Characterization of technical grade carbonic anhydrase as biocatalyst for CO2capture in potassium carbonate solutions

Reactive absorption promoted by carbonic anhydrase (CA, E.C. 4.2.1.1.) catalysis has been proposed for CO2 capture from exhaust gas as an alternative to the reactive absorption in amine solutions. Potassium carbonate solutions allow feasible CO2 capture assisted by CA. This paper reports on the characterization of a CA form supplied by Novozymes as a catalyst for CO2 capture in K2CO3 solutions at operating conditions relevant for industrial processes (2-3 M K2CO3, 298-313 K, 0-40% carbonate to bicarbonate conversion). CO2 absorption tests were carried out in a batch-stirred cell apparatus at constant gas volume and temperature by measuring gas pressure decay. The enhancement ratio between CO2 absorption rate with and without the enzyme at biocatalyst concentration above 0.3 kg m-3 ranged between 5 and 8 depending on the solvent composition and temperature. Active enzyme aggregates formed at protein concentration larger than 0.3 kg/m3 and their contribution to the absorption rate enhancement was remarkable. Assessment of enzyme kinetics in homogeneous solutions at enzyme concentration lower than 0.018 kg·m-3 showed that kcat/KM increased with both temperature and salt concentration. kcat/KM ranged between 9.0·103 and 7.0·104 m3·kg-1·s-1 in agreement with literature data on carbonic anhydrase kinetics. Reliability of the adopted method for kinetic characterization was confirmed. The activity of enzyme aggregates formed in the carbonate solvent was verified and CA aggregates can be used to select the proper CA based biocatalyst for CO2 capture application as an alternative to free and immobilized enzyme.