Catalytic degradation of ammonia and nitrate from wastewater - A critical review

The foibles in extant wastewater treatment technologies release untreated nitrate and ammonia containing compounds into different potable water sources. The toxicity and fatal effects of entrained nitrate and ammonia produce lethal health consequences upon being consumed. Novel methods obtained by combining light, electrical, and chemical energy have opened new frontiers of pronounced efficiency and reduced demerits. These methods lead to the destruction of nitrate instead of just removing it by adsorption on the surface of another material. Photochemical (PC) and electrochemical (EC) reactions offer a vast scope for degrading harmful ammonia and nitrates from wastewater. The catalyzed form of these processes has been found to be meritorious over non-catalyzed techniques due to several advantages like improved efficiency, lower energy input, lower reaction time, and product selectivity towards N2 gas over nitrate and nitrite. This paper presents a review of significant research that has been performed using PC, EC, and photoelectrochemical (PEC) to remove ammonia and nitrate from wastewater. Not much research is available on the combined and simultaneous use of PC and EC oxidation and reduction processes which have immense potential as future methodologies for treating municipal and industrial wastewater to remove these toxic inorganic nitrogenous compounds. High ammonia and nitrate removal efficiencies at the laboratory scale have been reported using specific combinations of catalysts, pH, cell composition, electrodes, electrical input, and reaction time by electrochemical denitrification. However, they lack practical viability. Catalyzed photochemical processes are successful in removing ammonia and nitrate to a large extent and are practically viable if carried out using natural sunlight. Combined PC and EC, i.e., PEC oxidation and reduction processes, eliminate the occurrence of toxic intermediates and give about 90 % to 98 % conversion of ammonia and nitrate in the form of nitrogen gas.