A solution to stripping problems caused by organophilic anion impurities in crown-ether-based solvent extraction systems: a case study of cesium removal from radioactive wastes

A problem related to stripping efficiency has been identified in the use of crown ether derivatives to extract alkali metal salts, and a simple solution is proposed. Focusing on the specific case of cesium extraction from simulants of alkaline nuclear waste by a calix-crown ether, calix[4]arene-bis(tert-octylbenzo-crown-6) (BOBCalixC6), it has been shown that low concentrations of a common surfactant, dodecylsulfonate, seriously impairs stripping. This surfactant has been identified as a trace component in laboratory simulants and was subsequently studied in experiments in which it was added in controlled amounts. Computer modeling of stripping behavior is consistent with the formation of a 1:1:1 organic-phase complex of the calix-crown with cesium and its nitrate counterion. In the presence of an organophilic surfactant anion, cesium ion can only effectively be stripped from the solvent until its organic-phase concentration becomes equivalent to that of the surfactant anion. Cleanup of nuclear waste requires a high decontamination factor for 137Cs, and insufficient stripping therefore leads to process failure. This difficulty raises a generic issue for use of crown ethers for waste decontamination or for other hydrometallurgical applications. However, remediation is possible by simply adding an alkylamine to the solvent. The alkylamine in its ammonium form acts as a counterion of the organophilic anion, suppressing the deleterious effects of the organophilic anion and allowing the cesium cation to be stripped efficiently. Trioctylamine (TOA) at a concentration of only 1 mM was found effective at restoring stripping performance while not affecting extraction. Ultimately, this solvent amendment enabled the development of a robust solvent for the Caustic-Side Solvent Extraction (CSSX) process and its successful demonstration on actual nuclear waste.