Progress 04/01/02 to 09/30/04
Outputs
To gain insight into the mobility of sulfonamide antimicrobials in soils and subsurface environments, we investigated the adsorption of sulfamethazine, sulfapyridine and sulfamethoxazole to the clay minerals montmorillonite and kaolinite. Use of radiolabeled sulfamethazine allowed investigation of adsorption at environmentally relevant concentrations. Sulfonamide antimicrobial adsorption to clay minerals was examined as a function of pH, ionic strength and type of exchangeable cation. We employed the General Linear Model to obtain species-specific equilibrium distribution coefficients for the cationic, neutral and anionic forms of each sulfonamide. Distribution coefficients for adsorption of cationic species to montmorillonites were one to two orders of magnitude larger than those for neutral species. Increases in solution ionic strength suppressed adsorption of the cationic species and increased adsorption of the anionic species. The affinity of the exchangeable
cation to the clay minerals significantly impacted the adsorption of the cationic but not the neutral sulfonamide species. X-ray diffractometry indicated that sulfonamide antimicrobials do not intercalate in the interlamellar spaces of montmorillonite clays. Adsorption of cationic sulfamethazine to Na-kaolinite was negligible, while that of the neutral species was comparable to that observed for Na-montmorillonite. Our results suggest that sulfonamide antimicrobials are moderately to highly mobile in soil and subsurface environments and highlight the importance of considering speciation in predicting the transport of these compounds.
Impacts
Our results suggest that sulfonamide antimicrobials should be relatively to highly mobile in soil depending on the pH, fraction of organic carbon and clay mineral content, having potential to be transported to groundwater and surface water. Because of relatively low degree of sorption, a large fraction of sulfonamide antimicrobials would be available to exert selective pressure on microorganisms.
Publications
- Gao, J.; Pedersen, J.A. Adsorption of sulfonamide antimicrobial agents to clay minerals. Environ. Sci. Technol. 2005 (to be submitted February 2005).
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Progress 01/01/03 to 12/31/03
Outputs
During the past year we investigated the association of three sulfonamide antimicrobial agents with smectite clay and an organic soil and the oxidative cross coupling of sulfonamides with natural organic matter constituents. The bulk of our sorption studies utilized radiolabeled sulfamethazine allowing us to obtain sorption coefficients at environmentally relevant concentrations. Sorption to both smectite clay and natural organic matter was highly pH dependent with maximum sorption at lower proton activities. Sorption to Ca-saturated smectite was greater than for Na-saturated smectite at pH values at which the neutral species dominated aqueous speciation. The nature of the exchangeable ion did not appear to significantly influence sorption at pH values at proton activities at which the cationic or anionic form dominated aqueous speciation. Equilibrium distribution coefficients for the neutral species ranged from 5.3 to 16.5 L/kg depending on the nature of the
exchangeable cation. Anionic species of sulfonamides did not appear to interact significantly with smectite clay surfaces. X-ray diffractometry indicated that sulfonamide antimicrobials did not significantly intercalate into the smectite. Sorption to natural organic matter was significantly greater than to smectite clays with Kd and Koc values of 130 and 184 kg/L for the neutral species. This Koc value is in the range of those reported by previous investigators examining sorption to whole soils. We also examined the cross coupling of sulfamethazine and model humic constituents in the presence of birnessite and oxidoreductive enzymes. Incubations with hydroxybenzoic and hydroxycinnamic acids in the presence of peroxidase, tyrosinase or d-MnO2 resulted in significant reductions in sulfamethazine concentrations relative to controls. Birnessite and tyrosinase also mediated the oxidation of sulfamethazine with hydroxybenzenes. These results imply that ortho-hydroxylated constituents may
have a preferential role in sulfamethazine transformation. 1H-13C heteronuclear single-quantum correlation nuclear magnetic resonance experiments provided direct spectroscopic evidence for peroxidase-mediated covalent bond formation between sulfamethazine and syringic acid. These experiments suggest that cross coupling of sulfonamide antimicrobial agents with humic substances may result in their immobilization and reduced bioavailability. In the remainder of the project period we intend to examine sulfonamide sorption to kaolinite as a function of pH and will use our sorption edge data to derive Kd values for the cationic and neutral species using the General Linear Model procedure. We also plan to examine the reactivity of additional sulfonamide antimicrobials toward constituents of natural organic matter in the presence of manganese oxide and oxidoreductive enzymes.
Impacts
Information on the association of sulfonamide antimicrobial agents with specific soil constituents will aid in the prediction of the mobility of these compounds in soils and subsurface environments. The covalent binding of these antimicrobials with natural organic matter may serve as a natural means of decreasing their bioavailability and potential to select for antimicrobial resistance.
Publications
- No publications reported this period
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Progress 04/01/02 to 12/31/02
Outputs
Understanding the sorption of sulfonamide antibiotics to soil and sediment particles is essential for assessing their potential to leach into groundwater and be transported in aquifers and surface waters. During the last 10 months we have been investigating the sorption of three sulfonamide antibiotics to clay minerals and soils with high organic matter contents. Apparent sorptive equilibrium was achieved rapidly (within several hours) in both cases. As expected, the extent of sulfonamide sorption was pH dependent, with more significant sorption of the neutral species. Distribution coefficients for the Na-montmorillonite employed were quite low (< 10 L/kg), indicating little adsorption to clay mineral surfaces. We are currently examining the effect of exchangeable cations (e.g., K+, Ca2+) on sorption to clay minerals. Initial results from experiments employing high organic content soil indicate that sorption of unionized sulfonamide antibiotics to natural organic
matter (NOM) may be more important than adsorption to clay mineral surfaces and that the extent of sulfonamide sorption to NOM is greater than would be predicted from the hydrophobicity of the compounds. This suggests that mechanisms in addition to hydrophobic partitioning may be important in the association of sulfonamide antibiotics with NOM. To better understand the interaction of sulfonamide antibiotics with NOM, we intend to examine sorption to dissolved humic substances in batch experiments and with 13C-nuclear magnetic resonance (NMR) spectroscopy. NMR studies will allow us to investigate the interaction of 13C-labeled sulfamethazine with humic substances at the molecular level.
Impacts
This project will increase our ability to predict the mobility, fate and effects of sulfonamide antibiotics. Initial results indicate that depending on soil properties, sulfonamide antibiotics are moderately to highly mobile with respect to their ability to leach through the soil column and into groundwater.
Publications
- Pedersen, J.A. Sorption of sulfonamide antibiotics to clay minerals. In Proc. Soil Science Society of America Annual Meeting, Indianapolis, IN, November 10-14, 2002 (invited).
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