Study on the Adsorption Behavior of Typical β-blockers on Microplastics in Water

Microplastics, as vectors of contaminants, can adsorb organic pollutants in aquatic environments such as beta-blockers, which may affect their environmental fate and also cause unpredictable ecological risks. The adsorption performances of typical β-blockers propranolol (PRO) and atenolol (ATL) on microplastics polypropylene (PP) and polyethylene (PE) were studied in this paper. The two types of microplastics were characterized by scanning electron microscopy and Fourier transform infrared absorption spectroscopy. The adsorption behaviors of PRO and ATL by PP and PE microplastics were comparatively analyzed based on batch adsorption experiments, and the influences of different environmental factors on the adsorption process were investigated. The experimental results show that PP had higher adsorption capacity for PRO and ATL than PE because of its rough and porous surface (specific surface area of 2.625 2 m²·g^-1) and lower crystallinity (51.44%). PRO exhibited a higher adsorption capacity on hydrophobic PP than ATL due to its high hydrophobicity (lg K_ow ≈ 3.48), whereas the adsorption capacity of PRO on PE is not much different from that of ATL. Furthermore, the influence of environmental factors on adsorption behavior was explored and results indicated that the adsorption capacity of PRO was significantly increased with the increase of pH from 3 to 9 (the dominant adsorption mechanism shifted from electrostatic attractions to hydrophobic interactions), and the adsorption capacity did not obviously change when pH > 10. But ATL adsorption was less affected by solution pH. The PRO and ATL adsorption were inhibited by salinity with Ca²⁺ exhibiting a stronger inhibitory effect than Na⁺ (cation competitive adsorption). Humic acid has a dual effect on PRO adsorption. When the concentration of humic acid is low (< 100 mg·L^-1), PRO adsorption could be promoted through bridging, while its adsorption was decreased due to site competition when the concentration of humic acid is high (> 150 mg·L^-1). But humic acid had little effect on hydrophilic ATL adsorption. The adsorption kinetics, isothermal characteristics of two different hydrophobic β-blockers by PP and PE, and key environmental impact factors were discussed in this study. These results will provide an important theoretical basis for evaluating the migration and transformation of β-blockers and potential ecological risks in aquatic environments containing microplastics.