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Cho K, Hoffmann MR.
Environ Sci Technol. 2014 Oct 7;48(19):11504-11.
Linde+Robinson Laboratories, California Institute of Technology , Pasadena, California 91125, United States.
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 (Ea) 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 Ea of +3.0 V than at +2.2 V, presumably because dichlorine radical (Cl2(-)·) ions facilitate the urea transformation primary driven by free chlorine. Quadrupole mass spectrometer analysis of the reactor headspace revealed that N2 and CO2 are the primary gaseous products of the oxidation of urea, whose urea-N was completely transformed into N2 (91%) and NO3(-) (9%). The higher reaction selectivity with respect to N2 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.
Significance Statement
Urea is the primary nitrogen source in domestic wastewater which often leads to the eutrophication and algal blooms in surface waters. A large number of suburban cities require a decentralized wastewater treatment practice to reduce the nitrogen loadings from urea. To this end, an electrochemical system can accomplish the urea degradation efficiently, in terms of fast reaction rates (when compared to biological processes) and operational flexibility. This work describes the reaction pathway and kinetics of urea degradation mediated by reactive chlorines (free chlorine and chlorine radicals), electrochemically generated on bismuth doped TiO2 anode. The urea oxidation involves chloramines and chlorinated urea as reaction intermediates, while mass/charge balance analysis revealed that N2, CO2, and NO3- are the major products. The significant contributions of this work include that a control of selectivity towards N2 (for nitrogen loading reduction) or NO3- (for fertilizer production) is feasible by changing operational reactive chlorine concentration or current density. Another scientific finding is that, under an anodic potential exceeding +3 V NHE, a generation of dichlorine radical would facilitate the rate limiting step of urea transformation, the initial chlorination of urea. Consequently, this paper provides scientific backgrounds for deployment of electrochemical systems to treat nitrogen-rich wastewater such as black water and landfill leachate among many others. This work has been carried out as a part of “Reinvent The Toilet Project” with financial support of the Bill and Melinda Gates Foundation which seeks towards sustainable sanitation methods for use in developing countries.
