Active chlorine mediated ammonia oxidation revisited: Reaction mechanism, kinetic modelling and implications

Ammonia nitrogen removal from wastewaters has gained much attention in recent decades as a result of the environmental problems associated with discharge of excessive amounts of this critical nutrient including eutrophication of receiving waters, generation of offensive odours as a result of organism decay and complications associated with the disinfection of water supplies. While removal via biological processes represents the principal means by which a reduction in dissolved nitrogen concentrations is achieved, an electrochemical advanced oxidation process has been proposed as a potentially effective alternate means of removing ammonia from wastewaters with the removal associated with the in situ generation of oxidants (particularly active chlorine) at the anode. Here we describe the influence of key factors on the rate and extent of ammonia nitrogen removal in an electrochemical cell with a Ti/IrO₂-RuO₂ anode and Ti cathode. The rate of ammonia removal was found to be dependent on both current density and initial chloride concentration with ∼95% ammonia removed from a 20 mM Cl^- solution within approximately 40 min at a current density of 3 mA cm^-2, resulting in an energy consumption of 126 kWh kg^-1 NH₄⁺-N. Additionally, we show that by-products formation is effectively suppressed during the electrolysis process. A mechanistically-based kinetic model incorporating the key processes operating in the ammonia electro-oxidation process was developed with particular attention given to (i) anodic generation of active chlorine and other chloride-related by-products, (ii) active chlorine mediated ammonia oxidation. The results demonstrate that the electrochemical advanced oxidation process is a promising technology for treatment of ammonia-containing wastewaters with advantages including simplicity, safety and effectiveness.