Determining the Distribution of Pu, Np and U Oxidation States in Dilute NaCl and Synthetic Brine Solutions

Abstract

The effect of iron powder (Fe0) on the reduction of Pu(VI),Np(V), and U(VI) was investigated in dilute NaCl and synthetic brines. Thetotal concentrations and oxidation states of the actinides in these solutionswere monitored as functions of pC H +, Eh, and time using techniques includingVis/Near IR absorption spectrophotometry, solvent extraction, activity counting,and inductively coupled plasma spectroscopy-mass spectrometry (ICP-MS). Whenconcentrations were too low and the oxidation states could not be directlydetermined by spectrophotometry or solvent extraction, comparing the measuredconcentrations with the solubility of reference systems helped to define thefinal oxidation states. In general, the reduction was more rapid, and couldproceed further, in the dilute NaCl solution than in the brine solutions.The experimental observations can be summarized as follows: (1) in the diluteNaCl 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 Fe0; (2) insynthetic brines containing Fe0 (pC H + 8 to 13), the reductionof Pu(VI) was much slower than in the dilute NaCl solution. The dominant oxidationstate of Pu in the brine solution was Pu(V), the concentration of which wascontrolled by the electrochemical potential and could probably be representedby a heterogeneous redox reaction PuO2 . xH2O(s) PuO2 + +e —; (3) in synthetic brines containing Fe0 (pC H + 8 to 13), Np(V) was probably reduced to Np(IV) and precipitatedfrom the solution; (4) in synthetic brines containing Fe0 (pC H+ 8 to 13), no significant reduction of U(VI) was observed within 55 days.