Coprecipitation of Pu(IV) with Fe(III) and Cr(III) hydroxides from nitrate-acetate solutions under hydrothermal conditions simulating deep disposal of liquid radioactive wastes

Abstract

Precipitation of Fe(III), Cr(III), Ni(II), and Mn(II) from nitrate-acetate solutions and coprecipitation of Pu(IV) with Fe(III) and Cr(III) were studied. The degree of precipitation of 80–95% is attained for Fe(III) at 95–200°C and pH>0.5–0.6, and for Cr(III), at T=95°C and pH≥4.0 or T=200°C and pH≥1.0. The phase composition of the precipitates formed by thermal hydrolysis of iron nitrate in model solutions was analyzed. Depending on pH and temperature, the solid phase contains various modifications of Fe2O3, FeOOH, and amorphous phases. Noticeable coprecipitation of plutonium from nitrate-acetate solutions is observed at pH≥4, and it is incorporated in the precipitate only at formation of FeOOH. No coprecipitation of Pu(IV) with Fe2O3 was found. Under the given experimental conditions, plutonium in aqueous solutions occurs in the oxidation state +4 forming monoacetate (or, probably, hydroxo acetate) complexes.