Repeatable use assessment of silicon carbide as permanent susceptor bed in ex situ microwave remediation of petroleum-impacted soils

Efficiency of microwave-enabled ex situ soil remediation can be improved by dielectric susceptors. Cost, and environmental burden of these susceptors can be minimized if they are used repeatedly in a permanent bed set-up. In this study, carbon nanofibers, activated carbon, magnetite, and silicon carbide were tested at the lab scale for repeated use in permanent microwave-induced thermal soil remediation. Despite their superior ability to convert microwaves into heat, carbon nanofibers experienced electrokinesis and activated carbon partially combusted in the microwave cavity, which complicates their pragmatic use in remediation applications. Magnetite was also able to convert microwaves into heat effectively and it was relatively more stable; however, repeated heating/and cooling cycles changed its physicochemical properties, which was attributed to oxidation of iron oxides at the air-soil interface. Silicon carbide, on the other hand, was an efficient heating agent and was stable during repeatable heating and cooling cycles. Through 25 heating/cooling cycles, an average peak temperature of 329 ± 55 °C was achieved for a 29 cm³ sample and analysis of dielectric properties after every 10^th and 25^th cycle indicated that there were no significant losses in thermal conductivity or permittivity of the material. Subsequent remediation experiments with silicon carbide demonstrated that between 89 and 97 % of the total petroleum hydrocarbons were removed from soil including a marked fraction of heavy hydrocarbons when 20.2 kJ g⁻¹ of microwave energy was introduced. In addition, post-treatment experiments demonstrated that soil conditions were capable of supporting seed germination indicating that some conditions of soil were recovered after microwave remediation.