Steady-State Kinetic Investigation of Enzyme Assisted Carbon Capture

Abstract

Enzyme assisted carbon capture is attracting massive research interest, and absorbents composed of carbonic anhydrase and aqueous carbonate solutions have proven particularly promising. Here, we study basic capture mechanisms by a novel approach, which is based on the principles of comparative biochemistry. We determined capture rates in potassium carbonate at a range of conditions and found a characteristic saturation behavior at high concentrations of either enzyme or CO₂. These results could be rationalized by a Michaelis Menten approach applied to a thin film or “reaction zone” near the liquid surface. Thus, capture rates corresponded directly to enzyme reaction rates in the reaction zone calculated from published K_M and k_cat values.  This link between capture rates and enzyme kinetics explained the observed saturation behavior. It also allowed estimation of the depth of the reaction zone, which for the current setup was only about 7 µm. This meant that liquid equilibrium between CO₂ and HCO₃^- was obtained within this narrow film, and that enzymes deeper in the liquid had little or no influence on capture rates. This limitation of the enzyme reaction to a narrow reaction zone also allowed us to rationalize the effect of pH on enzyme assisted capture. Overall, we propose that steady state kinetics can be used in comparative and mechanistic analyses of enzyme accelerated capture. The approach is theoretically simple, requires limited experimental input and could become important e.g. in attempts to identify rate limiting steps in the network of reactions that underpins carbon capture in aqueous absorbents.

Keywords: Carbonic anhydrase, Comparative enzyme kinetics, Carbon capture rate, Two-film theory, Michaelis Menten theory, Saturation behavior