extraction Efficiency Experiments ◦ Evaluated after 24 hr equilibrium while in end-overend tumbler (CVAFS) ◦ 100 ppt to 700 ppt Hg2+ concentrations ◦ 92% average Hg sorption ◦ 96.5% average Hg extraction efficiency w/ HCl ◦ Demonstrates potential of resin for in-situ Hg detection
Laboratory Experiments (3-M) Sorption Efficiency
% Hg2+ Removed
100 80 60 40 20 0 0
200
400
600
Initial Hg2+ Conc (ppt)
800
Hg2+ Mass Extracted by HCl (ng)
Extraction Efficiency 30
y = 0.9967x R2 = 0.9471
25 20 15 10 5 0 0
10
20
Theoretical Hg2+ Mass in Resin (ng)
30
Laboratory Experiments (3-M)
Time dependent sediment experiment ◦ Purpose: optimize specific resin for particular site and validate use in sediments ◦ South River, VA site sediment ◦ 3 probes per 24 hour interval for 4 days ◦ Each probe elution measured twice
Laboratory Experiments (3-M)
Experiment Set-up
Time Dependent Sorption in Site Sediment 18
Avg Mass Measured by DGT 16
Hg Mass (ng)
14 12 10 8 6 4 2 0 0
1
2
3
Time (days)
4
5
Time Dependent Sorption in Site Sediment 18
Avg Mass Measured by DGT
M= DCbtA Δg
16
Hg Mass (ng)
14
Cb= MΔg DtA
12 10 8 6 4 2 0 0
1
2
3
Time (days)
4
5
Time Dependent Sorption in Site Sediment Cb= MΔg DtA
Avg Mass Measured by DGT
18
Model Prediction (D=8.7E-6 cm2/s)
130 PPT
Best Fit Line
16
Hg Mass (ng)
14 12
110 PPT
10 8 6 4 2 0 0
1
2
3
Time (days)
4
5
Laboratory Experiments (3-M)
Time dependent sediment experiment ◦ Data follows linear uptake ◦ Data approx. fits model with diffusivity of Hg in water ◦ 130 ppt (DGT Measured) vs. 110 ppt (Equilibrium Measured)
Laboratory Experiments (3-M) Methyl Mercury Initial research shows promise for measuring porewater Me Hg
DGT for MeHg 0.35
MeHg Accumulated in Resin (ng)
Deff=5.0E‐6 cm2/s Total MeHgpw=3.5 ng/L
0.3 0.25 0.2 0.15 0.1 0.05 0 0
2
4 Time (days)
6
Laboratory Experiments (3-M)
Conclusions ◦ 3-M demonstrates affinity for Hg and Me Hg, has a high extraction efficiency, and is compatible with the gel making process ◦ Method ◦ Best results if you are able to utilize a site specific sediment Deff for determining the pore water concentration ◦ Even w/o calibration, the 3-M DGT provides good estimate of pore water concentration
Future Work- South River
sediment experiments (sediment probe) Field deployment June 2010
DBL
Used equation proposed by Zhang and Davidson
Different thicknesses deployed for 6 hrs Plot 1/M vs. gel thickness: DBL=intercept/slope ≈ 0.4mm Estimating Diffusive Boundary Layer 0.03 y = 0.011x + 0.0047 R² = 0.987
1/M (ng‐1 )
0.02
0.01
0 0.6
0.8
1
1.2
Gel Thickness (mm)
1.4
1.6
SOURCES
www.dgtresearch.com
Zhang, H., & Davison, W. (1995). Performance Characteristics of Diffusion Gradients in Thin Films for the In Situ Measurement of Trace Metals in Aqueous Solution. Analytical Chemistry , 67 (19), 3391-3400.
Clarisse, O. & Hintelmann, H (2006). Measurements of Dissolved Methylmercury in Natural Waters Using Diffusive Gradients in Thin Film (DGT). Journal of Environmental Monitoring, 8, 1242-1247.
Harper, M., Davison, W., Tych, W. (2000). DIFS – A Modeling and Simulation Tool for DGT Induced Trace Metal Remobilization in Sediments and Soils. Environmental Modelling & Software, 15, 55-66.
Danny Reible, Tim Chess, YS Hong, Nate Johnson University of Texas at Austin
Outline Motivation- Mercury processes Porewater Sampling Techniques Background and Theory General DGT fabrication Laboratory Experiments
◦ Resin performance ◦ Measurement of Hg/MeHg
Future Work
Background
methyl mercury
Solid Phase
Aqueous phase
• Mercury vs. methyl mercury (MeHg) • Methylation is a byproduct of microbial activity sulfate reducing bacteria Inorganic Mercury
Methyl Mercury
Hg-NOM Hg(SH)20
HgS2HHgS22-
SO4
2
HS -
HgS0
Hg
2
OM
Hg2+
FeOOH FeS
uncharged, bioavailable mercury-sulfide
CH 3 Hg
Research Approach Aqueous speciation modeling -1
KS0 = 26.7 K-S2H = 31.5
-2
LogK
Hg2++2HS-=Hg(SH)20
37.7
Hg2++2HS-=HgS2H-+H+
31.5
Hg2++2HS-=HgS22-+2H+
23.2
Hg2++HS-=HgSH+
30.2
Hg2++HS-+H2O=HOHgSH0+H+
26.7
Hg2++RS-=HgRS-
0
Hg(SH)2 -3
-
HgS2H log ST
Equilibrium Reaction
-4 2-
HgS2 -5 +
HgSH -6
HgS
0
-7
22.4-23.8* -8 2
3
4
5
6
7
8
9
10
pH
• Equilibrium modeling for aqueous speciation based on sulfide and DOM complexes • Bioavailable complexes at ~20-100uM HS- neutral pH
11
Results
in-situ model application
model results & sensitivity analysis In-situ BGC observations
Methyl mercury model predictions MeHg [% Hg-tot]
Sulfide [mM],MeHg [%Hgtot]
2
4
6
2
4
-2
-2
-1
0
0
2
1
Depth [cm]
Depth [cm]
0
0
4 6 8
2 3 4 5 6
7.0
8.0
9.0
pH Total Total Measured Measured Sulfide Sulfide [mM] [mM] Obs % MeHg pH pH
7
Obs % MeHg Predicted MeHg Range (Model B')
Selected Characteristics South River Fine Bank Deposit Sample
Sediment
◦ Hg 9.7 ± 1 mg/kg ◦ AVS – 8.85 µmole/g ◦ OM – 9.2 ± 0.7
Porewater ◦ pH – 6.97 ◦ Hg – 106 ±7.9 ng/L aqueous equilibrium filtered
Goal
Primary Goal ◦ Insitu measurement of porewater concentrations Mercury 10-100 ng/L Methyl mercury 0.1-10 ng/L
Secondary Goal ◦ Couple with direct or indirect biogeochemical measures Sulfate reduction rate Sulfide concentrations
Porewater Sampling Techniques
Active sampling techniques ◦ Centrifugation and Filtration ◦ Displacement ◦ Direct water sampling (Henry’s sampler)
Passive sampling techniques ◦ Diffusive gradient in thin films (DGT) ◦ Advantages Minimal disturbance Suspension of particles Redox conditions Flexible Vertical Resolution
Diffusion Gel Thin Film Device
Resin – Chelex 100 ◦ Hg, MeHg – thiol (3mercaptylpropyl silica resin) ◦ Acrylamide gel base
Diffusion layer ◦ Agarose gel
Background and Theory
Davison & Zhang – Lancaster, UK Based on Fick’s 1st Law of Diffusion ◦ Measures flux, not an equilibrium device
Diffusion of metal = to that in pure water Cb
DBL
Resin Gel
Diffusive Gel
Concentration
Distance
Solution
Background and Theory
DGT theory also applicable to sediments ◦ Difference is solid phase influence
Ci
Resin Gel
Concentration
Diffusive Gel
Csoln
k1
Csolid
Sediment
Distance
Pseudo steady state achieved in ~ day deployments times
Background and Theory
Current Field Applications ◦ Bulk water, Sediment, and Soil ◦ Mn, Zn, Cu, Cd, Ni, Fe, Pb, Al, etc. (Chelex-100)
Developing Applications ◦ Hg & Me Hg ◦ Requires Different sorbing resin Strong and complete sorption of mercury species Complete extraction of mercury species during analysis
DGT Fabrication
Both diffusive and resin gel thickness controlled by glass plates/spacers Insert Picture
Probe Cover
0.45µm Filter Diffusive Layer Resin Gel Layer
Probe Base
Complete Probe
DGT Fabrication Probe Base Resin Layer Diffusive Layer 0.45 µm Filter Layer Probe Retaining Wall
Complete Sediment Probe
Laboratory Experiments (3-M)
Sorption and Extraction Efficiency Experiments ◦ Evaluated after 24 hr equilibrium while in end-overend tumbler (CVAFS) ◦ 100 ppt to 700 ppt Hg2+ concentrations ◦ 92% average Hg sorption ◦ 96.5% average Hg extraction efficiency w/ HCl ◦ Demonstrates potential of resin for in-situ Hg detection
Laboratory Experiments (3-M) Sorption Efficiency
% Hg2+ Removed
100 80 60 40 20 0 0
200
400
600
Initial Hg2+ Conc (ppt)
800
Hg2+ Mass Extracted by HCl (ng)
Extraction Efficiency 30
y = 0.9967x R2 = 0.9471
25 20 15 10 5 0 0
10
20
Theoretical Hg2+ Mass in Resin (ng)
30
Laboratory Experiments (3-M)
Time dependent sediment experiment ◦ Purpose: optimize specific resin for particular site and validate use in sediments ◦ South River, VA site sediment ◦ 3 probes per 24 hour interval for 4 days ◦ Each probe elution measured twice
Laboratory Experiments (3-M)
Experiment Set-up
Time Dependent Sorption in Site Sediment 18
Avg Mass Measured by DGT 16
Hg Mass (ng)
14 12 10 8 6 4 2 0 0
1
2
3
Time (days)
4
5
Time Dependent Sorption in Site Sediment 18
Avg Mass Measured by DGT
M= DCbtA Δg
16
Hg Mass (ng)
14
Cb= MΔg DtA
12 10 8 6 4 2 0 0
1
2
3
Time (days)
4
5
Time Dependent Sorption in Site Sediment Cb= MΔg DtA
Avg Mass Measured by DGT
18
Model Prediction (D=8.7E-6 cm2/s)
130 PPT
Best Fit Line
16
Hg Mass (ng)
14 12
110 PPT
10 8 6 4 2 0 0
1
2
3
Time (days)
4
5
Laboratory Experiments (3-M)
Time dependent sediment experiment ◦ Data follows linear uptake ◦ Data approx. fits model with diffusivity of Hg in water ◦ 130 ppt (DGT Measured) vs. 110 ppt (Equilibrium Measured)
Laboratory Experiments (3-M) Methyl Mercury Initial research shows promise for measuring porewater Me Hg
DGT for MeHg 0.35
MeHg Accumulated in Resin (ng)
Deff=5.0E‐6 cm2/s Total MeHgpw=3.5 ng/L
0.3 0.25 0.2 0.15 0.1 0.05 0 0
2
4 Time (days)
6
Laboratory Experiments (3-M)
Conclusions ◦ 3-M demonstrates affinity for Hg and Me Hg, has a high extraction efficiency, and is compatible with the gel making process ◦ Method ◦ Best results if you are able to utilize a site specific sediment Deff for determining the pore water concentration ◦ Even w/o calibration, the 3-M DGT provides good estimate of pore water concentration
Future Work- South River
sediment experiments (sediment probe) Field deployment June 2010
DBL
Used equation proposed by Zhang and Davidson
Different thicknesses deployed for 6 hrs Plot 1/M vs. gel thickness: DBL=intercept/slope ≈ 0.4mm Estimating Diffusive Boundary Layer 0.03 y = 0.011x + 0.0047 R² = 0.987
1/M (ng‐1 )
0.02
0.01
0 0.6
0.8
1
1.2
Gel Thickness (mm)
1.4
1.6
SOURCES
www.dgtresearch.com
Zhang, H., & Davison, W. (1995). Performance Characteristics of Diffusion Gradients in Thin Films for the In Situ Measurement of Trace Metals in Aqueous Solution. Analytical Chemistry , 67 (19), 3391-3400.
Clarisse, O. & Hintelmann, H (2006). Measurements of Dissolved Methylmercury in Natural Waters Using Diffusive Gradients in Thin Film (DGT). Journal of Environmental Monitoring, 8, 1242-1247.
Harper, M., Davison, W., Tych, W. (2000). DIFS – A Modeling and Simulation Tool for DGT Induced Trace Metal Remobilization in Sediments and Soils. Environmental Modelling & Software, 15, 55-66.
