Adsorption of oxytetracycline in hyporheic zone sediments mediated by microplastics: Experimental revelations and mechanistic insights

Microplastics (MPs), serving as carriers, possess a mediating effect on altering antibiotics (ATs) migration, yet this novel effect has not received due attention in the vulnerable hyporheic zones where surface water and groundwater converge. This study collected sediment layers from the typical hyporheic zone of the Weihe River, focusing on oxytetracycline (OTC) detected therein, and employed three types of MPs (polyethylene (PE), polypropylene (PP), and polymethyl methacrylate (PMMA) to establish various mediated adsorption systems. Through batch equilibrium adsorption experiments, morphological and spectroscopic characterizations combined with the impact of external environmental factors, the changes in OTC adsorption characteristics and the interactions before and after MPs mediation were systematically examined. The results indicate that MPs-mediated adsorption led to the formation of a rough, porous amorphous structure on the sediment surface, with potential internal hydrogen bonds. This exhibited an additional micropore diffusion step that delayed the adsorption kinetics equilibrium time, increased the isothermal maximum adsorption capacity along with desorption hysteresis effects, shifted thermodynamic enthalpy from physically dominated to semi-chemical adsorption participation, and resulted in an increased tendency towards disorder in entropy. The pH, ionic strength, and aging of MPs all affect the charge balance within the adsorption system, while MPs, OTC, and sediments can interact through the negative-charge-assisted hydrogen bond ((-)CAHB). The mediating effect of MPs stems from their role as receptors, facilitating the formation of (-)CAHB and thus crosslinking an amorphous structure available for OTC micropore filling. Compared to PE and PP, PMMA provides receptors through an additional free radical pathway, hence possessing a stronger mediating effect. These findings contribute to reevaluating the environmental fate of ATs and MPs from a new perspective, offering theoretical references for risk assessment and management of emerging pollutants.