[M-IS26] Gas hydrates in environmental-resource sciences

[MIS26-P08] Carbon isotope fractionation of carbon monoxide hydrate between gas and hydrate phases

*Hiromi Kimura¹, Akihiro Hachikubo¹, Satoshi Takeya² (1.Kitami Institute of Technology, 2.National Institute of Advanced Industrial Science and Technology (AIST))

Keywords:gas hydrate, stable isotope, carbon monoxide, isotopic fractionation

It is possible that carbon monoxide hydrates exist in the icy bodies of outer planets of the solar system (Zhu et al., 2014). Isotope signatures of carbon monoxide hydrates may provide useful information to understand formation processes of gas hydrate. Kimura et al. (2019) reported at the past JpGU conference that the carbon isotopic difference between hydrate-bound and residual gases distributed between between 1.2‰ and 1.5‰ in the case of carbon dioxide hydrate, suggesting that the hydrate rather concentrates light molecules (¹²CO₂). Besides, the equilibrium pressure of ¹³CO₂ hydrate is slightly higher than that of ¹²CO₂ hydrate (Kimura et al., 2020) and agreed with the result of carbon isotopic fractionation. On the contrary, there no information about carbon monoxide hydrate. In this study, we measured carbon isotopic difference between hydrate-bound and residual gases at the formation of carbon monoxide hydrate.
Samples of carbon monoxide hydrate were synthesized in a pressure cell (volume: 42 mL). Fine ice powder was put in a pressure cell and introduced the guest gas. The temperature was controlled by a liquid bath (253-273 K) and cold rooms (223-243 K) to form hydrate crystals from gas and ice powder. After their formation, isotopic compositions of both hydrate-bound and residual gases were measured by an isotope ratio mass spectrometer (CF-IRMS, DeltaV, Thermo Fisher Scientific).
The results of PXRD pattern showed that the crystallographic structure of carbon monoxide hydrate was the cubic structure I. The carbon isotopic differences between hydrate-bound and residual gas was 0.6 ± 0.1‰ at 223-273 K. In contrast to the result of carbon dioxide hydrate, the hydrate of carbon monoxide concentrated heavier molecules (¹³CO), indicating that the equilibrium pressure of ¹³CO hydrate is lower than ¹²CO hydrate.

References
Kimura H, Matsuda J, Kikuchi Y, Hachikubo A, Takeya S (2019) Carbon isotope fractionation of CO₂ during the formation of clathrate hydrate. Japan Geoscience Union Meeting 2019, MIS21-P07
Kimura H, Hachikubo A, Takeya S (2020) Equilibrium pressure of gas hydrate enclathrated carbon dioxide isotopologues. JpGU-AGU Joint Meeting 2020, MIS32-11
Zhu J, Du S, Yu X, Zhang J, Xu H, Vogel SC, Germann TC, Francisco JS, Izumi F, Momma K, Kawamura Y, Jin C, Zhao Y (2014) Encapsulation kinetics and dynamics of carbon monoxide in clathrate hydrate. Nat Commun 5, 4128, doi: 10.1038/ncomms5128