Site remediation of an industrial waste dump: fenton treatment of PCB contaminated soil

PCBs represent an important class of organic pollutants, due to their toxicity and ubiquity in contaminated sites. They are found in the environment mainly as a result of uncontrolled disposal of used oils and transformers. PCBs are refractory to most of current remediation technologies such that only thermal treatments can be effective in removing PCBs from contaminated soils. Advanced oxidation processes (AOPs) have become one of the most interesting and promising remediation techniques. Their operational principle is based on the idea of generating a pool of oxidizing species in the subsurface environment. The different AOP processes differ simply in the way this pool is produced. In Fenton’s reaction hydroxyl radical (OH) is generated by an iron-catalyzed reaction. The present work deals with the treatment of a PCB-contaminated soil, collected from a dump near Rome, by Fenton’s reagent. The soil was a complex matrix characterized by a very high organic matter (TOC = 6.57%), with an average PCB concentration equal to 47 mg/kg(dry soil). All the experiments were carried out in batch reactors using a 1:10 soil:water ratio at hydrogen peroxide concentration of 6%, 10% and 20% wt/wt, and different catalysts (FeCl3 or FeSO4) and concentrations (2mM, 8mM and 12mM). All experiments were carried out at acidic pH (between 2 and 3). Hydrogen peroxide decomposition kinetics and residual PCBs concentrations after 8-hour treatment were measured. As far as H2O2 decomposition kinetics are concerned, a linear correlation between H2O2 decomposition rate and Iron/H2O2 molar ratio was found when FeSO4 catalyst was used. On the contrary such a relationship was not found when FeCl3 catalyst was used. This different behaviour was ascribed to the different role played by Iron(III) ions with respect to Iron(II) ions within the Fenton’s reaction. The maximum removal efficiency using FeCl3 as catalyst was equal to 46% at [H2O2] = 10% and [FeCl3] = 8mM; The maximum removal efficiency using FeSO4 as catalyst was equal to 38% at [H2O2] = 10% and [FeSO4] = 2mM. Based upon the obtained results, it can be concluded that Fenton’s reagent may represent a valid process for treatment of PCB-contaminated soils. Clearly, optimisation of the operatives conditions is required in order to increase the removal efficiency and also to possibly reduce the oxidant requirements.