Feasibility study of storing CO₂ in the ocean by marine environmental impact assessment

Since the industrial revolution, which was accompanied with the use of fossil fuels as an energy source, the content of carbon dioxide (CO₂) in the atmosphere has increased. To mitigate global warming, industries that utilize fossil fuels have continuously explored new approaches to reduce CO₂ emissions and convert it to alternative fuels. The ocean is a vast source of absorbed CO₂ on Earth, and various studies have been conducted on the use of the ocean to reduce global CO₂. This study focused on reducing CO₂ in the atmosphere by storing it as bicarbonate, a form of CO₂ that exists in the ocean. The optimum condition for the conversion of CO₂ into bicarbonate was investigated by considering the dissolved inorganic carbon (DIC; HCO₃⁻, CO₃²⁻, H₂CO₃) concentration and pH. To confirm the biological impact of this conversion, biological impact experiments were conducted under various DIC concentrations using Skeletonema japonicum, a phytoplankton present in most areas of the sea. Based on the DIC concentration (2.09 mM) of the seawater, the DIC concentrations used in the Lab-scale experiment ranged from 2.5 mM to 18.75 mM, and the concentration with the highest conversion rate (< 6.38 mM) was applied in the pilot plant. Marine environmental impact modeling was performed to observe the effect of discharge to the ocean and its movement. The results revealed a slight growth inhibition of phytoplankton at DIC concentrations higher than the base concentration. Nevertheless, the change in the DIC concentration exerted no effect on the phytoplankton growth except at extremely high concentrations. Moreover, the high DIC concentration can be diluted by the ocean current flow rate, thus counterbalancing the growth inhibition effect. The results obtained in this study demonstrate the feasibility of CO₂ storage in the form of DIC, and will be helpful for further development of CO₂ mitigation.