Remediation of Mixed Chromium and TCE Releases

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Remediation of Mixed Chromium and TCE Releases

• Paula R. Chang
• Richard A. Brown, Ph.D.
• ERM, Inc.

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Distribution of Cr and TCE in Desert Soils
Vadose Zone Dry, oxidized environment. Low
carbon. Moderate to high calcium, pH 6-8.
Retention of Cr VI. Little retention of TCE
Cr
Saturated Zone Oxidized environment. Low Carbon.
pH 6 8. Some retention of Cr. Little retention
of TCE large dissolved plume
TCE
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What Controls Cr VI Distribution and Persistence?

• Eh-pH
• Mineralogy
• Binding to Calcium/Iron
• Recharge
• FOC
• Quantity spilled
• Depth to Water
• Recharge

What Controls TCE Distribution and Persistence?
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Speciation of Chromium
Cr2O7 H2O ? 2CrO4 2H Acid
Base
Desert Soils
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Binding to Calcium/Fe

• Ca2 CrO4 ? CaCrO4 Ksp 7.1 x 10-4
• Na/K 3Fe3 6H2O CrO4 ? KFe3(CrO4)(OH)6
  6H (Jarasite)

Ca/Fe Content of Soil
Plume length
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Ion Exchange with Sulfate in Minerals
Ettringite
Ca6Al2(SO4)3(OH)12 CrO4 ? Ca6Al2(SO4)2(CrO4)(OH
)12 SO4
Chromium(VI) hydrocalumite.
Ca2Al(OH)4.5Cl0.5(CrO4) SO4 ?
Ca2Al(OH)4.5Cl0.5(SO4) CrO4
Ion Exchange of Minerals is a significant cause
of rebound!
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Ion Exchange with Sulfate in Minerals
Before and after submergence in 3 mM sulfate
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Chromium VI Remediation

• Processes
• Flushing
• Biological
• Chemical
• Permanence
• Re-oxidation?

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Flushing of Chromium VI

• Other Factors Affecting Flushing Efficiency
• pH
• Lithology
• Mineralogy

Ca/Fe Content of Soil
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Pore Volumes required for complete removal
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Biological Reduction of Chromium VI

• Direct Metabolism
• Coincidental Reduction of Cr(III) to Cr(VI)
• Both Require Carbon Addition

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Direct Metabolism

• Cr VI is the Electron Acceptor
• Some Sulfate Reducing Bacteria can utilize
  chromate as the electron acceptor
• 2CrO4 -CH2- 10H ? 2Cr3 6H2O CO2

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Coincidental Reduction

• Cr(VI) is reduced by products generated by other
  anaerobic pathways
• Iron reduction
• 6Fe3 -CH2- 2H2O ? 6Fe2 CO2 6H
  Iron metabolism
• 6Fe2 Cr2O7 14H ? 6Fe3 2Cr3 7H2O Cr
  VI Reduction
• Sulfate reduction
• SO4 -CH2- 2H ? S CO2 2H2O
  Sulfate metabolism
• 3S Cr2O7 14H ? 3S0 2Cr3 7H2O Cr VI
  Reduction

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Why Biological Reduction Fails

• Requisite Bacteria not present
• Chromate-utilizing SRBs
• SRBs
• Iron reducing bacteria
• Requisite Electron Acceptor not available
• Ferric iron
• Sulfate
• Chromium levels are toxic to bacteria
  (inhibition)
• pH too low needs to be between 5.5 - 8
• Cannot maintain reducing conditions (vadose zone)

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Chemical Reduction of Chromium VI

• Cr VI is a strong Oxidant
• Electrode Potential (Eo) 1.33 V
• It reacts with reductants with a reducing
  potential gt -1.33 V
• Reduced Sulfur Compounds
• Reduced Iron
• Reduced (oxidizable) organics
• The issue is kinetics not reactivity
• Reaction times vary from lt 5 min to gt 5 days

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Screening of Reductants
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Comparison of Biological and Chemical Reduction
Injection of Molasses
Injection of Polysulfide
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Comparison of Chemical and Biological Chromium VI
Treatment

• Chemical
• Applicable over wide pH range
• Fast reaction
• Able to maintain residual
• No reoxidation

• Biological
• Narrow pH range
• 5.5-8
• Slow reaction
• Competitive consumption of carbon
• Possible reoxidation

Chemical reduction is simpler, faster and more
reliable
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TCE Remediation

• Vadose zone
• SVE
• Chemical Oxidation
• Saturated
• Air sparging
• Pump and treat
• Dual Phase
• Recirculation wells (ART)
• Anaerobic reductive dechlorination
• Zero Valent Iron
• Chemical Oxidation

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ART Recirculation Well
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Available Oxidants

• Ozone
• O3 2H 2e- ? O2 H2O Eo 2.07v
• Key Issues Stability, low mass delivery
• Persulfates
• S2O8 2e- ? 2SO4 Eo 2.01v
• Key Issues Needs activation, Long term
  stability, reaction rate
• Hydrogen Peroxide
• H2O2 2H 2e- ?2H2O Eo 1.77v
• Key Issues Decomposition, needs activation
• Permanganate
• MnO4- 4H 3e- ? MnO2 2H2O Eo 1.695
• Key Issues Soil Demand (low for low FOC)

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Applicability of ISCO
Adsorbed
S2O8
O3
MnO4-
H2O2
Dissolved
DNAPL
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Conclusions

• Remediation of mixed spills is achievable
• Remediation of vadose zone and saturated zone may
  require different packages of technology
• Reductive dechlorination in vadose zone is
  difficult