Oxalate precipitation of lanthanides and actinides from radioactive waste in the nitric acid media

Abstract

Oxalate precipitation for separation of lanthanide and actinide from radioactive waste was examined. The solubilities of rare earth(Y, La, Ce, Nd, Sm, Eu, Gd, Dy, Er and Yb) oxalates were investigated in nitric acid and oxalic acid media. The oxalate precipitation of americium/neptunium and neodymium with oxalic acid was investigated in the simulated radwaste, which was composed of 10 elements in nitric acid solution. The decomposition of oxalate by hydrogen peroxide was investigated. The rare earth oxalate was precipitated by addition of oxalic acid to rare earth nitrate in nitric acid solution. The concentration of rare earth in filtrate was determined by ICP. The experiment revealed that the solubility decreased as the oxalic acid concentration increased and the nitric acid concentration decreased. Based on the assumption, rare earth ions can react with oxalate ion to form $RE(C_{2}O_{4})_{n}^{3-2n}(n=1,2,3)$ complex, we have proposed a solubility model of rare earth oxalate. In order to estimate the solubility of rare earth oxalate, oxalic acid, hydrogen ion concentration and ionic strength were calculated by stoichiometric material balances of chemicals in precipitation reaction. The activity coefficients of ions were calculated by the modified Debye-Huckel equation. Within the experimental range of oxalate ion activity $1 × 10^{-8}M to $1 × 10^{-5}M, solubility products $K_{sp}$ and equilibrium constants $β_{1}$ for rare earth oxalates, were obtained. The oxalate precipitation of americium and neodymium by oxalic acid was investigated in the simulated radwaste, which was composed of 10 elements(Cs, Sr, Fe, Ni, Pd, Ru, Mo, Zr, Nd and Am) of alkali, alkaline earth, and transition metals in nitric acid solution. The effect of concentrations of oxalic acid and nitric acid in the simulated radwaste on the precipitation yield and purity of Am and Nd was examined. The precipitated fraction of each element increased with increasing concentration of oxalic acid ...