Sur l'importance des hétérogénéités du sous-sol en électrodépollution

Résumé

Des études expérimentales de dépollution par méthode électrocinétique de sols contaminés nous ont révélé que d'excellents résultats peuvent être obtenus avec des sols argileux homogènes et pollués par du Cd, tant lors d'essais en cellules qu'en s'assujettissant de l'effet d'échelle lors d'essais pilotes conduits sur plus de 3 tonnes de matériaux. Par contre l'introduction d'une discontinuité de stratification, par exemple la superposition de deux couches d'argile légèrement différentes, toutes deux polluées de manière identique, nous montre que le taux de dépollution chute immédiatement, la cause en étant l'introduction d'une hétérogénéité de stratification.

D'autres types d'hétérogénéités fortes ou diffuses vont probablement poser problème. Ceci tend à atténuer l'intérêt de la méthode d'électrodépollution dans son application directe sur le terrain.

Abstract

 Recently, attention has focused on electrokinetic techniques that make it possible to move and extract contaminants from fine-grained soil under an electric field (Fig. 1). The application of a constant electric current has several effects: (1) electrolysis of water, plating reactions and gas formation occur at the electrodes. H⁺ is produced at the anode, and OH^– at the cathode (2) the electric potential difference leads to electro-osmosis, the pore water flow is toward the cathode, since most soils have a negative surface charge (3) the electric field initiates electro-migration of species available in the pore fluid and of those introduced at the electrolytes. These phenomena change the chemical pore fluid composition and induce sorption reactions in the soil. Bench scale studies showed that inorganic species and heavy metals such as arsenic, cadmium, chrome, copper, iron, lead, mercury, nickel and zinc can be efficiently extracted from polluted or spiked soils.

The feasibility and efficiency of transporting cadmium with this treatment are investigated at pilot scale in a 3.25 t clayey medium specimen spiked with cadmium nitrate solution and at an electrode spacing of 100 cm. Sulphuric acid is added to catholyte to neutralize the hydroxides generated at the cathode. A constant current density of 0.3 mA/cm² is applied. The test is conducted with cadmium at a concentration of 882 mg/kg of dried matter. Cadmium is transported towards the catholyte and plated on the cathode (Fig. 2). After 3,259 h of processing and an energy expenditure of 159 kWh/m³, 98.5% of the cadmium has been removed (Fig. 3).

A second treatment test is investigated at pilot scale in one 3.437 t clayey medium specimen. It is composed of two different layers spiked with cadmium at level of 919 mg/kg of dried matter (Fig. 4). An electrode spacing of 100 cm is used. Catholyte pH is maintained near 3 and a constant current density of 0.3 mA/cm² is applied. Cadmium is transported toward the catholyte and plated on the cathode (Fig. 5). Subsequent to 3,260 h of processing, it appears that a part of the second layer has been isolated during the test. There, cadmium concentrations are still important, whereas they are low everywhere else (Fig. 6). The result shows that the rate of removal is not the same in the two beds. This simple heterogeneity seems to play a major role.

There are a lot of heterogeneities in soil, such as stratification, mineralogic composition variation, or permeability variation. These major or diffuse heterogeneities play an important role in a electrokinetic treatment because they control the current density repartition, for example the water table (Fig. 7). As a result, some parts of the soil can be ignored by the electroremediation and remain polluted. For these reasons, it seems more realistic in most cases to excavate and to homogenize the polluted soil and to electroremediate it in tanks instead of using an in situ treatment.