TECHNICAL REPORTS 654 Antibiotics, such as sulfadiazine (SDZ), may enter arable soil by spreading of manure of medicated husbandry or directly by the excrement of grazing animals. Knowledge of the fate of antibiotics in soils is crucial for assessing the environmental risk of these compounds, including possible transport to ground water. Kinetic sorption of 14 C-labeled SDZ (4-amino- N-pyrimidin-2-yl-benzenesulfonamide) was investigated using the batch technique. he batch sorption–desorption experiments were conducted at various concentration levels (0.044–13 mg L −1 initial solute concentration) and time scales (0.75–272 d). Sorption of 14 C-SDZ in the investigated silty loam was time dependent and strongly nonlinear in the solution phase concentration. he time to reach an apparent sorption equilibrium was about 20 d. However, desorption was very slow, and 41 d were insufficient to reach the desorption equilibrium. An inverse modeling technique was used to identify relevant sorption processes of 14 C-SDZ during the batch experiments. Among the investigated two- and three-domain sorption models, adsorption and desorption of 14 C-SDZ were best described with a new model defining two sorption domains and four parameters. Whereas sorption in the first sorption domain was nonlinear and instantaneous, solute uptake in the second sorption domain was rate limited following first-order kinetics. Desorption followed the same rate law until an equilibrium distribution was reached. After that, desorption was assumed to be impossible due to partly irreversible sorption. Although the proposed model needs further validation, it contributes to the discussion on complex sorption processes of organic chemicals in soils. Long-Term Sorption and Desorption of Sulfadiazine in Soil: Experiments and Modeling Anne Wehrhan* Agrosphere Institute Thilo Streck University of Hohenheim Joost Groeneweg, Harry Vereecken, and Roy Kasteel Agrosphere Institute V eterinary pharmaceuticals may reach the soil environ- ment via the excreta of treated livestock. Dung is directly dropped by pasture animals, or the manure of housed animals is spread onto agricultural soils. Up to 96% of the administered dose of the antibiotic substance sulfadiazine (SDZ) is excreted by pigs as parent substance or metabolites within 10 d after admin- istration (Lamshöft et al., 2007). Although SDZ may be further degraded during manure storage, acetyl-SDZ, one of the major metabolites, reacts back to the parent compound in manure (Berger et al., 1986; Grote et al., 2004). After application of SDZ- containing manure on a grassland soil, SDZ was detectable in soil and pore water for more than 3 mo (Stoob et al., 2007). Fate and transport of a contaminant in the soil environment depends largely on its interaction with the soil solids (e.g., Kleineidam et al., 2004). Understanding the governing sorption processes is cru- cial to estimating the leaching potential of the contaminant (e.g., Pignatello and Xing, 1996; Altfelder et al., 2001). Because of the complex nature of the soil matrix and the wide variety of organic and inorganic xenobiotics, numerous processes on the molecular level contribute to overall sorption. Apart from the sorbent properties (e.g., hydration status, grain size, surface coatings of the grains, and surface charge), pH, ionic strength, temperature, and the presence of co-solutes influence overall sorp- tion (e.g., Luthy et al., 1997; Gao and Pedersen, 2005). hese various effects on sorption might be investigated separately for process elucidation. However, the overall sorption extent in the soil environment is unlikely to be the sum of the separate pro- cesses (Addiscott et al., 1995). herefore, macroscopic evidence, usually by means of batch experiments, is used to propose and validate suitable sorption concepts (Luthy et al., 1997). For sulfonamides such as SDZ, nonlinear sorption isotherms have previously been reported by hiele-Bruhn and Aust (2003), Gao and Pedersen (2005), and Sukul et al. (2008). he Freundlich sorption isotherm was a suitable model to describe the sorp- tion characteristics. However, the estimated Freundlich param- Abbreviations: 2S1R, two-stage, one-rate sorption model; 2SIS, two-stage irreversible sorption model; 3S2Rirrev, three-site, two-rate irreversible sorption model; LSC, liquid scintillation counting; SDZ, sulfadiazine. A. Wehrhan, J. Groeneweg, H. Vereecken, and R. Kasteel, Agrosphere Institute, ICG 4, Forschungszentrum Jülich GmbH, 52425 Jülich, Germany; T. Streck, Dep. of Soil Science and Land Evaluation, Biogeophysics, Univ. of Hohenheim (310), 70593 Stuttgart, Germany; A. Wehrhan, present address: Environmental Safety and Metabolism Contract Research Services, Harlan Laboratories Ltd., CH-4452 Itingen, Switzerland. Copyright © 2010 by the American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America. All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including pho- tocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Published in J. Environ. Qual. 39:654–666 (2010). doi:10.2134/jeq2009.0001 Published online 26 Jan. 2010. Received 4 Jan. 2009. *Corresponding author (wehrhananne@hotmail.com). © ASA, CSSA, SSSA 677 S. Segoe Rd., Madison, WI 53711 USA TECHNICAL REPORTS: VADOSE ZONE PROCESSES AND CHEMICAL TRANSPORT