4D freeze-driven purification of heavy-metal contaminated sandy soil

Heavy metal contamination in soil has been a global environmental concern. Taking advantage of the widely available natural condition, freezing can purge and isolate the toxic heavy metal contaminants in the soil. This study utilizes in-situ X-ray computed tomography (CT) to visualize pore-scale unidirectional freeze-driven purification processes in lead ions contaminated sand. The micron resolution observations of pore unfrozen water with various cooling rates and different freezing directions are presented. 3D CT images demonstrate the morphologies of unfrozen water trapped within the frozen zone include "saline columns" (top-down freezing) and dispersed unfrozen saline. Interestingly, a relatively pure ice layer with very low saline content emerges and thickens in frozen zones as the cooling pause duration increases. Further quantitative analyses show reducing the cooling rate can effectively increase purification fraction, reaching 52.24 % with a cooling rate of 0.125 degrees C/h, and enhance the saline enrichment towards the warm ends. A diagram is provided to demonstrate governing physical processes and their characteristics in four sections categorized according to the local temperature and phase diagram. These results can guide potential practices such as heavy-metal contamination remediation towards a high purification fraction.