Biocatalytic capture of CO2 with carbonic anhydrase and its transformation to solid carbonate

Atmospheric CO2 is well known to be a major contributor to the “green house” effect on earth and as such it deserves to be treated as any environmental pollutant. The present paper focused on its biocatalytic capture by an anhydrase carbonic enzyme to form HCO3− anions, followed by trapping as solid CaCO3 in basic conditions, in a “one pot” process. The kinetics of CaCO3 formation with and without enzyme were compared at 5 and 20 °C, as well as the crystalline nature of the solid formed. Depending on the temperature and the initial pH of the buffer used, two different solid phases were observed: metastable vaterite and stable rhombohedra calcite. The formation of vaterite was enhanced when a buffer stock solution at an initial pH of 10.5, without any enzyme, was used. The possible mechanisms to explain these observations are discussed. At 5 °C, the initial precipitation rate of solid CaCO3 increased by the addition of the enzyme, by a multiplication factor >10. However, this initial rate was also found to depend on the concentration of enzyme and the buffer capacity. Depending on these two parameters, an increasing formation rate of HCO3− in a first step, may lower the reaction medium pH so quickly, that the precipitation of solid carbonate in a second step may be highly hindered. As a consequence, the overall formation rate of solid CaCO3 may actually decrease, for instance when the mass of enzyme is increased.