AUTOMATED IN-LINE EXTRACTION OF URANIUM(VI) FROM RAFFINATE STREAMS WITH ONLINE DETECTION BY CATHODIC STRIPPING VOLTAMMETRY

An automated method for on-site monitoring of uranium(VI) in raffinate streams originating from nuclear fuel reprocessing plants is describe d. An in-line stripping procedure (based on liquid/liquid extraction) was developed to extract U(VI) from this stream, a solvent mixture of 20% tributyl phosphate and nitric acid in kerosene, into an aqueous so dium sulfate solution, Degradation products in the solvent mixture, es pecially dibutyl phosphate, give rise to very strong complexes and are responsible for moderate but constant U(VI) recoveries (similar to 50 %), Optimal conditions for in-line stripping comprise a mixing ratio o f extractant (0.5 M sodium sulfate in water)/solvent mixture of simila r to 3 and a pumping rate of similar to 0.4 mL min(-1) of the solvent mixture. The determination of U(VI) was by on-line cathodic stripping voltammmetry (CSV), preceded by adsorptive collection of the U(VL) as an oxine complex onto a hanging mercury drop electrode, Quantities of 1-2 mL of the aqueous extract were pumped into the voltammmetric cell and diluted (1/5 to 1/10) with a background electrolyte containing 0.1 M PIPES buffer, 2 x 10(-4) M oxine, 10(-4) M EDTA, and 0.2 M hydrazin e hydrate (pH 9.0), The CSV peak for U(VI) was obtained at -0.68 V wit h a detection limit of 20 nM in the raffinate stream using an adsorpti on time of 120 s, Both the inline stripping procedure and the on-line measurement were fully automated, with a relative standard deviation i n the measurements of < 5%.