Presentation Information
[MIS19-03]CO2 reduction utilizing mine drainage: estimation of interfacial tension via contact angle measurements
*Ken Nagashima1, Yoko Ohtomo2, Hisao Satoh3 (1.The Institute of Low Temperature Science, Hokkaido University, 2.Faculty of Engineering, Hokkaido University, 3.Low-Level Radioactive Waste Disposal Center, Japan Nuclear Fuel Limited)
Keywords:
contact angle,interfacial tension,calcite,PHREEQC,mine drainage,enhanced rock weathering
Ocean Alkalinity Enhancement (OAE) is gaining attention as a key technology for achieving carbon neutrality by 2050. This study focuses on an Enhanced Rock Weathering (ERW) technique that simultaneously neutralizes acid and achieves OAE using acid mine drainage. We investigated drainage from the Shojin River mine, Hokkaido (pH2-3) [1] as a model case. Geochemical modeling with PHREEQC predicted that more than 30 minerals would become supersaturated during the neutralization process. However, field tests showed that only specific minerals, such as schwertmannite, precipitated, and the mineral species did not depend on the degree of supersaturation. This suggests that mineral formation in multicomponent systems is strongly governed by kinetic factors, specifically solid-liquid interfacial tension (γSL), rather than thermodynamic driving forces (supersaturation). To evaluate the efficiency and sustainability of OAE, it is essential to correctly understand the γSL of various mineral species in multicomponent solutions.
Previous studies [2-3] have shown a strong negative correlation between γSL and the logarithm of the solubility product (Ksp). While this model aligns with the physical intuition that stable crystals have higher γSL, it assumes γSL is constant because Ksp is a material constant. However, because the solubility of minerals can change by orders of magnitude with pH during neutralization, it is unclear whether γSL remains constant under such environmental conditions. Therefore, we investigated the pH dependence of γSL by measuring contact angles on calcite (CaCO3) cleavage planes.
Contact angles were measured on calcite (10-14) cleavage planes under pH5-8 conditions using the sessile drop method. We developed a method to estimate the thermodynamically stable contact angle by continuously varying the droplet volume, thereby eliminating the effects of contact angle hysteresis (pinning). The results showed a significant dependence of calcite contact angle on pH, decreasing from approximately 90 degrees at pH5 (high solubility condition) to approximately 79 degrees at pH8 (low solubility condition). This change corresponds to a roughly 10% decrease in interfacial tension (estimated based on γSL = 170 mJ/m2 [4]). Theoretically, this reduction increases the nucleation frequency at pH 8 by more than 10 orders of magnitude compared to pH 5 under the same supersaturation conditions, indicating that pH variation causes non-negligible changes in interfacial tension.
The observed γSL variation cannot be explained solely by classical hydration models, suggesting ion adsorption at the interface. We examined three potential mechanisms: (1) Electrostatic adsorption (Zeta potential) and (2) Solubility effect (total ion hydration), both of which predicted trends opposite to the experimental results. (3) Specific adsorption: The activity of CO32- increases exponentially with pH, which correlates well with the decrease in γSL. We propose that divalent CO32- ions specifically adsorb onto surface Ca2+ sites, stabilizing the interface more effectively than HCO3-.
These results imply that pH elevation during neutralization provides a dual effect: increasing the thermodynamic driving force (supersaturation) and decreasing the kinetic barrier (γSL). The kinetic effect is particularly significant; a slight decrease in interfacial tension drastically increases nucleation frequency. This study highlights the need to treat γSL as a dynamic parameter based on ion-adsorption equilibrium rather than a constant to predict elemental behavior in multicomponent systems relevant to OAE.
[1] Chikanda F, Iwaki H, San HH, Kikuchi R, Ohtomo Y, Otake T, Sato T (2025), Research Square (Preprint). doi:10.21203/rs.3.rs-8066399/v1
[2] Nielsen AE and Söhnel O (1971), J. Cryst. Growth. 11, 233.
[3] Kashchiev D, van Rosmalen GM (2003): Cryst. Res. Technol. 38, 555.
[4] Liu XY, Tsukamoto K, Sorai M (2000) Langmuir. 16, 5499.
We also plan to present related poster presentations from different perspectives in other sessions: the formation mechanism of biogenic carbonates in the "M-GI35: Carbonate Biology (May 24)" session, and the prediction of CO2 mineralization in multicomponent systems in the "H-SC07: CCUS for Climate Mitigation (May 26)" session.
Previous studies [2-3] have shown a strong negative correlation between γSL and the logarithm of the solubility product (Ksp). While this model aligns with the physical intuition that stable crystals have higher γSL, it assumes γSL is constant because Ksp is a material constant. However, because the solubility of minerals can change by orders of magnitude with pH during neutralization, it is unclear whether γSL remains constant under such environmental conditions. Therefore, we investigated the pH dependence of γSL by measuring contact angles on calcite (CaCO3) cleavage planes.
Contact angles were measured on calcite (10-14) cleavage planes under pH5-8 conditions using the sessile drop method. We developed a method to estimate the thermodynamically stable contact angle by continuously varying the droplet volume, thereby eliminating the effects of contact angle hysteresis (pinning). The results showed a significant dependence of calcite contact angle on pH, decreasing from approximately 90 degrees at pH5 (high solubility condition) to approximately 79 degrees at pH8 (low solubility condition). This change corresponds to a roughly 10% decrease in interfacial tension (estimated based on γSL = 170 mJ/m2 [4]). Theoretically, this reduction increases the nucleation frequency at pH 8 by more than 10 orders of magnitude compared to pH 5 under the same supersaturation conditions, indicating that pH variation causes non-negligible changes in interfacial tension.
The observed γSL variation cannot be explained solely by classical hydration models, suggesting ion adsorption at the interface. We examined three potential mechanisms: (1) Electrostatic adsorption (Zeta potential) and (2) Solubility effect (total ion hydration), both of which predicted trends opposite to the experimental results. (3) Specific adsorption: The activity of CO32- increases exponentially with pH, which correlates well with the decrease in γSL. We propose that divalent CO32- ions specifically adsorb onto surface Ca2+ sites, stabilizing the interface more effectively than HCO3-.
These results imply that pH elevation during neutralization provides a dual effect: increasing the thermodynamic driving force (supersaturation) and decreasing the kinetic barrier (γSL). The kinetic effect is particularly significant; a slight decrease in interfacial tension drastically increases nucleation frequency. This study highlights the need to treat γSL as a dynamic parameter based on ion-adsorption equilibrium rather than a constant to predict elemental behavior in multicomponent systems relevant to OAE.
[1] Chikanda F, Iwaki H, San HH, Kikuchi R, Ohtomo Y, Otake T, Sato T (2025), Research Square (Preprint). doi:10.21203/rs.3.rs-8066399/v1
[2] Nielsen AE and Söhnel O (1971), J. Cryst. Growth. 11, 233.
[3] Kashchiev D, van Rosmalen GM (2003): Cryst. Res. Technol. 38, 555.
[4] Liu XY, Tsukamoto K, Sorai M (2000) Langmuir. 16, 5499.
We also plan to present related poster presentations from different perspectives in other sessions: the formation mechanism of biogenic carbonates in the "M-GI35: Carbonate Biology (May 24)" session, and the prediction of CO2 mineralization in multicomponent systems in the "H-SC07: CCUS for Climate Mitigation (May 26)" session.