Advanced oxidation processes (AOPs) as innovative technology for the remediation of contaminated sites

The contamination of soil and groundwater by means of bio-recalcitrant organic compounds, is becoming a matter of concern for scientific community and public opinion. The increase of the number of contaminated sites, is forcing to gradually switch from traditional dump disposal, towards innovative technologies which are capable of reaching the remediation goals, thus reducing the pollutant load to concentrations which are considered harmless for human health. Among the innovative ones, Advanced Oxidation Processes (AOPs) could represent a potential solution to be applied for remediating contamination by bio-recalcitrant organic compounds. Their operative principle is based upon the idea of generating a pool of highly-oxidative species. The AOPs differ only by the way in which this pool is generated. Once formed, these species are capable to effectively react with most of common pollutants such as hydrocarbons, chlorinated solvents, polycyclic aromatic hydrocarbons and polychlorobiphenyls until their complete oxidation to carbon dioxide and water, or at worst their transformation to more bio-degradable products. Moreover, some AOPs are able to effectively tackle sorbed compounds, since oxidative radicals can desorb these compounds from the soil surface, thus allowing their oxidation in aqueous phase. Besides, it is worth pointing out that the AOPs characteristics make them suitable to be applied as in-situ remediation technologies. In this configuration, the oxidant is injected directly into the subsurface without the need of soil excavation or groundwater extraction. The present study has been developed with the intention of achieving a two-fold objective: on the one hand, to better understand the fundamental mechanisms of AOPs, in order to develop innovative criteria for their design; on the other hand, to assess the feasibility of different AOPs to those situations which are somehow representative of the Italian contaminated sites. The first objective was pursued by developing a fundamental study aimed to identify the relationship between the process operating conditions and the formation of radical and non-radical species for Fenton’s process, activated persulfate and peroxy-acid oxidation processes. The second objective was instead pursued by developing, based on the experimental results of the fundamental study, a design approach based on the execution of feasibility studies. In case of Fenton’s process, a pilot-scale In-Situ Chemical Oxidation (ISCO) treatment for the remediation of an MtBE-contaminated site was developed, whereas in the case of activated persulfate and peroxy-acid oxidation technologies a lab-scale feasibility test was carried out. The design of the different experimental phases was performed, as much as possible, by applying the Rotatable Central Composite method (RCC), whereas the relationships between process performance and applied operating conditions was found by handling and interpolating the experimental results by proper statistical tools based on the Response Surface Method (RSM). In this Ph.D. thesis, each tested AOP is first discussed in a bibliographic part, where the process is introduced and its main features are explained, based on the available and updated literature. The main findings obtained in this part and the innovation introduced with respect to the state of the art is also described in this section of the Ph.D. thesis. The details of these results are shown in the second section of the thesis, which consists of four Appendices, where a selection of papers submitted either to international conferences and peer-reviewed journals during my Ph.D. research are included.