Effects of grain size and seawater salinity on brucite dissolution and secondary calcium carbonate precipitation kinetics: implications for Ocean Alkalinity Enhancement

Ocean Alkalinity Enhancement (OAE) is a carbon dioxide (CO₂) removal technology with one of the largest potentials, which simultaneously decreases the pressure of ocean acidification. Most of the current understanding of OAE stems from numerical models. However, two recent studies have shown that secondary calcium carbonate (CaCO₃) precipitation can occur at unexpectedly low saturation state, when using particle based OAE feedstocks. This is undesirable as it reduces OAE efficiency and can lead to “runaway CaCO₃ precipitation”.

Both mineral dissolution kinetics and secondary CaCO₃ precipitation are influenced by the physical and environmental properties of mineral feedstock and seawater. For example, the surface area of particles in suspension is an important factor for dissolution kinetics of minerals, as well as secondary CaCO₃ precipitation kinetics. Furthermore, CaCO₃ precipitation depends directly on the concentrations of calcium (Ca²⁺) and carbonate ions (CO₃^2-) in seawater. The higher their concentrations, the more likely CaCO₃ will precipitate. Since Ca²⁺ concentration have a positive correlation with salinity in the open ocean, variations in seawater salinity could be an important modifier.

Here, we present experimental data on the effects of grain size and salinity on the kinetics of brucite (magnesium hydroxide) dissolution and secondary CaCO₃ precipitation. Preliminary results on the effect of grain size suggest that CaCO₃ precipitation for medium sized particles (63-180 µm) is slower. At larger grain size, the slower dissolution rate, as of the smaller surface area, leads to more quickly measurable CaCO₃ precipitation. At smaller grain size, it is the greater surface area that seems to increase the CaCO₃ precipitation rate.

For salinity, first results suggest that dissolution rates increase towards lower salinities, while CaCO₃ precipitates quicker. The former finding is most likely related to higher brucite dissolution at lower ambient magnesium concentrations, due to lower salinity. The quicker CaCO₃ precipitation is also likely due to the lower magnesium concentration in lower salinity seawater. Magnesium ions are known to inhibit CaCO₃ precipitation, hence CaCO₃ precipitation is less likely inhibited at lower rather than higher salinities. Therefore, both feedstock grain size and seawater salinity are two key parameters for real-world OAE applications.