Microwave-Enhanced Thermal Desorption of Polyhalogenated Biphenyls from Contaminated Soil

The effect of microwave (MW) field on the rate of thermal desorption of polychlorinated biphenyls (PCBs) from long-term contaminated soil was examined in the laboratory environment. For these purposes a modified MW oven was used, with a uniformly extended MW field and a power consumption of 200–600 W. The weight of the soil samples was 100 g, the sum of concentrations of seven indicative congeners of PCB Nos. 28, 52, 101, 118, 153, 138, and 180 was, on average, 264 mg/kg of dry matter. It was experimentally proven that the efficiencies of PCB desorptions were high, over 99.9%. The maximal desorption temperature of $600°\text{C}$ was reached within 15–17 min. It came to light that the presence of alkaline additives in the soil (such as carbonates and alkaline metal hydroxides) did not have an apparent effect on the desorption of PCBs under those conditions. The results concerning the efficiency of PCB separations are in agreement with our previous findings regarding the efficiency of thermal desorption without using MWs in the pilot (input 35 kg) and industrial (input 15 t) desorption chambers with a stationary layer of the same type of contaminated soil. The main disadvantage of experiments with thermal desorption without the action of MWs was the low rate of heating up the soil. This study was focused to solve the problem of contamination in a complex way; we also studied the effect of MWs on the rate of the deterioration of the separated PCBs in aqueous condensates, the deterioration of PCBs in contaminated waste water containing alkaline agents (soda and lye), and on the catalytic reductive dechlorination of PCBs in the system Pd supported on active coal with sodium formate as the hydrogen donor. Experiments with the MW-enhanced catalytic reductive oxidation show over 98.5% reduction in the concentration of PCBs. An important and new finding is the more than 98% reduction in the concentration of PCBs when the MW field was applied on the contaminated water into which only 2.5% of ${\text{Na}}_2{\text{CO}}_3$ was added. Further studies on possible influences of MWs on the reaction kinetic are needed. Meanwhile we attribute the positive MW effects to their thermal properties rather than to their influence on the reaction mechanism.