Theoretical Study on Adsorption of Halogenated Benzenes on Montmorillonites Modified With M(I)/M(II) Cations

Abstract

Halogenated benzenes (HBs) are hydrophobic organic chemicals belonging to persistent organic pollutants. Owing to their persistence, they represent a serious problem in environmental contamination, specifically of soils and sediments. One of the most important physical processes determining the fate of HBs in soils is adsorption to main soil components such as soil organic matter and soil minerals. Smectites, layered clay minerals of the 2:1 type, are common minerals in clay-rich soils, of which montmorillonite (Mt) is a typical representative. This work focuses on a systematic modeling study of the adsorption mechanism of selected HBs interacting with the basal (001) surface, which is the dominant surface of Mt particles. The HB···Mt interactions were studied by means of a quantum chemical approach based on the density functional theory method. HBs were represented by five molecules, particularly C₆F₆, C₆Cl₃F₃, C₆Cl₆, C₆Br₃Cl₃, and C₆Br₆. In mixed HBs (C₆Cl₃F₃ and C₆Br₃Cl₃) Cl atoms are in 1,3,5 or rather 2,4,6 positions. The effect of a different cation type on adsorption was investigated for M⁺/M²⁺-Mt models with cations from alkali group (M⁺: Li, K, Na, Rb, Cs) and alkaline earth metal group (M²⁺: Mg, Ca, Sr., Ba). The calculations were also performed on the gas phase HB···M⁺/M²⁺ complexes for comparison. Adsorption energies and distances of the main HB molecular plane from the Mt surface were calculated as a measure of the adsorption strength. The results showed that the strongest HB adsorption is for the Na⁺-Mt and Ca²⁺-Mt surfaces. The strongest affinity was observed for hexabromobenzene, while the weakest adsorption was found for hexafluorobenzene. The decomposition of the adsorption energy showed that its dominant component is dispersion energy and less important is the cation-π interaction. The calculated adsorption energies showed a good correlation with experimentally determined log K_d values.