TECHNICAL REPORTS 822 Adsorption of antibiotics on the surfaces of common mineral sorbents plays a major role in determining their fate in soils and sediments. he mechanisms of these reactions are, therefore, important for understanding and predicting the environmental fate of antibiotics. We used in situ attenuated total relectance Fourier-transform infrared spectroscopy to elucidate the binding mechanisms of oxytetracycline (OTC) onto the surface of magnetite [Fe 3 O 4(s) ], a common Fe oxide mineral in soils and sediments, as a function of pH (3–9) and aqueous OTC concentration (5– 150 μmol L -1 ). Comparison of dissolved OTC spectra to those of OTC–magnetite surface complexes indicated strong interactions of OTC molecules with the Fe 3 O 4(s) surface via carbonyl (C=O) and amine (–NH 2 ) moieties of the amide group (–CONH 2 ) and the N atom of the dimethyl amino group [–N(CH 3 ) 2 ]. Increasing the aqueous OTC concentration led to increased OTC adsorption but did not notably alter the OTC binding mode at the magnetite surface. he results of this study would help to assess the importance of Fe oxide minerals in determining the environmental fate of OTC in soils and sediments. In Situ Attenuated Total Relectance Fourier-Transform Infrared Study of Oxytetracycline Sorption on Magnetite Sudipta Rakshit, Evert J. Elzinga, Rupali Datta, and Dibyendu Sarkar* O xytetracycline, which belongs to the tetracy- cline (TC) group of antibiotics, is widely used in live- stock production as a therapeutic agent and growth promoter (Arikan et al., 2007; Kümmerer, 2009a, 2009b). Due to low absorption in the animal gut, a large portion of antibi- otics is excreted through feces and urine (Kumar et al., 2005). Application of untreated animal manure onto agricultural land is one of the most common pathways of entry for antibiotics into the environment (Kong et al., 2012). Reported adverse efects of OTC include growth of antibiotic-resistant microbial strains, inhibition of microbial soil respiration, Fe(III) reduction, and nitriication (Halling-Sørensen, 2001; Boleas et al., 2005; hiele- Bruhn and Beck, 2005; Aristilde et al., 2010; Knapp et al., 2010). Adsorption of antibiotics on minerals in soils has been found to decrease their adverse environmental efects (Chander et al., 2005). A thorough knowledge of the eiciency and mechanisms of antibiotic retention on clay and oxide minerals is required to provide guidance on risk assessment criteria in soils and sediments (Aristilde et al., 2010). Hence, many studies have been performed to investigate the adsorption of antibiotics onto common clay and oxide minerals (Kulshrestha et al., 2004; Figueroa and MacKay, 2005; Gu and Karthikeyan, 2005; Trivedi and Vasudevan, 2007; Aristilde et al., 2010; Rakshit et al., 2010; Zhao et al., 2012). Macroscopic sorption studies have shown that Fe oxides such as goethite, hematite, and magnetite have a high ainity for OTC, exhibiting strongly pH-dependent sorption behavior (Figueroa and MacKay, 2005; Rakshit et al., 2010). For OTC adsorption on goethite and hematite, maximum adsorption occurs around pH 8, consistent with the mineral surface charge (i.e., goethite and hematite point of zero net proton charge is close to 8) and OTC speciation (-log acid dissociation constant pK a values 3.57, 7.49, and 9.44), and adsorption decreases at both lower and higher pH values, resulting in a bell-shaped pH envelope (Figueroa and MacKay, 2005). For magnetite, OTC adsorption is highest (approximately 90%) at low pH values and decreases with increasing pH >5.5 to 60% (at pH 9.4) (Supplemental Fig. S1). Macroscopic adsorption trends of OTC onto goethite and hematite surfaces have been described using surface complexation models (Figueroa and MacKay, 2005); however, studies providing direct characterization of Abbreviations: ATR, attenuated total relectance; FTIR, Fourier-transform infrared; IR, infrared; OTC, oxytetracycline. S. Rakshit and D. Sarkar, Earth and Environmental Studies Dep., Montclair State Univ., Montclair, NJ 07043; E.J. Elzinga, Dep. of Earth and Environmental Sciences, Rutgers Univ., Newark, NJ 07102; and R. Datta, Dep. of Biological Sciences, Michigan Technological Univ., 1400 Townsend Drive, Houghton, MI 49931. Copyright © American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America. 5585 Guilford Rd., Madison, WI 53711 USA. All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. J. Environ. Qual. 42:822–827 (2013) doi:10.2134/jeq2012.0412 Supplemental data ile is available online for this article. Received 25 Oct. 2012. *Corresponding author (sarkard@mail.montclair.edu). Journal of Environmental Quality ORGANIC COMPOUNDS IN THE ENVIRONMENT TECHNICAL REPORTS