Optimization of combined photocatalytic involving immobilized ZnO nanoparticles and electrochemical processes for ammoniacal nitrogen removal from aqueous solutions

A photocatalytic process consisting of a low-pressure ultraviolet light and ZnO nanoparticles immobilized on glass plates in combination with an electrochemical process using Pt as an anode and graphite as a cathode were used to remove ammoniacal nitrogen from an aqueous solution. A comparison of photocatalytic, electrochemical and combined process for ammoniacal nitrogen removal was performed. The results showed that ammoniacal nitrogen removal follows the increasing order: photocatalytic process < electrochemical process < combined photocatalytic and electrochemical processes. Response surface methodology (RSM) based on central composite design (CCD) was used to evaluate the individual and interactive effects of the four main independent variables, including ammoniacal nitrogen concentration, reaction time, current intensity and initial pH, on the removal of ammoniacal nitrogen. Analysis of variance (ANOVA) exhibited a high coefficient (R²=0.976 and adjusted R²=0.953), indicating a suitably close fit between the experimental and predicted values. Using a desirability function for the highest ammoniacal nitrogen removal of 76%, the optimum ammoniacal nitrogen concentration, reaction time, current intensity and pH were identified to be 188 mg NH₄-N L^-1, 116 min, 664 mA and 6, respectively. Based on the results, the present process can be used as an efficient means for the removal of ammoniacal nitrogen from aqueous media.