Stability assessment of calcium carbonate dissolution as a marine carbon dioxide removal mechanism

Ocean Alkalinity Enhancement (OAE) is a promising approach for removing carbon dioxide from the atmosphere by increasing seawater alkalinity. However, the effectiveness of OAE depends on avoiding the precipitation of calcium carbonate (CaCO3) that would reduce OAE efficiency by removing added alkalinity before CO2 uptake can occur, or in cases of more extensive precipitation, potentially release CO2 back to the atmosphere (e.g., ‘runaway’ precipitation). We examined the stability of alkalinity-enhanced seawater under conditions relevant to CaCO3-based OAE by dissolving CaCO3 in CO2-enriched seawater and testing three alkalinity additions (+3,000, +7,000, and +14,000 µmol kg-1) at two temperatures (5 °C and 25 °C) using two CO2 equilibration approaches. We also evaluated how mixing alkalinity-enhanced water with natural water from the Savannah River influences stability across different salinity levels and conditions. Enhanced alkalinity waters showed clear threshold behavior for stability. The highest alkalinity addition (+14,000 µmol kg-1) consistently caused rapid CaCO3 precipitation (within a day), while the lowest addition (+3,000 µmol kg-1) remained stable for more than one month. The intermediate addition (+7,000 µmol kg-1) showed delayed CaCO3 formation, with the timing strongly influenced by temperature and equilibration endpoint. Even in the highest alkalinity additions no ‘runaway’ precipitation was observed where more alkalinity was removed than was added to the seawater. Mixing alkalinity-enhanced water with natural water increased stability, particularly when estuarine water made up at least 60% of the mixture, with salinity of the mixing waters playing an important role in stability. Models showed that aragonite saturation state provides a useful indicator of precipitation risk, however, increases in calcium concentration must be taken into account when CaCO3 is used as the alkalinity source. Our results define practical limits for pre-equilibrated CaCO3-based alkalinity enhancement in coastal environments and emphasize the importance of site-specific conditions when determining effective alkalinity addition levels.