SPECTROSCOPIC STUDY OF THE HYDROLYSIS OF PUO22+ IN AQUEOUS-SOLUTION

The hydrolysis of Pu(VI) is investigated under Ar atmosphere in the pH range between 4.5 and 9.0 at 22 degrees C in 0.1 M NaClO4 solution in contact with an initial solid phase, PuO2CO3(s). Under the given conditions, PuO2CO3(s) is changed to PuO2(OH)(2)(s). The estimated equilibrium partial pressure for the transformation : PuO2CO3(s) + H2O(1) reversible arrow PuO2(OH)(2)(s) + CO2(g) can reach P-CO2 similar to 10(-3.6) atm. In contrast to earlier investigations in oversaturated solution, no polynuclear species formation is observed. Two mononuclear complexes, PuO2OH+ in near neutral and PuO2(OH)(2)(aq) in weak alkaline solution, are found to be the dominating hydrolysis species of Pu(VI). The stability constants of the two species, evaluated from UV/VIS spectroscopic measurements, are found to be: log beta(11) = 8.10+/-0.15 and log beta(12) = 14.25+/-0.18, respectively. The first equilibrium constant is in good agreement with the corresponding value for the formation of UO2OH+, log beta(11) = 8.4+/-0.3, indicating a close similarity in hydrolysis between the two actinide ions.