An apparatus and method for separating heavy isotopes of hydrogen from contaminated water. The apparatus includes a treatment chamber with an elongated hollow core fiber (HCF) extending within the chamber. A reservoir holds contaminated water mixed with beads formed of an exchange resin, the mixture of contaminated water and beads forming a flowable slurry. The slurry is continuously circulated through the hollow core fiber and the reservoir preferably by a pulsating peristaltic pump. The beads absorb a portion of the heavy isotopes from the slurry by exchange with waters of hydration of the beads while the hollow core fiber allows permeation of only light water from said slurry outwardly through the HCF wall as a permeate.