Nuclide Rejection Performance in Membrane Distillation Concentration Process for Simulated Radioactive Waste Liquid

In response to the need for minimizing the volume of radioactive waste during the operation and decommissioning of nuclear facilities, membrane distillation technology has attracted much attention due to its high rejection potential for nonvolatile nuclides, but the mechanism of nuclide rejection reduction in membrane distillation process is still lacking. The effects of feed temperature and feed flow rate on the nuclide rejection of PTFE, PVDF and PP hydrophobic membranes in membrane distillation for concentration of radioactive waste liquid were studied systematically by using a self-made air-gap membrane distillation unit. The mechanism of non-volatile nuclides across membrane and the quantitative relationship between rejection and permeate were revealed by microscopic characterization, rejection experiments and contact angle measurements. The results show that all three membranes exhibit high rejection rates, and the average rejection rates of PTFE, PVDF and PP membranes are 99.994%, 99.969% and 99.886%, respectively. When the flow rate increases from 80 L/h to 180 L/h, the rejection rates of the three materials decrease to 99.979%, 99.928% and 99.453%, respectively. The mechanism analysis shows that the non-volatile nuclides are mainly due to the permeation and migration of liquid droplets through membrane pores after membrane pores are wetted. When the flow rate increases, the pressure of liquid on the membrane surface increases, resulting in the increase of permeation flux of PTFE, PVDF and PP membranes. Under the same flow rate change, the permeation fluxes of PTFE, PVDF and PP membranes increase by 13.43, 8.26 and 34.16 times respectively. PP membrane has more tear structure changes due to uneven pore structure, and the change is most significant. In addition, the durability of hydrophobic performance is a key indicator worthy of attention in the engineering application of this technology in the field of radioactive waste concentration. The decrease of membrane hydrophobicity will lead to the decrease of liquid wetting pressure on the membrane surface, resulting in more feed liquid penetrating into the condensate, which will lead to the decrease of rejection rate. Durability test shows that PTFE membrane keeps good hydrophobic property under long-term high temperature and high salt scouring, while PVDF membrane loses hydrophobic property after 25 hours. Based on the above research, membrane distillation has excellent rejection performance for nonvolatile nuclides, but it is significantly affected by the uniformity of membrane pore structure and hydrophobic durability. Nuclides migrate across membranes by droplet permeation, which can be quantified by permeate flux and regulated by feed flow rate. PTFE membrane has the best performance in structural stability and hydrophobic durability, and has more potential for engineering applications. This study provides a key basis for the selection and operation of hydrophobic membrane used for concentration of radioactive waste liquid by membrane distillation technology.