Freeze-thaw effect on adsorption and transport of two sulfonamides in soil: Batch and column studies

Freeze-thaw cycles (FTCs) have significant impacts on soil physicochemical properties, subsequently altering the fate of contaminants in soil. However, studies investigating the environmental behavior of antibiotics in soil subjected to FTCs are limited. This study investigated the effects of FTCs on the adsorption and transport of two commonly used sulfonamide antibiotics (SAs), sulfamethoxazole (SMX) and sulfapyridine (SPY), in soil. The results revealed that FTCs alter the adsorption behavior of SMX and SPY on the soil. Initially, after 1 FTC, the adsorption of both SMX and SPY decreased; however, subsequently, this adsorption gradually increased as the number of FTCs increased. This is because, during the FTCs, the increased soil pH hindered the adsorption of SAs by intensifying electrostatic repulsion between anionic SAs and soil particles. Subsequently, the increases in clay content, specific surface area (SA), small pores, and dissolved organic matter (DOM) provided more adsorption sites, overriding the initial pH effects and ultimately dominating the adsorption process. FTCs altered soil properties, which not only changed the adsorption of SAs but also induced the alteration of pore structure and the generation of preferential flow. During the vertical transport process, such changes in pore pathways played a dominant role, facilitating SMX and SPY transport in soil. The addition of heavy metals (Cd²⁺ and Cu²⁺) contributed to facilitating the transport of SMX and SPY in both unfrozen and freeze-thaw-treated soil columns. In the context of global climate change, this study offers valuable insights into the fate and environmental risks associated with pollutants in soil.