(Total and)Methyl Mercury Export from Denitrifying Bioreactors in Tile-Drained Fields of Central...
-
Upload
emmett-wilkenson -
Category
Documents
-
view
214 -
download
0
Transcript of (Total and)Methyl Mercury Export from Denitrifying Bioreactors in Tile-Drained Fields of Central...
(Total and)Methyl Mercury Export from Denitrifying Bioreactors in Tile-Drained Fields of Central Illinois
Robert J.M. Hudson, NRESRichard A.C. Cooke, ABEUniversity of Illinois at Urbana-Champaign
Acknowledgements• Illinois Sustainable Technology Center
• Funding for the project• Patience with our pace
• Brian Vermillion (foremerly NRES)• MeHg analysis
• Richard Cooke group• Access to field sites and assistance with sampling• Expertise
A Tale of Two Elements
Both have complex biogeochemical cycles involving chemical species in multiple phases and oxidation
states:
Nitrogen
N2(g) , NO3-(aq), NH4
+(aq), Soil Organic N, etc.
Mercury
Hg(g) , Hg2+(aq), CH3Hg+(aq), HgS(s), etc.
A T
ale
of T
wo
Elem
ents
Pollutant Sources and Impacts
Nitrogen• Non-point source pollutant derived from fertilizer overuse
• Nitrate exported from agricultural fields impacts:• Gulf of Mexico ecosystem (hypoxia)• Water quality for communities that obtain drinking water from rivers in
agricultural areas
Mercury• Non-point source pollutant generated via coal combustion and waste
incineration
• Methylmercury strongly bioaccumulates in food webs:• Fish consumption is main source of human exposure to Hg.• Leading cause of freshwater fish consumption advisories in the U.S.• Significant percentage of U.S. women of childbearing age have blood Hg higher
than the USEPA “Threshold Level”.
A T
ale
of T
wo
Elem
ents
Anaerobic Processing
Key processes that determine the environmental impacts of each element occur in anaerobic environments:
Denitrification (alleviates impacts)
NO3-(aq) NO2
-(aq) N2(g)
Mercury methylation (greatly increases impacts)
Hg2+(aq) CH3Hg+(aq)
A T
ale
of T
wo
Elem
ents
O2Respiration
Photosynthesis
Reduction
O2
NO3-
MnO2(s) FeOx (s) SO4
2-
Sediments&Hyporheic
Groundwater
Hg(SH)2 Methylation MeHg
NO3-
H2O N2 Mn2+
Fe2+
H2S
MeHg
Oxidation
SRB
HgII
Plants Trophic Transfer
HgII
Anaerobic Transformations in Aquatic Ecosystems
Nitrate in surface waters inhibits Hg methylation since denitrifiers can out compete Fe- and Sulfate-reducing bacteria for energy (reduced carbon compounds).
A T
ale
of T
wo
Elem
ents
Methylation Theoretical ConsiderationsBioavailability of HgII in Natural Ecosystems
RSH + Hg(OH)2 + 2 H+ RS-Hg (unavailble)
H2S + RS-Hg Hg(SH)2
Sulfate-reducing bacteria control this reaction by producing H2S. SRB also have the right enzymes to methylate. Fe reducers do as well, but may depend on SRB to make HgII bioavailable.
Clear Pond, Kickapoo State Park Profile (August 2010)
MeHg Analysis at UIUC by W.Y. Chen
Landscape-Scale Effects of Natural Anaerobic Ecosystems on Mercury Cycling
Brigham et al. (2009)
Constructed Anaerobic Ecosystems
Denitrifying Bioreactors: Constructed Anaerobic Ecosystems
• Pioneered by Richard Cooke, ABE-UIUC.
• Designed to reduce nitrate export from tile-drained fields.
• Economical• Small footprint• Wood chips are inexpensive• Minimal cost of water control structures (<$10k) • Little time required to operate/maintain
• Field tests show their high efficiency at NO3- removal.
Den
itri
fyin
g B
iore
acto
rs
Monticello Bioreactor Site
Den
itri
fyin
g B
iore
acto
rs
Den
itri
fyin
g B
iore
acto
rs
CapacityControlStructure
Woodchips
DiversionStructure
Second Generation Bioreactors
Den
itri
fyin
g B
iore
acto
rs
Andrus et al. (2010)
Den
itri
fyin
g B
iore
acto
rs
Capacity controlstructure
Up
to
so
ilsu
rfac
e
Side View
Trench bottom 1’ below tile invert
5’ section of non-perforated tile
Length dependent on treatment area
Diversionstructure
Top View
5’ Soil backfill
10’ W
ide
Den
itri
fyin
g B
iore
acto
rs
LR = 95.524 LD-0.361
R² = 0.93
30
40
50
60
70
80
90
100
0 1 2 3 4 5 6 7 8 9
Load
Red
uctio
n, L
R (%
)
Loading density, LD (Acres per 100 sq. feet of the bioreactor)
BIOREACTOR EFFICACY CURVE
Den
itri
fyin
g B
iore
acto
rs
Biogeochemistry
Den
itri
fyin
g B
iore
acto
rs
Abrus et al.
Bioreactor Methylmercury Study Objective
1. Determine whether denitrifying bioreactors produce methylmercury due to anoxic environments formed within bioreactor.
2. Determine total Hg export flux.3. Compare to natural levels in environment.
Bioreactor Methylmercury Study Design• Synoptic sampling design
– Collect samples after storm events and other periods when tiles are flowing– Inlet and outlet sampled simultaneously– Sampled periodically from summer 2008-June 2009– Sampled again in summer 2010.– Preserved by filtration/acidification or freezing.
• Analyze– Dissolved Methylmercury (MeHg)– Dissolved organic carbon– Sulfate, nitrate, chloride
– Only a limited number of samples have been analyzed to date.
Sampling Bioreactors
Figure 1. Schematic diagram of a sub-surface bioreactor (R. Cooke).
Inlet Samples
Outlet Samples
UIUC Method for Hg Speciation Analysis• Shade and Hudson (2005) Environmental Science and
Technology
• Shade, Hudson, et al. patent (circa 2007)
• Vermillion and Hudson (2007). Analytical and Bioanalytical Chemistry. Preparation method for water samples.
pH-Modulated Thiol-Thione Switch After Laws (1970)
+
Thiol R
esin
-S-H
-S-H
-S-HN
C S
N H
HH
Hg
H
H
H
H
+
Thiol R
esin
N
C S
N H
HH
Hg
H
H
H
H
-S-
-S-H
-S-HN
C S
N H
HH
Hg
H
H
H
H
+
pH<2: Desorption Favored
pH > 3: Adsorption Favored
H+
H
H
University of Illinois at Urbana-Champaign
Thiourea Complexes of Hg2+ and CH3Hg+
N
C S
N H
HH
Hg
H
H
H
H
N
C S
N
Hg
H
H
H
H
N
CS
N
H
H
H
H
+
2+
MeHg+
Hg2+
University of Illinois at Urbana-Champaign
Mercury-thiourea complex ion chromatography
Shade and Hudson, ES&T 2005
University of Illinois at Urbana-Champaign
Dissolved methylmercury:Thiourea-catalyzed
solid phase extraction
Vermillion and HudsonAnalytical and Bioanalytical Chemistry (2007)
Brian Vermillion
University of Illinois at Urbana-Champaign
Validation of Sediment and Biota Sample Preparations
0.001
0.01
0.1
1
10
0.001 0.01 0.1 1 10
Mea
sure
d M
eHg
Valu
e (u
g/g-
dw)
Certified MeHg Value (ug/g-dw)
Biota
Sediments
1:1
Biota: Leaching of tissues in acidic TU (Shade, ES&T 2008)
Sediments: H2SO4+KBr Digestion/Toluene Extraction(Vermillion, Shade, and Hudson, in prep)
Comparison of Methods for Analysis of Dissolved MeHg
Ultraclean sample
collection
Sample preservation
Extraction from Sample
Matrix
Pre-concentration/
Loading
Chromatographic Separation of
Hg species
Sensitive detection
Distillation
________
TU-Catalyzed SPE
Ethylation/Purge & Trap__________
Elute SPE/Online Trap
Gas Chromatography
_____________
Ion Chromatography
AFS
_________
AFS
University of Illinois at Urbana-Champaign
0.01
0.1
1
0.01 0.1 1
[MeH
g]D
EID
(ng/
L)
[MeHg]TU (ng/L)
Peterborough
Calumet
Lake 658
Sulfidic
Sulfidic
Sulfidic
R=100%
R=50%
R=20%
Intercomparison with Distillation/Ethylation ICP-MSTrent University (Hintelmann)
y = 0.62xR² = 0.65
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
FMeH
g DE
(ng/
L)
FMeHgTU (ng/L)
0
5
10
15
20
25
N.D. 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
Num
ber o
f Sam
ples
FMeHgTU (ng/L)
Median = 0.21 ng/L
y = 0.71xR² = 0.72
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4
FMeH
g DE
(ng/
L)
FMeHgTU (ng/L)
0
5
10
15
20
25
N.D. 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4
Num
ber o
f Sam
ples
FMeHgTU (ng/L)
Median = 0.28 ng/L
Intercomparison with USGS Wisconsin District Mercury Lab (Krabbenhoft)
Bioreactor Study Results
Dissolved MeHg in Bioreactor Inlets
-Eight non-detects
-Six samples contained detectable MeHg
- Maximum: 0.16 ng/L
- Average: 0.09 ng/L
Dissolved MeHg in Bioreactor Outlets
Farm Progress City Bioreactor
Nitrate Removal
“Typical” Hg Levels• Groundwater
• Total Hg is very low (<0.5 ng/L)• MeHg is often non-detectable.
• Surface Waters• Total Hg is usually 1-5 ng/L in unpolluted systems• MeHg is usually <0.5 ng/L in unpolluted systems
Possible Relationship between Methylmercury Production to Sulfate Consumption in Bioreactors
Dissolved Organic Carbon• Dissolved organic carbon is known to be a carrier of mercury
and methylmercury.
• DOC in inlets: • Geometric mean of 1.6 mg-C/L
• DOC in outlets: • Geometric mean of 16 mg-C/L • Exhibits seasonality
Summary of Results to Date• Levels in bioreactor discharge are much higher than in tile
drain water entering bioreactors.• Methylmercury is clearly produced in bioreactors.• Some combination of increase in DOC and decrease in sulfate
may be responsible.• Levels in bioreactor discharge are much higher than typical
surface water values (0.1-0.3 ng/L).
What is source of Hg?• Tile water:
• Groundwater is typically very low except when preferential flow occurs
• Could accumulate on wood chips during high flow (aerobic conditions) and be released under high flow conditions
• Wood chips:• Trees accumulate Hg from air and soil water in the wood
Methylation Theoretical ConsiderationsBioavailability of HgII in Natural Ecosystems
RSH + Hg(OH)2 + 2 H+ RS-Hg (unavailble)
H2S + RS-Hg Hg(SH)2
Sulfate-reducing bacteria control this reaction by producing H2S. SRB also have the right enzymes to methylate. Fe reducers do as well, but may depend on SRB to make HgII bioavailable.
Questions
What controls the extent of methylation?– Amount of sulfate in tile drainage?– Extent of anoxia?– Flow rate?
Can reactors be designed to minimize MeHg formation?
Capacity controlstructure
Up
to
so
ilsu
rfac
e
Side View
Trench bottom 1’ below tile invert
5’ section of non-perforated tile
Length dependent on treatment area
Diversionstructure
Top View
5’ Soil backfill
10’ W
ide
Trench bottom at tile invert
Den
itri
fyin
g B
iore
acto
rs
Implications
• MeHg is produced in some bioreactors.
• Effect on MeHg fluxes in watersheds needs to be considered
An apparent trade-off:
Reducing nitrate levels to make water drinkable may
make the waters downstream unfishable.
Implications (cont.)
• Reduction of MeHg production likely can be attained by adjusting design:
• Eliminate ponding zones
• Use of low-Hg wood (hypothesis)