Modelling and preliminary technical, energy and economic considerations for full-scale in situ remediation of low-dielectric hydrocarbon-polluted soils by microwave heating (MWH) technique

Purpose: Microwave heating (MWH) has been recently proposed as a high-performance technique for the remediation of soils contaminated with organic pollutants. However, despite MWH potential advantages, it is scarcely applied due to the lack of full-scale in situ detailed studies. In this work, the in situ MWH applicability for the remediation of hydrocarbon-polluted soils was assessed by means of a specific energy and economic analysis. Essential technical information has also been purchased. Materials and methods: Energy and economic analysis was performed using data obtained from modelling for which a dedicated equation-based process computer code simulating MWH phenomena was adopted. Elaborations involved the assessment of the influence of soil texture and moisture as well as operating conditions (supplied power and time) on electric field penetration into the soils and soil temperature variation as a function of time and radial distance from the irradiation source. Results and discussion: Main results reveal that sandy soils are more penetrable by MW irradiation with respect to clayey ones. The soil MW penetrability was also observed to increase with decreasing the soil moisture. This was in turn reflected in the soil temperature profiles. However, the major effect on MWH effectiveness is ascribable by the changing of the operating power. In fact, the use of magnetrons with powers lower than 3 kW does not ensure enough microwave penetration into the soil and, therefore, is not suitable for in situ activities, whereas the application of a power of 6 kW led to a maximum treatable radius of 145 cm. In terms of energy consumption, calculation showed that almost 3 days more are in general required to remediate clayey soils with respect to sandy ones. Consequently, the economic analysis revealed that energy costs for sandy soils are about 3 € t−1 lower than those required for clayey soils. Furthermore, the application of a power of 6 instead of 3 kW results in a higher total energy cost, which, jointly with the higher soil volume treatable, leads to almost equal specific costs. Conclusions: The comparison of calculated costs with those of other available clean-up technologies for hydrocarbon-contaminated soils shows that very short remediation times and energy costs obtained (18–27 € t−1) make in situ MWH a deliverable alternative to conventional thermal desorption or physical-chemical techniques.