Precipitation of hydrated Mg carbonate with the aid of carbonic anhydrase for CO2 sequestration

Strategies for sequestering CO₂ directly from the atmosphere are likely required to achieve the desired reduction in CO₂ concentration and avoid the most damaging effects of climate change [1]. Numerous studies have demonstrated the accelerated precipitation of calcium carbonate minerals with the aid of carbonic anhydrase (CA) as a means of sequestering CO₂ in solid carbonate form; however, no study has examined precipitation of magnesium carbonate minerals using CA. Precipitation of magnesite (MgCO₃) is kinetically inhibited [2]; therefore, Mg²⁺ must be precipitated as hydrated carbonate minerals. In laboratory experiments, the uptake of atmospheric CO₂ into brine solutions (0.1 M Mg) was rate-limiting for the precipitation of dypingite [Mg₅(CO₃)₄(OH)₂-5H₂O] with initial precipitation requiring 15 days [3]. It was also found that dypingite precipitation outpaced the uptake of CO₂ gas into solution. CO₂ uptake is limited by the hydration of CO₂ to form carbonate ions [4]. Carbonic anhydrase (CA) enzymes are among the fastest known in nature and are able to catalyze the hydration of CO₂, i.e., converting CO_2(aq) to CO₃^2- and HCO₃^- [5]. CA plays an important role in the carbon concentrating mechanism of photoautotrophic, chemoautotrophic, and heterotrophic prokaryotes and is involved in pH homeostasis, facilitated diffusion of CO₂, ion transport, and the interconversion of CO₂ and HCO₃^- [6]. Introducing CA into buffered Mg-rich solutions should allow for more rapid precipitation of hydrated magnesium carbonate minerals. Batch experiments were conducted using 125 mL flasks containing 100 mL of Millipore deionized water with 0.2 M of MgCl₂-6H₂O. To buffer pH, 1.0 g of pulverized brucite [Mg(OH)₂] or 1.0 g of NaOH was added to the systems, which were amended with Bovine carbonic anhydrase (BCA) (Sigma-Aldrich). Solutions were stirred continuously and kept at room temperature (~22°C) with laboratory air introduced by bubbling. Temperature and pH were measured routinely and water was sampled for dissolved inorganic carbon (DIC) and magnesium concentrations. Final precipitates were collected for X-ray powder diffraction and determination of the percent carbon. The presence of BCA increases the concentration of DIC, thus accelerating the rate-limiting step. In alkaline Mg-rich solutions, disordered hydrated magnesium carbonate, resembling dypingite, rapidly precipitated within hours to form micron-wide flakes. At concentrations of 200 and 100 mg BCA/L, the rates of carbon uptake were ~7 and ~4.4 times that of the control system during the first 24 hours, respectively. BCA is able to catalyze the hydration of CO₂ thereby increasing concentrations of DIC relatively rapidly and allowing for the sequestration of atmospheric CO₂ as hydrated Mg carbonate minerals.