Effects of Effluent Stream Composition and Competing Reactions Effect on Wastewater Nitrate Reduction in a Photoelectrochemical Device

Abstract

More than 80% of nitrogen-contaminants are present in the form of nitrates in point sources of wastewater, including municipal wastewater effluents, ion-exchange brines, low-level nuclear wastes and in power generation. Wastewater nitrates represent an untapped source for the recovery of nutrients, value-added chemicals, and energy. Nitrates can be reduced to form ammonia, which can be used as a fertilizer or fuel, and nitrous oxide, which is a powerful oxidizer for the combustion and supercharging applications. Depending upon the source of the wastewater, the concentration of nitrates present in these streams can vary significantly. Ion-exchange brines and low-level nuclear wastes have large nitrate concentrations, on the order of > 0.1 M, whereas municipal and power generation effluents are more dilute (0.1 mM). However, the effect of varying contaminant concentrations and the competing redox reactions have not been quantified for these systems.In this study, a photoelectrochemical device is proposed to transform wastewater nitrates into ammonia, coupled with water oxidation. We expand on a previously developed numerical model to include the effects of the reacting nitrate species concentrations and the competing redox reactions on the solar-to-chemical efficiencies and nitrogen-removal rates. Notably, we implemented an equivalent circuit modeling approach to account for the influences of the competing reactions and the concentration-dependent mass-transfer limiting current densities. Model results predict that for a single light-absorber and a nitrate concentration of 100 mM, the optimal solar-to-chemical efficiency is 7% and the nitrogen recovery rate (as NH3) is 260 gN m-2 day-1. Oxygen reduction is more dominant as a competing reaction than hydrogen reduction at the cathode. However, this reduction is mass-transfer limited even while assuming an air-saturated (1 atm) device and therefore didn’t impact the performance significantly. Modeling results are complemented by experimental measurements of the effects of nitrate concentrations on the operating current densities and the rates of product formation with copper and platinum electrodes at the cathode and anode respectively.