Adsorption Of Oxytetracycline On The Different Fractions Of Soil Organic Matter

In recent years,the potential harm to the environment from the abuses of antibiotics has become a research focus.The adsorption function of soil is an important factor to affect the migration of antibiotics in the environment,and organic matter in the soil plays a key role in the adsorption of pollutants.However,researches on the adsorption characteristics of contamination on the different fractions of organic matter are still inadequate.Therefore,the further study on adsorption of antibiotics onto the different fractions of soil organic matter is important for understanding comprehensively the migration behavior of antibiotics in the environment.In this paper,humus soil(from plant residues in forest soils)and sedimentary soil(from the soil along the Weihe River)were selected as the original samples(OS)due to their large differences in organic matter,and were fractionated by water extraction,demineralization,organic solvent extraction and acid hydrolysis.Then the different fractions were obtained,namely,removal of dissolved organic matter(RDOM),removal of minerals(RM),removal of free fat(RLF)and nonhydrolyzable organic carbon(NHC).Using widely used oxytetracycline(OTC)as the target antibiotic,the kinetics,thermodynamics and isothermal adsorption experiments of OTC on each fraction were carried out respectively,and the structure of different fractions were characterized using elemental analysis,infrared spectroscopy and scanning electron microscopy.Then,based on these results,the adsorption characteristics of Oxytetracycline on the different fractions of soil organic matter were discussed,and the effect of endogenous dissolved organic matter(DOM)on OTC adsorption was explored by UV spectroscopy.The main results were summarized as follows:1.Adsorption behavior of different fractionsThe adsorption kinetics of OTC on different fractions of soil are more consistent with quasi-second order kinetic model,and the adsorption rate constant K of RM,RLF and NHC are significantly larger than that of OS and RDOM.The isothermal adsorption processes of OTC on the RM,RLF and NHC fractions can be well described by Langmuir and Freundlich models,while the HOS and HRDOM fractions of humus soil fit the Freundlich model better,and the SOS and SRDOM of sedimentary soil both follow the Langmuir model and Freundlich model.The adsorption capacity of each fraction for humus soil is in the following order: HNHC?HOS>HRDOM>HRLF>HRM,but for sedimentary soil: SNHC?SRDOM>SOS>SRLF>SRM.In addition,the thermodynamic results show that the adsorption of OTC on different fractions are spontaneous,and are mainly physical adsorption.2.Adsorption mechanism of different fractionsElemental analysis showed that the aromaticity and polarity of each fraction were significantly different.Although both aromaticity and polarity affect the adsorption capacity,the changes of aromaticity play a leading role.There is a good positive correlation between adsorption coefficient and aromaticity for the fractions(RM,RLF and NHC)which compositions are mainly of organic matter.The NHC has the maximum aromaticity,so the adsorption capacity of it is the largest.However,the change of adsorption capacity is related not only to aromaticity but also to the content and composition of inorganic mineral for the OS and RDOM comprised mostly of inorganic mineral.The infrared spectra revealed that there are some differences in the functional groups and radical groups of all fractions.And after adsorbing OTC,the peak position and vibration intensity of some characteristic functional groups and radical groups have varying degrees of change,indicating that the intensity of actions(hydrogen bond,charge transfer and surface complexation or coordination effect)are different,resulting in different adsorption capacity.Scanning electron microscopy demonstrated that the microstructure of each fraction is different,and the adsorption is less affected by the number of pores for organic-rich fractions.For example,although RM fractions have a large number of micropore structures,the adsorption capacity is not large,because the pore size is too large,which is unfavorable for adsorption.While the surface of NHC is smooth and the number of pores is less,the adsorption capacity is larger,this may be related to the adsorption sites which formed by a lot of organic reactive groups on the surface of NHC.3.Effect of endogenous DOM on OTC adsorptionIt was found by UV-visible spectroscopy that the endogenous dissolved organic matter derived from humus soil(HDOM)and sedimentary soils(SDOM)are different in composition,functional group type and structure.The aromatic ring substituents of HDOMhave many carbonyl,carboxyl,hydroxyl and so on,which is easy to react with OTC,promoting the adsorption of OTC on humus soil.However,the aromatic ring substituents of SDOM are mainly aliphatic chain,which is unfavorable to react with OTC.And SDOM occupies the adsorption sites,inhibiting the adsorption of OTC on sedimentary soils.