Study On Key Influence Factors During Thermal Desorption Of PCBs Contaminated Soils And Site Application

Recently, close attention was paid on soil contamination. Lots of site remediation technologies were proposed and applied. Thermal desorption is widely used on remediation of organics contaminated site in developed controies, while China lack related technologies and equipments. In this study, we investigated the levels and profiles of PCBs and PCDD/Fs in typical transformer oil contaminated soil, desorption and destruction mechanism of PCBs and effects of key influence factors during thermal desorption. A technical route for treatment of PCBs containing flue gas was proposed and investigated. The applicability and security of thermal desorption plant were carried out base on constrction and operation of two demonstration projects. Several valuable conclusions are as following:The PCBs concentration in typical transformer oil contaminated soil in Zhejiang Province could reach1705mg/kg. TrCB and TeCB lead the total PCBs (209congeners), and PCBs28/31is the main contributor. PCB77, PCB105and PCB118are the main contributors to the dl-PCBs. Significant positive correlations were found between the6ndl-PCBs/12dl-PCBs and total PCBs.With the increased heating temperature, removal efficiencies of PCBs increased. A removal efficiency of96.1%was achieved for1h at600℃. At low temperature, the distribution profiles of PCBs in flue gas were similar with that in untreated contaminated soil. Evaporation is the main mechanism for PCBs removal.With the increased heating temperature, the distribution profile of PCBs in treated soil changed a lot with obvious dechloration reactions occurring, while distribution profiles of PCBs in flue gas change a little. The destruction of PCBs molecule happened on the surface or inner of soil particles. Increase the heating temperature and extend the time of flue gas in high temperature zone could efficitively increase the removal and destruction efficiency of PCBs.500℃is a suitable temperature for PCBs removal.With the increased heating time, the residual PCBs in treated soil decreased and PCBs in flue gas increased. A removal efficiency of98.9%was achieved for2h at500℃.At the beginning of thermal desorption, evaporation and dechlorination were the main mechnisms for PCBs removal; with the increased heating time, destruction on the benzene ring gradually became marked.60minutes is a suitable time for PCBs removal.With the same temperature and heating condition, fine particles (d<250μm) have higher remval and destruction efficiency than coarse particles (425μm<d<840μm). Fine particles have larger specific surface area, which can provide more positions for reactions of PCBs molecule. When the rate-controlling mass transport mechanism is diffusion, smaller particles should desorb faster than large particles given identical heating profiles. To realize the same removal efficiency of PCBs, the heating tempertature for coarse particles could be100℃more than fine particles. Removal and destruction of PCBs by thermal desorption fit a first-order reaction.The distributions of PCBs in solid/gas phase were affected by that in untreated soil, and also affected by boiling point and vapour pressure of specific PCBs molecule.PCBs in solid phase of flue gas accounted for65%of PCBs in flue gas.Wet scrubber could realize TEQ removal efficiencies of86.2%to97.8%for PCBs, and82.7-95.0%for PCDD/Fs. Under the optimum condition,TEQ of the flue gas sprayed could be less than0.5ng TEQ/Nm3.The relationship between flow of spraying water and volume of flue gas affect PCBs removal. Increase spraying water flow and decrease flue gas volume will promote PCBs removal. Dust capture is the main PCBs removal mechanism when flue gas contains a lot of particles, and condensation and gas absorption worked when flue gas contains little particles.Activated carbon could effectively adsorp the PCBs in flue gas with adsorption efficiency more than99.8%. Significant correlations were found between adsorption efficiency of total PCBs and BJH mesopore area/volume. Selective adsorptions of PCBs were found between different activated carbons. Appropriate modification of acticated carbon will enhance the PCBs adsorption.Field demonstration showed thermal desorption could effectively treat highly PCBs contaminated soil with the residual PCBs in teated soil less than2mg/kg. Environmental impact accessment revealed inappropriate construction and operation of thermal desorption plant may cause adverse effects to the environment. During site remidation period, contaminated matrix should be strictly controlled.Environmental impact accessment and remidation should be carried out on thermal desorption plant once remidation action finished.