Environ. Sci. Technol. 2007, 41, 1928-1933
Sorption of Tetracycline and Chlortetracycline on K- and Ca-Saturated Soil Clays, Humic Substances, and Clay-Humic Complexes J U T T A R . V . P I L S * ,† A N D DAVID A. LAIRD‡ Agronomy Department, Iowa State University, 100 Osborne Drive, Ames, Iowa 50011, and United States Department of Agriculture/Agricultural Research Service National Soil Tilth Laboratory, 2150 Pammel Drive, Ames, Iowa 50011
Tetracycline (TC) and chlortetracycline (CTC) are used extensively for growth promotion and therapeutic purposes in livestock production. The sorption of TC and CTC on clays, humic substances (HS), and clay-humic complexes (clay-HC) derived from two agricultural soils was quantified using dilute CaCl2 (Ca) and KCl (K) as background solutions. In all systems, the soil components sorbed >96% of added tetracyclines. Strongest sorption was observed for clays, followed by HS, and then clay-HC. Greater sorption by the Ca systems than the K systems and decreased sorption with increasing pH suggests that cation bridging and cation exchange contribute to sorption. X-ray diffraction analysis showed that TC and CTC were sorbed in the interlayers of smectites and that the presence of HS reduced interlayer sorption of tetracyclines by smectites in clay-HC. The results indicate that tetracyclines are dominantly sorbed on soil clays and that HS in clayHC either mask sorption sites on clay surfaces or inhibit interlayer diffusion of tetracyclines.
Introduction The fate of antibiotics in the environment has been the focus of much research in recent years. Estimates in the literature about the amount of antimicrobial agents used and their applications are variable. Levy (1) reported that approximately 50 million pounds of antibiotics is produced annually in the United States, of which 50% is used by humans and 50% is used for agricultural applications, including 15.4 million pounds used as growth promoters in animal feed. In contrast, the Animal Health Institute reported that only 3.1 million pounds per year of antibiotics (6%) is used as non-therapeutic growth promoters, that 14.7 million pounds per year (29.5%) is used for therapeutic and prevention purposes in agriculture, and that human use accounts for 32.2 million pounds per year (64.5%) (2). Regardless of the exact amount and how it is used, it is clear that large amounts of antibiotics are used annually in the United States. Ultimately, most of these antibiotics enter the environment through municipal effluents, sewage sludge, solid waste, and manure applications. * Corresponding author phone: (302) 451-5872; fax: (302) 4515941; e-mail:
[email protected]. † Iowa State University. ‡ USDA/ARS National Soil Tilth Laboratory. 1928
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 41, NO. 6, 2007
Many antibiotics administered to livestock are excreted unmetabolized (3). Excretion of parent compounds for tetracyclines range from 70 to 90% (4-6). Winckler and Grafe (7) reported TC concentrations in manure up to 20 mg L-1. Because of the high excretion rates of bioactive compounds, there is growing concern that the use of antibiotics in livestock production will promote the evolution of microbial populations resistant to the antibiotics used by humans (8-13). The mobility of tetracyclines in soil environments is not fully elucidated. The class of tetracycline antibiotics consists of hydrophilic compounds (14) and hence have a high water solubility. However, several investigations have shown a high sorption of tetracyclines by whole soils (15, 16), which reduces their leaching potential. Soils receiving frequent manure applications have reported tetracycline concentrations ranging from 12 µg kg-1 (17) to 100 µg kg-1 (18). Reports of tetracyclines in surface waters generally indicate concentrations in the microgram per liter range (19-21). Sorption of tetracyclines on reference clays, humic acids, synthetic organo-clays, soil-slurry mixtures, manure, and marine sediments has been previously studied. Oxytetracycline (OTC) binds strongly to clay minerals via complexation with divalent cations (22). X-ray diffraction (XRD) analysis has demonstrated the expansion of montmorillonite d001 spacings on sorption, indicating intercalation of OTC and TC (23, 24). Treatment of reference clays with organic surfactants and the presence of dissolved organic matter in equilibration solutions have been shown to decrease sorption of OTC by reference clays (23, 25). Collectively, these studies demonstrate that tetracyclines are very strongly adsorbed by reference clays and that the addition of various organic compounds may reduce sorption of tetracyclines by reference clays. No studies, however, have sought to integrate these two observations to elucidate the nature of interactions between soil clays and natural HS and their impact on sorption of tetracyclines by soils, which is the focus of the present study.
Materials and Methods Chemicals. Tetracycline hydrochloride (purity >98%) and chlortetracycline hydrochloride (purity >98%) were obtained from MP Biomedicals (Irvine, CA) and stored at -20 °C. Calcium chloride, KCl, HCl, KOH, and Ca(OH)2 were reagent grade. Acetonitrile (ACN) and phosphoric acid (85%) were HPLC grade. Deionized water (18 MΩ) was used throughout the study. Whole Soil (WS). Samples of a Zook soil (fine, smectitic, mesic Cumulic Vertic Endoaquoll) and a Nicollet soil (fineloamy, mixed, superactive, mesic Aquic Hapludoll) were collected from surface horizons (0-40 cm) of pedons in Story County, IA. The soils were air-dried, crushed, sieved through a 2 mm sieve, and stored at 4 °C. Soil Components. To extract soil clay-humic complexes (clay-HC), the soils were mechanically dispersed in deionized water without pretreatment, and the
