Selective separation of hydroxide from alkaline nuclear tank waste by liquid-liquid extraction with weak hydroxy acids.

Recovery and recycle of caustic reagents in industrial processes offer potential means of pollution prevention, as investigated herein for particular needs related to the cleanup of alkaline nuclear waste. Specifically, the recovery of hydroxide from alkaline media by liquid-liquid extraction can be effected utilizing weak hydroxy acids, as demonstrated for NaOH utilizing a series of lipophilic fluorinated alcohols and alkylated phenols dissolved in 1-octanol. Extraction efficiency follows the expected order of acidity of the hydroxy acids, the phenols being the most efficient extractants among the compounds tested. After extraction, NaOH is effectively recoverable from the organic phase upon contact with water. The weakest hydroxy acids are the most efficiently stripped, NaOH recovery being nearly quantitative in a single contact. In competitive extraction experiments, good selectivity for hydroxide recovery over other anions such as nitrate and chloride was demonstrated. Since the order of extraction favors larger anions, the exceptional preference for hydroxide implies that the extraction occurs by deprotonation of the hydroxy acids in a cation-exchange process. Stripping therefore occurs by hydrolysis to regenerate the neutral hydroxy acid, liberating NaOH to the aqueous phase. Since hydroxide equivalents rather than actual hydroxide ions are transferred to the solvent, the process is termed "pseudohydroxide extraction." Hydroxide recovery from a simulant of alkaline nuclear tank waste (Hanford DSSF simulant) was also demonstrated in repeated extraction and stripping cycles.