Superior reduction of nitrate with simultaneous oxidation of intermediates and enhanced nitrogen gas selectivity via novel electrochemical treatment

This study reports an electrochemical reduction of the NO3- along with oxidation of the in-situ generatedNH4+ with maximum selectivity of the N-2 gas as the final-product. The use of aluminum as a cathode andTi/ RuO2as an anode showed enhanced electrochemical nitrate reduction at the cathode and oxidationof the ammonium ion at the anode. Effects of various parameters like initial NO3- concentration (C-0 = 100-400 mg L-1), a dose of the Cl- as NaCl (NaCl = 100-400 mg L-1), current density applied (j = 83.3-333.3 A m-2), solution pH (pH = 4-10) and electrolysis time (t = 0-120 min) were studied in terms of NO3- reduction and total nitrogen (TN) removal efficiencies. Current efficiency (CE) was elaborated with respectto end products like N-2, NO2- and NH4+. Specific electrical energy consumption (SEC) was calculatedin kWh kg(-1) NO3- removed for the electrochemical process. Electrochemical impedance spectroscopy ( EIS) and cyclic voltammetry (CV) were utilized for understanding the oxidation/reduction mechanismover electrodes and the characteristics of the electrodes in a different solution. The studied mechanismsuggested a circular conversion of NO3- through complex processes into the N-2 gas as the final product. The ultimate nitrate and TN degradation efficiency of similar to 95 % with N-2 selectivity of approximate to 100 % were achievedat the optimum condition of Co= 100 mg L-1, NaCl = 300 mg L-1, j = 333.3 A m(-2), pH = 6 and time = 120 minwith SEC = 927.4 kW h kg(-1) NO3- removed. The 1st, 2nd, and nth-order kinetic models were used for thereaction kinetics. FE-SEM, XRD, and AFM techniques were used for the characterization of the electrodesbefore and after all the electrochemical runs. The operating cost was calculated for lab-scale treatmentalong with a comparison with previous studies. No sludge or scum got produced for each electrochemicalrun. Finally, this study delivers a superior perceptive for electrochemical characteristics of Al at thecathode side and Ti/RuO2 at anode side as well as electrochemical NO3- reduction and oxidation of thegenerated NH4+, simultaneously. (C) 2020 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

Automated summary

This study demonstrates the electrochemical reduction of NO3- and oxidation of in-situ generated NH4+ with high efficiency using aluminum cathode and Ti/RuO2 anode. By investigating various parameters, the optimal conditions for nitrate reduction and nitrogen removal were determined. Impedance spectroscopy and cyclic voltammetry were used to understand the oxidation/reduction mechanism and electrode characteristics.