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Adsorption mechanism of typical monohydroxyphenanthrene on polyvinyl chloride microplastics
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Researchers studied how a pollutant derived from phenanthrene (a polycyclic aromatic hydrocarbon) adsorbs onto polyvinyl chloride microplastics in water. They found the organic compound binds tightly to PVC surfaces, adding to evidence that microplastics can accumulate and transport toxic chemicals in aquatic environments.
To enrich data related to the interaction mechanism between microplastics and organic pollutants, in this study, 3-hydroxy-phenanthrene (3-OHP, C14H10O), a phenanthrene derivative, was selected as a representative pollutant, and polyvinyl chloride (PVC) microplastics were chosen as the research objects. We investigated the adsorption behavior of 3-OHP on PVC microplastics in aqueous solutions and explored the adsorption mechanism in detail. The PVC microplastics were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), and Fourier transform infrared (FT-IR) spectroscopy. The standard curves of the ultraviolet (UV) absorption spectrum of the target pollutant were obtained using a UV spectrophotometer. The fitting coefficient values of all standard curves were higher than 0.99 (R2>0.99). To ensure the accuracy of the UV absorption spectrum, the pollutant concentration gradient was set according to the absorbance (Abs) values, which were higher than 0.438. The measured concentrations were calculated using a standard curve equation. The adsorption mechanism of 3-OHP on PVC microplastics in an aqueous solution was studied by combining adsorption models (adsorption kinetics model, adsorption isotherm model, and adsorption thermodynamics model) and density functional theory (DFT) calculations. The results are as follows: (1) From the adsorption kinetics experiment, the pseudo-second-order kinetic model had the best fitting degree, and the fitting coefficient of adsorption kinetics was 0.998 (R2=0.998). Hence, 3-OHP adsorption on PVC microplastics may be attributed to surface adsorption and external liquid film diffusion; the equilibrium adsorption amount was 36.866 μg/g after 24 h. (2) The adsorption isotherm experiment showed that the Langmuir and Freundlich isotherm models were more suitable for describing the adsorption mechanism of 3-OHP adsorption on PVC microplastics because of the satisfactory fitting coefficient (R 2=0.956 and 0.907), suggesting that the adsorption mode was mainly single-layer adsorption with a small amount of multilayer adsorption. The maximum adsorption amount of 3-OHP adsorption on PVC microplastics was 408 μg/g; (3) the adsorption thermodynamics results showed that the adsorption efficiency of 3-OHP adsorption on PVC microplastics decreased with increasing temperature, indicating that the adsorption of 3-OHP on PVC microplastics was a spontaneous and exothermic adsorption process; (4) the salinity experiment results showed that salinity had little effect on the adsorption efficiency of 3-OHP on PVC microplastics; (5) DFT calculations showed that PVC had a relatively low binding energy to 3-OHP. Therefore, we suggest that the main adsorption mechanism of 3-OHP on PVC microplastics may be the hydrophobic effect; weak hydrogen bonding, halogen bonding, and π-π conjugate action could also play a role in 3-OHP adsorption on PVC. These results reveal the interaction mechanism between PVC microplastics and organic chemicals, and enhance our understanding of the environmental behavior of PVC microplastics in aqueous solutions. To serve as a reference in scientific evaluations of the environmental impact of microplastics, future studies should focus on obtaining toxicological data for the microplastics.
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