Distribution Characteristics And Hydrolysis Mechanism Of Antibiotics In Interaction Hyporheic Zone

The river-groundwater hyporheic zone is an area with physical,chemical and biological gradients where material and energy are exchanged between surface water and groundwater.It is not only an important place for river ecological hydrological processes,but also an important place for pollutant migration,transformation and accumulation.Antibiotics is a new type of organic pollutant,which migrates through hyporheic zone with recycled water.This process poses a threat to groundwater safety.Thus,it is necessary to correctly understand distribution characteristics and environmental behavior of antibiotics in hyporheic zone.This provides a new understanding of interception mechanism and technical support of ecological risk assessment in the river-groundwater hyporheic zone.In this study,we choose Oxytetracycline(OTC),Sulfamethoxazole(SMZ)and Norfloxacin(NOR)as the target antibiotics.The distribution and environmental behavior of OTC,SMZ and NOR in the Zaohe River and Weihe River hyporheic zone were studied.The temporal and spatial distribution characteristics of antibiotics in hyporheic zone were analyzed by field continuous monitoring in three periods.Focusing on the hydrolysis of antibiotics,explored the characteristics of antibiotic hydrolysis behavior under the condition of aerobic and anaerobic.Combining chemical calculations and high-performance liquid chromatograpHy-mass spectrometry methods,analyzed hydrolytic products and pathways to reveal the hydrolytic mechanism of antibiotics.The following understandings have been obtained:1、Distribution characteristics and risk assessment of hyporheic zone antibiotics shows that:Between 2017 and 2019,the average concentration of OTC,SMZ and NOR were:1.55μg/L,0.86ng/L,104.38ng/L in hyporheic zone water.In the sediment,the average concentration were:0.724mg/L,0.118mg/L,0.003mg/L.Overall,OTC and NOR pollution is more serious than NOR.From the perspective of spatial distribution,concentrations of antibiotics were declined gradually along the river.The groundwater of perpendicular to the river,the further the distance from the river bank,the lower the concentration of antibiotics.From the perspective of time distribution,the pollution level of OTC decreased,while NOR and SMZ increased year by year.The concentrations of the three antibiotics in the sediments all showed a decreasing trend year by year.Correlation analysis shows that OTC and NOR may be homologous.Antibiotics will migrate and transform each other in surface water and groundwater.Risk assessment shows that:OTC and NOR are the major contributors to water risk level.In sediments,all of antibiotics were showed low risk.In the water,OTC,SMZ and NOR had a low level of non-carcinogenic risk to the exposed population.2、The hydrolysis behavior process of antibiotics shows that:In aerobic,the hydrolysis half-life of OTC was 15.90d,SMZ was 35.95d and NOR was 13.70d.In anaerobic,the hydrolysis half-life of OTC was 68.49d,SMZ was 76.25d,and NOR was 61.90d.Whether in aerobic or anaerobic,the order of hydrolysis rate was:NOR>OTC>SMZ.The presence of dissolved oxygen can produce oxygen free radicals.It not only accelerates the rate of antibiotic hydrolysis,but also the antibiotics hydrolysis is more complete.Hydroxyl radical(OH·)is the main active substance in the hydrolysis of OTC and SMZ.Superoxide radical(O2·)is the main active substance in the hydrolysis of NOR.In addition to OH· and O2·,there may be other active species.In aerobic,coexisting ions will inhibit the hydrolysis of antibiotics.In anaerobic,the situation is the opposite.In both aerobic and anaerobic,the hydrolysis rate of the three antibiotics under neutral and alkaline conditions is faster than that of acidic conditions.3、The hydrolysis products and the hydrolysis path of antibiotics show that:In aerobic,m/z452,m/z435,and m/z431 were the main hydrolysates of OTC.Epimerization,decarburization,ring cracking,and demethylation were the main hydrolysis pathways.In anaerobic,m/z433 and m/z461 were the main products.Epimerization and demethylation were the main hydrolysis pathways.In aerobic,m/z288,m/z274,m/z300,m/z519 were the main hydrolysates of SMZ.Oxidation of amino groups,hydroxylation and polymerization were the main hydrolytic pathways.The main hydrolysates of SMZ were m/z 519,m/z 288 and m/z 274 in anaerobic.Hydroxylation and polymerization were the main hydrolytic pathways.Whether in aerobic or anaerobic,m/z279 was the main hydrolysates of NOR.The m/z 279 would continue degrade to m/z 226 in the aerobic.Piperazine ring cracking and quinolone ring decomposition were the main hydrolytic pathways in aerobic.Piperazine ring cracking was the main hydrolytic pathway of NOR in anaerobic.