Insight into the Role of Dissolved Organic Matter in Sorption of Sulfapyridine by Semiarid Soils

Sorption–desorption behavior of sulfapyridine was studied with three distinct soil types low in organic carbon with or without the introduction of exogenous dissolved organic matter (DOM). Experiments with bulk soils yielded sorption coefficients equivalent to those obtained with soils richer in organic matter, indicating an important sorptive role for soil mineral matrices. Cointroduction of sulfapyridine with DOM significantly reduced sulfapyridine sorption. However, decreasing solution pH from ∼9 to ∼6 limited the effect of DOM and revealed the effect of ionic speciation of sulfapyridine on the sorption potential. Sulfapyridine sorption to soils precoated with DOM exhibited contrasting trends. Two of the coated soils exhibited decreased sorption of sulfapyridine probably due to blockage of sorption sites by DOM. Conversely, the third soil demonstrated cumulative adsorption of sulfapyridine. Precoating also enhanced sulfapyridine desorption, suggesting that sorption of sulfapyridine to mineral surfaces involves stronger chemisorptive binding as compared with interactions with sorbed DOM. The capacity of soil to sorb DOM as well as the chemical fractionation of DOM during sorption were found to significantly affect binding of sulfapyridine. Competition between preferentially sorbed DOM moieties (e.g., carboxyl, phenol) and sulfapyridine for sorption sites is proposed. This study suggests that the chemical nature of DOM can significantly affect the fate of sulfonamide compounds in soils.