Answer to Question #11425 Submitted to "Ask the Experts"

Category: Environmental and Background Radiation — Water

The following question was answered by an expert in the appropriate field:

Q

Is the selenium found in food (like Brazil nuts and turkey) radioactive? What about selenium found in vitamins? I am asking because I am about to install a reverse osmosis water purification system, and the specifications state that the system removes selenium, as well as other things like lead and arsenic. Thank you for your time.

A

Brazil nuts appear to be the most selenium-rich food and do contain relatively large concentrations of selenium, approximately 19 micrograms per gram (µg g-1) of nuts, from what I have read; the selenium concentration in turkey is about 50 times less. Naturally occurring selenium contains six isotopes, with five of them (atomic numbers 74, 76, 77, 78, and 80) being stable. The sixth isotope, with an atom abundance of 8.73%, is selenium-82 (82Se), a beta emitter that is very weakly radioactive, having a half-life of approximately 1.4 × 1020 years. This long half-life results in radioactivity so low that I cannot conceive of a situation in which sufficient 82Se would accumulate through your proposed reverse osmosis process to be a concern from any technical or health viewpoint.

Even if the selenium from a metric ton (1,000 kilograms [kg]) of Brazil nuts accumulated through your reverse osmosis process, the total radioactivity from the 82Se would be only about 2.0 × 10-6 becquerels (Bq), which is virtually undetectable by any practical radiation detection means and presents no concern from any realistic perspective. Some further context is provided by the fact that a metric ton of Brazil nuts would yield an expected mass of total selenium of about 19 g, of which the radioactive 82Se (whose activity would be 2 × 10-6 Bq) would represent about 1.7 g.

Regarding selenium in vitamins, typical multivitamin tablets contain on the order of 50 µg of selenium each, and a representative amount for selenium supplements is about 200 µg per tablet. I don’t know whether you’re considering a treatment system for your own home or whether it is perhaps for a municipality. If you are considering the latter, and perhaps 10,000 people a day were consuming a multivitamin and 1,000 per day were consuming a supplement, and it all ended up in the water and all the water got treated, the total expected daily accumulation of selenium might be about 0.7 g. I cannot envision that the accumulation over any reasonable time before the system would be recycled or refurbished would be sufficient to yield any measurable amount of 82Se.

Depending on the source(s) of the water, much more likely concerns, from a radioactivity viewpoint, would be natural radioactive decay products from the uranium-238 (238U) and thorium-232 (232Th) decay series. Radionuclides such as radium-226 (226Ra) and associated decay progeny radon-222 (222Rn), lead-210 (210Pb), bismuth-210 (210Bi), and polonium-210 (210Po) from the 238U series and possibly radium-224 (224Ra), radon-220 (220Rn), lead-212 (212Pb), bismuth-212 (212Bi), and thallium-208 (208Tl) from the 232Th series emit a variety of radiations, including numerous gamma rays. This gamma radiation is rather easy to detect as the concentrations of radionuclides, along with the stable salts, increase during the reverse osmosis operation.

In systems that capture and concentrate the dissolved solids, especially in areas (e.g., New England) where the well water contains somewhat elevated concentrations of the natural series radionuclides, the accumulation of these radioactive products can sometimes become a limiting criterion with respect to the need to shut down and/or recycle or refurbish the system. Radon removal systems can be very helpful in reducing the accumulation of the important progeny from radon decay.

Also depending on the water source, other stable elements (such as the arsenic that you mentioned) may be a concern from the point of view of accumulation and disposal.

Good luck with your new system.

George Chabot, PhD

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