Urea degradation by electrochemically generated reactive chlorine species: products and reaction pathways

Abstract

This study investigated the transformation of urea by electrochemically generated reactive chlorine species (RCS). Solutions of urea with chloride ions were electrolyzed using a bismuth doped TiO2 (BiOx/TiO2) anode coupled with a stainless steel cathode at applied anodic potentials (E-a) of either +2.2 V or +3.0 V versus the normal hydrogen electrode. In NaCl solution, the current efficiency of RCS generation was near 30% at both potentials. In divided cell experiments, the pseudo-first-order rate of total nitrogen decay was an order of magnitude higher at E-a of +3.0 V than at +2.2 V, presumably because dichlorine radical (Cl-2(-)) ions facilitate the urea transformation primary driven by free chlorine. Quadrupole mass spectrometer analysis of the reactor headspace revealed that N-2 and CO2 are the primary gaseous products of the oxidation of urea, whose urea-N was completely transformed into N-2 (91%) and NO- (9%). The higher reaction selectivity with respect to N-2 production can be ascribed to a low operational ratio of free available chlorine to N. The mass-balance analysis recovered urea-C as CO2 at 77%, while CO generation most likely accounts for the residual carbon. In light of these results, we propose a reaction mechanism involving chloramines and chloramides as reaction intermediates, where the initial chlorination is the rate-determining step in the overall sequence of reactions.