Monochloramination of oxytetracycline: Kinetics, mechanisms, pathways, and disinfection by-products formation

This study investigated the degradation kinetics, formation of disinfection by-products (DBPs), and degradation pathways during monochloramination of oxytetracycline (OTC). The degradation kinetics can be well described by a second-order kinetic model, first-order in monochloramine (NH₂Cl), and first-order in OTC. Reaction of OTC with NH₂Cl shows a high reactivity, with the apparent rate constant of 17.64/M/s at pH 7. The apparent rate constant declined as pH increased from 5 to 10. Six DBPs were detected during monochloramination of OTC, including chloroform (CF), 1,1-dichloro-2-propanone (1,1-DCP), 1,1,1-trichloro-2-propanone (1,1,1-TCP), dichloroacetonitrile (DCAN), trichloronitromethane (TCNM), and N-nitrosodimethlyamine (NDMA). CF, DCAN and NDMA had the maximum yields at neutral pH, while 1,1-DCP and 1,1,1-TCP had the maximum yields at pH 4. However, TCNM concentration increased as pH increased. Degradation pathways of OTC monochloramination were then proposed. Hydroxylation and Cl-substitution are found to be the dominant mechanisms in monochloramination of OTC. Two dominant mechanisms in oxytetracycline monochloramination were identified as hydroxylation and Cl-substitution. The formation of N-nitrosodimethlyamine and other disinfection by-products is due to the loss of dimethylamino structure and the formation of DMA or CDMA during the reaction.