Solubilization of Cs and K from Tetraphenylborate Precipitates in Highly Alkaline Environments

Abstract

Precipitated Cs tetraphenylborate, along with K tetraphenylborate and aqueous alkaline supernate, is present in certain high-level waste tanks at the Savannah River Site. Two possible processes for transferring the Cs+ (and K+) into the aqueous phase are described. One consists of displacement of the heavy alkalis from the solid tetraphenylborate by adding a tetraalkylammonium hydroxide. Almost 100% solubilization is obtained at ambient temperature using tetra-n-butylammonium hydroxide. The other consists of oxidative decomposition of the tetraphenylborate using persulfate. The extent of solubilization is found to increase with increasing temperature and persulfate:tetraphenylborate ratio. High yields of tetraphenylborate decomposition are obtained at elevated temperatures (55–90°C), and these yields increase with rising persulfate:tetraphenylborate ratios. The decomposition yields are lowered in the presence of nitrite, which is a component of the alkaline supernate, but are still significant, especially at elevated temperatures and high persulfate:tetraphenylborate ratios. The adverse effect of nitrite can be greatly reduced by lowering the pH.