Adsorption Behavior of Microplastics for Typical Psychoactive Drugs
Microplastics, as emerging environmental pollutants, can adsorb psychotropic drugs in aquatic environments, facilitating their migration and transformation, ultimately posing ecological risks. This study investigated the adsorption behavior and mechanisms of four common microplastics—polyethylene (PE), polypropylene (PP), polystyrene (PS), and polyvinyl chloride (PVC)—each with a particle size of 50 μm, toward three psychoactive drugs: diazepam, fluoxetine, and mianserin. Adsorption kinetics, isotherms, and the effects of pH and salinity were examined. Kinetic data fitted well to a pseudo-second-order model, indicating chemisorption as the rate-limiting step. Isotherm analysis using Langmuir and Freundlich models revealed that PE exhibited the highest affinity for fluoxetine, PP for mianserin, and PVC for diazepam, while PS showed linear adsorption for fluoxetine, suggesting partitioning. The adsorption of diazepam was maximal at pH 6.5–8.5, typical of natural surface waters, and increased with NaCl concentration, indicating that non-electrostatic interactions dominate and that higher ionic strength enhances adsorption. Mechanistic insights suggest that hydrophobic interactions, hydrogen bonding, π-π interactions (for PS), and halogen bonding (for fluoxetine) contribute to adsorption. These findings highlight the potential of microplastics to act as vectors for psychoactive drugs, necessitating further research on their environmental fate and ecological implications.