Microwave-induced decontamination of mercury polluted soils at low temperature assisted with granular activated carbon

In this study, a microwave-induced thermal treatment at relatively low temperature (300-350 °C) was introduced to the decontamination of polluted soils with high mercury concentration, in the interest of addressing the issues related to traditional thermal desorption methods i.e. high energy consumption, high cost, and irreversible damage to the soils. The addition of granular activated carbon (GAC) was found to significantly increase the heating rate and accelerate the decomposition of mercury pollutants. A total mercury (THg) removal efficiency of 86.9% could be achieved at microwave irradiation of 400 W (ca. 350 °C) for 40 min with a GAC/Soil mass ratio of 3:15. Moreover, the mutual transformation of five mercury fractions in the soils was also investigated to reveal the decomposition procedure. The environmental risk of THg remained in the soils was largely reduced by increasing the percentage of inert fractions from 88.1% to 96.1%. Of special importance, microwave irradiation greatly favoured the electric discharges of GAC, which produced molecular heating resources with excessively higher temperature than the overall soils to realize low damage to soils. Meanwhile, the specific bulk heating mode reduced the temperature barriers to allow easy release of decomposed mercury vapours. Furthermore, GAC additives showed negligible changes during the whole decontamination process and the increase of soil amounts did not affect the heating rate evidently. These results were remarkably promising to provide fundamental information for practical scaling-up remediation of contaminated soils with high mercury concentrations at relatively low temperatures that could be both convenient and energy-efficient.