SORPTION OF TETRACYCLINE AND CHLORTETRACYCLIINE ON K- AND CA- SATURATED SOIL CLAYS, HUMIC SUBSTANCES, AND CLAY-HUMIC COMPLEXES

Authors: PILS, JUTTA R.V. - IA STATE UNIVERSITY; Laird, David

Submitted to: Environmental Science and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/10/2007
Publication Date: 2/14/2007
Citation: Pils, J., Laird, D.A. 2007. Sorption of tetracycline and chlortetracycliine on K- and CA- saturated soil clays, humic substances, and clay-humic complexes. Environmental Science and Technology. 41(6):1928-1933.

Interpretive Summary: Large quantities of antibiotics are used in animal production both to fight disease and to promote growth. Most of these antibiotics pass right through animals without being chemically altered, and hence manure often contains detectable levels of antibiotics. There is concern that antibiotics present in manured soils will encourage the development of antibiotic resistent bacteria, which may pose a threat to human health. We discovered that >96% of tetracycline and chlorotetracycline (two commonly used antibiotics) was adsorbed almost irreversably by soil. Soil clay minerals were found to adsorbe more of the antibiotics than the soil organic matter and we also discovered that soil organic matter competes with the antibiotics for places to adsorbe on the surfaces of clay minerals. This information will help scientists to better understand the fate of antibiotics in soils and thereby to better assess the risk posed by the use of antibiotics in animal production.

Technical Abstract: Chlortetracycline (CTC) and tetracycline (TC) are used extensively for growth promotion and therapeutic purposes in livestock production. To elucidate the environmental fate of these pharmaceuticals, sorption isotherms were obtained using dilute CaCl2 or KCl background solutions at different pHs for clays (clays), humic substances (HS), and clay-humic complexes (clay -HC) derived from two agricultural soils. In all systems, the soil components sorbed > 96% of added tetracyclines. Strongest sorption was observed for clays, followed by HS, and least on clay-HC. Sorption isotherms indicated greater sorption in Ca-systems than K-systems and that CTC was more strongly sorbed than TC. Increasing the pH of the Ca-clay-HC from 5.8 to 7.0 decreased sorption of CTC and TC. Desorption of TC at pH 7 increased for clay < HS < clay-HC. At pH 7 little CTC was desorbed. The greater sorption in the Ca-systems and the decreased sorption with increasing pH suggested that cation bridging and charge neutralization contribute to sorption. X-ray diffraction analyses showed that TC and CTC were sorbed in the interlayers of smectites and that humic substances were in the interlayers of the clay-HC. The results indicate that tetracyclines (TET) are dominantly sorbed on soil clays and that HS either mask sorption sites on clay surfaces or inhibit interlayer diffusion of TET.