Effect of cation type in mixed Ca-Na systems on transport of sulfonamide antibiotics in saturated limestone porous media

Retention and transport of sulfonamides (SAs) in subsurface can strongly affect groundwater quality. In this work, a range of laboratory batch sorption and column transport experiments were conducted to determine the effect of cation type in mixed Ca-Na systems on the retention and transport of two typical SAs, sulfadimethoxine (SDM) and sulfacetamide (SCA), in saturated limestone porous media. Column experimental data showed divalent cation Ca 2+ played a more important role than monovalent cation Na + in decreasing the transport of only SDM in co-cation systems in the saturated limestone media. Further, in the single-cation (i.e., including either Ca 2+ or Na + ) system, increasing ionic strength (IS) of either NaCl or CaCl 2 had little effect on SCA transport; however, increasing of IS of CaCl 2 promoted the retention of SDM in the saturated limestone porous media. This is mainly due to the cation bridging effect of Ca 2+ on SDM and limestone. Overall, SDM showed much higher retention in the limestone columns than SCA, which can be attributed to the two SAs’ different physicochemical properties. Moreover, limestone showed stronger ability to retain the two SAs than quartz sand. Findings in this study suggest that cation type and the concentration of certain electrolyte (e.g., CaCl 2 ) as well as medium type play an important role in controlling the environmental fate and transport of antibiotics.