Sorption Kinetics of Sulfonamide Antimicrobials.

Sulfonamides are one of the classes of antimicrobials used in human therapy, animal agriculture and aquaculture industry. Substantial amount of partially metabolized sulfonamides enters the environment through diffused non-point sources. Sorption of sulfathiazole to soil material is one of the crucial mechanisms that determine the fate of sulfathiazole in the environment. However; the time dependent sorption behavior of sulfathiazole in different soil types is not fully understood. In this research study, time dependent sorption behavior of sulfathiazole was studied in loam, sandy loam and loamy sand soils collected from Iowa, Missouri and North Carolina. Sorption kinetics studies were conducted over a period of 96 hours using series and parallel sampling protocol with sampling duration varying from 0, 4, 8, 12, 24, 48 and 96 hours. Results indicate that in sandy loam, nearly 90% of sorption of sulfathiazole was observed within 24 hours, whereas only 60-75% sorption was observed for loam and loamy sand in the same duration. Unlike sandy loam, sorption rate in loam and loamy sand was observed to be low primarily because of significant differences in soil properties. Compared to other two soils, high sorption and nearly equilibrium conditions observed in sandy loam could be attributed to its relatively high percent organic carbon and cation exchange capacity. Only about 60% of sulfathiazole sorption was observed in 96 hours when similar studies were repeated for mixture of sulfonamides, indicating potential competitive sorption. Initially, sorption rate increased rapidly, but after 24 hours the rate of sorption was slow or stabilized indicating exhaustion of the sorption sites. Interestingly, series sampling protocol exhibited relatively higher percent sorption in the same duration, compared to the parallel sampling protocol, indicating the importance of having a constant mass, soil to solution ratio over time period.