Soil and groundwater clean-up by in situ chemical oxidation

In-situ chemical oxidation is a physico-chemical technique for the decontamination of groundwater and soil contaminated by organic compounds [J 3 981] . It involves injecting liquid, solid or gaseous oxidizing reagents into the groundwater or soil to react with the pollutants in the plume and/or source zone. The decisive point is the contact between the oxidizing reagent and the targeted pollutants, given that the pollutants are shared between the aqueous phase, the porous matrix and possibly a non-aqueous liquid phase (NAPL). Oxidation is aimed at relatively light pollutants such as volatile chlorinated compounds, benzene, toluene and xylenes, as well as higher molar mass compounds such as polycyclic aromatic hydrocarbons. The aim of the treatment is to degrade the pollutants into less toxic or more rapidly biodegradable substances, or in the case of complete oxidation, into carbon dioxide, water and possibly halogens.

This technique, well adapted and mastered for treating groundwater, is increasingly being developed for the unsaturated zone of the soil.

The most commonly used oxidants are potassium or sodium permanganate, hydrogen peroxide combined with iron(II) (Fenton's reagent), sodium persulfate, sodium percarbonate and ozone. The choice of oxidant depends on the pollutants and the characteristics of the site to be treated. As the usual oxidants are water-soluble, degradation takes place mainly in the aqueous phase, which favors dissolution of the free phases. The main condition for successful treatment is to ensure that contact between oxidant and pollutant lasts long enough for the reaction to take place.

To select and apply this technique, it is first necessary to characterize the site to be treated and carry out laboratory tests. Laboratory tests are designed to select the most appropriate oxidant and determine the quantities to be used. If the results are positive on this scale, i.e. if pollutants can be degraded with reasonable doses of oxidant, feasibility is tested on a pilot scale, in order to obtain data for sizing, set up monitoring, define health and safety conditions and forecast treatment costs. On-site treatment can then be implemented. In situ oxidation can be combined with finishing treatments, such as biodegradation.

A large number of books are available on in situ chemical oxidation