Veterinary antibiotics such as oxytetracycline (OTC) increasingly are found in the environment and often come into direct contact with soils via the release of animal wastes. Oxytetracycline is known to sorb strongly to soils by interaction with soil organic matter, clay minerals, and metal oxides. However, current knowledge of the influence of soil properties on OTC sorption is limited, as is our ability to predict OTC sorption to soils. This work was aimed at identifying properties that most influence the extent of OTC sorption in a suite of soils from the eastern United States representing a wide range in soil properties. Thirty soils were well characterized, an OTC soil-water distribution coefficient (Kd) was determined for each soil, and statistical analyses were employed to determine appropriate soil descriptors of OTC sorption. Soil texture, cation exchange capacity, and iron oxide content seemed to most influence the extent of OTC sorption in soils with organic carbon (OC) content between 0 and 4%. Thus, the knowledge of these three soil properties would be key to anticipating the extent of OTC sorption and gaining insight into OTC fate within a given soil system. Notably, OC content appeared to influence OTC sorption only in a soil with 9% OC.
Oxytetracycline (OTC) zwitterions sorbed to a greater extent than ciprofloxacin (CIP) zwitterions onto goethite and soils with moderate-to-low effective cation exchange capacities (ECEC < 10 cmol(c)/kg) because adjacent pairs of hydroxyl groups on the OTC molecule (absent in CIP) facilitated greater surface complexation to soil metal oxides and aluminosilicate edge sites. CIP sorbed to a higher extentthan OTC onto aluminosilicates and onto soils with "high" ECEC values (>10 cmol(c)/kg). The sorption of heterocyclic compounds structurally similar to CIP indicated that both positive charge localization on the cationic amine and the extent of charge delocalization to the heterocyclic ring influenced molecular orientation within the montmorillonite interlayers, van der Waals interactions, and the potential for sorption. The sorption of compounds structurally similar to OTC revealed that greater positive charge localization on the cationic amine facilitated sorption to montmorillonite, whereas ortho substituted anionic and cationic groups on a zwitterionic molecule resulted in unfavorable Coulombic interactions between the anionic moiety and the negatively charged surface and hindered sorption. Thus, greater CIP zwitterion sorption to aluminosilicates and "high" ECEC soils resulted from greater distance between the anionic and cationic groups, which maximized Coulombic attraction to the surface.
The extent of adsorption and the value of the adsorption equilibrium constant (K s intr ) for simple organic ligands is influenced by the identity of the ligand donor groups and other substituents on the aromatic ring. Catechols adsorb onto TiO 2 to a significantly greater extent than 2-aminophenols; the adsorption of 1,2phenylenediamines is negligible. The TiO 2 surface has a high ionic contribution to bonding; ligands possessing donor groups with the highest ionic contribution to bonding adsorb to the greatest extent. A covalent contribution to bonding can increase binding to Ti(IV)-containing surfaces, but only when the ionic contribution is already strong. Within each ligand class, substituents alter the competition between protons and surface sites for binding the deprotonated ligand. For this reason, pK a1 , pK a2 , and log K s intr are all important in determining the extent of adsorption. Additionally, substituents that impart hydrophobicity also raise the extent of adsorption and the value of log K s intr .
We investigated ciprofloxacin (a fluoroquinolone antibiotic) speciation as a function of pH in aqueous solution and in the presence of dissolved ferric ions and goethite using ATR-FTIR and UV-vis spectroscopy. The presence of dissolved and surface bound ferric species induced the deprotonation of the ciprofloxacin carboxylic acid group at pH < pKa1. The resultant ciprofloxacin zwitterions appeared to interact via both carboxylate oxygens to form bidentate chelate and bridging bidentate complexes within colloidal iron oxide-ciprofloxacin precipitates and bidentate chelates on the goethite surface. However, the structure of the aqueous ferric-ciprofloxacin complexes remains unclear. Our evidence for bidentate chelates (involving only the carboxylate oxygens) on the goethite surface was distinct from previous IR studies of fluoroquinolone sorption to metal oxides that have proposed surface complexes involving both the keto and the carboxylate groups. We find that the distinct ciprofloxacin surface complex proposed at the goethite-water interface may be a result of differences in metal oxide mineralogy or assignment of the carboxylate antisymmetric stretch in the metal oxide-fluoroquinolone spectra.
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