Effect of pH on carbon dioxide hydrate formation in mixed soil mineral suspensions

We investigated the effect of pH on CO2 hydrate formation in the presence of phyllosilicate mixtures. Different pH conditions of phyllosilicate suspensions (Na-montmorillonite-rich and phyllosilicate-rich suspensions) with and without NaCl (3.5%) were prepared and controlled by the addition of an acid or base before the dissolution of CO2. The formation of CO2 hydrates was observed in all phyllosilicate suspensions (30 bar and 273.45 K). The temperature-time plot results showed that hydrate formations were suppressed more in acidic mineral suspensions than in basic suspensions. The fastest hydrate induction time can be observed in Na-montmorillonite-rich and phyllosilicate-rich suspensions with and without NaCl at near neutral conditions (pH 6-8), followed by basic (approximately pH 12.0) and acidic (approximately pH 2.0) pHs. Hydrate induction time can be significantly affected by various chemical species forming under different suspension pHs. The distribution of chemical species in each mineral suspension was estimated by a chemical equilibrium model, PHREEOC, and used for the identification of hydrate formation characteristics in the suspension. Particle-particle and particle-water interactions may possibly contribute to the delay of hydrate formation. NaCl was not an efficient inhibitor but a possible promoter for hydrate formation when pH-dependent solid surfaces were present in the system.