- Despite decades of cleanup efforts, chlorinated solvents are some of the most common groundwater and subsurface contaminants of the industrialized world. These compounds include chlorinated ethenes (CEs) such as trichloroethene (TCE) and chlorinated methanes (CMs) such as carbon tetrachloride (CT). Dehalococcoides mccartyi belongs to a class of microorganisms called organohalide-respiring bacteria (OHRB) and is the only organism known to completely transform TCE, a chlorinated ethene, to harmless ethene via reductive dehalogenation, a process commonly exploited in bioremediation schemes. However, this process has been shown to be inhibited by the presence of chlorinated methanes. In order to gain strategic insight for sites co-contaminated with CEs and CMs, we explored the dynamics further to gain a better understanding of how CMs affect microbially-facilitated CE transformation.
The impact of CT and chloroform (CF), a CT transformation product, on microbial performance was assessed and compared by evaluating CE transformation rates. Transformation rates served as a proxy for microbial health and viability and were compared across experiments to establish trends in CM inhibition. Hydrogen production and consumption behavior was also monitored. Kinetic transformation experiments were conducted in triplicate batch reactors containing anaerobic TCE-dehalogenating cultures harvested from chemostats. Day 0 of each experiment began with the addition of TCE, formate, and either CT (“CT-exposed”) or CF (“CF exposed”). Further additions of TCE
and formate were delivered on days 1, 2, or 14 to establish the short and long term effects of CM exposure on CE transformation. Every addition of TCE was transformed to ethene, and the mass profile was analyzed to obtain zero-order transformation rates for each CE. Relative to controls, VC rates were decreased in reactors that were exposed to CT on day 0, and significant reductions in TCE, cDCE, and VC rates were achieved after 1 and 2 days of exposure, indicating an early time CT-related inhibition of OHRB. After CT transformation was complete, rates did not recover, and day 14 exposure rates were similar to those obtained after 2 days of exposure. Rates obtained from reactors exposed to CF without CT were slowed but not as dramatically, indicating the CF as a product of CT transformation was not primarily responsible for the CT toxicity. Increasing the CT concentration and adjusting the CT delivery scheme indicated a dependence of CE transformation inhibition on both concentration and the amount of CT mass transformed. Amendment of vitamin B12 to cultures prior to CT and TCE addition resulted in a faster VC transformation rate than in a control without B12 amendment and improved H2 consumption, indicating B12 as a key player in the CT mechanism of inhibition. Recovery potential of both CT-exposed and CF-exposed was assessed in select reactors sparged after 7 weeks of CT or CF addition. Reactors did not recover rates under either condition, indicating a permanent toxicity exerted by CT and CF for the time frame tested. The series of tests ultimately demonstrate that CT toxicity on OHRB is not explained by presence of the CF product alone, and that other mechanisms of toxicity contribute to the inhibition of CE transformation observed.