FWR Report FR/INV0003

Dec 1997

The water industry uses activated carbon to remove pollutants from raw water. A herbicide like atrazine uses only 5% of the total carbon surface. Optimising the adsorptive capacity of the carbon will increase bed life and so reduce costs. To do this we need to understand the mechanism of adsorption of organic residues by the active carbon used in water purification. Accordingly the aim of the research was to develop understanding of the interaction of organic residues with carbon surfaces. A computer-based molecular modelling approach was proposed. We also investigated the interactions of the chlorocarbon pollutants trichloroethene and l,l,l-trichloroethane with a carbon surface. They were chosen because of the water pollution problems which they present, because good experimental data on their adsorption are available, and because the molecules are sufficiently small to be amenable to accurate quantum mechanical calculations.

This Report summarises the outcome of work funded by FWR during 1995/1996. The aim of the work was to examine the utility of computer modelling in developing understanding of the interaction of carbon surfaces with pesticide residues and chlorocarbon pollutants encountered in the water industry. We have succeeded in calculating the interaction energies of the free and hydrated molecules with a carbon surface using a Monte Carlo procedure and a more computationally demanding quantum mechanical procedure. Characteristic molecular fragments of the herbicides were also studied.

Our main results are the following:-

• The interaction of molecules with carbon was enhanced by unsaturation (benzene vs cyclohexane; ethene vs ethane) and by alkyl and chlorine substituents.
• Molecules with aromatic rings were oriented with the rings parallel with the carbon surface.

 We also surveyed experimental data on the adsorption of the compounds on carbon and compared the experimental and computational results.

 We recommend future work should be directed towards improving the model of the carbon by incorporating oxo- and hydroxo-groups and by building three-dimensional structures to study porosity. The modelling of the herbicide and other molecules should include further work on hydration together with a systematic study of the surface interactions of the characteristic groups which make up the molecules.

Copies of the Report are available from FWR, price £25 less 20% to FWR Members.