The hydrolysis of neptunium(VI) and plutonium(VI) studied by the pulse radiolysis transient conductivity technique

The first hydrolysis constant of the NpO22+ cation has been determined by either oxidizing Np(V) to Np(VI) with radiation-produced hydroxyl radicals, or reducing Np(VI) to Np(V) with hydrated electrons, and studying the pH dependence of the asymptotic conductivity signals. The method is described in detail. The values obtained by both methods agree within experimental errors. Extrapolated to infinite dilution, pKsub(a)(NpO22+) = 5.45 +- 0.1. By reduction of Pu(VI) to Pu(V) with e-sub(aq), we determined pKsub(a)(PuO22+) = 6.3 +- 0.1. No evidence was found for a reduction of Np(VI) or Pu(VI) by H atoms. From the conductivity measurements one obtains an upper limit for the rate constants of both H atom reactions, of 107 M-1 s-1. With the optical pulse radiolysis technique, we could establish k(H + NpO22+) <106 M-1 s-1. Using the latter technique, we also determined k(e-sub(aq) + NpO22+) = 1011 M-1 s-1 +- 6% and k(e-sub(aq) + NpO2OH+) = (2.9 +- 0.8) x 1010 M-1 s-1, both for zero ionic strength. From the conductivity results, we derived k(OH + NpO2+) = (4.3 +- 0.1) x 107 M-1 s-1. (author)