Electrolytic Oxidation of Ce(III) to Ce(IV) in Nitric Acid Solution and Separation of Ce(IV) by Anion Exchange Method

摘要

To develop an efficient method to recover Ce from radioactive wastes containing concentrated nitric acid, we have studied the possibility of the combination methods of selective electro-oxidation and anion exchange. The redox reaction behavior of Ce(IV)/Ce(III) in nitric acid solution has been investigated. Anion exchange behavior of Ce(IV) at various nitric acid concentrations and temperatures was studied. Moreover, separation of Ce(IV) from other rare earth elements was attempted by anion exchanger column. The results of study by cyclic voltammetry indicate that the kinetics of the Ce(IV)/Ce(III) redox reaction on carbon electrode was significantly promoted by increasing HNO_3 concentration, while the formal redox potential remained at almost constant values between 1.34- 1.38V(vs.Ag/AgCl). Electrolytic oxidation of Ce(III) to Ce(IV) in 6 mol centre dot dm~(-3) HNO_3 solution was successfully performed with the conversion of 100 percent and current efficiency near 60 percent by using a novel flow type electrolysis cell containing a carbon-fiber column electrode as working electrode and Vycor Glass(porous silica glass) as separator. Ce(IV) showed relatively strong adsorption on AR-0l anion exchanger as the form of nitrato-complexes such as Ce(NO_3)_5~- and Ce(NO_3)_6~(2-) and the distribution coefficient increased with increasing HNO_3 concentration. However, the adsorbed Ce(IV) was reduced to Ce(III) and eluted into the solution through redox reaction with the anion exchanger. Lowering temperature (<288 K) can considerably suppress the redox reaction between Ce(IV) and the anion exchanger. It was found that the separation factor between Ce(IV) and other trivalent rare earths at 278 - 288K was as high as 25 - 60. Separation experiments by AR-0l packed column demonstrated that Ce can be separated with high purity from a nitric acid solution containing La(III), Ce(III) and Nd(III) after electro-oxidation using the flow type electrolysis cell. The recovery of Ce was 82 percent at 278 K, while La and Nd were not detected in the recovered Ce product solution.