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

Ammonia nitrogen (NH4+-N), which exists in the dominant form of ammonium ion (NH4+) 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 NH4+-N into harmless substances is urgently needed and also a great challenge. Herein we propose a new NH4+-N conversion pathway based on a catalytic ozonation process with birnessite-type MnO2 (δ-MnO2) catalysts. The δ-MnO2 exhibits a superior performance for NH4+-N (1.0 mg/L) removal in adsorption (8.26 %) and oxidation (44.6 %) as well as up to 95.2 % of N2 selectivity, surpassing almost all the reported O3-based catalysts. Unlike the traditional radical oxidation, a singlet oxygen (1O2)-dominated nonradical oxidation pathway is confirmed for the efficiently selective conversion of NH4+-N to N2 through the successive dehydrogenation, leading to a 15.0-fold enhancement in NH4+-N oxidation rate compared with ozone oxidation. The Mn(III)/Mn(IV) redox sites and surface lattice oxygen in δ-MnO2 act as the active sites promoting the decomposition of O3 into •O2− and then 1O2 via electrons transfer. In application, δ-MnO2 catalytic ozonation effectively removes NH4+-N in real drinking water and presents excellent stability. This work finds a nonradical reaction mechanism in catalytic ozonation for NH4+-N selective conversion to N2, and solidly solves the problem of low-concentration NH4+-N removal from drinking water under the limited pH of 6.5–8.5 condition.