[H-SC06] CCUS (Carbon Dioxide Capture, Utilization, and Storage) for Climate Mitigation

[HSC06-P01] Which dissolved magnesium ions choose to form carbonate or silicate in terrestrial environments?

*Kenichiro Tsujimoto¹, Reo Suma¹, Shunchou Kou², Tsutomu Sato¹ (1.Hokkaido University, 2.LIXIL HOUSING TECHNOLOGY)

As a crucial strategy for mitigating global warming, carbon capture and storage (CCS) has garnered increasing attention. When carbon dioxide (CO₂) is injected into underground reservoirs, a portion dissolves in the formation water, forming carbonic acid that interacts with surrounding rock minerals, leading to their dissolution. Among the released ions, divalent cations such as Mg²⁺, Fe²⁺, and Ca²⁺ can react with CO₂ to form stable carbonate minerals, thereby facilitating long-term carbon sequestration. However, conventional CCS reservoirs, such as sandstone formations, are typically deficient in these divalent cations, thereby limiting the potential for mineralization. In contrast, ultramafic and mafic rocks, which are rich in divalent cations, offer greater potential for carbonation. Among these, basalt has garnered particular attention due to its abundance in the crust of Earth, leading to large-scale demonstration projects such as the Deccan Traps initiative.
Basalt-based CCS environments not only contain dissolved divalent cations but also exhibit a high concentration of dissolved silica (Si). This raises concerns that magnesium ions, which are considered key cations for carbonate mineralization, may preferentially form magnesium silicate hydrate (M-S-H) instead of carbonate minerals.
To address this issue, this study investigates whether dissolved magnesium in near-surface environments of Earth preferentially forms carbonate minerals or silicate minerals. We conducted both M-S-H synthesis experiments and alteration experiments under CO₂-enriched conditions.
Initially, M-S-H (Mg/Si = 1.5) was synthesized under atmospheric conditions (25°C) at various pH levels (pH = 8, 10, 11, 12). Subsequently, the synthesized M-S-H was subjected to elevated carbon dioxide conditions (20% CO₂, 50% relative humidity) for 30 days to investigate potential mineral transformations. In the synthesis experiments, magnesium chloride dihydrate was utilized as the magnesium source, and tetraethyl orthosilicate (TEOS) as the silicon source. The resulting products were characterized using X-ray diffraction (XRD) and Raman spectroscopy.
Across all pH conditions, no Mg-carbonate minerals were detected. At pH 8, only amorphous silica was identified, while at pH 10 and above, M-S-H formation was confirmed. These results suggest that even under high dissolved Mg²⁺ conditions, the CO₂ concentration in atmospheric air is insufficient to promote Mg-carbonate formation. Previous M-S-H synthesis experiments were conducted in glove boxes filled with nitrogen to exclude atmospheric CO₂, but this study demonstrates that, even in its presence, Mg-carbonate formation does not occur under normal atmospheric conditions. Thermodynamic analysis supports these findings.
Under 20% CO₂and 50% relative humidity, M-S-H completely disappeared within 30 days, and the Mg-carbonate mineral nesquehonite was formed. This transformation is attributed to CO₂ dissolution in adsorbed water on the sample surface, leading to a decrease in pH, M-S-H dissolution, and subsequent Mg²⁺ release, which reacts with dissolved CO₂ to form nesquehonite. Thermodynamic modeling further supports this reaction pathway.
In addition to these experimental findings, this study incorporates thermodynamic modeling to explore Mg speciation under various temperature, pH, and dissolved Mg/Si concentration conditions, as well as in basalt interstitial water environments. Through these analyses, we provide insight into whether dissolved magnesium in near-surface environments preferentially forms carbonate minerals or silicate hydrates under CCS-relevant conditions. These findings contribute to our understanding of the geochemical stability of Mg²⁺ in basalt-based CCS environments and provide critical insights for optimizing CO₂ mineralization processes.