It is easy to find information about heavy-hydrogen water production, but not about heavy-oxygen water. How is heavy-oxygen water commercially produced (for both heavier isotopes)?
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As far as I know, while fractional distillation would be a suitable technique to obtain O-18 water (yet, in very low yelds and in a way that is hard to control), the most used method nowadays is the membrane distillation technique: basically, a semipermeable and hydrophobic membrane is employed during the distillation (tipically made in PTFE), and the isotopes are separated by their ability to permeate through the membrane: lighter isotopes (O-16 water) tend to flow through the membrane, while heavier isotopes (O-18 water) are trapped in the apparatus and recovered.
You can find many detailed descriptions online, for instance: