Clay minerals as sorbents for Hydrophobic Organic Chemicals

Hydrophobic organic chemicals (HOCs) include many halogenated aromatic pollutants that remain in the environment for decades and cause harmful effects on humans and biota. An understanding of HOC binding processes is needed to assess the transport, possible remobilization, and accumulation of these compounds. However, the structure-dependent mutual interactions of binding/adsorption between HOCs and clay minerals (CMs), as an essential component of soils, are poorly understood. The aim of the DFG-FWF bilateral project is to elucidate the mechanisms underlying HOCCM interactions with respect to the specific characteristics of both components. This aim will be achieved by combining laboratory experiments and computer simulations. The major objectives are: experimentally, (i) to quantify the adsorption of HOCs with different hydrophobicities to CMs of varying layer charge; (ii) to determine the impact of the exchangeable cation type on the adsorption of model HOCs to CMs; (iii) to determine sorption hysteresis for HOCCM interactions in terms of the specific surface and porosity of CMs; and, by using computer simulations, (iv) to reveal the molecular mechanisms underlying the formation of surface complexes and intercalates of HOCs with CMs; and (v) to quantify HOCCM interactions with respect to cation type and layer charge as well as determine the impact of the solvent on the stability of HOCCM complexes. HOCCM interactions will be investigated in laboratory adsorption experiments accompanied by advanced experimental techniques (e.g., X-ray diffraction, transmission electron microscopy, atomic force microscopy) to obtain a complex characterization of the samples. The experiments will be conducted using five halogenated HOCs and twenty CMs (mostly smectites) with a defined layer charge and chemical composition, including the type of exchangeable cation. For selected HOCs and CMs, adsorption/desorption experiments will be performed to investigate sorption hysteresis. The HOCs will be extracted by solventless, miniaturized solid-phase microextraction. Computer simulations will represent a combination of quantum chemical methods and classical (force field) molecular dynamics using models determined on the basis of the experimental CM characterization. Computer simulations will focus on elucidating the mechanisms of HOCCM interactions at the molecular scale and on determining the impacts of layer charge, cation type, and solvent effects on the stability of HOCCM complexes. In specific cases, Grand Canonical Monte Carlo is planned for the prediction of adsorption isotherms. The collaboration between the German group (laboratory experiments) and the Austrian group (molecular modeling) will yield positive synergistic effects regarding new insights into HOCCM interactions. The results will contribute to a better understanding of the environmental fate of HOCs and will improve assessments of the risk posed by these hazard compounds.

Clay minerals as sorbents for Hydrophobic Organic Chemicals (ClayHOC) Project coordinator: in Austria: Martin H. Gerzabek, in Germany: Dr. Leonard Böhm The project focused on a systematic theoretical and experimental investigation of interactions of hydrophobic organic chemicals (HOCs) with clay minerals (CMs). HOCs such as halogenated aromatic hydrocarbons, may be highly persistent in the environment and cause harmful effects on humans and biota. They can be bound in relevant amounts by clay minerals, but the structure-dependent mutual interactions between CMs (excess layer charge, specific surface, porosity, and cation type and distribution) and HOCs (molecular size, electronic structure, and hydrophobicity) are poorly understood. The aim of the DFG-FWF bilateral project was to elucidate the mechanisms underlying HOC-CM interactions with respect to the specific characteristics of both components. The methodological concept was to combine adsorption experiments (German partner) and molecular modeling methods (Austrian partner). The hypotheses postulated in the project claimed that the main factors affecting the strength of the binding of HOCs to CM are excessive layer charge of CMs and the hydration (water shell) of cations compensating the layer charge. Based on the defined goals and postulated hypotheses, the German partner collected and characterized a set of clay minerals, specifically of the smectite type (belonging to the so-called 2:1 clay minerals with two Si-tetrahedra layers and one Al-octahedra layer) that differed by composition and layer charge. The adsorption experiments were performed on selected CMs with halogenated benzenes (e.g., the well-known hexachlorobenzene, HCB). These experiments helped to prepare structural models for molecular simulations based on the density functional theory (computational chemistry). It was shown that the type of the compensating cation has a strong impact on the interaction of HCB with Mnt layers and the strength of the interaction correlates with physical parameters of the cations. These theoretical findings correlated well with the experimental observations. Further, numerous models of the Calcium-CM type with a varying layer charge were developed for investigating the impact of the charge on the binding of HCB. These models were also used in the simulation of the hydration (water shell) effect by two different approaches. It was shown that the layer charge has a smaller effect on the strength of the HCB adsorption. By calculating adsorption energies, it became obvious that the hydration significantly reduces the interaction of HCB with clay surfaces and strongly destabilizes the adsorption complex. The achieved theoretical results significantly contributed to the understanding of the adsorption mechanism of HOCs molecules to CMs and opened a new way for further theoretical investigation in this field, particularly for applying molecular dynamics (MD) and developing of extended structural models.