Evaluation of porous cathodes for the electrochemical reduction of nitrates and nitrites in alkaline waste streams

Abstract

This paper describes the mathematical modelling and experimental work performed to evaluate porous cathodes for the electrochemical reduction of nitrates and nitrites in alkaline waste streams. A dynamic model of a batch process was developed that included a divided cell with a porous cathode, a cation-selective separator, a planar anode, and reservoirs for electrolyte recirculation and gas–liquid separation. Constant current experiments were done using a divided cell with nickel foam as the porous cathode. The experiments were performed with a catholyte feed of either 0.6 M NaNO2 or 1.95 M NaNO3, both supported by a 1.33 M NaOH solution, a current density of 0.25 A cm−2 and a solution temperature of 32 ∘C. The experimental results showed that the ammonia production reaction is the dominant cathodic reaction (∼80% of the current). Estimates of the kinetic parameters were obtained using the experimental data and the model. The model was then used to simulate and study the performance of the porous electrode compared to the planar electrode for a range of operating currents. The results showed that at the optimum current density for a planar electrode of 0.25 A cm−2, use of a porous cathode results in one-third the energy costs and time required to achieve 95% destruction of nitrate and nitrite compared to a planar cathode. At 0.40 A cm−2, the energy and time required to achieve 95% destruction was an order-of-magnitude less for the porous electrode.