Co-hosting of Isotopic Water Species in Clathrate Hydrate Lattices: Implications for Isotopic Fractionation

Selective enrichment of deuterium oxide (D2O) from isotopic water mixtures remains challenging due to the nearly indistinguishable physicochemical properties of H2O and D2O. In this study, the feasibility of isotopic separation via hydrate crystallization was systematically evaluated using phase equilibrium measurements, in situ Raman spectroscopy, and high-pressure microdifferential scanning calorimetry. Phase equilibrium measurements of hydrates reformed from physically separated solid and liquid fractions of an equal-mass H2O/D2O mixture revealed no significant shift in phase boundaries relative to those of the initial mixture, indicating that the isotopic composition of the hydrate phase was governed primarily by the initial liquid composition rather than by the higher thermodynamic stability of D2O hydrates. Raman spectra exhibited merged vibrational features associated with HDO formation, without distinct spectral signatures corresponding to pure H2O- or D2O-based hydrates. In addition, calorimetric analysis revealed a single dissociation peak for the mixed systems, confirming the formation of a single hydrate phase rather than separate isotopic lattices. These findings demonstrate a fundamental limitation of thermodynamically driven hydrate crystallization for isotopic separation and highlight the necessity of developing alternative strategies, particularly those based on kinetic control, to suppress mixed lattice formation and enable effective isotopic fractionation.