Nonradical-dominated pathway toward selective catalytic ozonation of NH4+-N to N2 in drinking water over birnessite

Ammonia nitrogen (NH 4 + -N), which exists in the dominant form of ammonium ion (NH 4 + ) under the pH of 6.5–8.5, is one of the regular indexes and the most important pollutants in drinking water. The selective conversion of NH 4 + -N into harmless substances is urgently needed and also a great challenge. Herein we propose a new NH 4 + -N conversion pathway based on a catalytic ozonation process with birnessite-type MnO 2 (δ-MnO 2 ) catalysts. The δ-MnO 2 exhibits a superior performance for NH 4 + -N (1.0 mg/L) removal in adsorption (8.26 %) and oxidation (44.6 %) as well as up to 95.2 % of N 2 selectivity, surpassing almost all the reported O 3 -based catalysts. Unlike the traditional radical oxidation, a singlet oxygen ( 1 O 2 )-dominated nonradical oxidation pathway is confirmed for the efficiently selective conversion of NH 4 + -N to N 2 through the successive dehydrogenation, leading to a 15.0-fold enhancement in NH 4 + -N oxidation rate compared with ozone oxidation. The Mn(III)/Mn(IV) redox sites and surface lattice oxygen in δ-MnO 2 act as the active sites promoting the decomposition of O 3 into •O 2 − and then 1 O 2 via electrons transfer. In application, δ-MnO 2 catalytic ozonation effectively removes NH 4 + -N in real drinking water and presents excellent stability. This work finds a nonradical reaction mechanism in catalytic ozonation for NH 4 + -N selective conversion to N 2 , and solidly solves the problem of low-concentration NH 4 + -N removal from drinking water under the limited pH of 6.5–8.5 condition.