[M-IS32] ガスハイドレートと地球環境・資源科学

[MIS32-11] CO₂同位体分子種を包接するガスハイドレートの平衡圧

*木村 宏海¹、八久保 晶弘¹、竹谷 敏² (1.北見工業大学、2.産業技術総合研究所)

キーワード：ガスハイドレート 、安定同位体、CO2、同位体分別

Natural CO₂ hydrate was discovered at the Okinawa Trough, and it is expected that CO₂ hydrate exists in the polar ice caps of the Mars. Isotopic fractionation of CO₂ may provide useful information to understand formation processes of gas hydrate. Luzi et al. (2011) revealed that CO₂ δ¹³C in hydrate-bound gas is 0.9‰ lower than that of residual gas, suggesting that light CO₂ molecules prefer to be encaged into clathrate cages. We reported at the last JpGU conference (Kimura et al., 2019) that the isotopic difference between GH and residual gases distribute between between 1.2‰ and 1.5‰, agreed fairly well with the previous report. Ozeki et al. (2018) reported that the equilibrium pressure of CH₃D is larger than that of CH₄, causes isotopic fractionation in D between GH and residual gases as reported by Hachikubo et al. (2007). In this study, we measured the equilibrium pressures of ¹²CO₂ and ¹³CO₂ hydrates and checked the difference between them. CO₂ hydrate samples were synthesized in small pressure cells (volume: 5 mL). Fine ice powder was put in a pressure cell and introduced guest ¹³CO₂ gas. Hydrate crystals were formed by melting the ice powder at the temperature of the quadruple point. We also prepared normal CO₂ (mixture 98.9% ¹²CO₂ and 1.1% ¹³CO₂) hydrate as a reference, using the same preparation method. These pressure cells were placed in a temperature-controlled liquid bath, and measured their equilibrium pressures from 270 K to 278 K. The equilibrium pressure of ¹³CO₂ hydrate was about 0.01 MPa larger than that of normal CO₂ (mainly ¹²CO₂) hydrate, and it agreed with the previous results (Luzi et al., 2011; Ozeki et al., 2018).



References



Hachikubo A, Kosaka T, Kida M, Krylov A, Sakagami H, Minami H, Takahashi N, Shoji H (2007) Isotopic fractionation of methane and ethane hydrates between gas and hydrate phases. Geophys Res Lett 34: L21502. doi:10.1029/2007GL030557



Luzi M, Schicks JM, Erzinger J (2011) Carbon isotopic fractionation of synthetic methane and carbon dioxide hydrates. Proc. 7th International Conference on Gas Hydrates (ICGH2011).



Ozeki T, Kikuchi Y, Takeya S, Hachikubo A (2018) Phase equilibrium of isotopologue methane hydrates enclathrated CH₃D and CD₄. J Chem Eng Data 63(6): 2266-2270, doi: 0.1021/acs.jced.8b00203