Insitu Geochemical Fixation of Chromium in Groundwater in Arid Climates: A Comparison of Chemical Re

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In-situ Geochemical Fixation of Chromium in
Groundwater in Arid ClimatesA Comparison of
Chemical Reductant Technologies

• Peter Storch P.E.
• Andrew Messer, and Chris Lawrence
• URS Corporation, Arizona, USA
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In Memory of Roman Pyrih1946-2006
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Objectives

• One size does not fit all
• Identify key parameters for evaluating chemical
  reductant technologies
• Compare key parameters
• General recommendations

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Chromium Reduction Technologies

• Sodium Dithionite Na2S2O4
• Zero Valent Iron (ZVI) - Fe0
• Micron-scale
• Nano-scale
• Ferrous Sulfate Fe(SO4) - Fe(II)
• Calcium Polysulfide (CPS) - CaSx
• Bioreductants
• Molasses
• Vegetable Oil
• Corn Syrup
• HRC polylactate /organic sulfur

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General Technology Selection Criteria

• Effectiveness
• Subsurface Distribution
• Safety and Handling
• Regulatory Acceptance
• Cost

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Effectiveness

• Cr(VI) reduction per mass reductant
• Demonstrated in the field
• Persistence

100 mg/L Cr(VI)
0.02 mg/L Cr(VI)
precipitate Cr(III) 9,100 mg/Kg
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Cr(VI) Reduction per Mass Reductant

• ZVI
• Micron 2 g Cr(VI) /kg Fe1
• Nano 70 g Cr(VI) /kg Fe1
• Ferrous Sulfate
• CPS 135 g Cr(VI) /kg CaS52
• HRC/MRC 40 g Cr(VI) /kg HRC
• 1 Source DOE WMP-28124, Rev , Cao and Zhang
• 2Source URS, 2002

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Cr(VI) Reduction per Mass Reductant
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Comparison of Persistence

• Greater persistence means fewer injections

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Persistence of Reducing ConditionsOne CPS
Injection, Cr(VI) 50 mg/L
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Factors Affecting Distribution

• Transport Parameters viscosity, density
• Reaction Mechanism/Kinetics - stability
• Migration will it move with groundwater?

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Comparison of Distribution Factors
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Atomized Slurry in N2 gas Carrier (ARS
Technologies)
(Toda America)
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Safety of Reductants

• Risk to human health from direct exposure
• Risk to groundwater quality from byproducts

Add photo
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Typical Regulatory Concerns

• Dilution
• Degradation of groundwater quality from
  byproducts (NO3- NH4), SO42-
• Pore Space Plugging
• Displacement
• Preferential Pathways

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Preferential Pathways in fine-med sand
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Regulatory Acceptance

• Dithionite Hanford, Elizabeth City
• CPS full-scale in 12 US states, Australia
• ZVI numerous PRBs,
• Bio approved by EPA, most US states
• HRC approved by EPA, most US states

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Reagent Cost Comparison
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Reagent Cost as Percentage of Total Application
Cost
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Desert/Arid Environment

• Deep Groundwater
• Coarser Grains
• pH 7-9
• Low Organic Content
• Preferential Pathways

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Arid Environment
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Fracing
Conc./Fracing
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Arid Environment
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Summary

• Sodium Dithionite
• highly reactive, low persistence
• questionable cost-effectiveness

• Ferrous Sulfate
• low cost
• very reactive
• lower persistence
• history of lowering permeability

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Summary

• ZVI
• cost-effective, deep PRB
• shallow source area,
• mixed Cr/VOC,
• Nano-scale needs development,
• good choice for arid climates.

• CPS
• Low cost
• well-developed
• flexible
• combine with bio-reductants to offer low cost,
  long life
• good choice for arid climates.

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Summary

• HRC
• high unit cost
• good shallow, source-zone treatment,
• more difficult to distribute in deep aquifer
• mixed Cr/VOCs

• Bio-reductants
• low unit cost,
• multiple injections for long-term effectiveness
  and persistence,
• good for arid environment.

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2 CrO42- 3 CaS5 10H 2 Cr(OH)3(s)
15 S(s) 3Ca2 2H2O
In-situ Geochemical Fixation Groundwater Pilot
Test Metal Plating Shop, Arizona
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Chromium Non-detect After 600 Days
Injection Well MW-15
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• Application of CaSx through infiltration trenches

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Impact to Groundwater in 160 Days