Selective Recovery of Chromium from Precipitates Containing d Elements and Actinides: III. Effect of S2O82-

Reaction of Cr(III) hydroxides and mixed Fe(III)-Cr(III) and Ni(II)-Cr(III) hydroxides with persulfate ion in alkaline solutions was studied by spectrophotometry. The Cr(VI) yield (at oxidant deficiency) corresponds to the S₂O₈ ^{2 -} : Cr(VI) molar ratio close to 1.5. The initial reaction rate V ₀ is described by the kinetic equation -dCr(III)]/d = kCr(III)]S₂O₈ ^{2 -}]NaOH]. The activation energy is 53 kJ mol^{- 1} within the 41.5-95°C range. V ₀ is higher than the rate of thermal decomposition of persulfate ion, i.e., Cr(III) reacts directly with S₂O₈ ^{2 -}. Oxidation of NiCr₂O₄·nH₂O and mixed Fe(III)-Cr(III) hydroxides proceeds faster than oxidation of pure Cr(III) hydroxide. This is due to the catalytic effect of Fe(III) and Ni(II). Additions of Co(II) and Cu(II) also accelerate the process. Pu(IV) in alkali solution under the action of persulfate is converted into a more soluble oxidized species, which can be reduced back to Pu(IV) with appropriate reductants.