Influence of water saturation and soil properties on the removal of organic pollutants using microwave heating

Abstract

Properties of fluids and media, such as soil moisture, may play a significant role in the absorption of microwave and heat distribution during the remediation of soil contaminated with volatile and semi-volatile compounds. Previous studies have been performed on soil samples placed inside a microwave oven cavity in a reactor far from the waveguide outlet or directly inside the metal waveguides. These conditions are far from in situ applications where the unsaturated soil is directly exposed to microwaves through the antenna slots. The objective of this study was therefore to understand better how soil temperature and pollutant recovery change during microwave and conduction heating and how soil properties, liquid type, and saturation influence that. We developed a unique experimental setup that consists of a splittable soil column inserted inside the cavity of a modified domestic microwave oven (power 1000 W and frequency 2.45 GHz) so that the soil surface is in direct contact with the radiated microwaves. Experiments with electrical resistance heating using the same column but with a modified design were conducted for comparison. We used three types of soils spanning fine, medium, and coarse sands, and two semi-volatile pollutants (xylene and diesel fuel). The pollutants and water of different volumes (12% and 25%) were mixed with soils to make the artificially contaminated soils. Temperature values were measured at different points along the sand-packed column using fiber-based optical thermocouples. We evaluated treatment efficiency in space (soil analysis) and time (outlet phase decantation).The experimental results show that microwave heating technology is optimal for water saturation of around 12%, which gives the best compromise between the overall dielectric properties and allows rapid and efficient heating. The temperature increases fast at the beginning of the microwave heating and stabilizes because of the latent heat of the water and pollutant vaporization and then increases again but slowly for dry soil conditions. A maximum temperature of 170 °C was achieved after 140 min of microwave heating. The type of soil and pollution can drastically affect remediation efficiency through mechanical mechanisms (because of a pressure increase) in addition to physical mechanisms (evaporation) for pollutant removal. The removal efficiencies, using the outlet fluids decantation, were 67%, 73%, and 75% for fine, medium, and coarse sand, respectively, for the applied heating time. We found that microwave heating works better in coarser sand where classical conduction heating usually failed. Comparing the two types of heating (microwave and conductive heating) under the same conditions highlights that the use of microwaves makes it possible to reach very high temperatures in a shorter time than with thermal conduction heating.