Degradation behaviors of Nabumetone and its metabolite during UV/monochloramine process: Experimental and theoretical...

This study investigated degradation behaviors of a nonsteroidal anti-inflammatory drug Nabumetone (NMT) and its major metabolite 6-methoxy-2-naphthylacetic acid (MNA) in the coupling process of ultraviolet and monochloramine (UV/NH₂Cl). The second-order rate constants of the contaminants reacting with reactive radicals (HO•, Cl•, Cl₂•^⁻, and CO₃•^⁻) were determined by laser flash photolysis experiments. HO• and Cl• contributed predominantly with 52.3% and 21.7% for NMT degradation and 60.8% and 22.3% for MNA degradation. The presence of chlorides retarded the degradation of NMT, while promoted the destruction of MNA, which was ascribed to the photosensitization effects of MNA under UV irradiation. Density functional theory (DFT) calculations revealed that radical adduct formation (RAF) was dominant pathway for both HO• and Cl• reacting with the contaminants, and hydrogen atom transfer (HAT) preferred to occur on side chains of NMT and MNA. NMT reacted with NO₂• through single electron transfer (SET) with the second-order rate constant calculated to be 5.35 × 10⁷ (mol/L)^-1 sec^-1, and the contribution of NO₂• was predicted to be 13.0% of the total rate constant of NMT in pure water, which indicated that NO₂• played a non-negligible role in the degradation of NMT. The acute toxicity and developmental toxicity of NMT were enhanced after UV/NH₂Cl treatment, while those of MNA were alleviated. The transformation products of both NMT and MNA exhibited higher mutagenicity than their parent compounds. This study provides a deep understanding of the mechanism of radical degradation of NMT and MNA in the treatment of UV/NH₂Cl.