In vivo immobilized carbonic anhydrase and its effect on the enhancement of CO₂ absorption rate

Reactive absorption into aqueous solutions promoted by carbonic anhydrase (CA, E.C. 4.2.1.1.) has been often proposed as a post-combustion CO₂ capture process. The state of the art reveals the need for efficient biocatalyst based on carbonic anhydrase that can be used to further develop CO₂ capture and utilization technologies. The present study is focused on the use of a thermostable CA-based biocatalyst. The carbonic anhydrase SspCA, from the thermophilic bacterium Sulfurihydrogenibium yellowstonense, was in vivo immobilized as membrane-anchored protein (INPN-SspCA) on the outer membrane of Escherichia coli cells. The dispersed biocatalyst, made by cell membrane debris, was characterized in terms of its contribution to the enhancement of CO₂ absorption in carbonate/bicarbonate alkaline buffer at operating conditions relevant for industrial CO₂ capture processes. The amount of immobilized enzyme, estimated by SDS-PAGE, resulted in about 1 mg enzyme/g membrane debris. The apparent kinetics of the biocatalyst was characterized through CO₂ absorption tests in a stirred cell lab-scale reactor assuming a pseudo-homogeneous behaviour of the biocatalyst. At 298 K, the assessed values of the second-order kinetic constant ranged between 0.176 and 0.555 L∙mg^-1∙s^-1. Reusability of the biocatalyst after 24 h showed the absence of free enzyme release in the alkaline solvent. Moreover, the equilibration of dispersed cell membrane debris against the alkaline buffer positively affected the performances of the heterogeneous biocatalyst. These results encourage further studies on the in vivo immobilized SspCA aimed at optimizing the enzyme loading on the cell membrane and the handling of the biocatalyst in the CO₂ absorption reactors.