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NRSC 333 > 11. TCE Bioremediation Case Study > Flashcards

11. TCE Bioremediation Case Study Flashcards

1
Q

What is TCE? (5)

A
  • Trichloroethylene
    C=C with 3 Cl and one H around
  • Common industrial solvent, dry cleaning
  • Toxic, carcinogenic
  • Water soluble
  • Common groundwater contaminant
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2
Q

What is PCE?

A

Tetracholorethylene or perchloroethylene
C=C with 4Cl

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3
Q

TCE contaminated aquifer in Savannah River Site, South Carolina (4 points)

A
  • Located in a 320 square mile nuclear production facility in South Carolina.
  • One area was used to degrease target and fuel elements that were used in the reactors.
  • Degreasing solvents dumped into process sewer line that moved the contaminants to an unlined basin, process sewer line leaked, contaminating the soil subsurface.
  • The solvents from the subsurface and basin then continued to leach into the groundwater creating a plume over one square mile
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4
Q

How to biodegrade TCE and PCE?

A

Halorespiration / dehalorespiration

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5
Q

Halorespiration/dehalorespiration

A
  • Under anaerobic conditions, certain bacteria (ex. SRBs) can use PCE and TCE (and other compounds) as electron acceptors
    * PCE only degrades anaerobically
  • This reductive dehalogenation is a form of anaerobic respiration
  • They are sequentially knocking of the chlorine atoms, replace it with the H atom. And they do this anaerobically
  • Not the only anaerobic dehalogenation mechanism
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6
Q

What can biodegrade TCE and PCE?

A
  • Methanogens
  • Aerobic oxidation
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7
Q

How do methanogens biodegrade TCE and PCE?

A

Under anaerobic conditions PCE and TCE are dehalogenated by methanogens able to use these compounds as electron acceptors. This reductive dehalogenation yields ethene.

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8
Q

How does aeorobic oxidation degrade TCE?

doesn’t work for PCE bc aeorobic

A

Recently, some bacteria were shown to aerobically biodegrade / mineralize TCE, obtaining energy and carbon. C2HCl3 + O2 → energy + CO2 + H2O + biomass

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9
Q

Dehalorespiration (or organohalide respiration)

A

Dehalorespiration or organohalide respiration is the use of halogenated compounds as terminal electron acceptors in anaerobic respiration.

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10
Q

Bioremediation steps of PCE

A
  1. Using SRBs and methanogens, PCE becomes TCE
  2. TCE is dehalorespirated to DCE (only 2 Cl)
  3. DCE is dehalorespirated to Vinyl Chloride (only 1 Cl)
  4. Vinyl Chloride is dehalorespirated to Ethene (no Cl ; C=C with all Hs)
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11
Q

TCE Case study Plan A Air Stripping/Bioventing (alone)

Air stripping/bioventing

A
  • Drilled two horizontal wells, one above the other, for injecting (lower well → air stripping) and extracting gasses (upper well/vacuum → bioventing)
  • Drilled many vertical wells for sampling
    Through the lower well they added:
  • Air → air stripping / bioventing
  • Air stripping is a process where a constant air stream is sent through the contaminated ground water to force TCE from the water into its gas phase. The gas phase is then emitted into the atmosphere or in this case brought to the surface via bioventing.
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12
Q

TCE Case study Plan A Air Stripping/Bioventing (alone)

Hypotheses of air stripping / bioventing

A
  • Air will stimulate microorganisms to degrade TCE
  • Volatile organic compounds (VOCs) and TCE vapours will be extracted and incinerated
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13
Q

TCE Case study Plan A Air Stripping/Bioventing (alone)

Results

A

Not very successful at reducing TCE/PCE concentrations

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14
Q

TCE Case study Plan A Air Stripping/Bioventing (alone)

Why did air stripping / bioventing alone fail? (2)

A
  1. Volatility of TCE and PCE was too low under those conditions for them to be effectively removed by physical air stripping
  2. Air stripping will result in aerobic conditions whereas most microbes known for dehalogenation are anaerobes
    * The aerobes known for TCE degradation, methanotrophs, require methane as a carbon and energy source and for production of the methane monooxygenase enzyme responsible for the reaction
    * In addition to being devoid of CH4, the contaminated aquifer was also relatively low in available nutrients N and P.
    * PCE → TCE needs to be done anaerobically
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15
Q

TCE Case study Plan B

What was plan B?

A

To stimulate methanotrophs, they added:
* air
* methane at 1% or 4%

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16
Q

TCE Case study Plan B: enhanced in situ bioremediation

What was the hypothesis?

A

Methanotrophic bacteria will be stimulated by the addition of methane plus air and will break down TCE via cometabolism

Methanotrophs are aerobic so that’s why we continue to add air

17
Q

TCE Case study Plan B: enhanced in situ bioremediation

Rationale

A
  • Methanotrophs (i.e., Methylosinus trichosporium) produce an enzyme methane monooxygenase (MMO) which has broad specificity and can initiate the mineralization of TCE
  • i.e., cometabolism
  • PCE cannot be degraded by MMO, however it can be converted to TCE under anaerobic conditions (by sulfate-reducing bacteria (SRBs) and methanogens)
  • Despite pumping of air, there will be sufficient anaerobic microhabitats for these microorganisms to grow
18
Q

TCE Case study Plan B: enhanced in situ bioremediation

Results

A
  • Addition of air plus methane was successful for a while, however the rate of bioremediation began to slow down
  • Sampling indicated that methanotrophs were present but were nutrient (N,P) limited
19
Q

TCE Case study Plan C

What was the new plan to address the slowing of the bioremediation and the limited nutrients?

A

Adding:
* air
* methane at 4%
* N2O (nitrous oxide) 0.07%
* Triethylphosphate 0.007%

20
Q

TCE Case study Plan C

Results (5)

A
  • Addition of gaseous N and P into the air/ 4% methane mixture caused the rate of TCE degradation to increase once again
  • Increased number of methanotrophs from nearly undetectable to more than 106 per gram d.w.
  • Decreased [TCE] (TCE concentrations) to safe levels à down to 2 µg / L, below the drinking water standard of 5 µg / L
  • Decreased [PCE] significantly
  • Increased [Cl- ] and CO2 evolution
21
Q

Role of added methane

A
  • Stimulated growth of methanotrophs (enrichment)
  • Stimulated methanotrophs to produce methane monooxygenase
22
Q

Time required to reach 95% reduction in [TCE] for plan A and for plan B

A
  • Plan A = 10 years
  • Plan B = 4 years
23
Q

How did they characterize and ID the microbes? (6)

A
  1. Fatty acid signatures, phospholipid fatty acid analysis (PLFA)
  2. Gene probes
  3. Biolog (automated metabolic tests)
  4. Biomass
  5. In vitro assay of TCE biodegradation
  6. Microscopy

NRSC 333 (23 decks)

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