Study On The Transformation Mechanism And Disinfection By-products Of Fluoroquinolones During The Chlorination Of Water Containing Halide Ions

In the process of water disinfection,the disinfectant will react with organic matter in the water to generate toxic and harmful disinfection by-products,which has aroused widespread concern.The residual fluoroquinolones?FQs?in water can be also an organic precursor to react with disinfectants to form particular halogenated DBPs during the chlorinated disinfection process.The presence of halogen ions?i.e.,Cl?,Br?,I??in seawater will result in the generation of more toxic brominated/iodine by-products?Br/I-DBPs?.In this paper,based on a large amount of literature review,seven kinds of FQs reported in the marine aquaculture were selected as target compounds to carry out our research.The reaction kinetics and transformation mechanism of FQs with sodium hypochlorite?Na Cl O?in the presence of bromine ions and iodine ion were respectively investigate.Combined with the calculation of biological toxicity parameters and the detection of adsorbed organic halogen?AOX?in water,the toxicity of transformed products of FQs was evaluated preliminarily.The research results are summarized as follows:?1?In order to infer the relationship between compound structure and kinetic stability and the active site of chlorination reaction,the frontier orbital and charge distributions of FQs?i.e.,Ofloxacin?OFL?,Enrofloxacin?ENR?,Lomefloxacin?LOM?,Norfloxacin?NOR?,Ciprofloxacin?CIP?,Piperazine Acid?PIP?,Flumequine?FLU??were analyzed by density functional theory calculation.The results indicated that the piperazine ring was the electron-donating group and could be the optimum reaction center for electrophiles,and the quinolone group was easily attacked by nucleophiles.The N4 atom on the piperazine ring had the highest value of negative charges and would be the main reaction site for electrophilic reactions.According to the calculation results of each energy band gap(E_HOMO-E_LUMO),OFL had the smallest HOMO-LUMO gap and was easy to be activated relative to other six FQs.?2?In the bromide-containing synthetic water,the reaction rate constants were PIP<NOR?CIP<LOM<ENR<OFL,while FLU did not exhibit reactivity.Cl?and Br?as well as SO₄²?promoted the oxidation of FQs,but HA significantly inhibited the degradation of FQs.Eight emerging brominated DBPs?Br-DBPs?derived from FQs were detected in the bromide-containing synthetic water during chlorination.Their formation involved ring-opening/closure,dealkylation,decarboxylation and halogenation.Chlorine substitution occurred on the ortho-position of FQs's N4 and bromine substitution occurred on C8 position.According to the calculation results by EPI-Suite,the log Kow and log BAF values of Br-DBPs were larger than that of chlorinated analogues.?3?In the iodine-containing synthetic water,the reaction rate decreased in the following order:seawater>marine aquaculture water>fresh water.The results demonstrated that the enhancement of NOR oxidation in the presence of iodide ions.Four emerging kinds of iodinated DBPs?I-DBPs?were identified in iodine-containing marine aquaculture water during chlorination,and their formation mechanism was similar to that of Br-DBPs caused by FQs.Iodine substitutions occurred at the C3 and C8positions of NOR.The amount of AOX also increased with the increase in iodide concentration.The analysis results of toxicity and AOX indicated that the more iodo-groups the compound contains,the easier it is to accumulate in the organism.I-DBPs were more toxic than their brominated and chlorinated analogues as well as parent compound.In conclusion,the results provided theoretical basis for the rational use and risk management of antibiotics as well as disinfectants in marine aquaculture.