Environ. Sci. Technol. 2010, 44, 4651–4657
Transformation and Sorption of the Veterinary Antibiotic Sulfadiazine in Two Soils: A Short-Term Batch Study R O Y K A S T E E L , * ,† C H O M I L T I N M B O H , † MYRIAM UNOLD,† JOOST GROENEWEG,† J A N V A N D E R B O R G H T , †,‡ A N D HARRY VEREECKEN† Agrosphere Institute, ICG 4, Forschungszentrum Ju ¨ lich GmbH, Leo Brandtstrasse, D-52425 Ju ¨ lich, Germany, and Department of Earth and Environmental Sciences, K.-U. Leuven, Celestijnenlaan 200e, 3001 Heverlee, Belgium
Received January 14, 2010. Revised manuscript received March 25, 2010. Accepted April 1, 2010.
The worldwide use of veterinary antibiotics poses a continuous threat to the environment. There is, however, a lack of mechanistic studies on sorption and transformation processes for environmental assessment in soils. Two-week batch sorption experiments were performed with the antibiotic sulfadiazine (SDZ) in the plow layer and the subsoil of a loamy sand and a silty loam. The sorption and transformation parameters of SDZ and its main transformation products N1-2(4-hydroxypyrimidinyl)benzenesulfanilamide(4-OH-SDZ)and4-(2iminopyrimidin-1(2H)-yl)aniline (An-SDZ) were estimated using a global optimization algorithm. A two-stage, one-rate sorption model combined with a first-order transformation model adequately described the batch data. Sorption of SDZ was nonlinear, time-dependent, and affected by pH, with a higher sorption capacity for the loamy sand. Transformation of SDZ into 4-OH-SDZ occurred only in the liquid phase, with half-life values of 1 month in the plow layers and 6 months in the subsoils. Under the exclusion of light, An-SDZ was formed in substantial amounts in the silty loam only, with liquid phase half-life values of 2 to 3 weeks. Despite the rather large parameter uncertainties, which may be reduced using additional information obtained from sequential solid phase extraction, the proposed method provides a framework to assess the fate of antibiotics in soils.
Introduction Veterinary antibiotics are administered worldwide in animal husbandry for growth promotion and/or therapeutic use. The contamination of the environment with manure containing antibiotics has become of serious concern, since veterinary antibiotics have already been detected in surface waters, groundwater, and soils (1-3). The occurrence of antibiotic substances in the environment poses a potential threat, since it can augment antibiotic resistance in the microbial population (4) or endanger public health by contamination of surface water and groundwater. Pan et al. (5) noted that there is a lack of mechanistic studies on the * Corresponding author e-mail:
[email protected]. † Forschungszentrum Ju ¨ lich GmbH. ‡ K.-U. Leuven. 10.1021/es100141m
2010 American Chemical Society
Published on Web 05/14/2010
fate of antibiotics and their transformation products, and they stressed their urgent need for risk assessment and environmental exposure models. Besides tetracyclines, sulfonamides are the most widely used veterinary antibiotics in the European Union (1, 2, 6). We used the sulfonamide sulfadiazine (SDZ) as model compound, which is considered to be mobile in soils. 14 C-labeled SDZ is quickly eliminated after administration to animals, with only 4% of the total radioactivity remaining in pigs 10 days after administration (7). Lamsho¨ft et al. (7) found that N4-acetyl-SDZ and 4-OH-SDZ were the main transformation products in manure, accounting for about half of the total radioactivity. Transformation of SDZ occurred during metabolism in animals, by photodegradation in solutions (8, 9), during anaerobic fermentation with manure (10), and in soils (11-14). However, complete mineralization to CO2 is marginal in soils, with reported values of less than 2% (11, 15), even after 218 days. Transformation processes generally reduce the environmental impact of antibiotics, with a substantial reduction in the antimicrobial activity for 4-OHSDZ (10) and no activity for N4-acetyl-SDZ (16), although the latter can be transformed back into SDZ by deacetylation. To our knowledge, no antimicrobial activity of any other transformation product of SDZ is known. Sorption is the partitioning of a compound between the liquid and the solid phase. For sulfonamides in soils, this partitioning is best described by the Freundlich adsorption isotherm, in both the presence and the absence of manure (9, 17-20), indicating that high-energy sorption sites are preferably occupied. In general, solute partitioning is a kinetic process, which was experimentally observed for SDZ (20) and sulfonamides (19, 21), where the sorption capacity increased over time. Ignoring kinetic effects leads to misinterpretations of the long-term fate of chemicals. Furthermore, the sorption of sulfonamides to soils is pH dependent. Because of their amphoteric nature, sulfonamides undergo rapid dissociation/protonation which results in cationic, neutral, and anionic species with different sorption capacities to soils (19), clay minerals (22, 23), and organic matter (21). Generally, sorption capacity increases with decreasing pH. Speciation does not only have implications for sorption behavior, but also for the uptake of the substance into bacterial cells (24). Other than the presentation of “equilibrium” sorption parameters, there is only limited quantitative information on the kinetic sorption and transformation of veterinary antibiotics in soils. Zarfl et al. (25) proposed a detailed kinetic sequestration and transformation model for SDZ by adopting a pool concept using data from soils amended with fresh manure. Wehrhan et al. (20) presented a more realistic mathematical model to account for the observed much slower desorption of 14C-labeled SDZ compared to its adsorption. The convection-dispersion equation has been used in a few studies to quantify kinetic sorption and transformation of SDZ from column leaching experiments in the laboratory (13, 14, 26, 27) and at the field scale (28). In this study, we present a method for simultaneously quantifying the sorption and transformation processes of SDZ and its main transformation products for four soil horizons in short-term batch experiments applying commonly used mathematical models for kinetic sorption and transformation. Although single aspects of sorption and transformation have been addressed in the literature, a comprehensive quantitative approach, including nonlinear sorption and a parameter estimation VOL. 44, NO. 12, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Selected Physical and Chemical Properties of the Silty Loam from Merzenhausen (MRZ) and the Loamy Sand from Kaldenkirchen (KAL)a particle size distributionb
pHc
Cocd
Cace
θig
CECf
-1
Feth -1
soil
sand (%)
silt (%)
clay (%)
(-)
(%)
(%)
(g g )
(cmolc kg )
(%)
KAL Ap KAL Bw MRZ Ap MRZ Bt
68.5 75.6 5.9 1.4
26.7 21.6 78.7 75.2
4.9 2.8 15.4 23.4
5.7 6.1 7.0 7.4
1.07 0.25 1.24 0.37
0.048 0.005 0.083 0.017
0.086 0.067 0.17 0.15
7.8 n.d.i 11.4 n.d.
0.81 0.54 1.53 2.58
a Soil was air-dried and sieved to 2 mm prior to all analysis. All weight fractions are based on the total dry mass of soil. Pipette method (sand 63 µm-2 mm, silt 2-63 µm, clay
