Spectroscopic investigation of the formation of PuO2Cl+ and PuO2Cl2 in NaCl solutions and application for natural brine solutions

Abstract

The chloride complexation of the PuO22+ ion has been studied in acidic NaCl solutions with electrolyte concentrations as high as 5 mol kg−1 at 23°C by using conventional absorption spectrophotometry. Plutonyl and its complexes have ionic strength-dependent molar absorptivities that were determined in NaClO4, the first essential step in the quantitative analysis of chloride complexation. The distributions of species for the Pu complexes, PuO22+, PuO2Cl+, and PuO2Cl2o, formed under the conditions investigated, were determined by peak-fitting of optical absorption spectra. The apparent stability constants of the Pu(VI) chloro complexes were calculated at each NaCl concentration. Specific ion-interaction theory parameters were determined for the plutonyl chloro complexes and the electrolyte constituents, then compared with the literature data. The calculated values for log β° were determined to be 0.23 ± 0.03 and −1.7 ± 0.2 for the mono and bis chloro complexes, respectively. Spectra of Pu(VI) in brines representative of waters at the Waste Isolation Pilot Plant, the licensed nuclear waste repository in a salt formation at Carlsbad, NM, USA, were measured and modeled by using the thermodynamic data and ion interaction parameters were determined. In these brines, less than 10% of the total Pu(VI) concentration exists as the Pu(VI) aquo ion, whereas about 90% is present as Pu(VI) chloro complexes.