Effect of MgCl2 on CO2 sequestration as hydrates in marine environment: A thermodynamic and kinetic investigation with morphology insights

Oceanic hydrate-based CO 2 sequestration (HBCS) holds great promise for achieving carbon neutrality. However, the presence of inorganic salts, particularly MgCl 2 besides NaCl, in seawater can significantly impact the formation rate and the stability of CO 2 hydrate. In this study, experimental investigations were conducted to examine the thermodynamics, kinetics, and the resulting morphological features of CO 2 hydrate in the presence of MgCl 2 , covering mass fractions ranging from 0 to 5.0 wt%. The experimental findings reveal that MgCl 2 exerts a thermodynamic inhibitory effect with its inhibitory capacity increasing with higher mass fractions. The solubility model of CO 2 in MgCl 2 solution was modified, demonstrating a gradual weakening of CO 2 solubility as MgCl 2 mass fraction increases. Additionally, the growth kinetics of CO 2 hydrate decreases with increasing MgCl 2 mass fraction. Regarding CO 2 hydrate morphology, it was observed that at low mass fractions of MgCl 2 (<1.0 wt%), a dense hydrate film rapidly formed at the gas-liquid interface after CO 2 hydrate nucleation, hindering the further conversion of CO 2 into hydrate. Conversely, at higher mass fractions (>3.0 wt%), CO 2 hydrate exhibits a more porous and slurry-like structure, facilitating more gas-liquid contact and mass transfer, thereby enhancing the conversion of CO 2 into hydrate. During hydrate dissociation , a salt-removal effect associated with CO 2 hydrate formation was observed, leading to the accumulation of concentrated electrolyte (MgCl 2 ) and facilitating CO 2 hydrate dissociation. These findings have implications for understanding the CO 2 hydrate formation and dissociation in the presence of MgCl 2 relevant in the subsea environment and can contribute to the development of effective hydrate-based CO 2 sequestration strategies.