The dissolution of CO₂ in seawater in the form of bicarbonate ions is an attractive alternative to storage in geological formations, on the condition that the storage is stable over long periods and does not harm the marine environment. In this work, we focus on the long-term chemical stability of CO₂ absorbed in seawater as bicarbonate by monitoring the physico-chemical properties of the solutions (pH, dissolved inorganic carbon and alkalinity) in six different sets of experiments on both natural and artificial seawater lasting up to three months. The bicarbonate treatment of natural seawater consists of mixing it with pre-equilibrated solutions obtained from the reaction of CO₂ and Ca(OH)₂, with the same pH as natural seawater. This was achieved with a pilot plant working with tons of seawater, while small-scale laboratory experiments were carried out by adding sodium bicarbonate to artificial seawater solutions. If the increase in the overall carbon concentration in the final mixture does not exceed a critical threshold (about 1000–1500 μmol/L), the resulting bicarbonate-rich solutions are found to be stable for over three months.

Ocean Alkalinity Enhancement (OAE) is a technique proposed to address ocean acidification and global warming. This study examines rainbowing (i.e. water jets pumped into the air from ships) for the slaked lime slurry distribution. The fluid dynamic behaviour—both the in-air trajectory and subsurface—was studied using the Smoothed Particle Hydrodynamics method. Various outflow velocities were simulated for calm seas; different dilution factors and environmental slaked lime concentration spikes have been found. Linking slurry concentration to pH, discharge conditions that avoid damage to marine biota have been identified. A preliminary cost analysis compares rainbowing to ship-wake discharge. Rainbowing requires high power consumption to project large volumes of water far from ships. Very high dilution is needed, mainly because of the artificial requirement not to momentarily exceed 24-hour environmental pH limits; the effect of ocean turbulence on instantaneous pH peaks is negligible and is not considered in the analysis. Emissions due to energy consumption almost equalize carbon dioxide removed during discharge, making the method inefficient (only 176 kg of CO₂ removed per ton of slaked lime spread). Previous ship-wake discharge analysis shows greater OAE performance and lower cost (5.5 € versus 388 € per ton of CO₂ removed). In conclusion—based on the conservative environmental limits imposed, and the specific modeling undertaken—rainbowing is not demonstrated as an efficient method for OAE.

Increasing CO ₂ emissions have led to the development of CO ₂ removal strategies to counteract ocean acidification. Among these, ocean alkalinity enhancement techniques, particularly ocean liming, may represent a promising approach to restore seawater pH and boost CO ₂ sequestration. Yet, the impact of liming on plankton communities remains underexplored. In the framework of a mesocosm experiment, we conducted three dilution incubations to assess natural plankton response (abundance, composition, growth, grazing, diet, and food selectivity) to liming, achieved with calcium hydroxide (Ca(OH) ₂ ) additions. Experiments included two liming treatments (low concentration, and high concentration) and a control treatment without Ca(OH) ₂ . The community was dominated by small‐sized plankton (bacteria, Synechococcus and pigmented picoflagellates), outnumbering larger diatoms, dinoflagellates, and ciliates. While chlorophyll a , heterotrophic bacteria, and pigmented picoflagellates remained stable across treatments, the abundance of Synechococcus and dinoflagellates increased, whereas diatoms and ciliates declined particularly under high Ca(OH) ₂ . Growth and grazing rates were largely unaffected by alkalinity, except for increased growth in pigmented picoflagellates upon liming. Microzooplankton showed low ingestion of Synechococcus and pigmented picoflagellates and higher intake of diatoms, dinoflagellates, and ciliates. Food selectivity was unresponsive to liming, as the grazers selected prey based on size, regardless of Ca(OH) ₂ concentrations. Increased alkalinity and pH, and the parallel effect of trophic cascades might have driven nutrient fluctuations and shaped downstream trophic interactions. Despite positive responses highlighted in this study, further research is needed to explore liming potentiality on a wider range of food‐web components and larger scales in the framework of ocean alkalinity enhancement research.

The deployment of carbon dioxide storage is a challenge that must be overcome in order to reach the net zero emissions objective. Ocean alkalinity enhancement (OEA) is a promising method for removing carbon dioxide from the atmosphere and storing it pemanently in seawater as bicarbonates, with the co-benefit of counteracting ocean acidification. The challenge for future applications is ensuring a stable storage, avoiding adverse side effects on the environment or phenomena that can reduce efficiency, such as degassing of carbon dioxide and precipitation of alkaline minerals.The work presented in this study investigates the stability of the carbonate system of seawater, after adding alkalinity by two different pathways. One is the simple addition of NaHCO3 to artificial seawater, on a laboratory scale. The other is a test on a more complex system, consisting of a pilot plant that uses natural seawater, CO2 and calcium hydroxide and produces a carbon-enriched solution at the same pH of natural seawater. We suggest safe levels for the increase of alkalinity and considering the dilution of the solution with seawater in natural environments. The results represent important steps towards the achievement of safe and efficient ocean-based carbon storage and OAE.

The dissolution of CO₂ in seawater in the form of bicarbonate ions is an attractive alternative to the storage in geological formations, on the conditions that the storage is stable over long times and does not harm the marine environment. In this work, we focus on the long-term chemical stability of CO₂ absorbed in seawater as bicarbonate, by monitoring the physico-chemical properties of the solutions (pH, dissolved inorganic carbon and alkalinity), in six different sets of experiments on both natural and artificial seawater lasting up to three months. The bicarbonate treatment of natural seawater consists in pouring pre-equilibrated solutions obtained from the reaction of CO₂ and Ca(OH)₂. If the pre-equilibrated mixture does not exceed a critical threshold (ca. 1000-1500 mmol/L), the resulting bicarbonate-rich solutions can be stable for over three months.