# Presentation: 161391

## Remediation of Mixed Chromium and TCE Releases 

**Remediation of Mixed Chromium and TCE Releases**

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

## 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

**Saturated Zone: **Oxidized environment. Low Carbon.  pH 6 – 8. Some retention of Cr. Little retention of TCE; large dissolved plume

**Cr**

**TCE**

## 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?

**Notes:**
Iron complex- ferric chromate

## Speciation of Chromium

**Cr****2****O****7****=**** + H****2****O ****↔**** 2CrO****4****=**** + 2H****+**
**    ****Acid                     Base **

**Desert Soils**

**Notes:**
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

## 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 **

## Ion Exchange with Sulfate in Minerals

- Ca6Al2(SO4)3(OH)12 + CrO4= **↔** Ca6Al2(SO4)2(CrO4)(OH)12 + SO4=

**Ettringite **

**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!**

**Notes:**
Chromate ore processed residuals (COPR) 

## Ion Exchange with Sulfate in Minerals

**Before and after submergence in 3 mM sulfate**

**Notes:**
SEM – scanning electron microscope

## Chromium VI Remediation

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

## Flushing of Chromium VI

**Ca/Fe Content of Soil**

**Pore Volumes required for complete removal **

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

**4**

**Notes:**
Chromium exchange with sulfate in minerals – very slowly, addition of sodium sulfate to replace and mobilize chrome

## Biological Reduction of Chromium VI

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

## Direct Metabolism

**Cr VI is the Electron Acceptor**
- Some Sulfate Reducing Bacteria can utilize chromate as the electron acceptor
- 2CrO4= + -CH2- + 10H+  2Cr+3 + 6H2O + CO2

**Notes:**
Microbes not necessarily present

## Coincidental Reduction

**Cr(VI) is reduced by products generated by other anaerobic pathways**
**Iron reduction**
- 6Fe+3  + -CH2-  + 2H2O  6Fe+2 + CO2 + 6H+        Iron metabolism
- 6Fe+2 + Cr2O7=  + 14H+  6Fe+3 + 2Cr+3 + 7H2O Cr VI Reduction
**Sulfate reduction**
- SO4=  + -CH2-  + 2H+  S= + CO2 + 2H2O     Sulfate metabolism
- 3S= + Cr2O7=  + 14H+  3S0 + 2Cr+3 + 7H2O  Cr VI Reduction

## 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)**

**Notes:**
10  mg/L or greater – toxicity level Cr6+

## 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 > -1.33 V **
- Reduced Sulfur Compounds
- Reduced Iron
- Reduced (oxidizable) organics
**The issue is kinetics not reactivity**
- Reaction times vary from < 5 min to > 5 days

**Notes:**
Typical Eo = .3 to .9 of reductants

## Screening of Reductants

**Notes:**
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.

## Comparison of Biological and Chemical Reduction 

- Comparison of Biological and Chemical Reduction

**Injection of Molasses**

**Injection of Polysulfide**

**Notes:**
Likely toxicity issue

## 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**

## 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

## ART Recirculation Well

## 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)

**Notes:**
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  

## Applicability of  ISCO

**Dissolved**

**DNAPL**

**Adsorbed**

**MnO****4****-**

- H2O2

- O3

- S2O8=

## Slide 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 |

## Slide 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 |

## 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