Co-sorption of sulfamethoxazole and Cu onto several soils with different properties and their binding mechanism

Influence of metal cation and surface iron oxide on the transport of sulfadiazine in saturated porous media

Sulfadiazine (SDZ) is an antibiotic frequently detected in soil and groundwater. The transport of SDZ in subsurface environment is a critical process affecting its retention in soil. To date, the effects of iron oxide and metal cation on the transport of SDZ remain largely unknown, so we investigated the transport properties of SDZ in cleaned and iron oxides coated quartz sand, as affected by the presence of conventional cations (Ca²⁺, Mg²⁺, K⁺, and Na⁺) and Cd²⁺ through column experiments and simulation. We found that iron oxide coating on sand surface inhibited the transport of SDZ, mainly due to hydrophobic effect, complexation, and electrostatic attraction. The inhibitory effect became more marked with increasing concentration of Cd²⁺. It favors the transport of Cd²⁺ due to the electrostatic repulsion between positively charged iron oxide and Cd²⁺. Ca²⁺ promoted the transport of SDZ in coated sand, while all the conventional cations had no effect on the transport of SDZ in cleaned sand. The increase in the concentration of Cd²⁺ favors the transport of SDZ in cleaned sand. However, in iron oxide coated sand, the influence of Cd²⁺ on the transport of SDZ was dependent on the concentration of Cd²⁺. At lower concentration of Cd²⁺ and by competition, the transport is favored. At high concentration, the transport is inhibited mainly due to the formation of ternary surface complexes. A convective-dispersive transport model was applied to simulate and interpret experimental data. Breakthrough curves fitted well with a one-site model (OSM), indicating that SDZ adsorption on the sand experiences reversible kinetic. A low level of K_F values with nearly linear sorption isotherm shows high mobility of SDZ and a high potential risk of surface and groundwater contamination. However, such high mobility can be reduced by increasing the content of iron oxides in porous media.