Adsorption of chlorophenols on polyethylene terephthalate microplastics from aqueous environments: Kinetics, mechanisms and influencing factors

Microplastics have received growing attention as carriers of organic pollutants in the water environment. To better understand the contribution of hydrophobic interaction, hydrogen-bonding interaction, π-π interaction and electrostatic interaction on the adsorption of hydrophilic compounds on microplastics and their adsorption behavior in natural waters, polyethylene terephthalate (PET, <150 μm) was used as an adsorbent and 4-chlorophenol (MCP), 2,4-dichlorophenol (DCP) and 2,4,6-trichlorophenol (TCP) were used as adsorbates. The results of batch adsorption experiments showed that chlorophenols (CPs) reached adsorption sites of PET through film diffusion and intra-particle diffusion. pH greatly affected the adsorption capacity. Hydrophobic interaction was the main adsorption mechanism of undissociated CPs on PET. Hydrogen-bonding interaction was also an adsorption mechanism between undissociated CPs and PET, and the contribution of hydrogen-bonding interaction to adsorption decreased with the increase of chlorine content. Meanwhile, the increase of chlorine content was favorable to the hydrophobic interaction between undissociated CPs and PET. However, higher chlorine content CPs with lower pK values tended to dissociate at neutral pH condition and resulted in stronger electrostatic repulsion with PET. The increase of solution ionic strength and fulvic acid content negatively affected the adsorption of DCP and TCP on PET, but did not show significant impacts on MCP adsorption. Similarly, the adsorption capacity obtained using Taihu lake water and Bohai seawater as matrices was much lower than that using laboratory water for both DCP and TCP, while the adsorption coefficient (K) of MCP remained at approximately 10.6 L/kg to 11.4 L/kg in the three different solution matrices. The K values exhibited using natural water matrices consistently followed the order of DCP > MCP > TCP. This study provides insights into the fate of CPs in the presence of microplastics and suggests that the potential risks posed by CPs and microplastics to aqueous ecosystems merit further investigation.