EAGER: Coupling of Gas-Liquid Plasma Chemical Reactors with Bioengineered Microbes

Non-thermal plasmas are low-temperature, atmospheric-pressure ionized gases that are generated by high-voltage electric discharges. When these plasmas are in contact with liquid water, free radical species are formed that effectively decompose toxic compounds or transform organic waste into value-added products. While non-thermal plasmas alone can be used to completely degrade target pollutants into mineralized products, the total electrical power required may make the process economically nonviable.

Biological treatment processes, on the other hand, can completely transform some chemical waste species with greatly reduced power demands. However, these processes are limited to biodegradable compounds and may require lengthy processing times. Previous work by this research team demonstrated that the sequential combination of using a plasma reactor to initiate the breakdown of waste compounds followed by a bioreactor to complete the process can lead to significant energy savings.

It was also found, however, that the slower bioreactor dynamics results in a mismatch in time scales that ultimately led to very large reactor systems. To overcome this problem, in this study microbial cells will be genetically engineered to: a) survive in the liquid water contacting the non-thermal plasma, and b) increase enzyme production so that the rates of the biological reaction pathways compare to those of the plasma.

This matching of the two process time scales will make possible the design and operation of integrated, compact, and energy efficient plasma-bioreactor systems for degrading hazardous and toxic compounds from wastewater streams or to produce useful compounds from waste materials. Fundamental knowledge on coupling a non-biological (abiotic) system such as the non-thermal plasma reactor with the cellular metabolism of a bacterium also will be generated by this work.

This project seeks to develop non-thermal plasma gas-liquid bioreactors, research that couples plasma chemistry and chemical reaction engineering with bioengineering. Plasma reactions occur over time scales of less than 1 second while conventional bioreactors often operate on time scales of hours to days to weeks. These large differences in time scales suggest that it may be possible to attain greater efficiency and reaction process synergy by coupling non-thermal plasma to bioreactors containing microbes bioengineered for both plasma resistance and specific metabolic tasks.

The overall goal of the proposed work is to test the hypothesis that microbes resistant to the plasma-produced oxidizing compounds can be genetically engineered to perform other useful biochemical transformations within the reactive environment of gas-liquid non-thermal plasma reactors. Plasma resistant E. coli will be engineered to enhance their inherent capacity to metabolize carboxylic acids, specifically glycolic, formic and oxalic acids.

Likewise, the plasma reactor will be engineered to promote effective contact between the cells and the gas-liquid plasma environment. The specific hypotheses of this proposal are: 1) that the newly developed cells will be able to function while exposed to the plasma, and 2) the incorporation of these cells in the plasma reactor will lead to a faster overall mineralization of a target organic compound.

The first practical applications to be investigated in this work involve treatment of toxic and difficult to biodegrade organic compounds. The demonstration that microbes can be created to perform useful chemical transformation within the highly oxidizing plasma environment will constitute a novel example of the effective use of electrical energy in chemical processing.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.