Chapter 5: In situ technologies for hazardous materials contaminated soil and groundwater: the case of MtBE

In this chapter, experimental tests of different techniques for contaminated soils and groundwater remediation, are presented and discussed. The contaminant of concern is the MtBE, a common antiknock additive in the production of unleaded fuel. MtBE’s great water solubility, low partitioning into soils and low tendency to be degraded by microbial activity, are responsible for a considerable mobility that allows rapid dispersion and strong persistency in groundwater. To remove or/and destroy this contaminant, three in situ technologies are evaluated and compared: Air Sparging (AS), soil flushing (ISSF), and chemical oxidation (ISCO). AS, which mainly involves the stripping of the contaminant by injected air in the subsurface, is currently one of the most commonly used, due to the simplicity of the plant installation, operation and maintenance and, consequently, to its low associated costs. Other systems, like ISSF and ISCO can be also successfully implemented in MtBE contaminated soils and groundwater treating. Both ISSF and ISCO technologies deal with the chemical oxidation of the pollutant, with the difference that for ISSF the oxidant does not come into contact with the soil matrix. In the experimentation presented in this chapter, Fenton oxidation was performed in either technologies used. For each technology after the description of the basic principles and environmental applications, the results of lab scale experiments conducted on samples of saturated soil artificially contaminated, are discussed. AS tests were performed both in a cylindrical column (small scale test) and in a large tank of 1 m3 (intermediate scale test). These two testing set-ups were used to study the optimal air flow rate for MtBE transfer to the gas phase and the influence of the geometry around the injection well. ISSF tests were performed by flushing the soil with an aqueous solution of a surfactant agent (ethanol). The pH, the ethanol concentration and the flow rate of this extracting solution were optimized to achieve the complete removal of MtBE from the matrix. The recovered solution was then oxidized with Fenton’s reagent to destroy the pollutant. The effect of ethanol concentration towards MtBE oxidation was also evaluated, considering the non selectivity of the oxidant. ISCO tests were performed by injecting Fenton’s reagent directly into the soil, both at the natural pH of soil and under acidic conditions.