SORPTION OF SULFONAMIDE ANTIMICROBIAL AGENTS TO SMECTITIC CLAYS AND NATURAL ORGANIC MATTER

Sulfonamide antimicrobial agents are widely used in animal husbandry and are among the most commonly prescribed human drugs. Recharge of groundwater with treated municipal wastewater, effluent irrigation and application of animal manures to agricultural fields represent routes by which sulfonamides enter soils and subsurface environments. The primary concern with the introduction of antimicrobial agents into the environment is the potential for antibiotic resistant microorganisms to proliferate in response to increased selective pressure. Association of sulfonamide antimicrobials with geosorbents in soils, sediments and subsurface environments influences their mobility and susceptibility to degradation; both of which represent critical factors in determining risk of exposure and the selective pressure exerted by these compounds. We investigated the sorption of sulfamethazine (SMZ) and sulfamethoxazole (SMX) to smectitic clays and natural organic matter (NOM) in batch sorption experiments. Sulfonamide sorption was strongly pH-dependent with neutral species exhibiting maximal association with both smectitic clays and NOM. SMZ adsorption to smectite depended on the nature of exchangeable cations and decreased in the order of Ca⁺² > Mg⁺² > Na⁺ > K⁺. The type of exchangeable cation did not significantly affect the extent of SMX adsorption to smectite. The degree of SMZ sorption to NOM was comparable to that for Ca- and Mg- saturated smectite. SMX adsorption to clay was much less than to NOM. The ability of these antimicrobials to engage in cross-coupling reactions with NOM was investigated by incubating SMZ with model humic constituents in the presence of birnessite (i.e., d-MnO₂) or horseradish peroxidase (HRP). Initial experiments indicate SMZ is readily cross-linked to syringic acid in the presence of HRP or MnO₂.