350001 Kinetics and Rate Modeling of Chlorinated Aliphatic Hydrocarbon (CAH) Dehalogenation Using Anaerobic Cultures

Monday, November 4, 2013
Grand Ballroom B (Hilton)
Jenny Green, Oregon State University, Corvallis, OR and Lew Semprini, School of Chemical, Biological and Environmental Engineering, Oregon State University, Corvallis, OR

Kinetics and Rate Modeling of Chlorinated Aliphatic Hydrocarbon (CAH) Dehalogenation Using Anaerobic Cultures

Jenny Green, Dr. Mohammad Azizian, Dr. Lewis Semprini

Oregon State University; School of Chemical, Biological, and Environmental Engineering

Corvallis, OR 97331

ABSTRACT

Chlorinated aliphatic hydrocarbons (CAHs) were commonly used organic solvents. Their improper storage and disposal has resulted in widespread subsurface contamination. Bioremediation of CAHs is a promising solution to this problem and the in-situ option has been, and continues to be, investigated at Oregon State University. Some CAHs are under special scrutiny; perchloroethene (PCE), trichloroethene (TCE), cis-dichloroethene (cis-DCE) and vinyl chloride (VC) in particular. PCE and TCE are common groundwater contaminants, and cis-DCE and VC are their anaerobic transformation products. By the process of dehalogenation, PCE can potentially be transformed to ethene, a non-toxic end product. The species of bacteria known as Dehalococcoides mccartyi is recognized for its ability to degrade PCE completely into ethene by means of halorespiration. These are the only known organisms with the ability to do so. Two cultures (EV and VS) containing strains of Dehalococcoides mccartyi are maintained under chemostat growth conditions in the bioremediation lab at Oregon State University and are the focus of this study.   

            The objectives of this study were as follows: 1) To determine the rates that the culture in each chemostat can dehalogenate the CAH in question, and 2) To evaluate whether the initial CAH affected the transformation rates of the transformation products.

The rate studies were conducted as batch kinetic tests with cells harvested from the chemostats. The methods described were used for each batch experiment with variation in the mass of CAH and hydrogen (H2) gas additions. Batch experiments began by fitting three 125-mL glass reactors with screw-on caps and rubber septa in an anaerobic glove box (90:10 N2/H2) and then removing them from the system. Using anaerobic methods, 50 mL of culture was added to each reactor and purged with anaerobic gas to eliminate residual CAHs and ethene. After purging, additions of CAH and H2 were made depending on the experiment in question. The reactors were shaken for several minutes to equilibrate the liquid and gas concentrations. At fairly regular time intervals, 100 μL samples from the headspace of each reactor were analyzed using a HP-6890 GC to determine the CAH and ethene concentrations. The total masses of individual compounds in the reactor were calculated by mass balances using Henry's Law.

Mathematical analysis and modeling of the experimental results was done using the methods described by Berggren (2011). The model determines the rate of transformation, KmX, for each step of the transformation process by simultaneously fitting the complete temporal data set. Minor adjustments were made to the modeling spreadsheets used for Berggren's thesis to allow their use with other compounds. A process of Excel Solver utilization and value optimization provided an average Ks and Kcorrected­ value for each CAH tested and the best fit KmX value for each CAH that was transformed.

In addition, linear fitting was also done to estimate KmX values. Rates for each compound were calculated by summing all of the rates of product formation at each instant of time. Linear regressions were run and the slopes were corrected using culture volume in order to compare with KmX values obtained from the modeling.

Testing confirmed that, in general, cultures grown in 5-liter chemostats with excess electron donor added (formate) performed better in transforming VC to ethene than cultures fed limited donor in 2-liter chemostats. This is consistent with more complete transformation to ethene in the chemostats fed excess formate. In addition, the testing revealed that the culture harvested from the VS-5L chemostat performed better than the EV-5L chemostat culture. The VS-2L chemostat also performed better than the EV-2L chemostat. Overall, linear fitting rates were lower than modeling rates, since the linear method does not account for CAHs inhibiting the transformation of other CAHs, while the modeling-derived rates do. Although the modeling method functions well and fits the data well, it has a higher level of complexity and more variables are involved. A more in-depth analysis of the model-estimated rates is still needed.

            A focus of this study was to determine the affect that the initial CAH introduced to the reactors had on the rate of dehalogenation. The modeling method will confirm any observations when it is improved, but after a simple analysis, both linear and modeling patterns emerged suggesting that VC rates are similar regardless of initial CAH added. Another important observation is that both EV and VS have slower rates for all CAHs when beginning with PCE. An explanation for this is that all the chemostats were fed TCE, and therefore they may not be equipped with the enzymes that can rapidly dehalogenate PCE.

            The most variation in data occurs in the triplicates when PCE and TCE were added,  where inhibition has the greatest affect on formation and degradation rates. Modeling methods will be improved so that the effects of initial CAH on dehalogenation rates can be solidified. Further testing and analysis are required to confirm all conclusions.

REFERENCES

Berggren, D. (2011). Kinetic and Molecular Effects of Sulfate Reduction on a Dechlorinating

Culture under Chemostat Growth Conditions. Masters thesis. Oregon State University, Corvallis, OR.


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