Separation of dissolved Kr from a water sample by means of a hollow fiber membrane

Abstract

We investigated extraction of the krypton (Kr) radioisotope 85Kr from groundwater having an ultra-low concentration with a Kr extraction system using an external inflow type hollow fiber membrane. Our aim was to develop a system able to effectively extract Kr isotopes from a large volume of groundwater (104 L) in field studies for measurement of 85Kr radioactivity. Before testing our Kr extraction system in the field, we determined experimentally the conditions necessary for effective extraction of Kr isotopes from groundwater. We investigated the efficiency of Kr extraction from water by extracting trace amounts of Kr in the laboratory. With our system, we were able to extract 99% of Kr and O2 dissolved in water at water temperatures ranging from 11 to 29 °C and with a flow rate of less than 24 L/min, confirming that the separation efficiency of the system was sufficient for extraction of Kr isotopes from groundwater. We then used our method to extract 85Kr from groundwater at sites at Kyoto University in Osaka and on the Kumamoto Plain in central Kyushu, Japan. We showed that this technique for extracting 85Kr from the groundwater can be an effective tool for investigating the residence time of young groundwater.