Molecular Mechanisms Of Oxytetracycline Sorption On Goethite, Hematite, and Magnetite: In Situ ATR-FTIR Study.

Oxytetracycline (OTC) is an important veterinary antibiotic, which is widely utilized in livestock production, mainly in concentrated animal feeding operations (CAFO), as a therapeutic agent and growth promoter.  Due to frequent application of heavy doses of this antibiotic and low absorption in animal gut, a large unmetabolized portion is excreted through urine and feces, which in turn end up in agricultural land via application of manure.  Fate of antibiotic in the environment largely depends on their retention in soil particles.  Recent studies highlighted the importance of iron oxides in adsorbing antibiotics in the soil environment. However, the study of molecular level interactions using in-situ spectroscopic techniques to decipher the binding mechanisms is lacking.  Here we report in-situ ATR-FTIR study of OTC adsorption on goethite, hematite, and magnetite.  Binding mechanisms of OTC on goethite, hematite, and magnetite have been studied as a function of pH (3-9), and aqueous OTC concentrations (5-200 µM).  Spectra collected at different loadings of OTC (5-200 µM) indicated contrasting results for goethite and hematite compared to magnetite.  Our study indicates that  loading affected surface complexation mechanisms for goethite and hematite.  Currently, more experiments are underway to understand the contrasting OTC binding mechanisms on these iron oxides.