1. Remediation of Mixed Chromium and TCE Releases
19-Apr-17
Remediation of Mixed Chromium and TCE Releases
Paula R. Chang
Richard A. Brown, Ph.D.
ERM, Inc.

2. 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

3. What Controls Cr VI Distribution and Persistence?
19-Apr-17
What Controls Cr VI Distribution and Persistence?
Eh-pH
Mineralogy
Binding to Calcium/Iron
Recharge
FOC
Quantity spilled
Depth to Water
What Controls TCE Distribution and Persistence?
Iron complex- ferric chromate

4. Speciation of Chromium
19-Apr-17
Speciation of Chromium
Cr2O7= + H2O ↔ 2CrO4= + 2H+
Acid Base
Desert Soils
Chromic acid baths– typical source from metals plating
Hydrogen chromate, dichromate – acid conditions - CrVI
Chromate – basic conditions
Chromic dioxide, insoluble
Chromite – under basic conditions, < 11.5, insoluble

5. Binding to Calcium/Fe Ca+2 + CrO4=  CaCrO4 Ksp= 7.1 x 10-4
Na/K+ + 3Fe+3 + 6H2O + CrO4=  KFe3(CrO4)(OH)6 + 6H+ (Jarasite)
Ca/Fe Content of Soil
Plume length

6. Ion Exchange with Sulfate in Minerals
19-Apr-17
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=
Chromate ore processed residuals (COPR)
Ion Exchange of Minerals is a significant cause of rebound!

7. Ion Exchange with Sulfate in Minerals
19-Apr-17
Ion Exchange with Sulfate in Minerals
SEM – scanning electron microscope
Before and after submergence in 3 mM sulfate

8. Chromium VI Remediation
Processes
Flushing
Biological
Chemical
Permanence
Re-oxidation?

9. Flushing of Chromium VI
19-Apr-17
Flushing of Chromium VI
Other Factors Affecting Flushing Efficiency pH
Lithology
Mineralogy
Ca/Fe Content of Soil
Chromium exchange with sulfate in minerals – very slowly, addition of sodium sulfate to replace and mobilize chrome 4
Pore Volumes required for complete removal

10. Biological Reduction of Chromium VI
Direct Metabolism
Coincidental Reduction of Cr(III) to Cr(VI)
Both Require Carbon Addition

11. Direct Metabolism Cr VI is the Electron Acceptor
19-Apr-17
Direct Metabolism
Cr VI is the Electron Acceptor
Some Sulfate Reducing Bacteria can utilize chromate as the electron acceptor
2CrO4= + -CH H+  2Cr+3 + 6H2O + CO2
Microbes not necessarily present

12. Coincidental Reduction
Cr(VI) is reduced by products generated by other anaerobic pathways
Iron reduction
6Fe CH H2O  6Fe+2 + CO2 + 6H Iron metabolism
6Fe+2 + Cr2O7= + 14H+  6Fe+3 + 2Cr+3 + 7H2O Cr VI Reduction
Sulfate reduction
SO4= + -CH H+  S= + CO2 + 2H2O Sulfate metabolism
3S= + Cr2O7= + 14H+  3S0 + 2Cr+3 + 7H2O Cr VI Reduction

13. Why Biological Reduction Fails
19-Apr-17
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
Cannot maintain reducing conditions (vadose zone)
10 mg/L or greater – toxicity level Cr6+

14. Chemical Reduction of Chromium VI
19-Apr-17
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 > V
Reduced Sulfur Compounds
Reduced Iron
Reduced (oxidizable) organics
The issue is kinetics not reactivity
Reaction times vary from < 5 min to > 5 days
Typical Eo = .3 to .9 of reductants

15. Screening of Reductants
19-Apr-17
Screening of Reductants
Important for the vadose zone, due to lack of moisture difficult to maintain contact between reductant and Cr. Need faster reactions.
CaSx is cheapest and fastest for vadose treatment.

16. Comparison of Biological and Chemical Reduction
19-Apr-17
Comparison of Biological and Chemical Reduction
Injection of Molasses
Injection of Polysulfide
Likely toxicity issue

17. Comparison of Chemical and Biological Chromium VI Treatment
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

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

19. ART Recirculation Well
19-Apr-17
ART Recirculation Well

20. Available Oxidants Ozone O3 + 2H+ + 2e-  O2 + H2O Eo 2.07v
19-Apr-17
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
Key Issues: Soil Demand (low for low FOC)
There are a number of oxidants that are potentially usable. They are, in order of decreasing oxidative strength, ozone, persulfate, hydrogen peroxide, permanganate, oxygen nitrate and iron. Of these, ozone, peroxide and permanganate are the most widely used and are generally commercially available.
Ozone is a very strong oxidant. It reacts directly as well as through the generation of hydroxyl radicals. The hydroxyl radical formation can be enhanced by adding hydrogen peroxide. This is known as the “dark” AOP process because it does not require UV light.
Persulfates are strong oxidants but generally have a high activation energy. They are typically thermally activated in industrial use. A potential use is to supplement permanganate if there is a lot of reduced metals. Persulfates are less expensive than permanganate.
Hydrogen peroxide is

21. Applicability of ISCO
Adsorbed
S2O8= O3
MnO4-
H2O2
Dissolved
DNAPL

22. TCE Remediation Technology Impact on Chromium VI
Pump and Treat – Captures dissolved TCE. Recovered water is treated by air stripping, carbon, or bioreactors
Chromium VI would also be recovered since it is soluble. It would also require treatment in the recovered water. Air stripping has no effect on Cr VI. Carbon may reduce the Cr VI. Bioreactors would reduce Cr VI.
SVE/Air Sparging, ART – Removes TCE by volatilization
Has no effect on Cr VI.
Reductive Dechlorination – A carbon amendment is added which stimulates the biological reductive dechlorination of the TCE
The carbon amendment and biology will also reduce the Cr VI. High levels of Chromium can be inhibitory to biological processes.
Chemical Reduction – uses zero valent iron or ferrous iron minerals to dechlorinate TCE.
Zero valent iron and ferrous iron will both reduce Cr VI.
Chemical Oxidation – A chemical oxidant - peroxide, permanganate, or persulfate is added to oxidize the TCE to carbon dioxide and chloride ion.
The oxidants can potentially reoxidize chromium III to hexavalent chromium. Sequencing of oxidation with any chromium reduction needs to be considered. Generally chromium reduction would follow TCE oxidation. Excess oxidant consumes reductant

23. Chromium VI Remediation
Technology
Impact on TCE
Pump and Treat – dissolved chromium VI is recovered and removed from the water by reduction and precipitation
Dissolved TCE would also be recovered and would have to be treated by air stripping and/or carbon. Generally chromate is removed separately by reduction.
Chemical Reduction – a chemical reductant is added to convert Cr VI to Cr III.
Reductants will reduce iron present in soils. The reduced iron minerals can dechlorinate TCE
Biological reduction of Cr VI to Cr III
Reductive biological processes are generally compatible with TCE dechlorination. They may need to be done sequentially. Chromate reduction may occur before CE reduction

24. 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