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

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 PuO2Cl2degree, 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 βdegree were determined to be 0.23 ± 0.03 and minus1.7 ± 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