Adsorption of the hydrophobic organic pollutant hexachlorobenzene to phyllosilicate minerals

Abstract

Hexachlorobenzene (HCB), a representative of hydrophobic organic chemicals (HOC), belongs to the group of persistent organic pollutants (POPs) that can have harmful effects on humans and other biota. Sorption processes in soils and sediments largely determine the fate of HCB and the risks arising from the compound in the environment. In this context, especially HOC-organic matter interactions are intensively studied, whereas knowledge of HOC adsorption to mineral phases (e.g., clay minerals) is comparatively limited. In this work, we performed batch adsorption experiments of HCB on a set of twelve phyllosilicate mineral sorbents that comprised several smectites, kaolinite, hectorite, chlorite, vermiculite, and illite. The effect of charge and size of exchangeable cations on HCB adsorption was studied using the source clay montmorillonite STx-1b after treatment with nine types of alkali (M⁺: Li, K, Na, Rb, Cs) and alkaline earth metal cations (M²⁺: Mg, Ca, Sr, Ba). Molecular modeling simulations based on density functional theory (DFT) calculations to reveal the effect of different cations on the adsorption energy in a selected HCB-clay mineral system accompanied this study. Results for HCB adsorption to minerals showed a large variation of solid-liquid adsorption constants K_d over four orders of magnitude (log K_d 0.9-3.3). Experiments with cation-modified montmorillonite resulted in increasing HCB adsorption with decreasing hydrated radii of exchangeable cations (log K_d 1.3-3.8 for M⁺ and 1.3-1.4 for M²⁺). DFT calculations predicted (gas phase) adsorption energies (- 76 to - 24 kJ mol^-1 for M⁺ and - 96 to - 71 kJ mol^-1 for M²⁺) showing a good correlation with K_d values for M²⁺-modified montmorillonite, whereas a discrepancy was observed for M⁺-modified montmorillonite. Supported by further calculations, this indicated that the solvent effect plays a relevant role in the adsorption process. Our results provide insight into the influence of minerals on HOC adsorption using HCB as an example and support the relevance of minerals for the environmental fate of HOCs such as for long-term source/sink phenomena in soils and sediments.

Keywords: Cations; Clay minerals; Environmental fate; Halogenated aromatic hydrocarbons; Hydration enthalpy; Molecular simulations; Montmorillonite; Persistent organic pollutants (POP).