Environ. Sci. Technol. 2009, 43, 3135–3141
Fate of the Antibiotic Sulfamethoxazole and Its Two Major Human Metabolites in a Water Sediment Test M I C H A E L R A D K E , * ,† CHRISTOPH LAUWIGI,† GEORG HEINKELE,‡ ¨ RDTER,‡ AND THOMAS E. MU MARION LETZEL§ Department of Hydrology, University of Bayreuth, 95440 Bayreuth, Germany, Dr. Margarete Fischer-Bosch Institute of Clinical Pharmacology, Auerbachstrasse 112, 70376 Stuttgart, Germany and University of Tu ¨ bingen, Germany, and Bavarian Environment Agency, Referat 76, Lazarettstr. 67, 80636 Mu ¨ nchen, Germany
Received January 28, 2009. Revised manuscript received March 18, 2009. Accepted March 19, 2009.
Sulfonamide antibiotics are widely used in human and veterinary medicine. After their application, they are excreted in unchanged as well as in metabolized form. Due to incomplete elimination in wastewater treatment plants, they can be emitted into surface water. The environmental fate of both parent compounds and metabolites is currently unknown. The aim of this study was to determine the biodegradation potential of river sediment for the sulfonamide sulfamethoxazole (SMX) and its two major human metabolites N4-acetyl-SMX and SMX-N1-glucuronide using a water sediment test system. Each compound was tested in a separate series together with sterile and “water only” controls. All three compounds were efficiently removed from the test system by biodegradation in the sediment. Only for SMX-N1-glucuronide, a substantial removal in the absence of sediment was determined. Dissipation times from the aqueous phase (DT50) between 8.5 and 17.2 days were measured. Sorption to sediment was of minor importance due to the slightly basic pH of the test system. By the application of a mathematical model, biodegradation halflives in sediment between 3.3 and 25.6 h were calculated for SMX and its metabolites. The results of this study highlight the capability of native river sediment for degrading sulfonamide antibiotics, but also the potential of human metabolites to retransform into parent SMX under environmental conditions. Based on this study, it is unlikely that SMX or its metabolites will persist or accumulate in river sediments under pH conditions where sorption is of minor importance.
Introduction Pharmaceuticals are an essential part of human healthcare. Many pharmaceuticals are not completely metabolized in * Corresponding author phone: +49/921/552297; fax: +49 /921/ 552366; e-mail:
[email protected]. † Department of Hydrology, University of Bayreuth. ‡ Dr. Margarete Fischer-Bosch Institute of Clinical Pharmacology and University of Tu ¨ bingen. § Bavarian Environment Agency. 10.1021/es900300u CCC: $40.75
Published on Web 04/06/2009
2009 American Chemical Society
the human body; thus, both unmodified parent compound and metabolites are excreted and can enter the water cycle via wastewater. Due to incomplete removal of many pharmaceuticals in wastewater treatment plants (WWTPs), these micropollutants are emitted into the aquatic environment (1). An additional source of pharmaceuticals in the aquatic environment is their application in livestock followed by fertilization with manure (2). In spite of their intensive use in modern medicine and their ubiquitous presence in aqueous media, however, knowledge on the behavior and fate of pharmaceutically active compounds in the aquatic environment is still limited. The sulfonamide sulfamethoxazole (SMX) is one of the top-selling antibiotics. In 2001, 53.6 t of SMX were sold in Germany (3). After oral application, it is partly metabolized in the human body. Approximately 45-70% of a SMX dose is excreted via urine within 24 h (4). However, only 15-25% is present as unchanged drug while 43% is present as N4acetyl-sulfamethoxazole (Ac-SMX), and 9-15% is present as sulfamethoxazole-N1-glucuronide (SMX-Glu) (5-7). Three additional metabolites sum up to 4-10% of total dose (5, 7). A detailed scheme of SMX metabolism is provided in the Supporting Information. The ecotoxicological effect of SMX is well-studied for short time tests of acute toxicity. For a number of aquatic organisms, LC50 or EC50, respectively, were reported between 26.8 µg L-1 (8) and 35 mg L-1 (9). Lowest-observed effect concentrations (LOEC) down to 30 µg L-1 (L. Gibba) were reported (10), and for chronic toxicity a predicted no-effect concentration (PNEC) of 0.59 µg L-1 was determined (8). Ferrari et al. (8) calculated a ratio of predicted environmental concentration (PEC) to PNEC of 2.72 for Germany. At environmentally relevant pH, SMX can be present as neutral (1.8 < pH < 5.7) or anionic species (pH > 5.7) (11). This also holds true for Ac-SMX with a pKa of the secondary amine group of 5.6 ( 0.5 (12), whereas SMX-Glu is expected to be present as anion over the whole pH range of most aquatic systems (pKa ) 2.7 ( 0.7 (12)). It has been shown by several authors that sorption of sulfonamides to organic or inorganic materials is strongly dependent on pH and is lowest when the compounds are present as anionic species (13, 14). Ambiguous information is available on the biodegradation behavior of SMX in aquatic systems. While it was classified as resistant to biodegradation in some studies (15, 16), its biodegradation in sewage sludge and laboratory WWTPs (11) as well as in full size WWTPs (17) has also been reported. Go¨bel et al. determined an elimination rate of SMX and AcSMX of 62% in a Swiss WWTP (18). In laboratory WWTPs an elimination rate for SMX of 84% has been measured (19); this elimination could be attributed to microbial primary degradation. Only 40 µg L-1 during the incubation (see Figure S2). We do not have a straightforward explanation for these results; based on the recovery of the internal standards both for Ac-SMX and SMX, analytical errors are unlikely. However, from this control we still can deduce that in the absence of sediment no transformation of Ac-SMX to SMX occurred. This is different from the behavior of SMX-Glu. In contrast to SMXGlu, the Ac-SMX will only be retransformed to parent SMX when there is substantial interaction between surface water and river sediment. Since Ac-SMX concentrations in WWTP effluents can be in the range of 20-50% of SMX concentrations (19, 28), retransformation of this metabolite in sediments could be a continuous source of SMX downstream of WWTP effluents. Depending on the exchange rate between surface water and sediment, this could cause a release of SMX over some distance in the river. Dissipation Times and Kinetic Modeling. In Table 1, the kinetic parameters for all experiments are summarized. The DT50 of the three test compounds was between 8.5 and 17.2 days. For the sterile controls, DT50 of 147 d (SMX) and 273 d (SMX-Glu) were determined; for Ac-SMX, the slope of the regression line was not significantly different from zero and therefore no DT50 could be determined (Table 1). The DT50 for SMX and its human metabolites in the water-sediment system are in the same range than those reported for several other pharmaceuticals by Lo¨ffler et al. (33). From the results of our experiments, we conclude that in the tested river sediment (primary) biodegradation is much more important than abiotic processes such as sorption or hydrolysis. The comparatively low adsorption to the organic rich sediment can be attributed to the negative charge of all test compounds at the experimental pH g 7.6. This is in agreement with observations that sorption of sulfonamides to soil or clay is low (34), especially at pH > 7 (13, 14). Since sorption of sulfonamides to both organic and inorganic sorbents is much more effective for the neutral or cationic species than for the anionic ones (13, 35), the importance of sorption as removal process from the aqueous phase should be higher in systems with more acidic pH. Using the model Hydrus-1D for SMX a biodegradation rate constant of 0.54 d-1 (w/o MeOH) and 1.46 d-1 (w MeOH), respectively, was calculated; for SMX-Glu (w MeOH) and Ac-SMX (w/o MeOH), rate constants of 1.27 d-1 and 5.08 d-1, respectively, were determined. This corresponds to biodegradation half-lives in sediment between 3.3 and 25.6 h (Table 1 and Table S1). Since these rate constants are not depending on the nature of transport of water and/or solutes from the channel into the sediment, they can be applied in physically
based process models for river water quality. However, these rate constants are currently only a rough estimate since their dependence on the biogeochemical boundary conditions in the test system (e.g., type of sediment, availability of nutrients and carbon, temperature, etc.) so far is widely unknown. The calculated SMX biodegradation half-life in sediment determined by inverse simulation is in the same order of magnitude as photolysis half-lives between 2.8 and 6 h in synthetic field waters (36). It should be noted though that the relative importance of both processes is not directly accessible from these half-live times since they were determined under specific laboratory conditions and can not easily be transferred to the field scale. Implications for Fate of SMX and its Metabolites in Rivers and Sediments. This study highlights the intrinsic potential of river sediments for microbial degradation of SMX and its human metabolites. Based on the results of the sediment water tests, it is unlikely that SMX and its metabolites will persist and/or accumulate in river sediments at pH > 7.6. To determine the contribution of sedimentary biodegradation to the total attenuation of SMX and its metabolites in rivers, knowledge of the exchange rate of solutes between surface water and sediment compartment and the residence time of solutes in the sediment is crucial. Currently, reliable methods to determine these parameters on the scales of river stretches (10-50 km) are not available and therefore the relevance of sedimentary biodegradation of SMX on larger scales cannot be evaluated properly. Under conditions where river water is infiltrating into the groundwater or during bank filtration, an efficient removal of SMX and its metabolites can be expected. For both metabolites, a rapid retransformation to the antibiotic SMX was observed. Since Ac-SMX can be present in WWTP effluents at concentrations similar to those of SMX (28), this process is a potential in-stream source of SMX. To assess the environmental behavior and fate of SMX, it is therefore necessary to take the human metabolites into account. This also holds true for other sulfonamides having a similar metabolic pathway in the human or animal body that leads to the excretion of acetyl or glucuronide metabolites. In the absence of sediment, however, only SMX-Glu was transformed back to parent SMX while both SMX and Ac-SMX were only biodegraded from the test system in the presence of river sediment. Thus, an emission of SMX-Glu to surface water will result in an immediate retransformation to SMX, whereas the release from Ac-SMX will be delayed by the time necessary for transferring Ac-SMX into the sediment and the released SMX back into the river. An important scientific challenge is to determine the nature and fate of SMX degradation products in rivers and sediments. Studies on the behavior of sulfonamides in soil VOL. 43, NO. 9, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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revealed that these compounds were not completely mineralized. Instead, a high proportion was bound to the soil matrix as nonextractable residues (37) with a low bioavailability to microorganisms and plants and low remobilization potential (38). This is expected to be similar in river sediments but has not been addressed so far. In this study, the fate of SMX and its human metabolites has been determined using one specific river sediment and water. To address the variability among aquatic systems, future studies should compare the elimination kinetics in a variety of different river sediments and at different biogeochemical boundary conditions. Since the primary degradation of SMX and its metabolites is most likely a cometabolic process, elimination kinetics should be closely related to the biogeochemical state (e.g., availability of nutrients and organic carbon, microbial activity) of the specific water-sediment system. Moreover, tests at lower concentration levels and under more sorptive conditions (lower pH) are suggested to determine the relation between degradation kinetics and bioavailability of SMX and its metabolites.
Acknowledgments We thank Peter Gessner, Elfriede Grill, Adriana Rippberger, Monika Hanf, Karl-Heinz Stenger, and Jutta Eckert for their assistance during this study. The help of Wolfgang Durner is acknowledged for the initial setup of the Hydrus-1D model. Two anonymous reviewers are acknowledged for constructive comments on an earlier version of the manuscript.
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Supporting Information Available A schematic of human SMX metabolism, a description of the modeling procedure, and detailed results of all tests including the results of the Ac-SMX “water only” control. This material is available free of charge via the Internet at http:// pubs.acs.org.
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