Relating Clay Structural Factors to Dioxin Adsorption by Smectites: Molecular Dynamics Simulations

Dioxin adsorption by smectite clays can greatly exceed that by soil organic matter when compared on an equal mass basis. Bulk‐phase adsorption experiments demonstrated that sorption of dioxin by smectites depended strongly on (i) the hydration properties of the exchangeable cations, (ii) the surface charge density of the smectite clay, and (iii) the location (tetrahedral vs. octahedral) of isomorphous substitution in the clay mineral structure. Molecular dynamics simulations of saponites saturated with different exchangeable cations (Na + , K + , Cs + , and Ca 2+ ) and one, two, or three layers of water in the interlayer regions were conducted to study the molecular environment of adsorbed dioxin in each system. The simulated radial distribution functions were computed to quantify the spatial correlations between O on dioxin (O dd ) and interlayer cations as well as water molecules. The magnitudes of the O dd –cation coordination numbers followed the order Cs + > K + > Na + > Ca 2+ , which agrees with the observed trend of dioxin adsorption to the smectites from water. The estimated adsorption enthalpies follow the trend Cs + < K + < Na + ∼ Ca 2+ , further supporting the hypothesis that O dd –cation complexation plays an important role in adsorption of dioxin in clay interlayers.