Sorption of Ciprofloxacin and Oxytetracycline Zwitterions to Soils and Soil Minerals: Influence of Compound Structure ANTHONY J. CARRASQUILLO, † GREGORY L. BRULAND, ‡ ALLISON A. MACKAY, § AND DHARNI VASUDEVAN* ,† Department of Chemistry, Bowdoin College, Brunswick, Maine, Department of Natural Resources and Environmental Management, University of Hawai’i, Ma ˜noa, Honolulu, Hawaii, and Department of Civil and Environmental Engineering, University of Connecticut, Storrs, Connecticut Received May 20, 2008. Revised manuscript received July 22, 2008. Accepted July 24, 2008. 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 extent than 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. Introduction Study of the fate and transport of veterinary antibiotics has been motivated by several factors, including the high volume of veterinary antibiotic sales in the United States and Europe (1), the large extent (up to 72%) of excretion of unmetabolized antibiotics (2), the detections of veterinary antibiotics at subinhibitory concentrations in soils, surface waters, and ground waters in the U.S. and Europe (3, 4), and the global increase in antibiotic resistant strains of microbes (5). The potential for antibiotic sorption and desorption within soil systems plays a key role in their environmental fate. Current understanding of fluoroquinolone and tetracy- cline sorption to soils and pure phase minerals is relatively well advanced, and similarities in the sorption behavior of these compounds are well understood. For example, both compound classes exist as cations, zwitterions, and anions at environmentally relevant pH values (pH 4-8) (Figure S1, Supporting Information) and display pH dependent sorption to soils (6-8). In addition, the extent of fluoroquinolone and tetracycline sorption to soils was found to be strongly influenced by the soils’ cation exchange capacity (CEC) (6, 8, 9) and, to some extent, by the soils’ metal oxide content (8, 9). Studies of sorption onto model soil components (such as metal oxides and aluminosilicate clays) have revealed similarities in the mechanisms of tetracycline and fluoro- quinolone sorption; both compound classes were found to sorb onto aluminosilicate clays via cation exchange and cation bridging (electrostatic attraction or complexation of the anionic moiety on the antibiotic zwitterion to exchangeable cations) (10-13) and to participate in surface complexation or ligand exchange with metal oxides via the bonding of carboxyl or hydroxyl groups to surficial metal ions (7, 14-16). Comparison of two independent studies of oxytetracycline (a high-use tetracycline) and ciprofloxacin (a high-use fluoroquinolone) zwitterion sorption onto the same soils pointed to a large number of soils exhibiting greater extent of oxytetracycline sorption and a smaller group of soils exhibiting greater ciprofloxacin sorption (8, 9). This cursory analysis led to our hypothesis that differences in the extents of fluoroquinolone and tetracycline sorption to the same soil matrix result from key differences in compound structure and availability of surface sites for sorption via a particular mechanism. Our goal was to determine structural criteria responsible for differences in oxytetracycline and ciprofloxacin zwitterion sorption to soils and soil minerals. Objectives included understanding the influence of cationic amine group struc- ture on the extent of cation exchange to aluminosilicates and the effect of ligand group (e.g., carboxyl, hydroxyl) identity and positioning on the extent of complexation to surface- bound metal ions. Experimental Section To achieve our objectives, earlier independent measurements of ciprofloxacin and oxytetracycline zwitterion sorption to the same set of 30 soils (8, 9) were reanalyzed to identify the differences in the soil factors influencing the extents of ciprofloxacin and oxytetracycline sorption and to hypothesize preferred sorption mechanisms. Subsequently, specific struc- tural factors, such as electronic, delocalization, proximity, solvation, and steric effects, were evaluated by determining the extent of ciprofloxacin and oxytetracycline sorption to pure phase minerals (montmorillonite, kaolinite, and goet- hite) and sorption of structurally similar compounds (related in structure to ciprofloxacin and oxytetracycline) to mont- morillonite using methods detailed below. Materials. Detailed information on the sorbents (kaolin, Ca-montmorillonite, and goethite), sorbates (ciprofloxacin, oxytetracycline, and structurally similar compounds), and rationale for compound selection are found in the Supporting Information. Chemical structures and abbreviations for all compounds are found in Figure S1. All solutions were prepared in deionized water (18 MΩ), and all glassware was acid-washed prior to use. * Corresponding author phone: (207)725-3548; fax: (207)725-3017; e-mail: dvasudev@bowdoin.edu. † Bowdoin College. ‡ University of Hawai’i. § University of Connecticut. Environ. Sci. Technol. 2008, 42, 7634–7642 7634 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 42, NO. 20, 2008 10.1021/es801277y CCC: $40.75  2008 American Chemical Society Published on Web 09/05/2008