Enhanced precipitation of magnesium carbonates using carbonic anhydrase

Abstract

Carbonate precipitation, as part of the carbon dioxide (CO₂) mineralization process, is generally regarded as a high-temperature, high-pressure, and high-purity CO₂ process. Typical conditions consist of temperatures around 120 °C and a pressure of 100 bar of pure CO₂, making the process costly. A major challenge facing carbonate precipitation is performing the reaction at low temperatures and low partial pressures of CO₂ (p_CO2) such as 25 °C and CO₂ flue gas concentration. In this work, we investigated the effect of carbonic anhydrase (CA) to favor magnesium (Mg) carbonate precipitation at low temperatures and low p_CO2. CA is an enzyme that accelerates CO₂ hydration promoting its conversion into HCO₃^- and then CO₃^2-. This increases supersaturation with respect to Mg-carbonates. A geochemical model was implemented and used to identify supersaturated conditions with respect to Mg-carbonates. Tests were run at 25, 40, and 50 °C and at 1 bar of either pure CO₂ or 10 vol% CO₂ and 90 vol% N₂. The concentration of 10 vol% CO₂ was chosen to resemble CO₂ concentration in flue gas. In selected tests, the CA enzyme was added directly as bovine CA or through microalgae (Scenedesmus obliquus). Experiments were run for 48 hours; 24 hours to reach equilibrium, then another 24 hours until the supersaturated conditions were established. After 48 hours the experiments were interrupted and the solids were characterized. Results show that the addition of CA, either directly or through Scenedesmus obliquus, enhances Mg-carbonate precipitation. Regardless of the temperature, the precipitates were made entirely of nesquehonite (MgCO₃-3H₂O) when pure CO₂ was used. Otherwise, a solid solution containing brucite (Mg(OH)₂) and MgCO₃-3H₂O was formed. Overall, these findings suggest that CA can promote carbonate precipitation at low temperatures, pressures, and CO₂ purity. The enzyme is effective when added directly or supplied through microalgae, opening up the possibility for a CO₂ mineralization process to be implemented directly at a combustion plant as a CO₂ storage option without preliminary CO₂ capture.