Considering hydrous carbonates for ocean alkalinity enhancement

Ocean Alkalinity Enhancement (OAE) emerges as a promising strategy capable of sequestering several gigatons of CO2 annually from the atmosphere and store it in the ocean for extended periods (>1000 years). To achieve this objective, artificial alkalinity is introduced into the surface ocean through alkaline solutions or the spontaneous dissolution of alkaline solids. When contemplating alkaline solids for OAE, a primary challenge lies in generating substantial quantities of fine grained (<10 µm), soluble solids at low energy and cost. The hydration of carbonates presents a potentially less energy-intensive method, yielding products that exhibit favorable thermodynamics leading to their spontaneous dissolution in seawater1. We investigated the stability and dissolution kinetics of two hydrous carbonates, Ikaite (CaCO₃·6H₂O) and water-bearing amorphous calcium carbonate (CaCO3.nH2O), labelled hereafter as w-ACC. Both phases can be created from dissolving limestone at high CO2 pressures. An engineering concept using a CO2 pressure swing in a reactor has been recently published1. Once created,  the hydrous carbonate phases are unstable at temperatures higher than the formation temperature and transform to anhydrous polymorphs after a certain period of time. Thus, we have investigated the temporal stability of either phase at different temperatures in order to contribute to their life cycle assessment. Moreover, the transformation of ikaite and w-ACC to an anhydrous polymorph obliterates the effect of releasing alkalinity during spontaneous dissolution, which needs to be avoided. Our results show that at room temperature both phases dehydrate within hours when stored as wet powders after simple filtration. However, their stability extends to days when the physical adsorbed water is removed e.g. by rinsing with ethanol. A quantitative estimate of the kinetic rate of the hydrous-to-anhydrous phase transformation is currently being analyzed by Raman spectroscopy . Our results also indicate that w-ACC has a higher dissolution rate than ikaite in seawater due to its higher specific surface area (>90m2/g). However, the efficiency of both hydrated carbonates in releasing alkalinity will be further analyzed to elucidate the effect of particle coagulation, particle sinking, and secondary precipitation phenomena. Nonetheless, our pilot results demonstrate that both ikaite and w-ACC are promising candidates for OAE, considering their potential in augmenting ocean alkalinity and CO2 sequestration.   1             Renforth, P., Baltruschat, S., Peterson, K., Mihailova, B. D. & Hartmann, J. Using ikaite and other hydrated carbonate minerals to increase ocean alkalinity for carbon dioxide removal and environmental remediation. Joule 6, 2674-2679 (2022). https://doi.org/10.1016/j.joule.2022.11.