Speciation of Pu(Vi) in Near-Neutral to Basic Solutions via Spectroscopy
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SPECIATION OF Pu(VI) IN NEAR-NEUTRAL TO BASIC SOLUTIONS VIA SPECTROSCOPY
SHIGEO OKAJIMA, DONALD T. REED, JAMES J. MAZER, AND CARMEN A. SABAU Argonne National Laboratory, 9700 South Cass Avenue, Argonne, IL 60439 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 (Pu0 2 ) 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. INTRODUCTION The hydrolysis of Pu(VI) in the pH region of 6-8 was investigated utilizing the high-sensitivity method of laser photoacoustic spectroscopy (LPAS) and UV-visible-IR absorption spectrometry. This is a continuation of previous work published [1]. Emphasis of the work reported herein was on higher pH, temperature-dependent speciation and modeling the Pu(VI) speciation observed experimentally. All of these are important considerations in evaluating the performance of the waste package in the proposed high-level waste repository at the Yucca Mountain site. This work is a continuation of past studies to utilize both laserinduced fluorescence and LPAS [2-5] to Investigate radionuclide speciation in groundwater-relevant systems.
The application of laser photoacoustic
spectroscopy to the study of radionuclide speciation in groundwater systems was first identified by Kim and coworkers [6-8]. In our work, we have developed an LPAS system with a sensitivity of 3 x 10-7 absorption units/cm in the blue-green region of the spectrum, and modified the LPAS cell assembly to obtain spectra at elevated temperatures. The role of Pu(VI) in the migration of plutonium in natural systems is uncertain. Its high solubility in natural systems relative to Pu(IV), combined with and the high and well-documented mobility of U(VI) in natural systems suggest, however, that greater importance may need to be placed on its role. The presence of Pu(V) in association with organics known to be reducing, typically observed in far field studies, also suggests that the Pu(VI) oxidation state had an important role in the initial mobilization of Pu into the environment. An understanding of its environmental chemistry is clearly applicable to system modeling at the source term, where plutonium may be released as Pu(VI) into solution and in environments, such as near-surface groundwaters, where oxidizing conditions may exist. Hydrolysis data for plutonium species has been obtained elsewhere [9-13] and was reviewed by Baes and Messmer [14]. The UV-visible spectrum of Pu(VI) was first published by Kraus and coworkers [9,13]. This spectrum, in acid medium, consisted
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