Effects Of Fluorine Substituents On The Partition Of Florfenicol And Norfloxacin In An Ice-water System

Fluorinated antibiotics have been widely used in recent years due to their high efficiency and stability.However,they were frequently detected in the aquatic environment due to their strong anti-metabolism and refractories,which could easily cause harm to the aquatic ecosystem.Most rivers,lakes,reservoirs,and other surface water bodies in mid-latitude and high-latitude regions are covered by ice in winter.The seasonally frozen phenomenon of the water bodies can influence on the transport and transformation of the pollutants in them.Previous studies indicated that ice was a major sink for pollutants in winter and played an important role in their partition process.However,few studies focused on the partition of fluorinated antibiotics in ice-water systems.Therefore,florfenicol（FFC）and norfloxacin（NFX）were selected as the models of fluorinated antibiotics in this study.Based on establishing a simulated ice-water partition system,the partition characteristics of FFC and NFX in the ice-water system were studied.The effects of cooling area,freezing temperature,and dissolved organic matter（DOM）on the ice-water partition characteristics were also investigated.Meanwhile,thiamphenicol（TAP）and pipemidic acid（PPA）,similar to the structure of FFC and NFX except without fluorine substituents,were compared to analyze the differences in the partition characteristics.The roles of fluorine substituents in the ice-water partition of antibiotics were finally explored.The study reached the following conclusions:When the initial antibiotic concentrations increased from 10 to 90 ng·L^-1,the ice-water partition coefficient(K_IW)of FFC,TAP,NFX,and PPA decreased by 39.9%,31.2%,26.6%,and 28.2%,respectively.The order of K_IW was K_IW-FFC>K_IW-TAP,K_IW-PPA>K_IW-NFX.The K_IW values decreased with the increase of the initial concentration of antibiotics.With the cooling area doubling,the K_IW values of FFC,TAP,NFX and PPA increased by 34.8%,33.7%,33.2%,and 38.0%,respectively.The K_IW values increased with the increase of the cooling area.As the freezing temperature decreased from-10to-20^oC,the degree of undercooling and freezing rate of the solutions increased significantly,and the values of solute partition coefficient(K_bs)increased with the decrease of freezing temperature.The freezing temperature affected the amount of antibiotics entering the ice phase by affecting the freezing rate of the solution and the growth morphology of ice crystals.When DOM existed in the ice-water system,the K_IW values of FFC,TAP,NFX and PPA increased by 2.0%-8.2%,1.4%-6.2%,0.9%-3.8%,and 0.7%-3.0%,respectively.Compared with TAP and PPA,FFC and NFX contained the fluorine substituents were easier to combine with DOM.During the freezing process of the antibiotic solution,antibiotics were discharged from the ice phase and accumulated in a high-concentration zone at the ice-water interface,and then further diffused into the water phase through the high-concentration zone.Under the same freezing conditions,the concentrations of antibiotics in the two phases of ice and water were mainly controlled by their rate of diffusing into the water phase.The solubility of antibiotics controlled the diffusion rate by affecting the concentration of antibiotics in the high-concentration zone.The single fluorine substituent in FFC reduced its water solubility,which led to the rate of diffusing into water phase of FFC slower than TAP,and the amount of FFC retained in ice phase greater than TAP.PPA with a nitrogen-containing heterocycle had higher water solubility.But the solubility of NFX which replaced N on the nitrogen-containing heterocycle with a C–F bond was further increased,which led to less NFX retained in ice phase than PPA.The fluorine substituents affected the binding ability of antibiotics with DOM and the antibiotic solubility,which futher affected the partition characteristics of antibiotics in the ice-water systems.The results indicate the fluorine substituents play a key role in the partition of fluorinated antibiotics in ice-water systems.It is beneficial to support the subsequent studies on the environmental behaviors and effects of fluorinated antibiotics in the seasonally freeze-thaw waters.