Experimental Quantification of the Effects of Concentration and pH on Nitrate-to-Ammonia Reaction Selectivity for Copper Electrodes

Abstract

Nitrates constitute more than 80% of nitrogen-contaminants in point sources of wastewater like municipal wastewater effluents, ion-exchange brines and low-level nuclear wastes. They represent an untapped source for nutrient and energy recovery, as nitrates (NO3 -) can be electrochemically reduced to ammonia (NH3), which can then be used as a fertilizer or fuel. The concentration of NO3 - and the pH of waste streams are strongly influenced by the source. For example, ion-exchange brines are highly concentrated in NO3 - and have a neutral pH as the brine results from the regeneration process of the ion-exchange resin, which does not require a high pH solution, whereas low-level nuclear wastes are highly concentrated in NO3 - and highly alkaline due to the chemical processes involved in their formation. Therefore, we have performed experiments to quantify the influences of NO3 - concentration (0.1 M – 1 M) and solution pH (8, 10 and 14) on the current-potential behavior and reaction selectivity towards NH3 production. We test the performance of Cu electrodes, as its behavior in neutral and mildly alkaline conditions—pH 8 and pH 10—has not comprehensively studied as compared to its widely established behavior in more strongly alkaline conditions, pH 12-14. Cyclic voltammograms were obtained in a standard 3-electrode cell with a copper disk (0.07 cm2) and a platinum wire as the working and counter electrodes respectively. Polarization curves from this experiment were used to inform applied potentials in the galvanostatic tests performed with a planar copper electrode (0.3 cm2) to quantify reaction selectivity to ammonia production. Electrochemical Impedance Spectroscopy (EIS) was used in situ to correlate surface changes with catalytic performance for the galvanostatic measurements. UV-Vis spectroscopy was used to identify and quantify concentrations of species present in the solution (NO3 -, NO2 -, and NH3 using the salicylate method). Our results indicate that higher currents can be reached for higher concentrations of NO3 - and higher pH solutions. The onset potentials for NO3 - to NO2 - shifted towards smaller values vs. RHE with increase in pH. Additionally, as the pH decreases from pH 14 to pH 8, new and distinct peaks appear in the cyclic voltammograms that are different from the NO3 --to-NO2 - peak. This was confirmed by running a separate measurement with only NO2 - at equivalent concentration and pH. Ammonia quantification in galavanostatic measurements points to decreasing rates of ammonia formation with pH. However, at pH 14, a smaller NO3 - concentration of 100 mM resulted in a larger NH3 selectivity compared to a 1M case, indicating the effects of competing surface reactions. EIS measurements for charge-transfer resistance and capacitance provides a useful framework to correlate observed changes in measured currents with dynamic changes to the catalytic surface. Results are further interpreted to deduce empirical correlations for the concentration and pH dependence for the exchange current density for the rate determining NO3 --to-NO2 - step on the tested Cu electrodes.