METALS TREATMENT AND IN SITUPRECIPITATION
Mike HaySeptember 30, 2016
© Arcadis 2016
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© Arcadis 2016
About the Presenter
o 720-409-0684
MICHAEL HAY, PHDSenior Geochemist and the Geochemical Modeling focus area lead within Arcadis
© Arcadis 2016
Learning Objectives
After attending this session, participants should be able to:
• Identify challenges unique to in situ metals treatment
• Recognize in situ treatment approaches: injection-based vs. barriers
• Explain the importance of precipitate stability
• Describe geochemical processes/principles that can be used to yield metals precipitation
• Explain the importance of long-term precipitate stability and geochemical parameters that may affect stability
© Arcadis 2016
In Situ Metals Precipitation• In situ treatment of metals vs. organics: What is the fundamental difference?
• Metals cannot (“metals” = metals/metalloids/oxoanions, etc…)o Removed from solution via adsorption, precipitation: Reversible processeso Requires long-term alteration/stabilization of geochemical environment
• Organics can be irreversibly altered, destroyedReductive Dechlorination
Carbon source CO2
e-
TCE Ethene
Hydrocarbon Bio-Oxidation
O2 H2O
e-
C6H6 CO2
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Implications of Fixation
Metals Plume
Organic Plume
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Implications of FixationOriginal Plume Boundary
Mobile Contaminant Mass
Immobilized Contaminant Mass
Metals Plume
Organic Plume
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Implementation Approaches2) Permeable Reactive Barriers1) Soluble Injections
Aqueous phase delivery of treatment reagents through injection wells Excavation and direct emplacement
of solid-phase reagent
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Injection-Based Approaches• Alteration of geochemical environment to yield mineral saturation
o pH Reduce solubility of the metal: Al3+ Al(OH)3 (solid) Pb2+ Pb(OH)2 (solid)
Reduce solubility of other constituents: Fe3+ Fe(OH)3 (solid) HAsO42-
o Redox environment Reduce/oxidize the metal: U(VI) U(IV) UO2 (solid)
Reduce/oxidize other constituents: SO42- S2- Ni2+ + S2- = NiS (solid)
o Dissolved ion concentrations Example: Sulfide Addition Pb2+ + NaHS = PbS (solid) + Na+ + H+
Example: Phosphate Addition UO22+ + PO4
3- + Ca2+ = Ca(UO2)2(PO4)2 (solid)
© Arcadis 2016
Challenges• Long-term effectiveness
• pH rebound?• Ability to maintain redox environment?• Wash-out of added reagents?
• Secondary effects• Geochemical alteration Release of other constituents
• Reagent injectability/radius of influence• Balance between reagent distribution and reagent deposition
Fe2+
CO2
Organic Carbon
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Field Examples
1) Microbial reduction of Cr(VI)• Direct-redox effect
2) Arsenic coprecipitation with iron• Indirect-redox effect, pH adjustment
3) Uranium Precipitation with Phosphate• Precipitation with co-solute addition
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1) In Situ Treatment of Cr(VI)• Organic carbon injection
• Microbial reduction of Cr(VI) to Cr(III)
• Iron reduction enhances Cr precipitation
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In Situ Treatment of Cr(VI)• Organic carbon injection
• Microbial reduction of Cr(VI) to Cr(III)
• Iron reduction enhances Cr precipitation
© Arcadis 2016
In Situ Treatment of Cr(VI)• Reoxidation of Cr(III) by O2
• Thermodynamically favorable• Kinetically limited!• In practice, reoxidation by O2 is not observed
• Reoxidation of Cr(III) by Mn(III/IV) oxyhydroxides
• Primary known/environmentally-relevant oxidant of Cr(III)
• Kinetically limited: Solid-solid interaction
• Cr(III) extremely stable upon redox rebound
O2 H2O
NO3- N2
MnO2 Mn2+
SeO42- HSeO3
-
HCrO4- Cr(OH)3
Fe(OH)3 Fe2+
HAsO42-
SO42- H2S
HCO3- CH4
H3AsO3
UO2UO22+
Oxidized Reducedn O
HSeO3- Se
1.0
0.8
0.6
0.4
0.2
0.0
-0.2
-0.4
Eh (volts)pH = 7
© Arcadis 2016
In Situ Treatment of Cr(VI)Pilot testing
• Cr(VI) effectively reduced• Reductive dissolution/release of Fe, Mn, As
0
40
80
120
160
200
0
1,000
2,000
3,000
4,000
5,000
0
200
400
600
800
1000
1200
1400 To
tal O
rgan
ic C
arbo
n (m
g/L)
Hexa
vale
nt C
hrom
ium
(ppb
)
Elapsed Time (days)
Hexavalent Chromium
Total Organic Carbon
0.0
0.5
1.0
1.5
2.0
2.5
3.0
0
200
400
600
800
1,000
1,200
Mar
-06
Apr-
06
May
-06
Jun-
06
Jul-0
6
Aug-
06
Sep-
06
Oct
-06
Nov-
06
Dec-
06
Jan-
07
Feb-
07
Mar
-07
Nitra
te (m
g/L)
Sulfa
te (m
g/L)
Date
Sulfate
Nitrate
0
5
10
15
20
0
200
400
600
800
1000
1200
1400
Arse
nic (
ppb)
Elapsed Time (days)
Dissolved Arsenic
PT-1D
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Controlling Byproduct GenerationField-scale predictions: Attenuation of As and Mn before river discharge
Chromium Manganese
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2) Arsenic Removal with IronStrategy:
• Addition of iron as soluble Fe(II)• Addition of chemical oxidant to achieve:
– Fe(II) Fe(III)
– As(III) As(V)
• Alkaline oxidant (e.g., CaO2) balances acidity• Arsenic “co-precipitation” with iron
Challenges:• In situ mixing of soluble iron and oxidant• Retardation/consumption of reagents:
Fe(II), oxidant• Achieving target “ROI”
Sorption,Coprecipitation
As(III) As(V)
CaO2 Ca(OH)2
Arsenic Oxidation
CaO2Ca(OH)2
Fe(III) Fe(II)
Iron Oxidation
Iron oxyhydroxide
As(V)
As(V)
© Arcadis 2016
Field Pilot TestGroundwater arsenic at a coal ash site
View along highway of arsenic “hot spot” area Well installation
Completed well network, with IW-1 in center
© Arcadis 2016
Field Pilot TestGroundwater arsenic at a coal ash site
Reagent tanks Injection well (IW-1) below grade
Ferrous sulfate injection, with tube added to provide additional head for gravity injection
Fluorescein tracer and ferric iron precipitate in observation well
© Arcadis 2016
Coal Ash Site Field Pilot Test
• Specific capacity loss with solid-phase CaO2 injection
• Capacity regained with acidic ferrous sulfate injection
0.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.80
0.90
1.00
0 1000 2000 3000 4000 5000
Spec
ific
Cap
acity
(gpm
/ft)
Cumulative Volume Injected (gal)
Ferrous sulfate Calcium peroxide
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Coal Ash Site Field Pilot Test• Iron and calcium largely retained in 10-ft ROI
• “Emplaced reactive zone” established
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
-30 0 30 60 90 120 150 180 210 240 270
Rea
gent
and
Tra
cer R
ecov
ery
Days Post-injection
OW-3
Calcium - Dissolved Iron - Dissolved Bromide
GW Flow
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Coal Ash Site Field Pilot Test• Arsenic treatment at injection well
• OW-3: As treated to 20 ppb, gradual rebound
• Rebound due to change in flow direction, bypass of narrow treatment zone
GW Flow
0102030405060708090
100110120130140
-30 0 30 60 90 120 150 180 210 240 270
Dis
solv
ed A
rsen
ic C
once
ntra
tion
(µg/
L)
Days Post-injection
GW-41 IW-1 OW-3 OW-5
Method and laboratory reporting limit changed
5
6
7
8
9
10
11
12
13
14
-30 0 30 60 90 120 150 180 210 240 270
pH
Days Post-injection
GW-41 IW-1 OW-3 OW-5
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3) Uranium Precipitation with Phosphate
Uranium Mill Tailings Site, New Mexico Former Uranium Mine, Colorado
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Uranium Precipitation with Phosphate• U(VI) highly soluble
• U(VI) U(IV) reduction: Effective, but sensitive to reoxidation
• U(VI) insoluble in phosphate precipitates
Autunite: Ca(UO2)2(PO4)2·3H2O
Chernikovite: H3O(UO2)(PO4)·3H2O
MCL
Mehta et al., 2014
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Uranium Precipitation with Phosphate
Injection strategies• Direct injection as orthophosphate
• Injection of polyphosphate, timed release of orthophosphate
P
P
P P
P
P
Tripolyphosphate Orthophosphate
© Arcadis 2016
Uranium Mine ImplementationUnderground Mine Workings Waste Rock Piles
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Underground Mine Workings Implementation“Push-Pull” Test Results
0
5
10
15
20
25
30
0 2000 4000 6000 8000 10000
Conc
entr
atio
n (m
g/L)
Gallons Extracted from P-11
Phase-2b Pull: Uranium
Uranium(dissolved)
Uranium(total)
MixingPrediction
InjectateAverage
P-11Baseline
Theoretical concentration based on mixing of injectate and groundwater (calculated using fluorescein tracer)
Actual measured concentration
Actual < Theoretical = REMOVAL
0
10
20
30
40
50
60
70
80
0 2000 4000 6000 8000 10000
Conc
entr
atio
n (m
g/L)
Gallons Extracted from P-11
Phase-2b Pull: Fluorescein
Pull-PhaseMeasured
InjectateAverage
0
200
400
600
800
1000
1200
1400
1600
1800
0 2000 4000 6000 8000 10000
Conc
entr
atio
n (m
g/L)
Gallons Extracted from P-11
Phase-2b Pull: Phosphate
Phosphate(total)
Phosphate(dissolved)
MixingPrediction
InjectateAverage
© Arcadis 2016
Waste Rock Dump Implementation• Sustained uranium removal observed in
downgradient well (~60 ft)
• Additional results pending
GW flow
RD-01Injection Well
RD-02,03Extraction Wells
RD-04Monitoring Well
0
0.5
1
1.5
2
2.5
3
6/19
/201
6
6/29
/201
6
7/9/
2016
7/19
/201
6
7/29
/201
6
8/8/
2016
8/18
/201
6
8/28
/201
6
Conc
entr
atio
n (m
g/L)
Uranium (dissolved)
RD-01
RD-02
RD-03
RD-04
InjectionsActive
© Arcadis 2016
Learning Objectives
After attending this session, participants should be able to:
• Identify challenges unique to in situ metals treatment
• Recognize in situ treatment approaches: injection-based vs. barriers
• Explain the importance of precipitate stability
• Describe geochemical processes/principles that can be used to yield metals precipitation
• Explain the importance of long-term precipitate stability and geochemical parameters that may affect stability
© Arcadis 2016
Arcadis.Improving quality of life.
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