Aqueous complexation of palladium to prevent precipitation and extraction in a group actinide extraction system

It has recently been reported that a cyclohexanone based solvent containing the extractant, CyMe4-BTBP (Bis-Triazine,BiPyridine) together with tri-butyl phosphate intended for use in a GANEX (Group Actinide Extraction) process also extracts the fission product palladium. When cyclohexanone was used as a diluent it was also observed that without any extractant present in the organic phase, palladium rapidly formed a fine black precipitate of metallic palladium. In this work, two water soluble complexing agents were identified and found to be able to stabilize the palladium in 1 M nitric acid, preventing it from precipitating, namely the amino acid methionine and its derivative (2S,2′S)-4,4′-(ethane-1,2-diylbis(sulfanediyl))bis(2-aminobutanoic acid) (bimet). Of these two, however, only bimet could reliably prevent the precipitation of palladium from the process aqueous solution (4 M nitric acid) and stop its extraction by the organic solvent. It was also found that an addition of bimet to the aqueous phase did not alter the actinide extraction by the GANEX solvent, thus making it a feasible candidate for masking the palladium extraction in a future process.

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► Cyclohexanone and BTBP cause unwanted Pd precipitation and extraction from HNO3.
► Cyclohexanone and BTBP are used in spent nuclear fuel reprocessing (GANEX process).
► Pd precipitation can be prevented by the amino acid methionine and its dimer, bimet.
► Pd extraction with BTBP can be prevented by bimet in both 1 & 4 M HNO3.
► Bimet addition does not affect BTBP extraction of actinides and lanthanides.