Organic contaminants present a persistent concern to human and ecological health. Remediation <br/>efforts at sites contaminated with legacy pollutants produced and released prior to 1970 are ongoing, and newer emerging contaminants used widely in agriculture and consumer products are increasingly being recognized as damaging to environmental systems. A key measure of our understanding of a compound's fate and transport in the environment that also informs the development of removal and remediation methods is its behavior at the water-air interface. Here, we use molecular dynamics (MD)  simulations to determine interfacial properties for a broad range of approximately 80 organic contaminants of different origins and sizes. The compounds examined span those categorized as legacy contaminants (benzene, formaldehyde, PCBs, tetrachloroethylene) to more recently produced contaminants of emerging concern (non-ionic surfactants, PFASs, phthalates, pyrethroid pesticides) as well as ubiquitous pharmaceutical products (acetylsalicylic acid, ibuprofen, naproxen) and compounds naturally present at the air-water interface (acetic acid, glyoxal, isoprene). Each compound was simulated using the LAMMPS software in a water-air system consisting of a slab of water molecules between void space (air) for 40 nanoseconds. Umbrella sampling was used to bias the trajectory of each compound by applying a harmonic potential at regular intervals along the reaction coordinate, and a complete free energy landscape was assembled using the Weighted Histogram Analysis Method. The water-air partition coefficient and interfacial adsorption constant were determined from these results and compared to experimental values from the literature, when available. The agreement between simulated and experimental values provides confidence in the MD results for adsorption properties previously unreported. The library of compounds will be made publicly available for use in other systems such as mineral-water or biological systems.

Molecular dynamics simulations of organic contaminants at the water-air interface

ACS Spring 2022

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