Efficient Separation of Tritiated Water from Light Water Based on Membrane Distillation by Gas–Liquid Exchange

The removal of tritiated water (HTO) from “ALPS-treated water” originating from the disabled Fukushima Daiichi Nuclear Power Plant (FDNPP) is an important environmental issue. In this paper, we report a new method of separating HTO from light water (H2O) based on membrane distillation by gas–liquid exchange. HTO-containing water loaded on a hydrophobic membrane modified with a photothermal black dye was exposed to artificial sunlight. The heat generated from the dye upon sunlight exposure vaporized the water, with the HTO-containing water vapor penetrating through the membrane. The permeated HTO-containing water vapor was mixed with H2O added to hydrophilic filters under the membrane. Gas–liquid exchange occurred in these filters and significantly decreased the percentage of HTO in the water collected under all filters. The increases in both the filter number and H2O content in the hydrophilic filters increased the efficiency of HTO separation. When four hydrophilic filters containing H2O and four hydrophobic filters were set alternately, the ratio of HTO collected under these filters decreased to 12 ± 0.3%. This process was also effective for the removal of 137Cs from 137Cs-containing water.