The Controlling Factors and Ion Exclusion Mechanism of Hydrate-Based Pollutant Removal

Theoretically, impurities can be excluded from hydrate structure and pure water can be extracted during hydrate formation. We achieved the heavy metal (Cu2+ in this study) removal from aqueous solution by hydrate process. The hydrate samples were analyzed by X-ray computed tomography (CT). Boundary layer analysis was conducted to reveal the mechanism of ion exclusion. Moreover, the effect of initial concentrations on removal characteristics was analyzed and the solid–liquid separations were optimized. The main findings were as follows: (1) Solution pockets and trapped solution were identified as the limiting and controlling factors for the removal efficiency; (2) nonguest molecule exclusion and directed substance migration occur during hydrate formation; and (3) this method was suitable for relatively high concentration wastewater treatment application, and a removal efficiency of 96.63% could be achieved. We then proposed a theory in regard to the concentration gradient and directed substance migration during hydrate formation. Additionally, the concentrations near the hydrate front were calculated, which were used to further understand the mechanism of the process. Finally, an optimized heavy metal removal process was proposed. The results provide guidance for wastewater treatment, substance enrichment, and hydrate application improvements.