11. TCE Bioremediation Case Study Flashcards
What is TCE? (5)
- Trichloroethylene
C=C with 3 Cl and one H around - Common industrial solvent, dry cleaning
- Toxic, carcinogenic
- Water soluble
- Common groundwater contaminant
What is PCE?
Tetracholorethylene or perchloroethylene
C=C with 4Cl
TCE contaminated aquifer in Savannah River Site, South Carolina (4 points)
- 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
How to biodegrade TCE and PCE?
Halorespiration / dehalorespiration
Halorespiration/dehalorespiration
- 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
What can biodegrade TCE and PCE?
- Methanogens
- Aerobic oxidation
How do methanogens biodegrade TCE and PCE?
Under anaerobic conditions PCE and TCE are dehalogenated by methanogens able to use these compounds as electron acceptors. This reductive dehalogenation yields ethene.
How does aeorobic oxidation degrade TCE?
doesn’t work for PCE bc aeorobic
Recently, some bacteria were shown to aerobically biodegrade / mineralize TCE, obtaining energy and carbon. C2HCl3 + O2 → energy + CO2 + H2O + biomass
Dehalorespiration (or organohalide respiration)
Dehalorespiration or organohalide respiration is the use of halogenated compounds as terminal electron acceptors in anaerobic respiration.
Bioremediation steps of PCE
- Using SRBs and methanogens, PCE becomes TCE
- TCE is dehalorespirated to DCE (only 2 Cl)
- DCE is dehalorespirated to Vinyl Chloride (only 1 Cl)
- Vinyl Chloride is dehalorespirated to Ethene (no Cl ; C=C with all Hs)
TCE Case study Plan A Air Stripping/Bioventing (alone)
Air stripping/bioventing
- 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.
TCE Case study Plan A Air Stripping/Bioventing (alone)
Hypotheses of air stripping / bioventing
- Air will stimulate microorganisms to degrade TCE
- Volatile organic compounds (VOCs) and TCE vapours will be extracted and incinerated
TCE Case study Plan A Air Stripping/Bioventing (alone)
Results
Not very successful at reducing TCE/PCE concentrations
TCE Case study Plan A Air Stripping/Bioventing (alone)
Why did air stripping / bioventing alone fail? (2)
- Volatility of TCE and PCE was too low under those conditions for them to be effectively removed by physical air stripping
- 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
TCE Case study Plan B
What was plan B?
To stimulate methanotrophs, they added:
* air
* methane at 1% or 4%
TCE Case study Plan B: enhanced in situ bioremediation
What was the hypothesis?
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
TCE Case study Plan B: enhanced in situ bioremediation
Rationale
- 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
TCE Case study Plan B: enhanced in situ bioremediation
Results
- 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
TCE Case study Plan C
What was the new plan to address the slowing of the bioremediation and the limited nutrients?
Adding:
* air
* methane at 4%
* N2O (nitrous oxide) 0.07%
* Triethylphosphate 0.007%
TCE Case study Plan C
Results (5)
- 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
Role of added methane
- Stimulated growth of methanotrophs (enrichment)
- Stimulated methanotrophs to produce methane monooxygenase
Time required to reach 95% reduction in [TCE] for plan A and for plan B
- Plan A = 10 years
- Plan B = 4 years
How did they characterize and ID the microbes? (6)
- Fatty acid signatures, phospholipid fatty acid analysis (PLFA)
- Gene probes
- Biolog (automated metabolic tests)
- Biomass
- In vitro assay of TCE biodegradation
- Microscopy
