Study On Synergetic Thermal Desorption Mechanism And Characteristics Of PCBs Contaminated Soil

In recent years, close attention was paid on soil contamination, thus the research on soil remediation technology has become a hot issue. Thermal desorption is widely used on the site remediation of organic pollutants, of which the removal efficiency is mainly affected by temperature. In order to increase the efficiency and cut the cost of thermal desorption, the synergetic thermal desorption is raised for remediation of PCBs contaminated soil, collected from typical soil contaminated by waste capacitors and transformers. In this paper, the effect of iron and base additives on PCBs removal, destruction and detoxication was investigated during synergetic thermal desorption. The accelerating effect, chemical reaction kinetics, and different mechanisms were discussed. Based on the pilot tests, the economic analysis was conducted. The physical model on thermal desorption was built, also the heat and mass transfer was analyzed. The main conclusions were obtained as follows:The nano zero-valent iron (nZVI) was benefit for PCBs removal, destruction and destoxication. With nZVI addition, the soil particles were modified to loose structure so that PCBs desorption was much easier. At different temperatures, thermal desorption process was effectively promoted by nZVI, especially at lower temperature. Removal efficiency and destruction efficiency of PCBs increased and TEQ decreased obviously. At a certain temperature, increment of nZVI content was helpful for thermal desorption, gradually decreasing the concentration of PCBs homologues in soil and gas. The optimum content of nZVI was 5%. At different time, nZVI accelerated the thermal desorption process. The presence of nZVI had a positive effect on PCBs detoxication and the optimum time was 60min.The first-order kinetic model was applied to describe the elimination of PCBs by nZVI (R2 >0.94). There was an undeniable accelerating effect with nZVI during thermal desorption. The accelerating effect was more notable at 300℃, while it reduced when the temperature rose. The increase of nZVI content strengthened the accelerating effect. When nZVI content was low, the effect took a linear start, yet it rose asymptotically to a limit value with incremental nZVI addition. The accelerating effect by nZVI realized the rapid thermal desorption of organic pollutants.The first-order rate constant of PCBs with nZVI was knZVI=0.0428 min-1, higher than the rate constant without nZVI. The accelerating effect factor was knzvi/kblank=1.22. It was effective of nZVI to improve the removal rate and reduce the activation energy. The activation energy for PCBs removal decreased from 3771cal/mol to 2540cal/mol due to nZVI addition. The effect of nZVI was to increase the specific surface area, modify the soil structure, improve the conductivity, enhance heat and mass transfer, and participate in the chemical reaction of PCBs molecules. In conclusion, the synergetic mechanism of nZVI was to promote:(1) the evaporation and desorption of adsorptive PCBs from soil surface, (2) the diffusion and transfer of internal PCBs combined with soil, (3) the destruction and decomposition of PCBs, include the dechlorination and ring rupture.Alkaline compound was also enhanced thermal desorption. At different temperatures and with different content, sodium hydroxide promoted PCBs removal, destruction and detoxication, especially dechlorination. The main effect of NaOH during thermal desorption was dechlorination and modification, which modified soil particles into porous structure, accelerated the PCBs removal and decreased the activation energy. The chlorination degree of PCBs in soil and gas decreased dramatically. Moreover, the ratio of DiCB increased and replaced TrCB as main contributor under certain conditions, which was shown NaOH was involved in the dechlorination reaction of PCBs molecule. The effect of NaOH on PCBs removal, composition and dechlorination was attributed to base catalyzed decomposition (BCD) during thermal desorption. The hydrogenation/dechlorination mechanism was proposed to explain the PCBs pathways in the process of BCD. The rotary kiln was benefit for PCBs removal at higher temperature. Compared with the static heating, the rolling heating increased the heated area of the soil, raised the coefficient of heat and mass transfer, and accelerated the evaporation and decomposition of PCBs. According to the pilot test in rotary kiln, the PCBs removal, dechlorination and detoxication were promoted by Ca(OH)2. The RE of PCBs at 600℃ was equal with the soil heated at 500℃ and mixed with 1% Ca(OH)2. The synergetic thermal desorption reduced the energy consumption, cut the disposal cost so the technique was economical feasible for industrial application. Based on some assumptions and empirical formula, desorption model of heat and mass transfer in granular bed was established.