Structure-dependent degradation of nitroimidazoles by cobalt-manganese layered double hydroxide catalyzed peroxymonosulfate process.

The increasing concentration of nitroimidazoles antibiotics (NIs) in the water environment has great threat to human and ecosystem security. Herein, the degradation rates of four NIs were found to vary with their molecular structures using Co3Mn-layered double hydroxide (LDH) catalyzed peroxymonosulfate oxidation process. Specifically, the degradation efficiency of secnidazole (SNZ) was determined to be the highest with a reaction rate of 0.24 min-1, which was 3.6, 2.3 and 1.8 times to that of menidazole (MZ), metronidazole (MTZ) and ornidazole (ONZ), respectively. During the reaction, 8.3% of Co2+ and 8.4% of Mn3+ transformed to Co3+ and Mn4+ after reaction, respectively. The conversion of bimetallic valence in Co3Mn-LDH donated electrons (e-) for PMS activation, resulting in the production of 1O2, OH, SO4- and O2-. Density functional theory (DFT) calculation showed that the presence of electron-donating groups (-CH3 and -OH) and the absence of electron-withdrawing atom (Cl) leaded to the richest active sites in the molecular structure of SNZ, which thus contributed to the highest degradation efficiency of SNZ. By deducing the structure-dependent degradation pathways of four NIs, the carbon chain of SNZ was found to be more easily attacked to form MTZ and MZ because of its unique active sites, resulting in the faster degradation rate of SNZ than MTZ and MZ. This study may provide a valuable insight into the effects of molecular structures on the degradation rates and transformation pathways of NIs.