Electrochemical oxidation of ammonia on nickel oxide nanoparticles

NiO and NiO–TiO2 nano-catalysts were synthetized using solution combustion synthesis (SCS) method and tested toward ammonia oxidation in synthetic and real wastewater. As-synthesized NiO nano-catalyst showed a tightly agglomerated nano-porous spherical structure with sizes ranging from 10 to 50 nm. NiO–TiO2 nano powders have homogenous structure with an average size of 19.5 ± 0.03 nm and a lattice spacing of 0.22 ± 0.03 nm corresponding to cubic planes of NiO and 0.25 ± 0.01 nm corresponding to TiO2. Cyclic voltammetry under alkaline condition at low potential ranging from 0.95 to 1.35 V vs. HgO/Hg improved the electro-chemical activity of the nano-catalysts by the formation of Ni(OH)2 film on the surface of catalyst as confirmed by XPS measurements. Ammonia electro-oxidation on nano-catalysts occurred at approximately 1.28 V vs. HgO/Hg and was highly pH-dependent. Ammonia removals up to 92.9 and 96.4% were achieved by NiO and NiO–TiO2, respectively. Total nitrogen material balance showed that the electro-chemical oxidation of ammonia produce small amounts of NO2− and NO3 and the balance N2. Ammonia oxidation at concentration less than 150 mM followed direct electron transfer mechanism, whereas at higher concentrations, the oxidation mechanism shifted to the indirect oxidation regime. Ammonia electro-oxidation kinetics followed zero order reaction at ammonia concentration ≤100 mM and first order kinetics at higher concentrations. More than 93% of ammonia, 35% of organic matter and 40% phosphorous were removed from real wastewater samples using electro-oxidation process confirming the suitability of this technology as advanced wastewater treatment.