Effects of Initial Nitrate Concentrations and Photocatalyst Dosages on Ammonium Ion in Synthetic Wastewater Treated by Photocatalytic Reduction

Abstract

Ammonium ( ${\text{NH}}_4^{+}$ ) is an undesirable by-product of photocatalytic nitrate ( ${\text{NO}}_3^{-}$ ) reduction since it is harmful to aquatic life once it converts into ammonia (NH3). This research investigated the removal efficiency of ${\text{NO}}_3^{-}$ and for the first time quantified the relationships of initial nitrate concentrations ([ ${\text{NO}}_3^{-}$ ]0) and photocatalyst dosages on the remaining ammonium ( ${\text{NH}}_4^{+}$ ) in synthetic wastewater using photocatalytic reduction process with either nanoparticle titanium dioxide (TiO2) or 1.0%Ag-TiO2 under Ultraviolet A (UVA). The experiments were systematically carried out under various combinations of [ ${\text{NO}}_3^{-}$ ]0 (10, 25, 50, 80, and 100 mg-N/L) and photocatalyst dosages (0.1, 0.5, 1.0, and 2.0 g). The ${\text{NO}}_3^{-}$ removal efficiency of both photocatalysts was 98.96-99.98%, and the catalytic selectivity products were nitrogen gas (N2), nitrite ( ${\text{NO}}_2^{-}$ ), and ${\text{NH}}_4^{+}$ . Of the two photocatalysts under comparable experimental conditions, 1.0%Ag-TiO2 provided better ${\text{NO}}_3^{-}$ removal efficiency. For both photocatalysts, the remaining ${\text{NH}}_4^{+}$ was predominantly determined by [ ${\text{NO}}_3^{-}$ ]0; higher [ ${\text{NO}}_3^{-}$ ]0 led to higher ${\text{NH}}_4^{+}$ . Multiple linear regression analysis confirmed the dominant role of [ ${\text{NO}}_3^{-}$ ]0 in the remaining ${\text{NH}}_4^{+}$ . The photocatalyst dosage could play an essential role in limiting ${\text{NH}}_4^{+}$ in the treated wastewater, with large variation in [ ${\text{NO}}_3^{-}$ ]0 from different sources.