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SBI Research Feature: Modeling Bioremediation of High Concentrations of TCE
Posted: October 4, 2010
Former environmental engineering and Subsurface Biosphere IGERT graduate student Andrew Sabalowsky and Distinguished Professor Lewis Semprini have two articles in the October issue of Biotechnology and Bioengineering. The papers propose a new model for why bioremediation of the groundwater contaminant TCE slows when contaminant concentrations are very high. In this short web interview, Andy, who is now a post-doc at the Montana State University Center for Biofilm Engineering, describes their research and its relevance for groundwater remediation.
Your research focuses on groundwater contaminants TCE and PCE. What are these substances and why are they a concern?
TCE and PCE are two solvents that were used extensively in the 1950s for degreasing engines and other equipment and also for dry cleaning. They are dense non-aqueous phase liquids (DNAPLs ) so they are heavier than water and at many contaminated sites they percolated down through soils and pooled at the bottom of groundwater aquifers. Use of TCE and PCE stopped in the 1970s but they are slow to degrade and contaminated aquifers exist in many parts of the world. PCE, TCE and the compounds they degrade into are a health concern. They are hepatotoxins (substances that target the liver) and they and their daughter products are either suspected or known carcinogens.
Today, a common strategy for treating aquifers contaminated with PCE and TCE is bioremediation. Engineers try to create conditions that enable bacteria – either native to the site, or introduced – to degrade the pollutants. Our focus in these papers was on anaerobic bioremediation – it utilizes bacteria that thrive in anoxic zones, the type typical in source zones of contaminant plumes where concentrations of PCE, TCE and their daughter products can be very high. The goal is to treat the plume at its source zone rather than at its fringes and therefore cut down on the remediation time and cost.
What is the contribution of your two research papers to this problem?
In order to improve bioremediation methods, we need to understand the biological processes that are taking place. In the literature there have been observations that at very high concentrations of PCE, TCE and its immediate daughter product, cis-DCE, the ability of microbes to transform the contaminants declines with time. Researchers have discussed this loss of activity as an “inhibition process” – something is inhibiting the microbes ability to transform the contaminants. Inhibition implies reversibility – if you lowered the concentration of TCE and cis-DCE, the rate of transformation should rise again. But in our experiments, we found that this was not the case and we propose a new model – one that indicates it is irreversible toxicity that is happening, not inhibition. High concentrations of TCE and cis-DCE cause a certain percentage of cells to stop working and they cannot recover. Our model also proposes that the extent of the toxicity (the number of cells affected) is proportional to the contaminant concentration, hydrophobicity (the degree to which it partitions into cell walls) and the duration of the exposure. We tested our model with three different experimental set ups – batch systems, chemostats, and recirculating columns with biofilms and more or less the same model fit all three of these different scenarios.
Can you describe a little more about these experimental systems?
Batch grown cells are a closed system in a bottle with suspended cells. A chemostat is a well-mixed reactor with suspended cells, a continuous steady flow of growth medium moving in, and an effluent flowing out at the same rate. In this type of system, the TCE is continuously mixed so concentrations of TCE would in theory be lower and more constant than in a batch system where TCE is added in pulses. But we didn’t measure steady concentrations because the system was failing – as the bacteria stopped being able to transform the contaminants, the concentration of TCE and cis-DCE began to rise. Modeling showed that the failure was much more rapid and dramatic than could be explained by inhibition alone; and we could only explain the failure when we include a term for toxicity of both cis-DCE and TCE in the mathematical model we created to describe the system.
The biofilm experiments were done in recirculating columns packed with glass beads where cells grew as attached cells in an interconnecting network, called a biofilm. One pump was mixing the medium around at a relatively high rate and a second pump introduced contaminants to the system. Our model again matched the changes in contaminant concentrations that we observed over time. We were able to model not only the failure of the system with increasing TCE and decreasing cis-DCE concentrations, but also the temporal history of the failure. The concentration of TCE did not rise suddenly, rather the concentration went up a little and then plateaued and then increased again, and only the model with our toxicity term fit this observation.
These two papers were part of your dissertation – what was one of your most important lessons learned from this project?
One thing I learned from the development of these two papers was the utility of models to point toward hypotheses. Our model for toxicity developed out of this research because none of the existing models fit our experimental data. So it was an indirect approach to develop a hypothesis for a biological process that we were then able to go back and demonstrate with some followup batch experiments.
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