Diffusion coefficients of polycyclic aromatic hydrocarbons in supercritical carbon dioxide: A molecular dynamics simulation study

Supercritical carbon dioxide (SC-CO₂) extraction technology is an effective way to deal with soil contamination problems. Among them, polycyclic aromatic hydrocarbons (PAHs) are one of the main pollutants in soil. Studying the diffusion coefficients of PAHs in SC-CO₂ can help us better understand the mass transfer characteristics during the SC-CO₂ extraction process. Molecular dynamics simulation was applied in this work to study the diffusion behavior of PAHs in the SC-CO₂ environment. Under different SC-CO₂ thermodynamic states (temperature: 313–373 K, pressure: 10–30 MPa) and molecule sizes, the diffusion coefficients of PAHs were calculated. The trend of the interaction energy between PAHs and SC-CO₂ molecules as temperature, pressure, and molecule size changed was also investigated. The findings demonstrated that compared to heavy PAHs, light PAHs had a higher diffusion coefficient in SC-CO₂. Changes in the system's temperature and pressure have an important effect on the diffusion coefficients of PAHs in SC-CO₂. The high-temperature and low-pressure conditions had a promoting effect on the diffusion behavior of PAHs molecules. The van der Waals energy accounts for the largest proportion of the intermolecular interaction energy, which was about 80 %. It can be concluded that the van der Waals interaction energy played an important impact on the diffusion process of PAHs molecules in SC-CO₂.