Progress toward a better understanding of the urea oxidation by electromediation of Ni(III)/Ni(II) system in alkaline media

Abstract

Current treatments of wastewater containing urea are energy intensive and release significant quantities of gaseous nitrous oxide. The anodic oxidation of urea by electromediation of Ni(III)/Ni(II) system in alkaline media is a promising alternative: the nitrogenous pollution is decreased while producing hydrogen at the cathode. Firstly, this study aims to deeply investigate the phenomena occurring at the interface, between electrode and electrolyte, in particular by proposing an original methodology enabling to separately study both steps,i.e. the Ni(II) electro-oxidation and the subsequent indirect heterogeneous urea oxidation. Indeed, the variation the potential scan rate, enables (i) to study the effect of the operating parameters (concentration, temperature, angular velocity of a rotating disk electrode) on the rate of each mechanism step, and to (ii) determine key parameters of the overall process (activation energy, diffusion coefficient, anodic charge transfer coefficient and heterogeneous electron transfer rate constant). In a second time, results of urea electrolysis exhibiting high (∼ 80%) urea conversions, carried out at laboratory scale (1.4 g of urea) are discussed. The effect of some operating parameters (alkalinity, temperature, presence of an anionic separator or not) is examined, thanks to a set of analytical methods developed to establish complete mass balances of the products on the liquid phase. Cyanate, ammonium, carbonate and nitrite ions are identified to represent more than 80 % of the destroyed urea. Their respective quantities appear to be dependent of the operating conditions. This study constitutes an essential preliminary step before designing urea electrochemical process operating at larger scale.