In situ chemical oxidation of the unsaturated zone of soils contaminated with coal tar: from the laboratory to the field

In France, hundreds of brownfields are currently polluted with coal tar, a complex and recalcitrant mixture of organic compounds such as polycyclic aromatic hydrocarbons. When the unsaturated zone of soils is highly contaminated, it is commonly excavated and backfilled. This work deals with an alternative treatment, in situ chemical oxidation, that allows a potential reuse of soils directly on site. This technique has already provided encouraging results at the lab scale but has rarely been tested in the field. Research was made around three scales - bibliography, laboratory and pilot (lysimetric plots) - in order to respond to the main problem: is there an oxidative treatment able to meet technical, economic and environmental criteria quite acceptable to be applied at brownfield level? The laboratory research phase showed that the addition of a moderate thermal support prior or during oxidation significantly increased treatment effectiveness, by i) an increase in pollutant availability in the case of permanganate, or ii) an activation of the oxidant in the case of persulfate. At the pilot scale, a homogeneous contact between the oxidant and the pollutants was possible only by a partial flooding of the soil with the oxidizing solution. The latter was used at high concentration in order to limit the injected volumes. The thermal activation of persulfate turned out to be difficult to implement, unlike the increase in pollutant availability by soil preheating before permanganate oxidation. Indeed, in the first case heating concentrated solutions of persulfate led to a parasite decomposition of the oxidant, whereas in the second case the use of concentrated solutions of permanganate led to an even higher degradation than in the laboratory. (author)