ELECTROCHEMICAL DENITRIFICATION OF WASTEWATER CONTAINING NITROGENOUS COMPOUNDS

Abstract

Nitrogen as a pollutant can be found mainly in three forms in water bodies, i.e., NO3−, NH4+, and NO2−. The primary industrial sources of these pollutants are industries such as fertilizer, pharmaceutics, and explosive manufacturing units. The treatment of such wastewaters can't be performed by traditional methods such as biological treatment owing to the toxicity of various pollutants present in the wastewater. In recent years, electrochemical treatment of NO3− contaminated wastewater has gained much attention. However, a limited number of studies are available for electrochemical denitrification of synthetic and actual wastewater. Also, a detailed physicochemical and electrochemical characterization of the fresh and used electrodes is missing in the available literature. Physico-electro-chemical characterizations are required to analyze the electrode characteristic (before and after use) and analyze redox reaction phenomena. Very scarce studies report detailed mechanistic kinetic and thermodynamic modeling. The mechanistic kinetic study could help analyze the combination of the various possible reaction mechanistic steps through the electrodes. Thermodynamic modeling could be an essential study to predict the equilibrium condition of the multi-ion system in the aqueous medium. Part I: Electrochemical denitrification of synthetic nitrogenous wastewater This study reports an electrochemical reduction of the NO3− along with oxidation of the in-situ generated NH4+ with maximum selectivity of the N2 gas as the final product. The use of Al as cathode and Ti/RuO2 as anode showed enhanced electrochemical nitrate reduction. Effect of various parameters like initial nitrate concentration (Co), electrolyte (NaCl), applied current density, initial pH, and electrolysis time (t) was studied in terms of NO3− reduction and total nitrogen (TN) removal efficiencies. Oxidation/reduction mechanism and characteristics of the electrodes were established. The nitrate degradation into N2 was observed to be a cyclic conversion via the formation of nitrite and ammonium ion by cathodic reduction and anodic oxidation mechanism. The operating cost was calculated to be $210 m−3 wastewater for lab-scale, and it was found to be competitive when compared with previous studies. Part II: Electrochemical denitrification of actual nitrogenous wastewater Electrochemical denitrification of explosive manufacturing industry wastewater containing a high concentration of NO3− and NH4+ was performed using Ti/RuO2 as an anode; and Fe and Al as cathode, respectively. The approximate operating cost was calculated to be ≈$177 m−3 wastewater for the lab-scale batch reactor treatment of the explosive manufacturing industry wastewater. This study helped to understand parametric, i Abstract mechanistic, and kinetic aspects of electrochemical nitrate reduction and oxidation of previously present and produced ammonium ions in the explosive manufacturing industry wastewater. Part III: Mechanistic kinetics and thermodynamic modeling of electrochemical denitrification Two mechanistic kinetic models, i.e., scheme SND (surface adsorbed nitrogen as divergent) and SNOD (surface adsorbed NO as divergent), were developed to model simultaneous electrochemical reduction of the NO3− and oxidation of byproducts. The key assumption of schemes SND and SNOD is that the NO2− ion reduces to adsorbed nitrogen or NO, respectively, which acts as divergent towards NH4+ and N2 production. Desorption of NO2− from the cathode surface was the rate-determining step for the SND kinetic model, while adsorption of NO3− on the cathode surface and its reduction into surface nitrite was the rate-determining step for scheme SNOD kinetic model. The calculation of individual ions' activity coefficient (γi) and total minimum Gibb's energy (Gtotal) was performed for the electrochemical denitrification of nitrate ions in synthetic and actual wastewater. The γi was examined by applying Pitzer, Lin, and Lee modified Debye-Huckel theory which is based on the interaction of ion-ion (long-range interaction-electrostatic effect) and ion-water (short-range interaction-solvation effect) in the aqueous solution. The equilibrium composition of these ions was calculated by minimizing the error between Gtotal calculated by using experimental and predicted compositions. The experimental and calculated composition of NO3−, NO2−, and NH4+ ions were found to be in good agreement. The biological nitrate reduction is slow and too sensitive towards many parameters like pH, temperature, organics, and toxic level of wastewater. Ion-exchange and reverse osmosis methods are physical separations or transfer processes that generate a concentrated stream that requires additional treatment. The catalytic and photocatalytic denitrification have limitations towards the separation and reusability of the catalyst and photo-catalyst. The selection of adsorbent and its reusability are major issues for the denitrification of wastewater. The electrochemical process has accomplished more attention in the last decades. It is easy to operate with a high rate of reduction of nitrate into N2 with no production of sludge. It requires only an electron as a reagent, which is known as the most effective and clean reagent. Therefore, it is an environmental-friendly process to treat nitrogenous pollutants contaminated wastewater. Overall, the developed method can be developed into a technology that could be used to treat toxic wastewater generated in the industries such as explosive manufacturing, pharmaceutical, fertilizer, pesticides, and aquaculture.