Separation of Selenium from Liquid Media
Selenium is below sulfur on the periodic chart so it forms many of the same compounds. Selenium is an interesting compound as it is classified as an essential nutrient so it appears in all kinds of health food supplements and yet there are severe limitations on the discharge of this compound. Selenium forms as many as five different oxidation states. Some are anions, some are neutral and some are cations.
Nanofiltration (NF) membranes have also been described to remove selenium from ground water. In laboratory tests, 95+% of the selenium from a sample of water obtained from the San Joaquin Valley was removed with a nanofiltration membrane. The Se6+ was preferentially removed (99%) vs. the Se4+ (~40%). However, since the predominant selenium species is the Se6+, the majority of the selenium was removed. The proportion of water that can be decontaminated by nanofiltration will vary depending upon its chemical composition. However, 90+% is expected. Cost of removal is in the range of $1/1000 gallon or $325/acre‑foot.1
If selenium is oxidized, it is present as the anion selenate (SeO42-) that can be removed from solution with an anion exchange resin.
AmberLyst™ A21 Weak Base Anion Resin will remove weak acids, like selenate from water. The influent should be weakly acidic, pH of 2-5. This weak base anion resin will also remove other weak acids like carbonic acid.
If the stream is not acidic, then a strong base resin like AmberSep™ 21K XLT Resin will be required. Here the resin will remove all of the anions along with the Se.
For streams that have a high organic content and are prone to surface fouling, AmberLite™ HPR9200 Cl Resin is recommended. For solutions with higher levels of sulfate or chloride, you may want to use the Type II anion exchanger, AmberLite™ HPR4100 Cl Resin.
Selenium also exists as selenium dioxide or the hydrated selenite; SeO32- or HSeO3-. While selenite is anionic, it is a weaker acid so it is more difficult to remove from solution. In some cases, removal may be improved by fully oxidizing the selenium.
Selective selenium removal has been described using a metal hydroxide loaded ion exchange resin. This approach is particularly valuable in high salt streams like brines.2,3,4
Selenium can also be selectively removed with chelating resins loaded with oxyanions.5