Feasibility studies on the separation of ruthenium from high level liquid waste by constant potential electro-oxidation

• Separation percentage of Ru increased with increase in applied E and T.
• Lower Faradaic efficiency at higher E and T was due to the rapid oxidation of water.
• Separation percentage of Ru from SHLLW was 58% due to simultaneous oxidation of Pd.
• Ea for the oxidation of Ru in 1 M nitric acid at 1.25 V was 75.69 kJ/mol.

Ruthenium is one of the most abundant fission products in fast breeder reactors. It creates problems during the reprocessing of spent fuel and in the management and disposal of wastes, owing to the formation of highly volatile and radiotoxic 106RuO4. Cyclic voltammograms of [Ru(NO)]3+ solution recorded at Pt electrode revealed that surge in anodic current occurs at about 1.25 V (vs Ag/AgCl) due to oxidation of Ru and water. Batch experiments were carried out for the separation of Ru as RuO4 vapour from pure nitrosyl nitrate and simulated high level liquid waste (SHLLW) solutions by electro-oxidation method as a function of nitric acid concentration, temperature, applied potential and concentration of the redox mediator, Ce. The results indicated that separation percentage of Ru increased with increase in applied potential as well as temperature and decrease in acidity. In the presence of 0.04 M Ce at 1.65 V (vs Ag/AgCl), separation of 97% was obtained in 1 M HNO3 after 10 h of electrolysis; however, only 58% could be separated from SHLLW solution under similar experimental conditions. Rate constants were calculated for the oxidation reaction of ruthenium and the energy of activation was determined to be 75.69 kJ/mol.