Toxicity Evaluation of Microplastics Exposure in Water Contaminated with Pyrene and Methyl Chlorpyrifos Using Daphnia magna

Abstract In freshwater environments, microplastics (MPs) have emerged as important organic contaminants vectors. This study investigates the individual and combined toxicity of microparticles of polystyrene (PS) and polylactic acid (PLA) with two contaminants of major concern in Spanish waters─pyrene (PYR) and methyl chlorpyrifos (CPF). Adsorption behavior was assessed using adsorption isotherms fitted to mathematical models such as Henry, Langmuir, Freundlich, and Temkin. PYR adsorption on both polymers followed monolayer behavior (Langmuir model), while CPF adsorption was best described by the Freundlich model, indicating heterogeneous multilayer adsorption. Toxicity of these particles was assessed in Daphnia magna using acute and chronic toxicity assays. Acute toxicity tests showed that coexposure to MPs and contaminants increased immobilization, suggesting that adsorption onto MPs may influence contaminant partitioning and exposure. When organisms were exposed only to the contaminant remaining in the aqueous phase of CPF and PYR (after adsorption), EC50 values increased due to this reduced concentration. Regarding chronic exposure, significant effects were observed on Daphnia survival and reproduction, particularly when PS contained adsorbed contaminants on its surface. Therefore, this study indicates that MPs modify contaminant behavior and provides mechanistic insight relevant for understanding contaminant exposure and hazard processes in aquatic systems.