(312d) Investigating the Potential of Boron Nitride Materials for PFOA Removal from Water Using Molecular Dynamics

Per- and polyfluoroalkyl substances (PFAS) represent a pervasive class of synthetic organofluorine chemicals that have garnered significant global concern due to their environmental persistence, bioaccumulation potential, and adverse health effects. Their unique combination of hydrophobic and lipophobic properties has led to their widespread use in numerous industrial and consumer applications, ranging from non-stick cookware and firefighting foams to textiles and food packaging. The recalcitrance of PFAS to natural degradation processes and their ability to migrate readily through environmental media pose a significant challenge to remediation efforts and risk assessment. Consequently, PFAS are ubiquitous contaminants found in water sources, soil, air, and even human and animal tissues.

This work delves into the application of molecular modelling methodologies to study the removal of PFAS from aqueous streams. In particular, we explore the potential of Boron Nitride (BN) porous materials to adsorb perfluorooctanoic acid (PFOA) [[1]]. PFOA is a prototypical PFAS produced and used worldwide as an industrial surfactant in chemical processes and as a material feedstock. PFOA is considered a surfactant, or fluorosurfactant, due to its chemical structure consisting of a perfluorinated, n-heptyl "tail group" and a carboxylic acid "head group". BN materials have been proposed [[2]] as potential photocatalysts for the in-situ destruction of PFOA; hence, the combined adsorption/removal process could potentially be performed in a single unit operation.

We employ the pcff+ forcefield, validated against experimental contact angles. We report large-scale molecular dynamics (MD) simulations of water-PFOA systems under ambient conditions. We consider bulk system (solubility) and static adsorption unto BN nanotubes of different pore widths. It is seen that while PFOA molecules have a high solubility in water they have a propensity to adsorb unto the BN surfaces. After the initial adsorption, PFOA molecules cluster on the surface, but do not spread across the entirety of the available surface area. On higher surface concentrations PFOA self-assembles into structured liquid-crystal - like fluids and even forms micelle-like structures.

Lastly, we have considered boundary-driven non-equilibrium MD simulations [[3]] where a BN membrane composed of aligned nanotubes is placed in contact with a stream of water with PFOA. Transport diffusion coefficients and adsorption are monitored throughout the simulation. It is observed that even under flow conditions, BN membranes are effective way in removing PFOA from aqueous streams.

References

[1] Dasetty, S. et al. Data-Driven Discovery of Linear Molecular Probes with Optimal Selective Affinity for PFAS in Water. J. Chem. Eng. Data 68, 3148–3161 (2023).

[2] Duan, L. et al. Efficient Photocatalytic PFOA Degradation over Boron Nitride. Environ. Sci. Technol. Lett. 7, 613–619 (2020).

[3] Frentrup, H., Avendaño, C., Horsch, M. & Müller, E. A. Modelling Fluid Flow in Nanoporous Membrane Materials via Non–equilibrium Molecular Dynamics. Procedia Engineer 44, 383–385 (2012).