Environ. Sci. Technol. 2007, 41, 132-138
Sorption of the Veterinary Antimicrobial Sulfathiazole to Organic Materials of Different Origin M A R E N K A H L E * ,†,‡ A N D CHRISTIAN STAMM‡ Agroscope Changins-Wa¨denswil Research Station ACW, Schloss, Postfach 185, 8820 Wa¨denswil, Switzerland, and Swiss Federal Institute of Aquatic Science and Technology, Ueberlandstrasse 133, 8600 Duebendorf, Switzerland
Sulfonamides (SA), ionizable, polar antimicrobial compounds, may reach the environment in substantial amounts by the spreading of manure. The environmental behavior of SA is still difficult to predict. We investigated the influence of the main factors supposed to control SA sorption to organic materials: composition of sorbent, solute chemistry, and contact time. For that purpose, sulfathiazole (STA) sorption to compost, manure, and humic acid after 1 and 14 d was studied under sterile conditions. The experiments demonstrated that sorption was most strongly affected by contact time and pH. Irrespective of sorbent and pH, sorption continued substantially after the fast initial sorption within 1 d. For all sorbents and both contact times, STA sorption exhibited a pronounced pH dependence. Speciesspecific Koc values decreased in the order Koccation . Kocneutral > Kocanion. Differences in sorbent composition influenced STA sorption weaker. For the neutral STA species, NMR chemical shift regions assignable to ketonic, carboxylic, and phenolic C as well as aromatic C-H and methoxyl/N-alkyl C seemed to control sorption. For the cations, sorption followed the cation exchange capacities of the sorbents. STA sorption to manure and humic acid increased with higher ionic strength (0.31 M compared to 0.06 M) at pH 7.5.
Introduction Sulfonamides (SA) are a widely used group of antimicrobial compounds in the European Union utilized for the prevention and therapy of infectious animal diseases (1). They exhibit a high excretion rate by animals as parent compound or as acetylated metabolite in urine and faeces (1). In pig manure, Haller et al. (2) found SA in total concentrations of up to 20 mg kg-1. During storage of manure, the excreted acetyl conjugates were cleaved back to the parent compound (3). Therefore, SA may reach the environment in substantial amounts (tens to hundreds of g ha-1 y-1) through grazing livestock or the spreading of manure on agricultural soils (4). A further pathway into the environment is the direct use in aquacultures. Residues of SA have been found in soils and adjacent environmental compartments (1, 5). As for all antimicrobials, the spreading of SA is of great environmental concern as it bears the risk of enhancing microbial resistance * Corresponding author phone: +41-44-783 6384, fax: +41-44780 6341, e-mail:
[email protected]. † Agroscope Changins-Wa ¨ denswil Research Station ACW. ‡ Swiss Federal Institute of Aquatic Science and Technology. 132
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by prolonged exposure of microorganisms to antimicrobials (6). The environmental fate of SA is difficult to predict since they are molecules that ionize depending on matrix pH. Although physicochemical properties of neutral SA (e.g., water solubility, polarity, refs 7, 8) suggest a high mobility in the environment, SA were shown to undergo cross-coupling with model organic matter constituents (9) that potentially render them immobile. Field and plot studies yielded an inconsistent picture of SA mobility depending on soil and experimental conditions (7, 8, 10, 11). In soils, about 15% of added SA were retained over months (8). The identification of factors affecting sorption is essential for a reliable assessment of SA mobility and bioavailability. Knowledge regarding SA sorption remains limited but recent results suggest complex behavior of SA. In general, sorption studies to soils and soil particle size fractions at natural pH demonstrated low soil-water partition coefficients (Kd < 40 L kg-1) (1, 12-15). For sulfachloropyridazine, it was observed that an increase of pH resulted in decreasing sorption to soil because of substance speciation (10, 14), while no influence of ionic strength was found (14). In contrast, Gao and Pedersen (16) demonstrated that ionic strength as well as exchangeable cations had an effect on the sorption of sulfamethazine to montmorillonite. In sorption experiments, the addition of manure to soil samples resulted in decreasing sorption of SA compared to soil without manure (10, 13). Thiele-Bruhn et al. (12) suggested that for SA sorption in soil, not only the content of organic material (OM) is important but also OM composition. They tentatively assigned phenolic and carboxylic groups, N-heterocyclic compounds, and lignin decomposition products as preferred binding sites (12). Apart from OM, clay minerals and pedogenic oxides may contribute to SA sorption as well (12, 16). Results from extraction and field experiments (8, 17) support the conclusion that after an initial fast sorption of SA to soils (3), sorption may further increase with contact time. However, no sorption study investigated contact time as an important parameter for SA sorption so far. The objective of this study was the assessment of the relative role of the main factors affecting SA sorption to organic substances, namely sorbent characteristics, solution chemistry and contact time, under conditions relevant for their environmental fate. We investigated the sorption of sulfathiazole (STA) to three organic sorbents of deviating composition under different solution conditions (pH, ionic strength) and contact times (1 d, 14 d). Additionally, the influence of exchangeable cations (Ca2+, K+) on the sorbents was analyzed. Experiments were conducted under sterile conditions to prevent biological transformation. Therefore we were able to investigate sorption for an extended contact time of 14 d.
Materials and Methods Organic Sorbents. The organic sorbents applied in these studies include compost from garden waste (obtained from the commercial composting plant of Zurich, Switzerland), the solid phase of liquid pig manure (provided by the ETH research station Chamau, Switzerland) and a standard humic acid (IHSS Leonardite Humic Acid Standard 1S104H-5). Sorbent Pretreatment. Dissolved organic carbon was removed from compost fraction previously dry sieved (2 mm by dry sieving. Similar to the compost sample, manure was split, saturated with Ca2+ and K+ ions respectively, freeze-dried, ground in a mortar, and kept dry in a desiccator. Humic acid was employed as delivered by the IHSS. The characterization of the sorbents can be found in the Supporting Information (SI). Carbon-13 Cross Polarization Magic-Angle Spinning NMR Spectroscopy. Solid-state 13C NMR spectra were obtained on a Bruker DSX 200 spectrometer (Bruker BioSpin GmbH, Rheinstetten, Germany) operating at a 13C resonance frequency of 50.3 MHz by using the cross polarization magicangle spinning (CP-MAS) technique. Manure and compost samples were pretreated with HCl to remove inorganic carbon traces. Afterward they were dialyzed against deionized water, freeze-dried, and ground. Samples were packed into a cylindrical zirconia rotor (diameter 7 mm) with Kel-F caps, and spun at a frequency of 6.7 kHz. A contact time of 1 ms and a pulse delay of 300-5000 ms were used as well as ramped 1H-pulse during contact time to circumvent inexact Hartmann-Hahn conditions. After accumulation of 0.9-15 × 103 scans and before Fourier transformation, a line broadening of 20-50 Hz was applied. The 13C chemical shifts were referenced to tetramethylsilane ()0 ppm) and calibrated with glycine. The spectra were divided into eight chemical shift regions (18). Relative C distribution for regions was determined via an integration routine supplied with the instrument software (Table 1). In the case of humic acid, spinning side bands of the aromatic C-H and carboxylic C regions were integrated and added to the respective value. Carbon-13 NMR analyses confirmed the different composition of the organic sorbents (Table 1, for details see SI). Sulfonamide. Sulfathiazole (STA, Sigma-Aldrich, St. Louis, MO, see Figure S2 in SI) was used for the sorption experiments. STA has two pKa values at pH 2.4 and 7.1 (8) resulting in varying speciation (cation, neutral, and anion) depending on pH. A minor zwitterionic species is in tautomeric equilibrium with the neutral species (16). Experimental Kow of neutral STA is one and water solubility exceeds several g l-1 at 25 °C (7, 8). STA was dissolved in methanol to achieve a stock solution of 1.26 g kg-1. Spike solutions (25 mg kg-1) were diluted from stock solution with 0.01 M CaCl2 just before the experiment.
Isotope labeled STA (d4-STA, Toronto Research Chemicals, North York, ON, Canada) was used as internal standard for the analysis of STA. A stock solution in methanol was diluted with HPLC water to get spike solutions of 10 mg kg-1. Sorption Experiments. The batch sorption experiments were performed according to OECD technical guideline 106 (19) but under sterile conditions. The organic sorbent samples were gamma irradiated after weighing out into glass tubes (dose 60-70 kGray). Solutions were autoclaved or sterile filtered through 0.22-µm syringe filters (polyvinylide-fluoride membrane). Further handling took place under a clean bench using sterile materials. Generally, sample:solution ratios (w: w) of 1:100 were used for manure and compost and 1:200 was used for humic acid. The routine procedure included two steps: (1) sample pre-equilibration for 48 h with background solution (0.01 M CaCl2) and further additions of various amounts of NaOH, HCl, and NaCl (1 M solutions) for pH adjustment. The salt addition for pH adjustment was kept constant on either 0.02 mol kg-1 solution (compost) or 0.03 mol kg-1 solution (manure, humic acid). Therefore, total ionic strength in the batch was about 0.05 M (compost) and 0.06 M (manure, humic acid). (2) After spiking appropriate amounts of STA spike solution, the samples were shaken for either 24 h or 14 d in the dark at 20 °C. Blanks without sorbent were included and showed no decrease of STA concentration. Blanks without STA revealed that none of the sorbents was contaminated with STA (limit of detection 8 (Figure 1). In general, species Koc decreased from Koc+ . Kocn > Koc(Table 2). The experiments clearly revealed that for pH < 5 cations strongly affect STA sorption to organic sorbents. This observation is reflected in the high Koc+ values obtained. Despite the character of this parameter, which is affected by experimental conditions, substance and sorbent properties (see SI), the Koc+ values demonstrated the high affinity of
STA cations to all three organic materials. The affinity of the cations may have been apparently enhanced by an enrichment of STA cations close to the negative surface of organic sorbents compared to solution. An enrichment of STA cations would result in STA species ratios close to the surface corresponding to a higher pKa1 than the known substance pKa1 in solution. A reasonable estimate of this surface effect is up to two pH units (12). Model fits proposed by eq 3 do not allow us to test this postulated surface effect because an increasing pKa1 is compensated by decreasing Koc+ values, while the Koc of the other species remain more or less stable. For anions similar surface effects (depletion of STA anions) may exist since they are repelled from the negative surface of organic sorbents, resulting in a changed STA species ratio close to the surface as well. A repetition of the fits using eq 3 and increasing pKa2 values showed courses of fit lines not as well as before. An exception was humic acid after 14 days, where the fit improved when pKa2 was increased by 0.5 pH units. This may indicate that humic acid, with the highest negative surface charge of all sorbents, for anions could have developed a surface effect. Despite the fact that extrapolation of our Koc values to real soils and sediments must be done with care because organic sorbents do not exist separately but aggregated in organo-mineral associations, the results from our 1 d experiments were consistent with literature reporting sorption to soils under equilibration from 16 to 48 h. Langhammer (3) observed a Koc value of 200 L kg-1 for the sorption of STA to a loamy sand (pH 5.2), while Thiele-Bruhn (1) reported a Koc of 97 L kg-1 for a clay loam (pH 6.2). For several other SA, Koc values between 30 and 325 L kg-1 were found for soils and soil particle size fractions in a pH range from 5.2 to 7.0 (1, 12, 13, 15). Drillia et al. (15) observed a Koc of 530 L kg-1 for sulfamethoxazole and a soil with pH 4.3. In pH experiments similar to ours, a decrease of sulfachloropyridazine sorption with increasing pH was observed for some soils (10, 14). ter Laak et al. (14) also fitted distribution coefficients of neutral species and anions to their pH experiment data, but omitted pKa1 value and cations. For two soils, they observed Kocn values of 370 and 535 L kg-1, respective Koc- were 0 and 35 L kg-1. In contrast to the pronounced effects of pH, changes in solution chemistry due to different ionic strengths were of minor importance (see SI, Figure S5). For manure and humic acid, a fivefold increase in ionic strength caused a maximum 2.5 fold increase in sorption. Influence of Contact Time. Our experiments demonstrated that contact time is a main factor controlling the extent of STA sorption. Irrespective of pH, a substantial increase of STA sorption from 1 d contact time to 14 d was observed for all sorbents (Figure 1). Accordingly, the fitted Koc values of all species increased with time (Table 2). Even Koc- values were above the limit of the analytical procedure error after 14 d. For all sorbents, the sorption process was fast during the first 24 h and apparently slowed down during the following 13 d. This supports results from a field study investigating the environmental behavior of several SA after manure application on grassland (8). Stoob (8) observed increasing distribution ratios (or apparent Kd) of SA with time in the top soil matrix based on measured pore water concentrations and the extractable content. Normalized to soil OC content, a distribution ratio of about 4000 L kg-1 was found for STA after 14 d contact time (soil pH of 5.8). This is higher than in the present study but was observed under real environmental conditions including climatic influences and transport. A detailed look at the mass fluxes between solution and solid-phase revealed differences for the first 24 h and the following period of 13 d. The strong pH dependence of sorption for all sorbents after 1 d contact time corresponds VOL. 41, NO. 1, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 2. Fitted Koc Values for the Sorption of Sulfathiazole (STA) Species (Koc+ ) Cation, Kocn ) Neutral, Koc- ) Anion) to the Organic Sorbents sorbent compost manure humic acid
1d 14 d 1d 14 d 1d 14 d
Koc+ (kg kg-1)
Kocn (kg kg-1)
Koc- (kg kg-1)
r2
12 640 ( 477 75 015 ( 1190 1543 ( 166 10 700 ( 1667 77 414 ( 4006 36 4661 ( 15 659
232 ( 13 654 ( 28 159 ( 6 1078 ( 64 352 ( 13 1142 ( 54
27 ( 17a 125 ( 36 32 ( 7a 371 ( 69 (15 ( 13)a,b (227 ( 54)b
0.973 0.995 0.932 0.833 0.992 0.991
a Values in the range of the analytical procedure error (difference between initial and final STA concentration compost > manure. This sequence reflects the cation exchange properties of the sorbents at pH 5 (see SI Table S1) indicating that cation exchange may be the main binding mechanism. Gao and Pedersen (16) also suggested that cation exchange contributed to the interaction of sulfamethazine cations with smectite surface. The Kocn values of STA for the three sorbents differed as well (Table 2). While after 1 day contact time, the order was humic acid > compost > manure, it changed to humic acid ) manure > compost after 14 d. The species Koc values of compost may have been slightly overestimated, since the inorganic material in compost (see SI) may have contributed to STA sorption. However, mineral phases seem to be much weaker sorbents for SA than organic materials as specific experiments on clay minerals revealed (16). To get information about the possible contributions of specific parts of organic substance to the sorption of neutral STA species, Monte Carlo simulations were run. They attributed different relative contributions to sorption to the different chemical shift regions distinguished by NMR (for details, see SI). For 1 d contact time, chemical shift regions assignable to ketonic, carboxylic, and phenolic C as well as to aromatic C-H and methoxyl/N-alkyl C seemed to be most important for the sorption of STA neutral species (see SI Figure S6). However, for none of the shift regions it could be demonstrated that they necessarily had to contribute to sorption. The results indicate sorption of neutral STA via hydrogen bonds but also via van der Waals forces to aromatic parts of the organic sorbents. Thiele-Bruhn et al. (12) suggested that the amino group of SA interacts with keto-, enol-, alcoholic, and phenolic OH and with carboxylic functions of organic matter. They observed positive correlations between Kd values of SA to soil particle-size fractions and pyrolysis-mass spectra signals assigned to phenols, lignin monomers, lipids, lignin dimers and N-containing compounds. Gao and Pedersen (16) suggested water and cation bridging as possible mechanisms of neutral sulfamethazine sorbing to montmorillonite surfaces. For a contact time of 14 d, no successful Monte Carlo simulations (see SI) could be obtained because manure had similar Kocn values as the humic acid. There was no linear combination of relative contribution values of the NMR shift regions that agrees with the observed data. Since the concept could be successfully applied for a contact time of 1 d, the failure after 14 d indicates a change of manure as sorbent compared to the other two sorbents. This could be explained by a specific change in the sorptive surface of manure important for neutral STA. The sorption of STA anions to organic sorbents was negligible for a contact time of 1 d. However, after 14 d, fitted Koc- values decreased in the order of manure > humic acid > compost. The course of the fit line for manure revealed an overestimation of Koc-, while the value for humic acid may
have been underestimated since increasing dissolution of solid sorbent took place with increasing pH. It is unknown if the dissolved humic acid sorbs STA to a similar extent as the solid form. Taking into consideration that the overall negative surface charge of organic sorbents hinders anion exchange, it is rather probable that STA anions sorbed via the same mechanisms as the neutral species, by means of the uncharged parts of the molecule. The observed sorption increase with higher ionic strength (see SI Figure S5) may also indicate sorption of neutral cation-STA- pairs as a mechanism for anion sorption (24). Environmental Implications. Our results demonstrate that sorption of STA to organic sorbents is mainly controlled by pH and contact time. Based on these two findings one may conclude that the risk of STA to be transported off-field after a manure application to, e.g., water bodies rapidly decreases. On the one hand, the high pH in manure will drop after contact with the soil solution and precipitation. Hence, speciation of STA will change from hardly sorbing anions to neutral molecules that interact much stronger with soil organic sorbents. On the other hand, the enhanced contact time will strengthen this effect. Actually, a field study on the fate of SA after manure application on grassland soil supports this expectation (8). The sorption extent of ionic xenobiotics like STA in environmental compartments is influenced by a large number of parameters. This paper intended to evaluate the relative importance of the main factors sorbent characteristics, solute chemistry, and contact time. Based on the species-specific Koc values one may infer that STA speciation is the main contributing factor to the observed variability causing a factor of 5-20 between the Koc values over a pH range of 4-9. Contact time was of equal relevance. However, for periods of several months the time effect may outweigh the effect of speciation. Less variability was attributed to differences in sorbent composition. Despite conspicuous differences between the materials (Table 1, Figure S1) the respective Koc of, e.g., neutral STA differed not more than a factor of two to three. Further aspects of solution chemistry like ionic strength (see SI) seem to be even less important. Our results suggest that, for modeling SA fate in terrestrial systems, it will be crucial to account for pH and buffer capacity of manure, soil, and rainwater. Furthermore the nonequilibrium conditions that may prevail in real-world situations for most of the time should be considered.
Acknowledgments This work was funded by the Swiss National Science Foundation (NFP 49). We thank Reinhold Jahn and Gudrun von Koch for the mineralogical analyses, Heike Knicker for recording the NMR spectra, Saskia Zimmermann, Heinz Singer, Martin Krauss, Krispin Stoob, Kathrin Fenner, Michael Burkhardt, Michael Sander, and many other colleagues for useful discussions on the content of the paper as well as three anonymous reviewers for their helpful comments.
Supporting Information Available Results and discussion of sorbent characterization, sorption isotherms, influence of ion concentration on STA sorption and the estimation of C type contribution to STA sorption. This material is available free of charge via the Internet at http://pubs.acs.org.
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Received for review May 18, 2006. Revised manuscript received October 19, 2006. Accepted October 20, 2006. ES061198B
