(70x) Physico-Chemical Limitations during the Electrokinetic Treatment of a Polluted Soil

Contaminated industrial sites represent an important danger for the environment and a treatment or a confinement of these sites is necessary. Polluted soils close to cities represent important economic opportunities for development and, in these cases, confinement is not an available solution. However, treatment processes usually employed are sometimes ineffective especially in the case of soils which have a very low hydraulic permeability or which are contaminated by heavy metals.

Electrokinetic remediation is being used in the United-States and in the Netherlands, for many years, for the treatment of low-permeable soils containing several inorganic (like heavy metals) or organic (like light hydrocarbons) chemical species. Electrokinetic processes are based on two transport phenomena promoted by the application of an electrical field on a porous media: electromigration and electro-osmosis. Electromigration is the transport of ions in solution with respect to the solution [1]. Electro-osmosis takes place at the solid/liquid interface of the medium where a displacement of the free water molecules is induced [1]. The electrochemical reactions at the electrodes are responsible for the electrical circuit continuity.

In addition to these mechanisms, the classical mass transport and the physical and chemical interactions of the species with the medium take place. For modeling, classical transport mechanisms are usually neglected : diffusion is very much slower than the electrokinetic transport and the processes concern usually non-permeable media.

Most of publications on electrokinetics are based on experiments carried out at laboratory or pilot plant scale, with artificially polluted media and, very often, species reactivity is not taken into account. For example, metallic pollutants in the solid phase are considered to be completely dissolved by the protons produced at the anode by water electrolysis [2][3]. Different industrial tests have failed or have obtained moderated results because of this point [4]. On the other hand, the authors are in general assuming that transport phenomena in soils are very slow and then chemical reactions can be considered at the equilibrium. Jacobs and Probstein [5] has showed that the characteristic times of the different phenomena can be similar enough to take into account possible chemical limitations for transport species. Finally, kaolinite is often used as a model for a non-permeable medium because of its light adsorption capacity, and chemical reactions are usually neglected.

Even when the reactions are fast enough to be considered as equilibrated, the electrokinetic process is strongly affected by these chemical reactions, almost in two ways: the characteristic time of the process is longer and the amount of the species recovered is controlled by the chemical reactions. The aim of this work was to illustrate the time delay induced on the species transport by the solid/liquid chemical reactions, through an experimental and theoretical work carried on for a model system.

The influence of the solid/liquid (S/L) reactions on the characteristic time of the process is demonstrated by the study of the lithium (Li+) electrokinetic transport on a sodium (Na+) pre-saturated kaolinite. Kaolinite has been selected as model low-permeable medium, even when it is usually considered to have low ion exchange capacity and weak selectivity.

Experimental results have been obtained in a device specially developed for the study of the electrokinetic transport and well characterized [6]. The tests have been performed by introduction of a solution containing Li+ at the anode, and measurement of the Na+ and Li+ concentrations at the cathode. Li+/Na+ ion exchange equilibrium has been determined by independent experiments on a laboratory column.

At the operating conditions, ion exchange equilibrium can be well described by a linear isotherm. A theoretical model based on the tanks-in-series model has been developed for modeling the experimental results of the Li+ electrokinetic transport. This model consider electromigration and electro-osmosis as the only transport mechanisms. The comparison between experimental and theoretical results shows that the chemical S/L reaction at least double the characteristic time of the transport.

References

[1] J.K. Mitchell, Fundamentals of soil behaviour, John Wiley & Sons, Inc. (Ed.), 2nd ed., (1993). [2] Y.B. Acar, J. Hamed, Electrokinetic Soil Processing in Waste Remediation and Treatment: Synthesis of Available Data, Transportation Research Record, N°.1312 Energy and Environment Issues (1991) 153-161. [3] Y.B. Acar, A.N. Alshawabkeh, Electrokinetic Remediation. Pilot-Scale Tests with Lead-Spiked Kaolinite, Journal of Geotechnical Engineering, March (1996) 173-184. [4] R. Lageman, W. Pool, G. Seffinga, Electro-Reclamation: Theory and Practice, Chemistry & Industry, September (1989) 585-590. [5] R.A. Jacobs, R.F. Probstein, Two-dimensional modeling of electroremediation, J. Aiche 42 (6) (1996) 2648-2650. [6] V. Pomès, A. Fernandez, D. Houi, Characteristic time determination for transport phenomena during the electrokinetic treatment of a porous medium, Chemical Engineering Journal, 3938 (2001) 1-11.