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 de grees C by using conventional absorption spectrophotometry. Plutonyl and it s complexes have ionic strength-dependent molar absorptivities that were de termined in NaClO4, the first essential step in the quantitative analysis o f chloride complexation. The distributions of species for the Pu complexes, PuO22+, PuO2Cl+, and PuO(2)Cl(2)degrees, formed under the conditions inves tigated, were determined by peak-fitting of optical absorption spectra. The apparent stability constants of the Pu(VI) chloro complexes were calculate d at each NaCl concentration Specific ion-interaction theory parameters wer e determined for the plutonyl chloro complexes and the;electrolyte constitu ents, then compared with the literature data. The calculated Values for log beta degrees 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 re presentative of waters at the Waste Isolation Pilot Plant, the licensed nuc lear waste repository in a salt formation at Carlsbad, NM, USA, were measur ed and modeled by using the thermodynamic data and ion interaction paramete rs were determined. In these brines, less than 10% of the total Pu(VI) conc entration exists as the Pu(VI) aquo ion, whereas about 90% is present as Pu (VI) chloro complexes. (C) 1999 Elsevier Science Ltd.