US EPA

8421555 

Meetings & Symposia 

The importance of pH in reductive dechlorination of chlorinated solvents 

Fogel, S; Findlay, M; Smoler, D; Folsom, S; Kozar, M 

2009 

Baltimore, MD 

10th International In Situ and On-Site Bioremediation Symposium, In Situ and On-Site Bioremediation-2009 

English 

It is generally assumed that pH values between 5 and 9 are favorable for most microbial activity associated with biodegradation in soils and groundwater. However, little published data exists on pH optima of anaerobic mixed microbial populations during reductive dechlorination of chlorinated solvents such as TCE. We will present data comparing the pH optima of functionally different bacteria involved in the biodegradation of chlorinated solvents including Dehalococcoides, Dehalobacter and methanogens. Dehalococcoides ethenogenes is the only bacterium capable of the anaerobic transformation of TCE to ethene, while Dehalobacter is responsible for the reductive dechlorination of TCA to chloroethane. Dechlorinating bacteria do not function as pure cultures under field conditions, but rather as members of interactive microbial communities. From our work and that of others, we know that such a complex microbial community is required for the successful degradation of TCE in situ. Members of the this consortia include fermentors, (such as Clostridium) to provide H2, sulfate-reducing bacteria to lower the ORP, acetogens and methanogens to provide vitamin B12, and D. ethenogenes to transform cDCE to ethene. Each type of bacteria has an optimum pH range, and depending on the narrowness of this range, one group of microbes may be less able to function than other members of the consortium at a particular field site. We will provide results from both microcosm studies and from field pilot studies which indicate that Dehalococcoides functions within a very narrow pH range. Results will be presented that show that the pH optima for Dehalococcoides is between 6.6 and 7.6. In contrast, we have found that other members of the consortium involved in the degradation of chlorinated ethenes have wider pH ranges at which they can function. For example, methanogens in certain environments can grow at pH's both lower and higher than Dehalococcoides, less than pH 6.2 and higher than 7.6. As a result, competition for molecular hydrogen could result in an advantage to methanogens at a pH outside the optimum range for Dehalococcoides. The importance of pH will be discussed in terms of growing bacteria in site groundwater, the need and challenges to adjust aquifer pH, and attempts to grow Dehalococcoides at pH's outside of its optimum pH range. We will also discuss the role of excess electron donor and its impact on groundwater pH.