Research program to determine redox properties and their effects on speciation and mobility of Pu in DOE wastes. 1998 annual progress report

'The goal of the authors research is to develop data to allow successful modeling of the effects of redox chemicals present in Hanford waste tanks on the oxidation state and chemical speciation of plutonium in the wastes. As of May 1998, scoping experiments on PuO2+ in a solution of 1.0 M NaCl and pH 8 in the dark showed no reduction of PuO2+ concentration over a week nor was reduction observed when this experiment was repeated with 10-5 M H2O2. Reduction was observed over hours/days as the H2O2 concentration was raised above 10-3 M. The rate of reduction in the dark was measured at pcH = 8, 1.0 M (NaCl) for [H2O2] = 10-2 to 4 x 10-2 and [PuO2+] = 1.2 x 10-10 to 3.5 x 10-10 M and first order kinetics observed. From the rate constants, they calculate for [H2O2] = 10-2 M and [PuO2+] = 10-6 M, a half life of reduction in the dark of 0.06 min and for [H2O2] = 10-6 M, [PuO2+] = 10-6 M, of 10 hr. These data indicate that radiolytic production of H2O2 in the wet tank wastes would serve to reduce PuO2+ (and PuO22+) to Pu(IV), thereby reducing the concentration of dissolved Pu due to the strong hydrolytic tendency of the Pu(IV). The oxidation of Pu(IV) by NaOCl is also being investigated. As of May, 1998, the methodology to measure the oxidation state speciation at tracer Pu(IV) concentrations has been developed and scoping experiments conducted. At pcH 4.7, I = 1.0 M (NaCl), [Pu(IV)] = 10-10 M and [NaOCl] = 10-5 to 10-3 M, no oxidation (i.e., PuO22+) has been observed. Experiments are now underway at pcH = 8. To understand the role of complexation by the strong organic ligand in the tank wastes in the redox speciation of Pu, they are measuring the stability constant of Pu(IV) with EDTA. Attempts to use arsenazo (III) as a chromophoric competing ligand were unsuccessful and as of May, 1998, the authors are studying competition experiments of Pu(IV) and Fe(III) with EDTA by absorption spectrophotometry. A value of log 101 = 24.0 261 0.1 has been obtained for I = 1.0 m (NaCl).'