Environ. Sci. Technol. 2005, 39, 9166-9173
Sorption of the Antimicrobial Ciprofloxacin To Aluminum and Iron Hydrous Oxides CHENG GU AND K. G. KARTHIKEYAN* Department of Biological Systems Engineering, Environmental Chemistry and Technology Program, 460 Henry Mall, University of Wisconsin, Madison, Wisconsin 53706
Solution chemistry (pH, ionic strength (I), and sorbate-tosorbent ratio) effects on ciprofloxacin sorption to hydrous oxides of Al (HAO) and Fe (HFO) were investigated using macroscopic and spectroscopic analyses. Sorption to both HAO and HFO showed a strong pH-dependent behavior, following the fraction of zwitterionic species over the entire pH range studied. Increase in I from 0.01 to 0.5 M had an insignificant effect on the extent of ciprofloxacin sorption, and isotherms were well-described by the Langmuir model. HFO possessed a higher sorption capacity (0.066 mmol kg-1) than HAO (0.041 mmol kg-1). Ligand-promoted dissolution of hydrous oxides, more pronounced for HAO, was observed in the presence of ciprofloxacin, but at a fairly high initial concentration (0.5 mM). Attenuated total reflectance Fourier transform infrared spectroscopy analysis indicated that different types of ciprofloxacin surface complexes are formed with HAO and HFO; while a monodentate mononuclear complex (with -COO-) appears likely between ciprofloxacin and HAO, keto O and one O from COO- seem to be involved in the formation of a sixmembered ring with Fe on the HFO surface. The study results are expected to increase our understanding of the environmental reactivity of fluoroquinolones, an important class of antimicrobial compounds.
Introduction Ciprofloxacin is a synthetic fluoroquinolone antimicrobial. As a second-generation fluoroquinolone, it is commonly used as a broad-spectrum antibacterial agent. The world annual sales of ciprofloxacin were over $1 billion by the end of 1990s (1). Like other antimicrobials, ciprofloxacin can leave target organisms unaltered and reach different water and soil environments (2). In both the U.S. Geological Survey’s national (3) and our Wisconsin (4) statewide surveys, ciprofloxacin was among one of the most frequently detected antimicrobial compounds. Soil is a major repository for contaminants in the environment; however, it can act as a source of antimicrobials to the aqueous environment when surface runoff or leaching occurs (5). Hence, it is important to understand the factors controlling the surface and subsurface mobility of antimicrobials. Sorption processes, particularly, are important, since association of antimicrobials with mineral particles and organic matter will determine their transportability in surface runoff, leachability through soils, and mobility in aquifers. Compared to other well-known xenobiotics (e.g., * Corresponding author phone: (608)262-9367; fax: (608)262-1228; e-mail:
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pesticides, PAHs, PCBs), very little information is available on the environmental fate of antimicrobials. Previous studies indicate that ciprofloxacin has a high affinity for soils and mainly accumulates in the topsoil (6). The concentrations of ciprofloxacin in sewage sludge and manure treated soils ranged in the mg kg-1 levels (7, 8). However, only limited information is available on the sorption mechanisms of ciprofloxacin with different soil components. Nowara et al. (2) studied the sorption of several fluoroquinolone derivatives, including ciprofloxacin, to soils and pure clay minerals and proposed that cation bridging was the major mechanism responsible for fluoroquinolone sorption. Lutzhoft et al. (9) observed that complexation of fluoroquinolones with humic acid occurred by both hydrophobic-type partitioning and electrostatic interactions. Hydrous oxides of aluminum (Al; HAO) and iron (Fe; HFO) are important mineral components of environmental particles, and in highly weathered soils HAO/HFO can account for up to 50% of the total soil mass (10). Poorly crystalline/ amorphous Al and Fe oxides are short-range-ordered minerals, and even when not present in large quantities in soils, they are considered major “sinks” for many organic and inorganic contaminants as they possess highly reactive surfaces. In addition, these hydrous oxides are capable of forming strong surface complexes with many metals (Me) and organic ligands. Furthermore, unlike the layered-clay minerals which are primarily negatively charged due to isomorphic substitution, hydrous oxides generally are amphoteric and can develop either a positive or negative surface charge depending on soil solution pH. Hence, these minerals can exert a profound influence on the fate and behavior of contaminants in the soil environment (11, 12). To our knowledge, there have been very few investigations (e.g., 13, 14) on the sorption of fluoroquinolone antimicrobials with oxide minerals. The structure of ciprofloxacin is shown in Figure 1. Ciprofloxacin possesses a carboxylic acid group (C-3, pKa1 ) 6.1) and an amine group in the piperazine moiety (C-7, pKa2 ) 8.7), both of which confer a marked pH-dependent behavior on solubility and lipophilicity. The ionization behavior can be expected to significantly influence ciprofloxacin sorption to soil components. The pKa1 of ciprofloxacin is higher than that typically observed for carboxylic acids (pKa ≈ 2-3), attributable to the intramolecular hydrogen bonding between the carboxylic acid and the neighboring keto functional group (Figure 1a) (15). Important physicochemical properties of ciprofloxacin are the following (16, 17): MW, 331; aqueous solubility, 0.21-9.89 mM; and logKow, -1.51 to -1.13. Ciprofloxacin is capable of forming strong complexes with multivalent Me ions. The structure has been described as a bidentate complex between Me and ciprofloxacin through the keto O (C-4) and one of the O’s of the carboxylic group (C-3a) (18-21). It is fairly well-known that sorption of chelating compounds, such as ethylenediaminetetraacetic acid and salicylic acid, containing carboxylic and phenolic functional groups capable of forming stable Me complexes will significantly increase the solubility of oxide minerals through the ligand-promoted dissolution mechanism (2224). Since ciprofloxacin possesses similar functional groups, ligand-promoted dissolution can be expected to occur during its interaction with HFO and HAO. In a previous study (25), we demonstrated the occurrence of ligand (tetracycline)promoted dissolution during tetracycline sorption to HAO and HFO. Fourier transform infrared (FTIR) spectroscopy has been widely used to provide insights on sorption mechanisms of 10.1021/es051109f CCC: $30.25
2005 American Chemical Society Published on Web 10/20/2005
FIGURE 1. (a) Structure and (b) pH-dependent speciation of ciprofloxacin (CIP). various organic ligands on the surface of hydrous oxides. However, conventional FTIR spectroscopy requires sample drying, which is likely to create artifacts and distort the sorbate-sorbent complex (26). Increasingly, researchers (2629) have used the attenuated total reflectance (ATR) infrared technique, which facilitates in situ investigation of the interfacial structures at the mineral oxide-solution interface. In this study, HAO and HFO were chosen as model sorbents to evaluate the sorption behavior of ciprofloxacin as a function of solution chemistry variables (pH, ionic strength (I), sorbate-to-sorbent ratio). The interaction mechanism between ciprofloxacin and HAO/HFO was investigated using ATR-FTIR spectroscopy in aqueous environments.
Materials and Methods Materials. Details on the chemicals used and suppliers, and preparation of hydrous oxides, are included in the Supporting Information. For all macroscopic experiments, radiolabeled [2-14C]ciprofloxacin (specific activity ) 13 mCi mmol-1) was thoroughly mixed with unlabeled ciprofloxacin to provide a stock with
