Activity coefficient of multi-ions and Gibbs free energy calculation during electrochemical nitrate reduction in synthetic and actual wastewater

Abstract

This study reports the calculation of individual ion's activity coefficient and total minimum Gibbs energy for electrochemical denitrification of nitrate ions in synthetic and actual wastewater. The concentration of nitrate ion continuously decreased during treatment via the electrochemical method. During the treatment, nitrate ions got reduced into nitrite and ammonium ion. The individual ion activity coefficient was examined by applying Pitzer, Lin and Lee modified Debye-Huckel theory which is based on the interaction of ion-ion (long-range, i.e., LR interaction-electrostatic effect) and ion-water (short-range, i.e., SR interaction-solvation effect) in the aqueous solution. The equilibrium composition of these ions was calculated by minimizing the error between total minimum Gibbs energy calculated by using experimental and predicted compositions. The individual ion activity coefficient, which was calculated through this model, was used to measure the total minimum Gibbs energy (Gtotal). Variation of individual ion activity coefficient and Gtotal with initial nitrate ion concentration, applied current density, initial pH, and concentration of NaCl as electrolyte has been studied. It was found that the experimental and calculated composition of nitrate, nitrite, and ammonium ions were in good agreement for electrochemical denitrification of synthetic and actual wastewater.