Determining the distribution of Pu, Np, and U oxidation states in dilute NaCl and synthetic brine solutions

The effect of iron powder (Fe-0) on the reduction of Pu(VI), Np(V), and U(V T) was investigated in dilute NaCl and synthetic brines. The total concentr ations and oxidation states of the actinides in these solutions were monito red as functions of pC(H+), E-h, and time using techniques including Vis/Ne ar IR absorption spectrophotometry, solvent extraction, alpha -activity cou nting, and inductively coupled plasma spectroscopy-mass spectrometry (ICP-M S). when concentrations were too low and the oxidation states could not be directly determined by spectrophotometry or solvent extraction. comparing t he measured concentrations with the solubility of reference systems helped to define the final oxidation states. In general the reduction was more rap id, and could proceed further, in the dilute NaCl solution than in the brin e solutions. The experimental observations can be summarized as follows: (1 ) in the dilute NaCl solutions (pC(H+) 7 to 12), all three actinides, Pu(VI ), Np(V) and U(VI), were reduced to lower oxidation states (most likely the tetravalent state) within a few days to a few months in the presence of Fe -0; (2) in synthetic brines containing Fe-O (pC(H+) 8 to 13), the reduction of Pu(VI) was much slower than in the dilute NaCl solution. The dominant o xidation state of Pu in the brine solution was Pu(V), the concentration of which was controlled by the electrochemical potential and could probably be represented by a heterogeneous redox reaction PuO2. xH(2)O(s) reversible a rrow PuO2++e(-); (3) in synthetic brines containing Fe-0(pC(H+) 8 to 13), N p(V) was probably reduced to Np(IV) and precipitated from the solution; (4) in synthetic brines containing Fe-0 (pC(H+) 8 to 13), no significant reduc tion of U(VI) was observed within 55 days.