(Invited) 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. 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. We propose to use Cu for this study as it is widely known for its selectivity towards reducing NO3 - to NH3. Cu has been exhaustively studied in alkaline conditions, but its behavior in pH 8 and pH 10 solutions has not been widely probed. 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. Additional experiments were also performed with only NO2 - in the solution to isolate the effects of the rate determining NO3 --to-NO2 - reduction step from the downstream NO2 - to NH3 transformations. Cyclic voltammograms were obtained in a standard 3-electrode cell with a copper disk (0.28 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 to quantify reaction selectivity. UV-Vis spectroscopy was used to identify and quantify concentrations of species present in the solution (NO3 -, NO2 -, and NH3 using the salicylate method). Additional experiments are performed on brine solutions with Cl- and SO4 2- ions to quantify the effect of competing anions on the NO3 - to NH3 transformations at various pH conditions. Our results indicate that higher currents can be reached for higher concentrations of NO3 - and higher pH of the solutions. This is most likely due to the improved kinetics of the reactions taking place with the increased presence of ions, as the final current densities reached increase as well as the onset potentials shifted towards smaller values vs. RHE. As the pH decreases from pH 14 to pH 8, more peaks appear that are distinct from the NO3 --to-NO2 - peak, which was confirmed by running a separate NO2 - trial at the same concentration and pH. This indicates for these lower pH solutions, the nitrate reduction process might not be as fully selective towards ammonia formation, thereby highlighting the dependence of the products formed on Cu on the pH of the solution. These 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 disk electrodes.