Electrochemical reduction of aqueous nitrate ion at tin cathodes

The remediation of nitrate-contaminated water using electrochemical reduction at a tin cathode has previously been shown to give almost quantitative denitrification (removal of dissolved nitrogen species) under highly cathodic polarization. A particular focus of this project was to identify specific role(s) for tin in the reaction in the context of the previous literature. The current efficiency for denitrification was enhanced in alkaline solution, and the reaction was accelerated by the presence of small concentrations of Sn(II) salts, which are in a dynamic exchange between cathodic deposition and corrosion of the cathode. Literature precedent indicates that Sn(II) salts promote the "dimerization" pathway of NO to hyponitrite in preference to reduction to ammonia. Hyponitrite is a known intermediate in the electrochemical reduction of nitrate, but its spontaneous decomposition gives predominantly N2O, which does not reduce further to N-2. We have shown that hyponitrite is reduced electrochemically in competition with its thermal decomposition, which provides a pathway to N-2 via the spontaneous dehydration of HO-NH-NH-OH. The possible role of surface-bound Sn-H species in the reduction mechanism is discussed, but further work is needed to substantiate this proposal.