[A-OS25] Oceanography (General)

[AOS25-P11] Dissolution Experiments of Alkaline Substances for the Establishment of Ocean Alkalinity Enhancement

*Hori Rintaro¹, Junsei Kugimoto¹, Yoshukazu Ohno¹, Atsushi Suzuki², Mayuri Inoue¹ (1.Graduate School of Environmental, Life, Natural Science and Technology, Okayama University, 2.National Institute of Advanced Industrial Science and Technology)

Keywords:

Ocean Alkalinity Enhancement / OAE,Marine carbonate system,marine Carbon Dioxide Removal / mCDR,Calcium carbonate precipitation / CaCO3 precipitation,Runaway precipitation,Total Alkalinity / TA

Ocean Alkalinity Enhancement (OAE) is gaining attention as a marine carbon dioxide removal (mCDR) approach to achieving carbon neutrality by increasing total alkalinity (TA) to promote oceanic uptake of atmospheric CO₂. However, a major challenge is the risk of reduced efficiency caused by the secondary runaway precipitation of calcium carbonate (CaCO₃) resulting from excessive alkalinity addition. The primary objective of this study is to identify the suitable alkaline substances to achieve OAE in addition to investigating possibilities for runaway precipitation during the enhancement of TA. We dissolution experiments using four alkaline substances: Na₂CO₃, NaHCO₃, a mixture of Na₂CO₃ and NaHCO₃, and Mg(OH)₂ and five levels of TA addition (+0, +500, +1000, +1500, and +2000 µmol/kg). Each substance was added to sterilized, filtered Ogasawara seawater in a closed system maintained at 25℃. We continuously monitored pH and pCO₂ for 18 hours. Subsequently, TA and dissolved inorganic carbon (DIC) were measured to evaluate the short-term behavior of the carbonate system and precipitation risks under each condition. Results of continuous monitoring of pH and pCO₂ indicated that while Na₂CO₃ and Mg(OH)₂ effectively reduced pCO₂, CaCO₃ runaway precipitation, that was aragonite based on the XRD analysis, occurred at higher TA levels. To prevent runaway precipitation, the recommended TA addition levels were estimated to be approximately +500 µmol/kg for both Mg(OH)₂ and Na₂CO₃. Based on the 18-hour continuous monitoring and the measurements of TA and DIC after the continuous dissolution experiments, Mg(OH)₂ seems to be the most suitable alkaline candidate among four alkaline substances. The results suggest that TA addition level of +500 µmol/kg for Mg(OH)₂ would be optimal to enhance CO₂ uptake while preventing secondary runaway precipitation.