Mechanistic studies on the conversion of carbon dioxide to calcium carbonate or bicarbonate ions using seawater and steel slag

Abstract

In the pursuit of mitigating atmospheric carbon dioxide (CO₂) levels, research on Carbon Capture Utilization and Storage (CCUS) technologies has gained considerable traction. This study focused on the indirect carbonation method, which has a faster conversion rate and involves reacting CO₂ with calcium (Ca²⁺) or magnesium (Mg²⁺) contained in natural minerals or alkaline industrial by-products to produce carbonates. In this study, we used natural seawater as a solvent and convert slag (CS) of steel slag, an alkaline industrial waste. Seawater was selected for its innate ability to capture CO₂, leveraging its abundant natural resources. Meanwhile, CS exhibits chemical reactivity, resulting in lower energy requirements for CO₂ absorption and carbonation reactions and a high Ca²⁺ content. Most previous studies using seawater and industrial by-products have focused on factors influencing the enhancement of CO₂ absorption rates for effective CO₂ storage, the removal of Ca²⁺ to mitigate scaling in desalination processes, or the improvement of calcium carbonate (CaCO₃) production by including additional chemical reagents. However, we aimed not only to convert CO₂ into CaCO₃ but also to consider the long-term storage of CO₂ in the stable form of HCO₃⁻ ions for immediate utilization without additional additives or costs, through the study of mechanisms. We analyzed the thermodynamic reactions of CO₂ storage, driven by the solubility differences among various minerals. In particular, this study emphasizes the role of naturally precipitated magnesium hydroxide (Mg(OH)₂) as an additional alkaline source that dissolves to facilitate the carbonation reaction upon CO₂ injection. The slag-to-seawater ratio (S/L ratio) significantly influenced the leaching of Ca²⁺ ions, which are crucial for CaCO₃ formation, with the highest Ca²⁺ ion leaching observed at a ratio of approximately 1:9. In this context, Mg²⁺ ions in the seawater were found to act as a facilitator for the leaching of Ca²⁺ ions from the slag. Moreover, quantitative measurements of the carbon chemistry system were employed to monitor the changes in various ions. Specifically, by utilizing naturally precipitated Mg(OH)₂ as a carbonation activator, we achieved the concurrent conversion of CO₂ into CO₃²⁻ and HCO₃⁻ ions. Despite using seawater and CS, both rich in impurities, we confirmed that the resulting CaCO₃ exhibited the most thermodynamically stable crystal structure, calcite. Ultimately, we aim to realize effective long-term CO₂ storage by substituting chemical solvents with natural seawater and repurposing industrial waste for eco-friendly solutions.