Sorption of sulfonamide antimicrobial agents to humic acid-clay complexes

The interaction of sulfonamide antimicrobial agents with smectite clay minerals and humic acid (HA)-clay complexes was investigated in batch experiments to assess the influence of adsorbed humic acid on sulfonamide sorption. Soil HA-clay complexes were produced at HA:clay ratios of 1:5, 1:50, and 1:100 (w/w). Vibrational and electronic spectroscopy indicated the preferential adsorption of polar and aliphatic components of HA to smectite surfaces, a phenomenon most readily discerned at the two lower HA:clay ratios (1:50 and 1:100). Humic acid adsorption to smectite clay minerals enhanced sulfamethazine [2-(p-aminobenzenesulfonamido)-4,6-dimethyl pyrimidine] sorption, especially at the highest HA:clay ratio (1:5). Sulfamethazine sorption to adsorbed HA generally increased with the abundance of carboxyl (and possibly other O- and N-containing) moieties and aliphatic carbon content. Both the smectite surface and the adsorbed humic acids contributed to sulfamethazine sorption-desorption hysteresis; hysteresis in sulfamethazine sorption to the adsorbed humic acids was equal to or larger than for adsorption to the smectites. Competitive sorption with a structurally similar sulfonamide antimicrobial was apparent only at high competitor concentrations. This study indicates that sorption to organic carbon and clay mineral surfaces as well as the hysteretic sorption behavior warrant consideration in predicting the transport of these antimicrobial agents in soils and subsurface environments.