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For minimizing the transport of antibiotics to groundwater, the migration of antibiotics in soils should be investigated. Soil organic matter can affect the migration of antibiotics. To date, the influence of aromatics and aliphatic content of organic matter on the adsorption of antibiotics has been controversial. To better understand the reaction mechanism of soil organic matter with antibiotics, this study investigated the adsorption of oxytetracycline (OTC) by humus soils (HOS) and their fractions. HOS were sequentially fractionated into four organic fractions, including the removal of dissolved organic matter (HRDOM), removal of minerals (HRM), removal of free fat (HRLF), and nonhydrolyzable organic carbon (HNHC). Moreover, batch experiments revealed that adsorption capacity was ordered by HNHC > HOS > HRDOM > HRLF > HRM. SEM images and N2 adsorption/desorption isotherms indicate that adsorption capacity is independent of the external structure. However, adsorption capacity is related to the internal structure and composition. Combination analysis with elemental composition and infrared spectroscopy showed that the adsorption capacity of HRM, HRLF, and HNHC had a good positive correlation with aromaticity, but a negative correlation with polarity and hydrophilicity. Additionally, the rule of binding affinity between OTC and functional groups with different properties was summarized as aromatic > polarity > hydrophilic.

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Antibiotics are widely used in medicine, agriculture and aquaculture [1][2][3]. Most antibiotics (about 25%-75%) cannot be completely metabolized and degraded by organisms, and are released into the environment in the form of parent or intermediate products [4,5]. These antibiotics will pose a great threat to the survival of humans and animals [6,7], so it is crucial to control the content of antibiotics in the environment. Antibiotics that enter the environment will be attenuated by biological and abiotic interactions [8][9][10]. Since antibiotics could inhibit or kill microorganisms at low concentrations [11][12][13], the biological attenuation effect that depends on the action of microorganisms