Speciation of Pu(Vi) in Near-Neutral to Basic Solutions via Spectroscopy

Abstract

The high sensitivity technique of laser photoacoustic spectroscopy and optical absorption spectroscopy were utilized to investigate the hydrolysis of Pu(VI) in perchlorate media. Absorption bands attributed to the hydrolysis of Pu(VI) were characterized in the pH = 6–8 regime which is of most interest to the proposed repository site at Yucca Mountain. Evidence was presented that supported the assignment of the polynuclear species (640 nm species) reported previously \1] as (PuO2)3(OH)5+1. Some temperature-dependent LPAS spectra are reported. The results obtained were compared to the predicted speciation of the geochemical code EQ3NR using the associated data base.

Initial Hydrolysis of Plutonium(VI)

“…The 638 nm species, once formed, does not dehydrolyze reversibly and takes at least weeks to completely dehydrolyze. We previously [3] suggested that this indicates the possible presence of polymerization, which may be affected by solution aging. Our data show that monomeric species are not important at concentrations as low as 10~4 M. Plutonyl appears to have a tendency toward the formation of polynuclear species that is comparable to the uranyl (UOl + ) species.…”

“…The spectra obtained at higher pH were deconvoluted in the same manner as the spectrometric data. The generalized reaction for the hydrolysis of Pu(VI) is: qPuO| + + pH20 <-+ (Pu0 2 )q(0H)<, 2q ~p ) + + pH + (3) and the stability constant βρ^ is defined as…”

“…This was done in a Pyrex vessel while the solution was bubbled with argon gas to minimize carbon dioxide uptake. Only upward adjustments in pH were made due to the pH hysteresis previously noted [2,3]. The Pu(VI) solution was then transferred into a quartz optical cell (1-cm optical pathlength), which was sealed with a gas-tight Teflon cap.…”

“…Plutonium(VI) hydrolysis is complex and does not proceed reversibly at high plutonium concentrations and pH [2,3], This has been attributed to the formation of polynuclear/polymeric species. The initial hydrolysis of Pu(VI), as reported by others, proceeds by one or both of the following reactions: (2)…”

Actinide(IV) and actinide(VI) carbonate speciation studies by PAS and NMR spectroscopies; Yucca Mountain Project: Milestone report 3031-WBS 1.2.3.4.1.3.1

“…The cornerstone technique is pulsed-laser photoacoustic spectroscopy (PAS). This technique is rapidly gaining acceptance as the premiere method for determining actinide speciation in solution (Bennett et al, 1992;Berg et al, 1991a and b;Beitz et al, 1990;Okajima et al, 1990;Klenze and Kim, 1988;Beitz et al, 1988;Pollard et al, 1988;Eiswirth et al, 1985;Stumpe et al, 1984). The theoretical and experimental bases for the technique have been summarized in a number of review publications (Patel and Tam, 1981;Tam, 1986).…”