Insights into sorption and leaching behavior of sulfadiazine in soil as affected by humic acid

Purpose Sulfonamides with high mobility in the soil pose a significant threat to environmental ecology and human health. Organic matter is known to affect the sorption and transport of these antibiotics in soil, although the mechanism remains unclear. In this study, the effects of humic acid (HA) on the transport of sulfadiazine (SDZ) in soil were investigated to advance the understanding on the fate of SDZ in soil. Materials and methods Column experiments were conducted in combination with batch sorption experiments. The sorption and transport of SDZ in soil were simulated by the convection-dispersion model coupled with a linear sorption isotherm and kinetic adsorption processes. Fourier transform infrared spectroscopy was used to evaluate the molecular interactions and mechanisms of sorption. Results and discussion The sorption isotherms conformed to a linear model with low distribution coefficients (K-d = 9.84 L/kg). Increasing the HA content led to increased sorption and inhibited transport of SDZ in soil. The sorption of SDZ on HA decreased with increasing pH and decreasing ionic strength. The mechanisms mainly comprised hydrophobic partition, hydrogen bonding, complexation, and pi-pi interactions. Mineral clays also played an important role in the sorption of SDZ on soil. The sorption and transport of SDZ were adequately described by the model, which demonstrated that as the HA content of the soil increased, the sorption capacity for SDZ and proportion of the time-dependent kinetic adsorption process both increased. Retained SDZ could also release slowly into the surrounding medium posing a longer-term risk to the environment. Conclusions Humic acid increases the retention of SDZ in soil due to its high sorption capacity for SDZ. The sorption of SDZ on HA is dependent on solution chemistry such as pH and ionic strength. However, the retained SDZ would release slowly into the soil solution later. Thus, time-dependent sorption should be prioritized over sorption capacity in studies assessing the environmental fate and eco-toxicity of SDZ.