| Antibiotics are often used to treat animal diseases caused by bacteria and other pathogenic microorganisms'infection and safegauard the growth of livestock and poultry.Because antibiotics cannot be fully metabolized by animals,a large number of antibiotics will be excreted following urine and manure.As a kind of important agricultural resources,livestock manures were usually used as organic fertilizer,but antibiotics were also brought in soil environment by fertilizer application.Antibiotics in soil environment may migrate to water environment under rainwater leaching,contaminating the surface water and groundwater.They will be absorbed and accumulated by crops and enter the food chain,affecting human health.Microorganisms exposed in antibiotics for a long time may produce anbiotic resistant genes and resistant bacteria,causing the potential risks to ecological and environment human health.The antibiotics pollution and its risks have become a significant environment scientific issue around the world.Fluoroquinolones antibiotics(FQs)are synthetic broad-spectrum antibiotics with strong antibacterial effect.They are widely used in animal husbandry and caused the accumulation of FQs in soil environment.Because of the difference of soil properties and environmental conditions,the environmental behaviors of FQs in soil were varied in different soil types and regions.Four commonly used FQs:ciprofloxacin(CIP),enrofloxacin(ENR),norfloxacin(NOR)and ofloxacin(OFL)were studied as the target compounds in this paper.The wastewater and manure samples from animal farms and soil samples from vegetable farms in Chongqing which usually fertilized by using manure for a long time were collected to monitor the FQs concentration and evaluate its ecology and human health risks.In the laboratory,the mechanism and influencing factors of sorption and degradation of FQs in purple soil widely distributed in Chongqing were studied.The leaching behavior of FQs in purple soil was also studied by soil column leaching test.The specific conclusions were as follows:(1)The wastewater and mannnure samples from pig farms(TN-P,DZ-P)and chicken farm(CS-C)were collected to determine FQs in a year.The FQs concentration in untreated wastewaters were varied from not detectable to 228 ng/L,and the detection frequency was from 68.2%to 90.9%.The FQs concentration in wastewater from chicken farm was lower than that from pig farms.In summer,the FQs concentration of pig wastewater were varied from 96.1 to 135 ng/L,higher than that in other seasons.This phenomenon may be related to the large use of FQs in spring and summer,which had the high incidence season of bacterial infection.The removal rate of FQs in all wastewater treatment system was more than 80%.However,the wastewater treatment system with multi-stage A/O process in TN-P had a higher FQs removal rate than that in DZ-P and CS-C.After alternating anaerobic treatment and aerobic treatment,sludge adsorption and aerobic microbial degradation could remove FQs in wastewater more effectively.The FQs detection frequency of manure from pig farms and a chicken farm was 100%,and the total FQs concentration of chicken manure(388.5?g/kg)was higher than that of pig manure(145.2?g/kg).After composting 15?20 days,43.6%?79.6%of FQs in manure was removed,which can reduce FQs in soil environment by organic fertilizer application.(2)The FQs in soils from vegetable cultivation areas using manure as fertilizer showed high concentration and detection frequency,which threatened to the soil ecologoly and human health.The FQs detection frequency of vegetable farm soil was100%.The detection frequency and average concentration of ENR were higher than those of the other three antibiotics.There are 15.9%of soil samples with total concentration of higher than the ecotoxic effect trigger value(100?g/kg),proposed by the Veterinary International Coordination.Due to the amount difference of organic fertilizer in different planting modes,the FQs concentration in soil of intensive vegetable planting area were higher than that of traditional planting.Vegetable types could affect the amount of fertilizer,soil microorganisms and the FQs bio-accumulation,so FQs concentration in soil from leafy vegetables,rhizomes and eggplant fruit vegetable planting areas were different.The concentrations of four antibiotics in soil in summer were lower than that in winter soil,which may be related to a large amount of fertilization in spring and winter and the change of soil microbial activity in different temperature.The ecological risk of FQs in farmland soil was evaluated.The ecological risk of CIP in some soil samples was in high level,and that of ENR and NOR are mostly in medium level.OFL had the lowest ecological risk level.The non carcinogenic hazard quotient of FQs in vegetable farm soils for adults was less than 1 through eating vegetables,respiratory intake and oral intake,meaning human health risk was acceptable.In addition,FQs in agricultural soil could also enter the surrounding water through soil erosion,but the predicted FQs concentrations in water environment were very low.(3)The adsorption kinetics of FQs in purple soil and the effects of environmental conditions and soil organic matters on the adsorption process were explored.The mechanism of FQs sorption in purple soil was analyzed by FTIR and XRD.Sorption process of FQs in purple soil achieved equilibrium within 6 h,and more than 95%of FQs was adsorbed by purple soil.Because of the difference of sorption sites and functional groups exposed on soil particle surface,the fast and slow sorption process were observed.Two compartments first order model had a good fit on FQs sorption in purple soil.The sorption coefficients(K_d)of CIP,ENR,NOR and OFL were 2566 L/kg,1685L/kg,3876L/kg and 1250 L/kg,respectively.The adsorption isotherm conformed to the Freundlich isotherm adsorption model,and the correlation coefficient R~2 was 0.987?0.992.The sorption free energy of FQs was ranging from 8.08 k J/mol to 8.61 k J/mol,indicating that the sorption mechanism of FQs in purple soil might be dominated by cation exchange.Futhermore,hydroxyl group in FQs molecules reacted with soil particle surface was found by FTIR.The differences of mineral composition,CEC and organic matter resulted a lower sorption coefficient of FQs in purple soil than that in red soil and black soil,and the adsorption mechanism is also different.Due to the ionization of carboxyl group and piperazine group in FQs,the adsorption of FQs by purple soil was strongly affected by the pH value of the solution.With the increase of ionic strength in solution,the equilibrium adsorption capacity of CIP,ENR,NOR and OFL on purple soil decreased obviously.Compared with raw purple soil,adsorption capacity and K_d of FQs were slightly decreased in purple soil without soil organic matters.The hysteresis coefficients of FQs desorption in purple soil were ranging from 0.49?0.61.The strong sorption and positive hysteresis of FQs in purple soil indicated that FQs could accumulate in surface soil and be hard for migration.(4)The degradation kinetics of FQs in purple soil under different environmental conditions and the changes of soil microbial community structure during the degradation process of FQs were investigated.The half-lives of CIP,ENR,NOR and OFL in purple soil were 14.8 d?36.9 d.Microbial degradation made main contribution to FQs degradation in purple soil.The initial concentration of FQs increased from 500?g/kg to5000?g/kg,and its degradation half-life increased by 1.9 to 2.8 times.The activity of soil microorganisms could be inhabited by low soil mosture and temperature,resulting in low degradation rate of FQs.Compared with the surface soil,the number of active microorganisms in deep soil was only 20?30%of that in surface soil,resulting in a long half-life of FQs in deep soil.After incubating the purple soil samples containing FQs for20 days,the richness and uniformity of the microbial community decreased,and the bacterial community structure in the soil also changed.Compared with raw purple soil,the relative abundance of gram negative-bacterias,such as thiobacillus,acinetobacter and thermomonas decreased 60%,and the relative abundance of bacillus increased obviously under FQs addition.Although FQs in purple soil can be continuously degraded by microorganisms,its degradation rate would be decrased under the condition of low temperature and low soil moisture,and organic fertilizer was used many times in agricultural production.Those factors may lead to the long-term existence of FQs in agricultural purple soil.(5)Based on the sorption-desorption and degradation process,the migration ability of FQs in purple soil and the influence of pH and initial concentration on the migration process were investigated by leaching experiment.According to Cohen and Gustafson models,CIP,ENR,NOR and OFL had low leaching potential in purple soil.FQs was difficult to migrate to deep soil in continuous leaching of simulated rainoff.No FQs was detected in leachate,indicating the low risk of FQs to groundwater.The initial FQs concentration had little effect on its migration in purple soil.Although the high initial concentration condition provided more FQs to migrate into deep soil under continuous leaching.But most of FQs were still accumulated in the surface layer of 0?20 cm soil,and the concentration of FQs had not changed in the deep layer(30?50 cm),which was mainly related to the strong adsorption of FQs by soil.Compared with normal rainoff(pH=6),FQs concentration was increased in 0?30 cm soil layer after acid rain(pH=4)leaching.The pH value change of soil after acid rain leaching changed the original microbial community structure and enzyme activity in the soil,resulting that the degradation rate of FQs in the soil column was decreased and promoted more FQs to migrate to deep soil.In conclusion,the migration ability of FQs in purple soil through irrigation and rainwater leaching was weak,caused little threat to groundwater environment. |
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