Molecular Dynamics Simulation of the Adsorption of Emerging Organic Contaminants by Smectite Mineral Surfaces

Abstract

The partitioning of organic contaminants between water and solid surfaces is a key process controlling their fate and transport in natural environments. In this dissertation we present a novel methodology to predict the adsorption of organic contaminants by smectite clay minerals (high specific surface area adsorbents abundant in natural soils) using molecular dynamics simulations. The methodology models a stack of flexible smectite lamellae in direct contact with a bulk aqueous reservoir and uses the metadynamics technique to facilitate the exploration of the free energy landscape. The methodology was tested and validated in the case of six phthalate esters, widely used chemical plasticizers, and later applied to study the behavior of three per- and polyfluoroalkyl substances (PFASs). The latter were simulated at various aqueous chemistry conditions to examine the effect of salinity and coordinating cation type on adsorption. Finally, soil organic matter proxies were introduced to the system at different loadings to examine how mineral surface coatings affect the extent and mechanism of adsorption. Simulation predictions reveal strong contaminant adsorption, especially for the larger and more hydrophobic molecules. Adsorption is observed primarily on the exterior basal surfaces with a strong inverse relationship between extent of adsorption and clay surface charge density as contaminant molecules preferentially occupy the more hydrophobic uncharged patches on each surface. Detailed analysis of the adsorption energetics reveals large favorable entropic contributions to adsorption. Overall, this research establishes a computational methodology capable of predicting water-clay partition coefficients, advances the mechanistic understanding of contaminant-smectite interactions, and provides new insights that could help inform fate and transport models and the development of adsorbents and remediation techniques.