Reinvestigation on the ozonation of N-nitrosodimethylamine: Influencing factors and degradation mechanism.

N-nitrosodimethylamine (NDMA), as a new disinfection byproduct, is a potential carcinogen. In this study, we focused on the role of ozone in NDMA degradation. We characterized the removal efficiency, influencing factors and degradation mechanism. Our results demonstrated that ozonation was an efficient process for NDMA degradation. The removal efficiency was affected by initial NDMA concentration; higher NDMA dosing required higher ozone utilization. NDMA oxidation was favored at high ozone dosage and high pH. NDMA ozonation under various pH as well as hydroxyl radical (OH) inhibition experiments verified that OH generated from ozone dominated NDMA oxidation. The main products of NDMA ozonation were methylamine (MA), dimethylamine (DMA), nitromethane (NM) and ammonium (AM). Their yields changed with the amount of ozone provided. A NDMA ozonation mechanism was proposed. It is suggested that NDMA degradation is induced by OH attacking through any of four pathways, with oxygen involving in the oxidation process. MA generation was due to OH attacking on amine nitrogen and methyl group. DMA formation was related to OH attacking on nitrosyl nitrogen via a parallel pathway. We speculate that supersaturated dissolved oxygen by ozone decomposition is responsible for NM generation by further oxidation of MA and DMA. AM formation may be favored due to MA degradation under OH exposure.