Electrochemical reductions of Hg(II), ruthenium-nitrosyl complex, chromate, and nitrate in a strong alkaline solution

The electrochemistry of low-level nuclear wastes, the reductions of Hg(II), [RuNO(NO2)4OH]2-, CrO42-, and NO3- in 1.33 M NaOH solution have been studied primarily at nickel electrodes for electrocatalytic advantages. Hg(II) was reduced to Hg metal [equilibrium potential of 0.099 V vs NHE (normal hydrogen electrode)]. The ruthenium-nitrosyl complex, [RuNO(NO2)4OH]2-, was reduced to Ru metal (half-wave potential of -0.38 V vs NHE). Chromate was reduced to chromic hydroxide (equilibrium potential of -0.24 V vs NHE). Nitrate reduction has two different regions of Tafel behavior from which reduction mechanisms are proposed. The exchange current densities for the nitrate reduction at Ni, Pb, Fe, and graphite were measured and the competition of the nitrate reduction with the hydrogen evolution investigated