Separation of Cs from acid radwaste solutions using substituted tetraphenyl borates

It was the aim of this study to selectively separate active caesium (Cs-134 and Cs-137) from acid, radioactive waste solutions (especially MAW and HAWC). The following strategy was designed for a separating process: synthesis of acid-resistant reagents selective to caesium; precipitation of Cs+ and separation of the precipitates by filtration or centrifugation; precipitation of Cs+ and separation of the precipitates by flotation; caesium separation by means of liquid-liquid extraction. Different fluorophenyl derivatives of sodium tetraborate were synthesized and tested with regard to acid stability and selectivity with regards to caesium ions. Furthermore, the solubility products of the caesium salts of these compounds also in dependence on the concentration of NaNO3, were determined. With regard to the separation of precipitates the best result for the MAW was obtained with sodium tetrakis (2,4 difluorophenyl) by which inactive Cs+ can be used as carrier. The pentafluorophenyl derivative, however, is the most suitable for the highly active waste concentrates (5M HNO3) due to its better acid stability; because of its comparatively high solubility the quantity of precipitants to be used is larger. In the pH range 7-10 the liquid-liquid extractions of these precipitates showed very good separation results with nitrobenzene. Apart from process-oriented aspects, also the physical and chemical processes are discussed. (orig./RB)