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

Abstract

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 (CO32-) 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.