(27c) Utilization of Carbonic Anhydrase-Displaying Escherichia Coli in the Foam Bioreactor to Capture and Sequester Carbon Dioxide

In recent years, global warming as the result of greenhouse gas emissions, especially that of CO₂, has become of great concern.  Many technologies have been proposed and employed to capture post-combustion CO₂.  Conventional methods such as absorption and adsorption have the problems of regeneration of spent solvents and media requiring additional energy input and expensive operating costs. For these reasons, the carbonic anhydrase enzyme (CA), which quickly catalyzes the conversion of CO₂ into bicarbonate ion and a proton, has been studied for use in carbon capture and sequestration (CCS) technologies.  This process would solve the limitations of current carbon capture technologies because it readily converts CO₂ into environmentally friendly products at mild conditions.  Furthermore, upon CO₂ removal, the aqueous bicarbonate can be precipitated and separated as calcium carbonate (bio-mineral) as a value-added product.  However, current CA-driven carbon capture remains expensive: the purified enzyme itself costs in the range of thousands of dollars per gram; the enzyme works best in slightly alkaline conditions, so some buffering capacity is necessary to combat the protons it generates; and CA must be replaced over time as it gradually loses its activity.  The major goals of this project are to employ genetically modified E. coli to continuously produce and display CA on their cell membranes and to use these bacteria in the Foam Bioreactor to capture and sequester CO₂.  The bacteria incubated in auto inducible media (AIM) have shown successful growth and enzyme expression as well as high activity and foaming capacity in the Foam Bioreactor.  Because of the huge gas-to-liquid surface area in the Foam Bioreactor, the CO₂ in gas phase was quickly transferred to many CA-displaying bacteria, allowing the CA to convert CO₂ to bicarbonate ions and protons.  Under ex situ pH control and an inlet stream of 0.5-2% CO₂ at 1 L/min, the increased cell concentrations enhanced the removal efficiency and elimination capacity of CO₂.  A decrease in gas flow rate from 1.0 to 0.4 L/min at an inlet stream of 0.5-2% CO₂ also enhanced the removal efficiency of CO₂.  Future work will include the effect of tertiary amine solutions (i.e., methyl diethanolamine) to enhance mass transfer and overall removal efficiency of CO₂, the in situ pH control in the foam for optimal CA activity inside the reactor, and the mathematical modeling of CA-driven carbon capture in the Foam Bioreactor.