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Thesis_Weilun Zhao5.25 MB
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Dissertation
Synergistic anaerobic and aerobic biodegradation of chlorinated ethenes with biochar
University of Iowa
Doctor of Philosophy (PhD), University of Iowa
Autumn 2024
DOI: 10.25820/etd.007548
Abstract
Chlorinated ethenes (CEs) such as tetrachloroethene (PCE) and trichloroethene (TCE) are common groundwater contaminants, and it is necessary to remove them from groundwater. Cisdichloroethene (DCE) and vinyl chloride (VC) are produced by incomplete anaerobic dechlorination of the CEs. PCE and TCE commonly accumulate in groundwater. TCE and VC are known human carcinogens that can cause adverse health effects, and PCE has been classified as likely to be carcinogenic to humans. The current PCE bioremediation strategy aims to create very strong reducing conditions in groundwater and drive the complete reduction of PCE to the environmentally friendly product ethene under anaerobic conditions. However, this strategy has proven ineffective due to the limited presence of essential microbes (Dehalococcoides) in the natural environment, leading to the quicker formation of cis-DCE and VC compared to their dechlorination. Therefore, it is imperative to enhance the current strategy.
It has been reported that biochar can enhance the growth and activity of dehalogenating bacteria, such as Dehalococcoides, which can aid in the elimination of TCE. Additionally, studies have shown that VC can be degraded rapidly in aerobic groundwater due to the presence of oxidizing bacteria, specifically etheneotrophs like Mycobacterium sp. strain JS60 and Nocardioides sp. strain JS614. Recent research has also revealed that aerobic and anaerobic VCdegrading bacteria coexist and express their functional genes in the anaerobic groundwater zones of contaminated sites. As a result, we investigated the impact of biochar on the biodegradation of PCE and VC, as well as the potential for aerobic VC-oxidizing bacteria and anaerobic dechlorinating bacteria to work on biodegradation simultaneously. This research could provide new insights to enhance existing PCE bioremediation strategies or foster the development of new strategies.
In our first study, we investigated the impact of biochar on the dechlorination of PCE by using a commercial anaerobic PCE-dechlorinating culture known as SDC-9 TM. We also explored the relationship between the properties of biochar and microbes. Our findings indicated that the dechlorination of PCE stalled at cis-DCE in the absence of materials and with sand, but the presence of biochar improved SDC-9's ability to dechlorinate PCE. We observed that the absence of biochar resulted in the absence of Dehalococcoides in SDC-9, and biochar expedited the revival of Dehalococcoides. Additionally, we found that Dehalococcoides could adhere to biochar. Furthermore, we found that the ethene production rate was positively correlated to pore size, vcrA abundance in attached cells, etc.
In the second study, we explored the influence of multiple carbon sources (VC, ethene, and organic acids detected in the third study) on the biodegradation of VC by VC-oxidizing bacteria, specifically Mycobacterium strain JS60 and Nocardioides strain JS614. Our findings showed that JS60 was able to grow on both VC and lactate, whereas JS614 only thrived on VC, leaving lactate behind. Furthermore, JS60 displayed diauxic growth, while JS614 was capable of utilizing multiple carbon sources simultaneously. These results support the notion that aerobic VC-oxidizing bacteria and anaerobic VC-dechlorinating bacteria can collaborate in degrading VC, with the anaerobic bacteria utilizing lactate as the electron donor.
The third study involved an experiment using the mixed culture of aerobic VC-oxidizing bacteria and anaerobic VC-dechlorinating bacteria along with biochar under low dissolved oxygen (DO) conditions. We observed that ethene production was initially hampered by oxygen exposure but gradually recovered. We found that ethene formation and recovery were accelerated in the presence of biochar. The expression of VC reduction and oxidation functional genes recovered after one or two weeks. We found that aerobic VC-oxidizing bacteria and anaerobic VC- dechlorinating bacteria have the potential to degrade VC simultaneously, and biochar could improve aerobic and anaerobic bacteria growth under low DO conditions.
In conclusion, the study presented here offers valuable insights for enhancing PCE bioremediation strategies. We have identified a promising aerobic bacterium, Nocardioides strain JS614, to be combined with the anaerobic PCE dechlorination culture SDC-9 for simultaneous action. Additionally, we have conducted a comprehensive evaluation of the correlation between biochar properties and dechlorination of PCE by SDC-9. Specifically, we have demonstrated that aerobic VC-oxidizing bacteria and anaerobic VC-dechlorinating bacteria have the potential to degrade VC simultaneously, thus providing new insights into PCE bioremediation.
Details
- Title: Subtitle
- Synergistic anaerobic and aerobic biodegradation of chlorinated ethenes with biochar
- Creators
- Weilun Zhao
- Contributors
- Timothy Mattes (Advisor)Craig Just (Committee Member)David Cwiertny (Committee Member)Gregory LeFevre (Committee Member)
- Resource Type
- Dissertation
- Degree Awarded
- Doctor of Philosophy (PhD), University of Iowa
- Degree in
- Civil and Environmental Engineering
- Date degree season
- Autumn 2024
- DOI
- 10.25820/etd.007548
- Publisher
- University of Iowa
- Number of pages
- xiv, 155 pages
- Copyright
- Copyright 2024 Weilun Zhao
- Language
- English
- Date submitted
- 12/08/2024
- Description illustrations
- Illustrations, tables, graphs, charts
- Description bibliographic
- Includes bibliographical references.
- Public Abstract (ETD)
Man-made products, such as metal degreasing and dry cleaning containing tetrachloroethene (PCE), can be dumped to the ground and move through the soil to the groundwater, posing a risk to the water we drink and threatening human health. These chemicals are tough to remove from the environment naturally. Bacteria can break down PCE but commonly stop at cis-dichloroethene (DCE) and vinyl chloride (VC) during the clean-up process. People usually clean up PCE in no-oxygen areas and believe that only bacteria that don't need oxygen can live in these areas, so they only use and provide the living conditions for the bacteria that don't need oxygen to break down the VC. However, the solutions people are using right now have not been very effective.
Recently, reports showed that the bacteria that need oxygen and bacteria that don’t need oxygen were found in the same place in the no-oxygen area, suggesting a possible unknown relationship between these two kinds of bacteria. Some reports suggest that black carbon materials from plants can help bacteria grow better, making it easier to remove PCE. My research aims to combine these two kinds of bacteria and to test whether they can break down chlorinated ethene (CEs) at the same time and whether black carbon materials improve bacteria growth.
Our findings revealed that biochar can improve the ability of certain bacteria to remove CEs. Additionally, we found that the combination of bacteria can collaborate effectively to degrade VC at the same time. Overall, our research provides new insights into how we can enhance strategies for cleaning up contaminated groundwater. By understanding how different bacteria interact with these harmful chemicals and how biochar can boost their activity, we aim to contribute to the development of more effective and efficient cleanup methods.
- Academic Unit
- Civil and Environmental Engineering
- Record Identifier
- 9984774959402771
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