644 / 2024-09-18 19:28:11

Design of Buoyant Mineral Carriers for The Application of Ocean Alkalinity Enhancement

Ocean alkalinity enhancement,Alkaline minerals,Buoyant foam carriers,Dissolution rate of CaCO3

Given that the ocean is the largest carbon reservoir in the world, absorbing 26% of atmospheric CO₂ annually, it is considered a crucial field for achieving carbon neutrality strategies. In recent years, ocean alkalinity enhancement (OAE) has garnered increasing attention as a promising approach for marine carbon dioxide removal (mCDR). However, overfast dissolution rates and sinking nature of alkaline minerals, such as magnesite, lime, and olivine, likely result in low carbon removal efficiency. For example, the fast release of alkalinity may trigger secondary precipitation of CaCO₃ decreasing alkalinity. In principle, foams can float on the sea surface for the sustained release of alkalinity due to its much smaller density than that of seawater, and thus has potentials to slow down both the dissolution and sinking rate of minerals. Here, we conducted a series of incubation experiments by adding a couple of alkaline minerals, with or without foams, into Pacific surface seawater for 16 days, in order to examine the alkalinity enhancement effect via mineral dissolution. Results showed that the salinity-normalized total alkalinity (NTA) and dissolved inorganic carbon (NDIC) in foam-carried groups increase at a nearly constant speed of 5.62 μmol kg^-1 d^-1, while both in groups without foams show a rapid increase of 116.39 μmol kg^-1 within the first two days and remain steady toward the end of experiment. In addition, salinity-normalized magnesium concentrations (NMg²⁺) markedly increase over time in all mineral addition groups, documenting the release of TA and Mg²⁺ via mineral dissolution. Salinity-normalized calcium concentrations (NCa²⁺) are nearly unchanged or slightly decrease in foam-carried groups, indicating the absence of secondary precipitation of CaCO₃, whereas NCa²⁺ concentrations in foam-free groups clearly decline. Therefore, we suggest that the incorporation of alkaline minerals with foam carriers can stabilize the alkalinity release and prevent the secondary precipitation, potentially contributing to the improvement of OAE efficiency.