Diffusion of sulphonamide antibiotics in agarose hydrogels enriched by humic acids

Many different pharmaceuticals can evince potentially dangerous behaviour in soil and aqueous environments. Their migration ability and toxicity can be significantly affected by interactions with organic matter. Sulfamethoxazole and sulphapyridine (used as representative commonly used sulphonamide antibiotics) are studied from the point of view of their transport in model hydrogels containing humic substances. Agarose hydrogel was used as a model medium for the transport experiments. Standard Elliot soil humic acid (4S102H) purchased from International Humic Substances Society was incorporated into the hydrogel as an active substance (and representant of organic matter) for interactions with antibiotics and potential suppression of their migration and toxicity in soil systems. Two different experimental arrangements were used: the diffusion from constant source, and the diffusion couple. Diffusion coefficients were determined for pure agarose hydrogels as well as for hydrogels containing humic acids and compared. It was found that the addition of humic acids into hydrogel resulted in the decrease in the effective diffusion coefficient of sulphapyridine and the increase in the effective diffusion coefficient of sulfamethoxazole. Humic acids can suppress the mobility of sulphapyridine and enhance the diffusion of sulfamethoxazole. Batch sorption experiments provided similar adsorption capacities of humic acids for both studied antibiotics. The ratio between adsorption and desorption rate constants were higher than 1 for sulphapyridine and lower than 1 for sulfamethoxazole. It indicated that adsorption predominated for sulphapyridine which resulted in observed lower diffusivity. Desorption experiments showed that a majority of sulphapyridine can be immobilized by interaction with humic acids whereas much more free movable sulfa-methoxazole was determined in the comparison with immobilized one.