Behaviors Of Antibiotics And Nitrosodimethylamine During Chlorine Dioxide Disinfection

Chlorine dioxide (ClO₂) is widely used for the disinfection of relatively high quality water like groundwater or used as preoxidation of surface water before chlorination or monochloramination for organic pollutants reduction. In recent years, Antibiotics as emerging pollutants posed a threat to drinking water safety, since it had been frequently detected in drinking water. Because of the poor oxidativity of monochloramine, ClO₂ become critical for antibiotics removal in the whole disinfection process. Furthermore, NDMA as a member of nitrosamines, a family of high hazardous disinfection by-products (DBPs) has been related to traditional disinfection processes. As preoxidant, ClO₂ can oxidate DMA, but generate NDMA. Therefore, it is more meaningful to take the preoxidation and disinfection as a whole process for evaluating the effect of ClO₂ preoxidation on NDMA formation.In current thesis, the behaviors of FQs, TCs and TMP antibiotics during ClO₂ oxidation are well investigated.The result shows that reaction kinetics are all highly pH-dependentand can be well described by a second-order kinetic model incorporating speciation of antibiotics. The specific second-order rate constant ofanionic FQs (k_tCs^- ) are from 2.44×10¹ M^-1s^-1 to 5.44×10² M^-1s^-1, and follow the trend of OFL > ENR > CIP NOR LOM >> PIP in reactivity.FQs'piperazine ring is the primary reactive center toward ClO₂. ClO₂ likely attacks FQ's piperazinyl N4 atom, leading to dealkylation, hydroxylation and intramolecular ring closure at the piperazine moiety. Overall, transformation of FQs during ClO₂ oxidation may not eliminate an tibacterial activity because of little destructionat the quinolone ring.The specific second-order rate constant of anionic (k_tCs^- ) and dianionic (k_tCs^-) TCs are from 3.05×10⁵ M^-1s^-1 to 7.15×10⁶ M^-1s^-1 and from 1.63×10⁷ M^-1s^-1 to 2.72×10⁷ M^-1s^-1. The oxidation of TCs by ClO₂ leads to (hydr)oxylation and breakage of TC molecules. Transformation of TCs may likely eliminate antibacterial activity because of the destruction of the phenonic-diketone group and the ring system.The specific second-order rate constant of cationic, netural and anionic TMP are 3.76 M^-1s^-1, 7.00 M^-1s^-1 and 31.4 M^-1s^-1. TMP reacts withClO₂ primarily via its 3,4,5-trimethoxybenzyl moiety at acidic pH, and with its 2,4-diaminopyrimidinyl moiety at circumneutral and alkaline pH. The oxidation of TMP by ClO₂ leads to amino group oxidation, (hydr)oxylation and (multi)chlorination. Transformation of TMP may also eliminate antibacterial activity because of the oxidation and chlorination of 2,4-diaminopyrimidinyl moiety.Secondly, The NDMA formation from DMA during ClO₂ oxidation are investigated.The results shows that the NDMA formation rate are much higherat the beginning of the oxidation and post-oxidation, which are 3.17 nmol/h and 0.56 nmol/h ([DMA]0 200?M, [ClO₂]0 200?M, pH 7.5). NDMA formation can be reduced by decreasing the initial concentration of DMA and ClO₂ synchronously or increasing the ratio of [DMA]0/[ClO2]0, while the effect of pH is insignificant. Comparison between the NDMA formation of FCNP and DMA/ClO₂ reaction indicates that the FCNP can not be the only NDMA formation pathway from DMA during ClO₂ oxidation. With 10mins(60mins) ClO₂-preoxidation, NDMA formation during monochloramination are reduced to 88.3% and 69.8%.