Carbon and nitrogen product distribution in the electrochemical degradation of nitrogen-heterocyclic compounds.

[Display omitted] • Electro-oxidation of N-heterocycles produces N 2 , N H 3 / N H 4 + , N O 3 - , C O and C O 2. • N H 3 / N H 4 + and an apparent N 2 yield correlate linearly with carbon removal. • Both apparent N 2 and N O 3 - form directly from organic intermediates rather than N H 3 / N H 4 +. • Higher concentrations of parent compounds promote a higher apparent N 2 selectivity. • N O 3 - dominates when oxidation of carbon is mass transfer-limited or at basic pH. We report for the first time the carbon and nitrogen product distribution during the electrochemical oxidation (EO) of pyrazine and related N-heterocyclic compounds (NHCs) present in the hard-to-treat wastewater from a biofuel production process. The experimental study was carried out using a boron-doped diamond anode. Identified and quantified intermediate products included oxamic acid, formamide, oxalic acid, and formic acid. Analysis of the C and N yields as a function of the extent of carbon removal reveals the influence of the current- and mass transfer-control operating regimes on product distribution. Carbon monoxide, which was detected among the gaseous products, suggests a possible decarbonylation pathway in the current-controlled regime. An apparent nitrogen gas and ammonium yields correlated linearly with the carbon removal, while nitrate yields significantly increased in the mass transfer-controlled regime. Nitrate and apparent nitrogen gas appear to form directly from organic nitrogen intermediates, rather than from the ammonium that also forms in the process. The research gives new insights into the NHCs degradation pathways, product distribution, and final water composition as a function of operating regime in the EO of recalcitrant wastewater. [ABSTRACT FROM AUTHOR]