As we count down to the 4th International Conference on Carbon Dioxide Removal in Milano, we are hosting a series of discussions on the research that will be shaping our sessions this June! 
This study investigates how CO2 ingassing, a factor often missing in lab settings, dramatically influences the stability of Ocean Alkalinity Enhancement. The researchers identify a “Critical Alkalinity Period” and argue that turbulent, open-ocean conditions might actually prevent the unwanted mineral precipitation that currently limits OAE efficiency. 
Read the full paper here: Link 
Full Abstract: Decreasing the Critical Alkalinity Period: a must to maintain ocean alkalinity enhancement’s efficiency
Authors: Charly A. Moras, Matias Saez Moreno, Peggy Bartsch, Jens Hartmann
Ocean alkalinity enhancement (OAE) is a marine carbon dioxide removal (CDR) strategy with high potential for carbon dioxide (CO2) capture. However, unwanted secondary mineral formation may occur, decreasing its CDR potential. The formation of such secondary minerals occurs mostly when high alkaline environments are maintained for sustained periods of time, e.g., in the diffusive boundary layers of dissolving particles or at the alkalinity release site. This is what is later referred to as the “Critical Alkalinity Period” – CAP. Recent suggestions to decrease such CAP involve quick dilution with untreated water, or filtration. However, one major process is not accounted for: the ingassing of CO2.
Most laboratory work assessing the feasibility of OAE omit the impact of CO2 ingassing, though it is a key process in natural settings. It is also determining in assessing the stability of alkalinity, and especially the risk for secondary mineral formation. To bridge the gap between laboratory work and real-world implementation, we conducted experiments investigating the dissolution kinetics of two minerals of interest for OAE, i.e., Ca(OH)2 and Mg(OH)2, as well as controls with sodium hydroxide. These were conducted open to the air, allowing for CO2 ingassing to occur, and under two different mixing regimes.
In all experiments, the generated alkalinity was consistently lower than expected, as low as ~5% in some cases. Furthermore, secondary mineral formation was identified, with formation of calcium carbonate (CaCO3) and magnesium hydroxide. Both led to a decreased CDR potential. Yet, despite such formation, these experiments identified a new precipitation pattern for CaCO3. Instead of occurring in a runaway fashion, it halted quickly within days. When CO2ingassing occurred, the CAP and saturation state of the water with respect to CaCO3 quickly decreased. The results from the study suggest that when allowing for CO2 ingassing, the saturation state may decrease fast enough to reduce and prevent unwanted secondary mineral formation. This is still largely overlooked in current laboratory-based experiments and has major implications for the applicability and implementation of OAE.
The CO2 fluxes measured over 6 months correlate with fluxes observed in open ocean conditions with low wind speeds of approximately 3 m s−1, corresponding to those recorded in calm seas. This suggests that deploying OAE in low turbulence systems may yield lower dissolution and alkalinity generation, but the subsequent CO2 ingassing may prevent sustained and unwanted runaway CaCO3 precipitation. Under such assumptions, we argue that implementing OAE in rougher seas will lead to a higher dissolution and alkalinity generation while enhancing CO2 ingassing rates, which in turns decrease the CAP significantly. The CDR efficiency of OAE may therefore be enhanced and secondary mineral precipitation less likely to occur compared to previously reported in laboratory observations.
Do you think we are underestimating OAE’s true real-world potential by ignoring CO₂ ingassing in standard laboratory settings? 
