Study On Detoxification Of Dioxins Contaminated Sediment By Pyrolysis

Dioxins (PCDDs, PCDFs and PCBs) are known to be carcinogenic, mutagenic, teratogenic and to possess endocrine, disrupting properties. There is an increasing concern about dioxins-contaminated sediment from dredged river, harbor or estuary since having direct toxic effects on aquatic life and indirect risk to humans through the bioaccumulation. The management of dioxins-contaminated sediment represents a challenging problem world wide on account of mismatch between available remedial technology options and scale and volume. Incineration is an effective way of reducing the organic content as well as the overall volume of the waste. However, in the presence of"O"atmosphere, some PCDD/Fs can be yielded and results to more serious environmental problems in traditional incinerators. The main objective of this study was the exploration and optimization of a pyrolysis process as an alternative to incineration for treatment dioxins-contaminated sediment.The contents of the research involved: (1) the investigation and assessment of dioxins concentrations in sediment from Tagonoura harbor of Japan and Shanghai Suzhou Creek of China; (2) the fate and transport of dioxins in sediment during single step pyrolysis and two step pyrolysis at lab-scale experiment; (3) an new approach of Oxygen-free Two-step Paralysis (OFTP) was proposed, and a pilot scale OFTP system was set up and conducted. Based on the lab-scale and pilot-scale experiments and theoretical analysis, the fates of dioxins containing sediment during pyrolysis process are addressed in this paper. The main results of the research are following as:1,The mechanism of dioxin removal from sediment during single step of pyrolysis is volatilization not decomposition of molecular. Results of the bench scale study of single step of pyrolysis indicated that: removal percentage of 99.9999% of total dioxins (PCDDs +PCDFs+ Co-PCBs) Toxicity Equivalent Quantity (TEQs) concentrations (pg-TEQ/g) was achieved at 800℃and retention time of 30 min. However, 99.4500% of TEQ value of raw sediment was transfered into volatile phase (tar and off-gas).2,"Oxygen"has a strong effect on PCDD/F and PCBs formation of dioxins. During cooling period following pyrolysis from 800℃to 400℃, the concentrations of total dioxins in solid residues increased 130 times. This confirmed that some complex reactions do occur to form PCDD/Fs and Co-PCBs under air atmosphere in the presence of oxygen.3,Pyrolysis temperature is also an important parameter for the formation of dioxins. (1) At 300℃and 400℃and retention time of 30 minutes, a removal efficiency of 89.6436% and 99.4482% for total dioxins were obtained at the solid residues of single stage pyrolysis, respectively; (2) Mass of Co-PCBs can be formed during the temperature range of 600℃~700℃. The temperature of 600℃~700℃is not suggested to apply for the treatment of PCBs-contaminated sediment.4,PCBs congener is more difficult decomposition than PCDD/Fs congener: (1) The Destruction efficiency (DE)of PCDD/Fs was close to 100% as compared to that lower 77.2% of the PCBs at 800℃and 30 min retention time of single-step pyrolysis; (2) Some PCBs congeners such as 2,3',4,4',5-PeCB(#118)and 3,3',4,4'-TeCB(#77)could possibly be formed at longer retention times extended to 60min at 800℃;(3) PCBs is the dominated parameter for the DE of total dixoins at the treatment of pyrolysis for the PCDDs, PCDFs and PCBs -contaminated sediment.5,Two-step pyrolysis system is very efficient to destroy vaporized dioxin in gas phase from first step pyrolysis. The 99.8028% DE of two-step pyrolysis was obtained as compared to 66.9757% DE of single-step pyrolysis. The dioxins molecules contained gas phase from first step pyrolysis was destructed at the 1000℃temperature of second step pyrolysis.6,Experimental results of pilot scale"OFTP"processes are shown that: (1) The 99.9915% removal efficencies of total dioxins TEQ concentrations was achieved from the solid residues. The removal efficencies of TEQ concentrations of total dioxins and PCDD/Fs were achieved 99.9915% and 100%, respectively; (2) All the dioxins components detected in solid residues were PCBs congener; (3) The solid residues of pilot scale"OFTP"processes have preferable cellular structure through SEM (Scanning Electron Microscope) and BET (Brunauer-Enmet-Teller) analysis.7,The surveys of PCDD/Fs and Co-PCBs Compounds in sediments of Shanghai Suzhou Creek are conducted and the results following as: (1) The TEQ concentrations of six sediment samples from different site along the creek were range from 5.2 pg-TEQ/g to 17 pg-TEQ/g. The TEQ concentration is lower and about 1/10 times of the environmental standards on dioxins of Japan; (2) The PCBs are the main components of total dioxins containing sediment from suzhou creek. The PCBs concentrations(pg/g)is about 2.1818~17.4603 times of PCDD/Fs concentrations. The TEQ of PCBs is main from the 3,3',4,4',5-PeCB(#126)congener, and the contribution to TEQ of PCBs is range from 83.4092% to 88.7286%.The main contributions of this study are following as: (1) Qualitative and quantitative description the fate of PCDD/Fs and Co-PCB congeners during pyrolysis has been investigated the temperature range of 300~500℃. The study found that dioxins components containing off-gas from pyrolysis can be destroyed by a seond step pyrolysis at over 1000℃temperature. (2)This research represents a new approach of Oxygen-free Two-step Paralysis (OFTP), and has developed a pilot scale system for the treatment of dioxins-contaminated sediment. This OFTP process may be applicable for the treatment of detoxification of dioxins-contaminated sediment or soil. (3) This work provides the information of the survey of PCDD/Fs and Co-PCBs polluted condition in sediment of shanghai suzhou creek. This data provides the information for the design of how to treat the dredge sediment from suzhou creek in the near future.