A solution for cesium removal from high-salinity acidic or alkaline liquid waste: the crown calix[4]arenes

The scientific literature abounds with articles on the extraction of cesium from liquid waste. From the 1940s, several classes of inorganic exchangers were used for cesium removal, such as zeolites, hexacyanoferrates, zirconium phosphate, ammonium phosphomolybdate, and, more recently, crystalline silicotitanate. Later, organic compounds like formophenolic resins, tetraphenylboron, or extractants such as dicarbollides or crown ethers were proposed. Generally, these compounds were only able to efficiently remove cesium under restrictive conditions (alkaline medium, low salinity, etc.) The performances of most of these compounds strongly decreased as the acidity (or salinity) of the liquid waste increased. Artifices had to be implemented to improve the performances of extractants. A restricted number of inorganic exchangers allowed the cesium removal, but often they could not be eluted or needed prohibitive volumes of saline eluent, leading to large volumes of secondary waste. Calix[4]arenes monocrown or biscrown, blocked in 1,3 alternative cone conformation, display an exceptional efficiency for cesium extraction, even from very acid or alkaline media. Moreover, they possess an important selectivity for cesium over sodium that makes possible the extraction of cesium from media containing high sodium nitrate loadings. Another advantage, since the extraction of cesium is reversible, is that the stripping of cesium can be carried out in deionized water, a property which leads to very high concentration factors.