Adsorption Behavior of Microplastics for Typical Psychoactive Drugs

• • Adsorption kinetics for all four microplastics (PE, PP, PS, PVC) toward diazepam, fluoxetine, and mianserin conformed to a pseudo-second-order model, indicating chemisorption as the rate-limiting step; this implies that microplastic-drug interactions are not merely physical but involve specific chemical forces, affecting the reversibility and potential release in the environment. • • PE exhibited the highest adsorption capacity for fluoxetine, PP for mianserin, and PVC for diazepam, with Langmuir and Freundlich model fits exceeding 0.9; this selectivity suggests that polymer type dictates the dominant adsorption mechanism (e.g., hydrophobic partitioning for PE/PP, polar interactions for PVC), which is critical for predicting contaminant transport in polymer-specific waste streams. • • Diazepam adsorption on microplastics was maximal at pH 6.5–8.5, the typical pH range of natural surface waters, and decreased under acidic or alkaline conditions due to electrostatic repulsion; this pH dependency implies that environmental pH fluctuations can modulate the bioavailability and mobility of psychoactive drugs, with potential implications for aquatic toxicity. • • Increasing NaCl concentration enhanced diazepam adsorption, with equilibrium adsorption capacity rising as ionic strength increased; this salting-out effect, driven by non-electrostatic interactions, indicates that in marine or brackish environments, microplastics may act as stronger sinks for such contaminants, altering their fate and transport.