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

Ammonium (NH4+) is an undesirable by-product of photocatalytic nitrate (NO3−) reduction since it is harmful to aquatic life once it converts into ammonia (NH3). This research investigated the removal efficiency of NO3− and for the first time quantified the relationships of initial nitrate concentrations ([NO3−]0) and photocatalyst dosages on the remaining ammonium (NH4+) 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 [NO3−]0 (10, 25, 50, 80, and 100 mg-N/L) and photocatalyst dosages (0.1, 0.5, 1.0, and 2.0 g). The NO3− removal efficiency of both photocatalysts was 98.96-99.98%, and the catalytic selectivity products were nitrogen gas (N2), nitrite (NO2−), and NH4+. Of the two photocatalysts under comparable experimental conditions, 1.0%Ag-TiO2 provided better NO3− removal efficiency. For both photocatalysts, the remaining NH4+ was predominantly determined by [NO3−]0; higher [NO3−]0 led to higher NH4+. Multiple linear regression analysis confirmed the dominant role of [NO3−]0 in the remaining NH4+. The photocatalyst dosage could play an essential role in limiting NH4+ in the treated wastewater, with large variation in [NO3−]0 from different sources.