Molecular dynamics simulation of organic contaminant adsorption on organic-coated smectite clay

Molecular dynamics simulations are used to examine the adsorption and aggregation of tyrosine and glutamate molecules on a stack of smectite particles at three different organic loadings. The results reveal a strong affinity of the smectite surface for the organic molecules with the zwitterionic tyrosine molecules coating the exterior surfaces (and entering the interlayer region to a lesser extent), whereas the negatively charged glutamate molecules are generally found near the clay edge sites and in coordination with aqueous and coordinating Ca ions. Additional simulations examine the effects of the tyrosine and glutamate organic coatings on the adsorption of two organic contaminants, dimethyl phthalate (DMP) and perfluorobutanesulfonic acid (PFBS). The addition of these coatings did not prevent the DMP and PFBS molecules from accessing previously identified favorable adsorption domains. An analysis of the adsorption energetics shows an initial decrease in contaminant adsorption relative to pure mineral surfaces as tyrosine and glutamate are introduced to the system, followed by increasing adsorption with increasing organic loadings. Overall, this research advances the mechanistic understanding of the interplay between smectite surfaces, organic coatings, and organic contaminants.