From Darcy to the Nanoscale: How Emerging Contaminants Challenge Existing Modeling Approaches in Contaminant Hydrology

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• Dr Kaveh Sookhak Lari, CSIRO
• Wednesday 13 November 2024, 14:30-15:30
• Unilever Lecture Theatre, Yusuf Hamied Department of Chemistry.

If you have a question about this talk, please contact Lisa Masters.

Traditionally, contaminant hydrology in the subsurface has relied on Darcy-scale models derived from upscaling the Navier-Stokes equations. These approaches have been effective for various complex scenarios, including multiphase and multi-component systems such as oil-contaminated sites undergoing microbial degradation. These models approximate fluid flow and contaminant transport based on bulk properties and averaged behaviors at the continuum scale. However, emerging contaminants like perfluoroalkyl substances (PFAS) present unique challenges that extend beyond the capabilities of these conventional approaches. PFAS are a large group of persistent and potentially carcinogenic chemicals characterized by their remarkable stability. They are prevalent in numerous contaminated sites, often at concentrations exceeding safe thresholds. Unlike typical passive solutes, PFAS can actively modify interfacial properties through solutocapillary effects, where concentration gradients change surface tension. Additionally, PFAS molecules can form micelles with diverse shapes, sizes, and aggregation numbers, depending on environmental conditions such as the types of counterions present. These nanoscale behaviors significantly affect transport properties, complicating the prediction of PFAS fate and mobility using traditional continuum models. Addressing these limitations necessitates a computational roadmap to upscale complex transport phenomena from the nanoscale to the Darcy scale. Molecular dynamics (MD) simulations have emerged as a valuable tool for characterizing PFAS interactions at the molecular level. However, most studies have focused on PFAS adsorption characteristics on solids as part of remediation strategies, leaving the behavior of PFAS at fluid interfaces underexplored. This is a critical gap that challenges the ability of Darcy-scale models to accurately represent PFAS transport and retardation. Even at the classical MD level, PFAS behavior in multiphase systems—such as micelle formation, adsorption at interfaces, and aggregation characteristics—varies significantly with environmental factors like counterion type. This variability raises questions about the reliability of fixed force fields traditionally used in MD simulations. It is necessary to validate the appropriateness of the force fields under given conditions, which can be achieved by comparing MD results with experimental data or through ab initio MD simulations. As a first step in this direction, we explore various options to study our system using ab initio MD. Given the computational demands of these methods, a hybrid QM/MM approach may be applied, where the quantum mechanical region captures chemically significant interactions while the molecular mechanics region accounts for the larger environment. The feasibility of Density Functional Theory (DFT) and wavefunction-based approaches will be assessed. These results can then be used to validate classical MD simulations and ensure the accuracy of the applied force fields under specific conditions. This multi-scale framework aims to bridge the gap between nanoscale molecular insights and macroscale modeling in contaminant hydrology, providing a more robust approach to addressing PFAS -related challenges.

This talk is part of the Theory - Chemistry Research Interest Group series.

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