A Suitable Combination of Electrodes for Simultaneous Reduction of Nitrates and Oxidation of Ammonium Ions in an Explosive Industry Wastewater

Abstract

The electrochemical reduction of nitrate and ammonium ion oxidation has been performed with the cathode (Al) and anode (Ti/RuO₂) in the explosive industry wastewater polluted with a significant quantity of NO₃– and NH₄⁺. For this, the influences of various other cathode materials (Fe, Cu, and C), the current density (j = 35.71 to 142.86 A m–²), wastewater pH (pH = 3–12), and treatment time (t = 180 min) were analyzed for nitrate ion reduction, ammonium ion oxidation, nitrite ion generation, and total nitrogen (TN) degradation. The maximum nitrate removal and TN removal efficiencies were found to be ≈51 and ≈63% at j = 142.86 A m–² and wastewater natural pH after 180 min treatment time, respectively. In this condition, SEC was calculated to be 74.4 kWh (kg NO₃– reduced)⁻¹. The mechanistic analysis suggested that the atomic hydrogen molecules, H⁺, and e– help in the nitrate ion reduction and the available Cl– ion in the wastewater helps in the oxidation of the ammonium ion into gaseous nitrogen through the electrochemical process. The new and used electrodes were characterized by FE-SEM, AFM, and XRD techniques. Electrodes were also characterized within untreated and treated wastewater through CV and EIS analysis. The in situ generated oxidizing species were identified by the p-nitrosodimethylaniline (RNO) bleaching test. Pseudo-first-order and nᵗʰ-order kinetic models were applied, and the reaction rate was calculated to understand the reaction rate of the nitrate reduction. The approximate operational cost estimation has been performed for the lab-scale batch reactor treatment of the explosive industry wastewater. No scum or sludge was produced throughout the treatment process. This study helps to understand the parametric, mechanistic, and kinetic aspects of electrochemical nitrate reduction and oxidation of the previously present and produced ammonium ion in the explosive industry wastewater.