Electrochemical reduction of nitrate and nitrite in simulated liquid nuclear wastes

The electrochemical reduction of nitrate and nitrite in simulated low-level nuclear wastes containing 1.8 M NaNO3 + 0.55 M NaNO2 + 1.16 M NaOH was studied under galvanostatic polarization on tin and bismuth cathodes. The rate of the reduction of nitrate was about the same on both metals. The selectivity (%S) to ammonia was similar on the two metals (12% at 450 mA/cm2) and that to nitrogen 82% on Sn and 72% on Bi. On the other hand, the %S to nitrous oxide was lower on Sn (8%) than that on Bi (18%) under the same conditions. The current efficiency (%CE) on both metals was 80% when the 99% of the initial nitrogen was removed. The %CE gradually decreases as the concentration of the nitrogen containing species in the solution decreases during the electrolysis. The energy consumption for the removal of 1 g of N was 100 W h at 450 mA/cm2 but it can be significantly reduced by a better design of the electrolysis cell.The presence of chromate in the electrolyte, which is a known inhibitor of the cathodic reduction of nitrate, has no influence on the rate of the reduction and the distribution of the products.