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N-nitrosodimethylamine (NDMA) is an emerging disinfection by-product which is formed during water disinfection in the presence of amine-based precursors.Ranitidine,as one kind of amine-based pharmaceuticals,has been identified as NDMA precursor with high NDMA molar conversion during chloramination.This study focused on the characterization of NDMA formation during ozonation of ranitidine.Influences of operational variables (ozone dose,pH value) and water matrix on NDMA generation as well as ranitidine degradation were evaluated.The results indicate high reactivity of ranitidine with ozone.Dimethylamine (DMA) and NDMA were generated due to ranitidine oxidation.High pH value caused more NDMA accumulation.NDMA formation was inhibited under acid conditions (pH ＜ 5) mainly due to the protonation of amines.Water matrix such as HCO3-and humic acid impacted NDMA generation due to.OH scavenging.Compared with.OH,ozone molecules dominated the productions of DMA and NDMA.However,.OH was a critical factor in NDMA degradation.Transformation products of ranitidine during ozonation were identified using gas chromatography-mass spectrometry.Among these products,just DMA and N,N-dimethylformamide could contribute to NDMA formation due to the DMA group in the molecular structures.The NDMA formation pathway from ranitidine ozonation was also proposed.