Abstract
In the bioremediation field, geophysical techniques are commonly applied, at lab scale and field scale, to perform the characterization and the monitoring of contaminated soils. We propose a method for detecting the dielectric properties of contaminated soil during a process of bioremediation. An open-ended coaxial probe measured the complex dielectric permittivity (between 0.2 and 20 GHz) on a series of six soil microcosms contaminated by diesel oil (13.5% Voil/Vtot). The microcosms had different moisture content (13%, 19%, and 24% Vw/Vtot) and different salinity due to the addition of nutrients (22 and 15 g/L). The real and the imaginary component of the complex dielectric permittivity were evaluated at the initial stage of contamination and after 130 days. In almost all microcosms, the real component showed a significant decrease (up to 2 units) at all frequencies. The results revealed that the changes in the real part of the dielectric permittivity are related to the amount of degradation and loss in moisture content. The imaginary component, mainly linked to the electrical conductivity of the soil, shows a significant drop to almost 0 at low frequencies. This could be explained by a salt depletion during bioremediation. Despite a moderate accuracy reduction compared to measurements performed on liquid media, this technology can be successfully applied to granular materials such as soil. The open-ended coaxial probe is a promising instrument to check the dielectric properties of soil to characterize or monitor a bioremediation process.
Highlights
Hydrocarbon biodegradation can cause changes in soil geophysical properties, like dielectric permittivity or electrical conductivity, for many reasons
We focused on the open-ended coaxial probe, which is a novelty in the bioremediation field
The real component of the complex dielectric permittivity was dependent on the moisture content and contaminant concentration, whereas the imaginary component was likely related to the decrease of the nutrients or some peculiar interaction between the solid phase and the liquid within the pore volume
Summary
Hydrocarbon biodegradation can cause changes in soil geophysical properties, like dielectric permittivity or electrical conductivity, for many reasons. The first and most evident is that, when hydrocarbon contamination is reduced, water or air takes its place. In this way, the relative quantities of the substances composing the soil matrix change, affecting the geophysical properties of the bulk [1]. More complex phenomena occur when biological processes are going on in soils. Soil weathering can induce variations in electrical conductivity [2], and the biological production of extracellular polymeric substances, known as biofilms, can cause changes in soil porosity as well as grain surface electrical interactions [3].
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