Cesium/sodium separation by nanofiltration-complexation in aqueous medium

Some nuclear effluents contain traces of radioactive elements in sodium salt media, from which radioactive cesium must be separated. Various processes (among them liquid/liquid extraction and ion exchange) can perform such separation, but they produce additional wastes. Therefore, nanofiltration has been selected as a new separation process that may allow a large volume reduction without generating additional wastes. Nanofiltration is a pressure-driven membrane process, between ultrafiltration and reverse osmosis, that can separate species at the ionic scale. To perform the separation of cesium from highly salted aqueous medium, a nanofiltration process was combined with a cesium-selective complexation step. Ligands that were known to be highly cesium-selective in organic solvents were synthesized and modified in order to make them hydrosoluble. Ligands such as resorcinarenes or calixarenes were then tested through a nanofiltration system. One of them allowed cesium removal of 90% in an aqueous medium containing 250 g/L of sodium nitrate. By combining two such nanofiltration-complexation stages, it is now possible to remove 99% of trace-level radioactive cesium.