Thermodynamic stability and formation kinetics of CHClF2 hydrates in the presence of NiCl2

Abstract A hydrate-based technology is suggested for the desalination of industrial effluents, especially electroplating effluents including NiCl 2 . To decrease the required energy for operating a hydrate-based desalination (HBD) process, chlorodifluoromethane (CHClF 2 ) was used as a guest for hydrate formation at relatively mild conditions. The phase equilibria of CHClF 2 hydrates formed in aqueous NiCl 2 solutions were investigated to identify the effect of NiCl 2 on the thermodynamic stability of CHClF 2 hydrates. The dissociation phase boundary of CHClF 2 hydrates in the presence of NiCl 2 was shifted to higher pressures and lower temperatures as the salinity of aqueous solutions increased. To confirm the feasibility of NiCl 2 separation from wastewaters via the HBD process, X-ray diffraction and Raman spectroscopic measurements were performed. The results showed that the Ni 2+ and Cl − ions did not participate in the hydrate framework, which revealed that the HBD process could be used to remove NiCl 2 from effluents. The formation kinetics of CHClF 2 hydrates were analyzed by a new kinetic model adopting transient time-dependent apparent rate constants, which significantly depends on the sub-cooling temperature and the concentration of NiCl 2 in the aqueous solutions. Thermodynamic approximations regarding the Ni 2+ and Cl − ion exclusion from the clathrate framework and the full occupancy of CHClF 2 molecules in the large cages of structure I clathrate hydrate were confirmed by comparing the calculated Ni 2+ concentrations in the liquid phase in equilibrium with CHClF 2 clathrate hydrates with the experimental measurements. These results provide good information for separating metal ions from wastewater streams by HBD processes.