Simultaneous deammoniation and denitrification under vacuum ultraviolet irradiation

The oxidative nitrate (NO₃⁻) and reductive ammonium (NH₄⁺) constitute the common nitrogen pollution in water. However, the high energy barrier of the comproportionation reaction makes it challenging for the deammoniation and denitrification reactions to occur simultaneously. This study evaluated the performance of simultaneous deammoniation and denitrification under vacuum ultraviolet (VUV) irradiation. The results demonstrate that the reduction reaction of NO₃⁻ and the oxidation reaction of NH₄⁺ conform to the pseudo-first-order reaction kinetics, with respective kinetic constants of 0.012 min⁻¹ and 0.002 min⁻¹. The presence of Cl⁻ and SO₄²⁻ inhibits the reduction of NO₃⁻, while HCO₃⁻ and CO₃²⁻ promote NO₃⁻ reduction and NH₄⁺ oxidation. The reaction mechanism of simultaneous NO₃⁻ and NH₄⁺ removal was proven through free radical capture and electron spin resonance (ESR) experiments. It was determined that NO₃⁻ was reduced to NO₂⁻, NH₄⁺, and N₂ by hydrated electrons (e_aq^–) in turn. While the free hydroxyl radical (·OH) produced by direct photolysis of water by VUV was identified as the key to the oxidation of NH₄⁺. When NO₃⁻ and NH₄⁺ coexist, they can capture e_aq^– and ·OH, respectively, thus avoiding the quenching of the two highly active species. Thus, energy waste is avoided and nitrogen removal efficiency is improved. The process study shows that the adjustment of the NO₃⁻ and NH₄⁺ ratio and the control of process conditions are essential for the synchronous conversion of nitrate and ammonium.