Remove mechanisms of sulfamethazine by goethite: the contributions of pH and ionic strength

Sulfamethazine (SMT) as an ionic compound can enter the soil environment via the application of livestock wastes in agricultural fields and via the abuse of pharmaceuticals. Goethite as one of the most important iron oxides in soil might interact with SMT and influence its environmental behavior and bioavailability in soil. In this study, the sorption properties of SMT on goethite under different solution conditions were investigated. The sorption isotherm exhibited a significant nonlinear shape and desorption hysteresis, while the data could be fitted well by the Freundlich model with the correlation coefficient R-2 = 0.991. Sorption capacity initially increased and then decreased as pH values increased from 3 to 13, while the strongest uptake occurred in neutral conditions. The sorption increased slightly and then kept relatively constant as the ionic strength of the solution increased. The results indicated that the sorption mechanism would be altered in different solution conditions. In acidic and neutral conditions, the p-p electron donor-acceptor interactions and outer-and inner-sphere complexions might be the dominating sorption mechanisms. The sorption in alkali conditions might be dominated by electrostatic interactions between SMT and goethite. It should be noted that the heterogeneous sorption affinity of SMT on goethite under various solution conditions will influence its environmental fate.