Adsorption of organic molecules on clay surfaces

Overview

A collaborative effort between researchers at the Universities of Bath and Cambridge, using two different modelling techniques, has led to the refinement of a model for the adsorption of polychloro-dibenzodioxins (PCDDs) (See Fig. 1), polychloro-dibenzofurans (PCDFs) and polychlorobiphenyls (PCBs) onto the pyrophyllite (001) surface.

Fig. 1: Fully chlorinated PCDD molecules above the pyrophyllite (001) surface

PCBs, PCDDs and PCDFs are not naturally present in the environment. Rather, they are a product of manufacturing processes such as the production of thermally insulating materials. PCBs are perfect products for such purposes, as they are extremely resistant to degradation. However, this also means that they persist in the environment, as there are no processes harsh enough to bring about their decomposition. Coupled with this, their toxicity and carcinogenic properties makes them a particularly important environmental problem. These chemicals are present in soils and aquatic systems, and move up the food chain until they reach humans, in whom they can bring about fertility problems, skin conditions, respitory illness and thyroid problems.

There are two common methods of remediation of areas polluted by PCBs. The first is bioremediation, and the scond is to trap the molecules on soil particles and thereafter remove them from the environment, requiring heating the soil particles to very high temperatures to decompose the PCBs. It is extremely useful, in this case, to understand the molecular processes governing the interaction of these chemicals with prevalent soil minerals, so that their behaviour in the environment can be modelled accurately and understood.

Scientific results

Two computational modelling approaches have been used to create a high quality model for the interaction of PCDDs with the (001) pyrophllite surface. Initially density functional theory (DFT) calculations were carried out using the SIESTA code. The DFT calculations provided information for the parameterisation of an accurate interatomic potential model for the adsorption of PCDDs on the pyrophyllite surface. Thereafter, these potentials were employed in the DL_POLY3 program and used to scan the potential energy surface to find the minimum energy configuration for the molecules. An unexpected result of these calculations is the strong dependance of adsorption energy on the degree of chlorination of the molecules, rather than the position of the chlorine atoms within the molecule (Fig. 2)

Fig. 2: Asorption energy versus number of chlorine atoms for PCDD molecules above the pyrophyllite (001) surface, calculated using empirical models

The geometries from the empirical potentail optimisations were taken and recalculated with DFT to explore the difference in the performance of the two models. It was found that there were differences in the potential energy surface between the DFT results and the empirical model. This is, in part, due to the fact that DFT allows the redistribution of electrons during optimisations, and also because DFT does not incorporate dispersive interactions, which are obviously important for the physisorbtion of these molecules to the clay surface.

Further work is needed to refine the model further. However, we have already learned a great deal about the magnitude of the adsorption of these molecules to clay surfaces, and the effect of chlorination on the adsorption energy.

Credits

This work was carried out by Arnaud Marmier and Steve Parker (Bath), Toby White, Kat Austen, Andrew Walker, Martin Dove and Emilio Artacho (Cambridge).