Spectroscopic and electrochemical studies of selected lanthanides and actinides in concentrated aqueous carbonate and carbonate-hydroxide solutions and in molten dimethyl sulfone

Abstract

Electrochemical and spectroscopic studies of neptunium, plutonium, americium, californium, and terbium in concentrated aqueous carbonate and carbonate-hydroxide solutions have been carried out. Changes in the absorption spectra of Np(VII), Np(V), Pu(VI), Pu(V), Am(VI), and Am(V) in concentrated Na/sub 2/CO/sub 3/ solution and in the formal potentials of the Np(VI)/Np(V) and Pu(VI)/Pu(V) couples as a function of pH were observed. Heptavalent neptunium in concentrated Na/sub 2/CO/sub 3/ solution could only be producted at pH values close to or greater than 14. Plutonium(VII) in 2 M Na/sub 2/CO/sub 3/ solution could only be produced at hydroxide ion concentrations in excess of about 2.5 M. The complexation of Np(VII) and Pu(VII) in Na/sub 2/CO/sub 3/-NaOH solution seems to be mainly by hydroxide ions. Neptunium(IV) and plutonium(IV) are insoluble in Na/sub 2/CO/sub 3/ solution above ca. pH 11-12. Neptunium(III) in carbonate solution is rapidly oxidized by water to Np(IV). Plutonium(III) is insoluble in Na/sub 2/CO/sub 3/ solution. In K/sub 2/CO/sub 3/ solution Pu(III) is stable to oxidation by water but is very sensitive to air oxidation. The redox properties of Cf(III) in Na/sub 2/CO/sub 3/ and K/sub 2/CO/sub 3/ solutions at pH values from 8 to 14 were investigated. The oxidation of terbium(III) in more » K/sub 2/CO/sub 3/-KOH solution was studied. Spectroscopic and electrochemical studies of cerium, samarium, europium, ytterbium, uranium, neptunium, plutonium, and americium in molten dimethyl sulfone (DMSO/sub 2/) at 400 K were performed. Differences in the DMSO/sub 2/ solution absorption spectra of trivalent Sm, Eu, and Yb and divalent Eu compared with those in aqueous solution were observed. Complexation effects on the spectra of Ce(III), Ce(IV), U(VI), Np(VI), Pu(VI), and Am(VI) are more noticeable in poorly coordinating DMSO/sub 2/ than they are in water. 123 references, 54 figures, 11 tables. « less