Device-Scale Modeling of a Photoelectrochemical Wastewater Nitrate Treatment Device: Effects of Competing Reactions and Effluent Stream Composition

Abstract

Nitrate contaminants dominate low-level nuclear wastes and agricultural runoff streams; nitrates and ammonium are present in industrial effluent streams from fertilizer production1, iron/steel making and finishing2,3, coal mining4,5, and municipal wastewater streams6. Most conventional wastewater treatment plants use the biological nitrification-denitrification treatment processes in which microbes consume and metabolize the dissolved contaminants to eliminate nitrates present in wastewater7,8. However, biological processes are energy intensive and are not effective in effluent streams that harbor conditions unsuitable for microbial growth9–11. We previously proposed a solar-powered photoelectrochemical device to treat wastewater nitrates12 and determined that reducing wastewater nitrates to ammonia and nitrous oxide facilitated an improved scope for energy recovery as compared to the traditional electrochemical denitrification processes that convert nitrates to dinitrogen (Figure, left). In this study, a device-scale model transport and kinetic model was developed to specifically investigate the influences of competing redox reactions and the effluent stream composition on the overall performance of this device. Competing reactions of interest were hydrogen evolution reaction, hydrogen oxidation reaction, oxygen reduction reaction, and reactions resulting from other ions present in the considered waste stream (Figure, right). The different effluent stream compositions considered were alkaline waste solutions from nuclear processes and ion-exchange brines from ion-exchange water treatment applications. The developed model predicts various performance metrics including the rates of product formation, solar energy conversion efficiency, the time required to reduce the initial concentration of nitrates to 0.1 mM, which is on the order of magnitude of what the US Environmental Protection Agency has deemed safe for human consumption, and faradaic efficiencies to quantify the effects of competing reactions. Model results demonstrate that the hydrogen evolution reaction competes with the nitrate reduction reaction at the cathode. However, the extent of competition was dependent on the cathode electrocatalytic parameters, the initial waste stream composition, and the light-absorber bandgap that dictated the operating potential. It was also found that the oxygen reduction reaction and the hydrogen oxidation reaction have negligible effects on the solar energy conversion efficiency of the device.