Relationship between land use, seasonality, hydrological conditions, and the adsorption of sulfamethoxazole, ciprofloxacin, and 17 β -estradiol onto polyethylene terephthalate microplastics in a small urban river system, Eastern Cape, South Africa

Conceptual diagram showing how land use, seasonality, and hydrological factors influence adsorption of SMX, CIP, and E2 onto PET microplastics in an urban river system Microplastics (MPs) are increasingly identified as contaminant vectors in aquatic systems. However, the role of land use, seasonality, and hydrological conditions in contaminant- microplastic (MP) interactions remains underexplored. This study investigated the adsorption of sulfamethoxazole (SMX), ciprofloxacin (CIP), and 17β-estradiol (E2) onto polyethylene terephthalate (PET) MPs in the Bloukrans River, South Africa, across four land use sites (municipal, wastewater, agriculture, and control) during summer and autumn. PET MPs were deployed in situ, retrieved, and analysed for adsorbed contaminants using LC-MS/MS. Sulfamethoxazole adsorption differed significantly by site (F(3,16) = 4.161, p = 0.0234) and season F(1,16) = 7.202, p = 0.016), with adsorption highest at wastewater and agriculture impacted sites, and autumn concentrations exceeding summer by 176.5 ng L−1. CIP adsorption varied significantly only by site (p = 0.0231) but not between seasons. E2 adsorption was non-significant (p > 0.05) for both sites and seasons. The hydrological analysis indicated that river width significantly influenced CIP, while SMX and E2 showed no hydrological dependence. By addressing the paucity of in situ evidence in small urban rivers of Africa, this study demonstrates that land use, seasonality, and hydrological conditions play an important role in influencing the adsorption of the selected compounds by PET MPs.