Adsorption/desorption behavior and mechanisms of sulfadiazine and sulfamethoxazole in agricultural soil systems

This study focused on the adsorption/desorption behavior and mechanisms of sulfadiazine (SDZ) and sulfamethoxazole (SMX) in two Chinese agricultural soils. The sorption on these two soil systems was investigated through a batch reactor at different initial concentrations (0.5-10 mg L-1), different pH values (4-9) and ionic strengths (0.01-0.5 M). The obtained isotherms were linear within experimental uncertainty. The reversibility of adsorption process was assessed through desorption experiments of SDZ and SMX loaded soil samples. The results demonstrated that sorption was mainly influenced by the physicochemical properties of soil and the adsorbable species. It means that the sorption ability is stronger at higher organic carbon, higher clay fraction, lower pH value and intermediate ionic strength. An apparent decrease in the sorption capacity of SDZ and SMX with increasing pH was primarily attributed to the electrostatic repulsion between anionic species and negatively charged soil particles. Under various ionic strengths, the sorption affinity of SDZ and SMX in two soils revealed that sorption first increased and then decreased with increasing ionic strength. The thermodynamic analyses suggested that the sorption of SDZ and SMX was a spontaneous process. The results from Fourier transform infrared spectroscopy (FUR) verified that the sorption of these sulfonamides was dominated by hydrogen bonds, electrostatic interactions and pi-pi interactions. This study contributed substantially to understanding the adsorption/desorption behavior and mechanisms of SDZ and SMX in the environment.