2.4.1. Overview of Nitrate Anion and Its Reaction Intermediates (Nitrite and Ammonium Ions)

Photocatalytic reduction of nitrate to molecular nitrogen presents a promising approach to removing nitrate from drinking water sources. However, unwanted products such as nitrite (NO2 −) and ammonium (NH4 +) result from the nitrate reduction reaction. The values of these unwanted products have been regulated over the years. The standard recommended levels of nitrates, nitrite and ammonium concentration in drinking water, 50 ppm NO3 −, 3 ppm NO2 − and 0.5 ppm NH4 +, respectively, are introduced by the World Health Organization (WHO) [118]. To reduce the recombination of photo-generated electrons and holes, metal loading or holes scavengers are generally needed, causing, in this way, secondary pollution of drinking water [119].

Continuous efforts have been devoted to the achievement of heterogeneous photocatalytic nitrate reduction, which is potentially a green and low-cost operation; however, achievements are not as much as expected. In general, photocatalytic nitrate reduction includes two successive reactions, from nitrate to nitrite and then from nitrite to nitrogen radical. The nitrogen radicals will further combine to form N2. Sometimes ammonia (NH3) may be generated as the over-reduced product. The unwanted NH3 is more toxic than NO3 −, and the problem is that the use of the majority of traditional photocatalysts cannot avoid the formation of NH3, leading to a low selectivity to N2 [120–130]. If the formation of undesired by-products (e.g., NO2 − and NH4 +) is prevented, photocatalytic technology could be one of the most promising options to solve the problems of environmental pollution and energy shortage due to its advantages such as adaptability, low cost, and no secondary pollutants [131]. Dark catalytic hydrogenation of nitrate has been considered a promising alternative to overcome the economic and environmental disadvantages of separation technologies without generating waste streams [132]. N2 selectivity and water matrix effects are important factors that should be assessed for any technology based on catalytic and/or photocatalytic processes proposed for nitrate removal [133,134].
