Enhanced HTO Separation from H2O by Membrane Distillation: The Crucial Role of Montmorillonite Clay in Gas–liquid Exchange for HTO Capture

Abstract The removal of tritiated water (HTO) from light water (H2O) is an important process not only for the processing of radioactive materials but also for securing nuclear fusion fuel. We previously reported that HTO can be separated from H2O by using an apparatus with a membrane distillation mechanism, with which H2O-impregnated glass fibers efficiently separate HTO from H2O by gas–liquid exchange. In this study, H2O-impregnated clays were installed instead of H2O-impregnated glass fibers. Montmorillonite, sepiolite, palygorskite, a synthetic zeolite, and a mesoporous silica were employed, and montmorillonite was found to be the most effective for the separation. The tritium (T) ratio in the water that was finally collected in the vessel below the apparatus decreased with the increase in the volume of H2O added to montmorillonite. When 4 mL of H2O was added to 1 g of montmorillonite, the T ratio was reduced by about 10%. X-ray diffraction analysis revealed that the layer structures of montmorillonite significantly varied with the volume of water added. The moderate removal of HTO from H2O was observed in condensed water vapor obtained from the montmorillonite impregnated with HTO-containing water in sealed-vial experiments at room temperature, suggesting that montmorillonite had the specific property of capturing HTO. On the other hand, although the T ratio in the water condensate from the HTO-impregnated montmorillonite remained unchanged during the heating treatment at temperatures higher than 100 °C, the ratio in that from the montmorillonite used for the separation process gradually decreased during the heating treatment. These observations showed that a portion of HTO was tightly captured in specific sites of montmorillonite. In this case, HTO in the vapor was loaded during the separation process by the membrane distillation mechanism. The method of loading HTO in the vapor to H2O-impregnated montmorillonite was found to be an important factor for the efficient removal of HTO from H2O. The improvement in the design of the apparatus for the effective separation of HTO from H2O was achieved using H2O-impregnated montmorillonite.