Techno-economic analysis of hydrocarbon-polluted soil treatment by using ex situ microwave heating: influence of soil texture and soil moisture on electric field penetration, operating conditions and energy costs

Microwave (MW) heating has been identified as a potential cost-effective technique to remediate hydrocarbon-polluted soils; however, the soil texture and properties could have a great impact on its full-scale treatment. In addition, very limited energy and economical data on MW treatment are available, and this lack makes its real application very limited. In this work, a first experimental phase was performed simulating a MW of several hydrocarbon-polluted soils. Obtained data were elaborated for a techno-economic analysis. Four soil textures, corresponding to medium, fine silica sand (at different soil moistures), silt as silica flour and clay as kaolin, were artificially contaminated with diesel fuel and irradiated by MWs using a bench scale apparatus. Soil samples were treated applying four specific power values at different times. At the end, soil temperature was measured, whereas residual contaminant concentrations were measured and fitted considering and exponential decay kinetic model. Temperature data, as well as kinetic parameters obtained, were used for the techno-economic analysis. The changing of the internal electric field was calculated for all the soils and operating conditions, then considering initial contamination values ranging from 750 to 5000 mg kg(-1), the minimal remediation time, specific energy and costs for the remediation were assessed. At low powers, MW effectiveness is limited by low soil moistures or fine soil textures due to a limitation of the electric field penetration, whereas when high powers are used soil properties have a limited effect. Remediation time, as a function of the initial contamination level, follows a linear trend, except for dry soils, for which an exponential trend was observed. For powers higher than 30 kW Kg(-1), remediation times lower than about 100 min are needed, for all the moisturized soils, in order to treat a contamination of 5000 mg kg(-1). The variation of soil moisture or soil texture results in the range 20-160 a,not sign ton(-1), and doubled costs are required for the treatment of clayey soils respect to sandy soils. The analysis performed suggests that soil layers lower than 70 cm should be considered for ex situ remediation. MW has been shown as a quick technique also for high hydrocarbon concentrations; however, for energy saving, the application of some powers should be avoid. Unmoisturized or fine texture soil treatment results in higher costs; however, a maximum cost of 160 a,not sign ton(-1) generally makes MW heating a quick and cost-effective ex situ technique.