Effect of electrolytes and soil minerals on nitrous oxide (N2O) hydrate formation kinetics

Abstract

The kinetics of greenhouse gas (GHG) hydrate formation is an important background for understanding the successful implementation of offshore GHG sequestration in seabed sediments in the hydrate formation zone. In this study, we investigated the effect of basic marine environmental factors (electrolytes and soil minerals) on N2O hydrate formation kinetics to evaluate N2O hydrate formation as a potential way to mitigate an increasing amount of N2O produced by human activities. The formation kinetics of N2O hydrate with and without electrolytes (KCl, NaCl, MgCl2, and CaCl2) and soil minerals (Na-montmorillonite (Na-MMT), kaolinite, sphalerite, and illite) was experimentally monitored at 273.4 K and 30 bar. In a pure water system, the formation of N2O hydrate was 2.44 times slower than that of CO2 hydrate due to its relatively less association with water molecules. The formation kinetics of N2O hydrate was decelerated as expected by all electrolytes compared to that in pure water. This is due to the decrease of driving force for crystallization of N2O hydrates caused by the chemical potential reduction of hydrate-forming molecules, penetration of electrolytes into open cavities of hydrate structures, and steric effects of electrolytes. The formation kinetics of N2O hydrate was influenced by the combined characteristics of each soil mineral in suspension (surface area, pore size, concentrations of dissolved cations, and solvation energy). We further confirmed the applicability of N2O hydrate formation to offshore GHG sequestration in heterogeneous marine environments, using synthetic seawater and real marine sediments. Our study could provide knowledge fundamental to successful offshore GHG sequestration under seabed sediments.