COMPARISON OF CHEMICAL EXTRACTIONS AND LASER PHOTOACOUSTIC-SPECTROSCOPY FOR THE DETERMINATION OF PLUTONIUM SPECIES IN NEAR-NEUTRAL CARBONATE SOLUTIONS

The solubility and speciation of plutonium(V) and plutonium(VI) in a p H = 6, 1.93 mM dissolved carbonate solution was measured to model the behavior of plutonium in a groundwater. We studied the solubility from supersaturation and compared two methods for the determination of oxi dation state distribution: an indirect chemical method using the extra cting agents PMBP (4-benzoyl-3-methyl-1-phenyl-2-pyrazolin-5-one) and HDEHP (di-(2-ethylhexyl)-orthophosphoric acid), and direct measurement using laser photoacoustic spectroscopy. The plutonium(V) solution had a soluble plutonium concentration of 2.3 +/- 2 x 10(-5) M over a time of 53 days and was greater than 95% PuO2+. Chemical extraction and sp ectroscopic results for the determination of PuO; were in excellent ag reement. Immediately after preparation of 1.9 to 3.7 x 10(-4) M PuO22 solutions, some of the plutonium precipitates and a new soluble speci es forms, probably PuO2(OH)(+). PuO2+ forms within hours, and within a few days, becomes the dominant soluble species. After several months, only 1-2% of the plutonium added initially remains in solution and it is essentially all Pu O-2(+). Chemical extraction and spectroscopic r esults for the determination of PuO22+ were in excellent agreement for two measurements and differed by 60% in a third determination.