Ammonium ion and organic nitrogen are primarly nitrogenous (N) pollutants in water and wastewater treatment processes. Electrochemically generated reactive chlorine species (RCS) undergo facile reactions with NH3 to form chloramines, subsequently transformed into N2 or NO3-, in terms of break-point chlorination which could be extended to model organic N compounds. Nevertheless, the mechanism (reaction pathways), products selectivity, and the effects of operational parameters (Cl/N ratio, current density and pH) have not been fully clarified, especially in comparison with chemical chlorination (i.e., addition of NaOCl). An electrochemical system equiped with IrTaOx anode effectively mitigated the ammonia-N (initial N: 20 or 50 mM, [Cl-]: 50 to 150 mM, pH: 4 to 10, current density: 200 to 400 A/m2) within several hours of reaction. An accelerated kinetics in a low pH (~ 4) indicated the active role of chlorine radicals (such as chlorine atom and dichlorine radical anion) during the electrochemical chlorination. The selectivity (or current efficiency) towards N2 and ClO3- was found to depend on the feed regime of RCS (chemical versus electrochemical chlorination) as well as the operational parameters such as Cl/N ratio and pH. In addition, a formation of chloramine mediated byproduct (N-nitrosodimethylamine) was retarded in the electrochemical chlorination at the same total exposure to RCS. The chlorine evolution rates on IrTaOx were found to be enhanced by a heterojunction with outer TiO2 layer. Nevertheless, depending on the operational conditions, the heterojunction did not accelerate the degradation rate of ammonia-N, due to the competition with ClO3- formation. The outcomes of this presentation would clarify the unknown aspects of electrochemical chlorination to address a wide array of N-pollutants.

Unraveling electrochemical chlorination of ammoniacal water

2019 Fall National Meeting

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