Removal of PCDD/Fs and PCBs from sediment by oxygen free pyrolysis

ISSN 1001-0742Journal of Envionmental Sciences Vol. 18, No. 5, pp. 989- -994, 2006CN11- 2629/XArticle ID: 1001-0742(2006)05-0989-06CLC number: X703Document code: ARemoval of PCDD/Fs and PCBs from sediment by oxygen freepyrolysisHU Zhan-bo', Saman Wijesekara R.G.3, Ronald R. Navarro', WU De-yil, ZHANG Da-leil,Masatoshi Matsumura?, KONG Hai-nan!.*(1. School of Environmental Science and Engineering, Shanghai Jiaotong University, Shanghai 200240, China. E-mail: hnkong@sjtu.edu.cn; 2.Graduate School of Life and Envionmental Sciences, UJniversity of Tsukuba, Tsukuba-shi 305-8572, Japan; 3. Guangxi University, Nanning 530004,China)Abstract: Few studies have dealt on the evaluation of volatilization and decomposition reactions of dioxins from sediment by oxygenfee pyrolysis. In this study, the performance of pyrolysis on the removal of dioxins from sediment was investigated. Dioxinconcentrations of the raw sediment and the solid residues after pyrolysis were analyzed at different conditions. Resuls showed aremoval eficiency of 9999% for total dioxins at 800C and retention time of 30 min. All the polychlorinated dibenzo-furans(PCDFs) have been removed and were not formed in the solid residues at the retention time range of 30- -90 min at 800C. Close to100% removal of polychlorinated dibenz0-p. dioxins (PCDDs) was als achieved. Only trace PCDDs were detected in the solid yieldsat a retention time of 60 min. The highest removal eficiency of polychlorinated biphenyls (PCBs) was more than 9.9994% at a .retention time of 30 min. During cooling period following pyrolysis, however, the concentration of total dioxins in solid residuesincreased 130 times as compared to that of the raw sediment under air atmosphere. This confirmed that some complex reactions dooccur to form PCDD/Fs and PCBs from 800 to 400C in the presence of oxygen. Oxygen-free atmosphere therefore can preventformation of dioxin during thermal process thus generating clean solid residues.Keywords: dioxins; pyrolysis; oxygen free; sediment; PCBsof oxygen are achieved at a temperature range of 650Introductionto 1200C (Tetra Tech, 1994). However, in theLarge quantities of dioxin-contaminatedpresence of C, H, 0 and CI, incineration can yieldsediments have been found in many rivers, harbourssome PCDD/Fs under suitable conditions of time andand estuaries in China and around the world and thesetemperature (Altwicker, 1991; Altwicker et al, 1994).has remained a signifcant problem (Sun et al., 2005;The biggest challenge for incineration remains on theOkumura et al., 2004; Paul et al., 2005). Thesetransfer of pollution from the waste to the air, whichcontaminated sediments pose direct toxic effects onresult in more serious environmental problems.aquatic life as well as contribute indirect risk toAlthough in the oxygen atmosphere, the PCDD/Fshumans through the bioaccumulation of toxiccontent is decreased when temperature is increasedcontaminants in the food chain (Rausa et al., 1999;(Kakuta et al., 2005)， however, regeneration ofNeigh et al, 2006). In order to reduce the amount ofPCDD/Fs has also been found in oxygen-containingsediment wastes as well as convert these intoatmosphere during treatment of MSW fly ash byrecyclable materials, the toxic substances of thecombustion (Chang and Huang, 2000; Benfenati et al,sediment should be removed or made more chemically1991). Incinerations can also emit harmful pollutantsstable. A great deal of work has been undertaken by(SOx, HCl, HF, NOx, etc.) and toxic volatile organicresearchers to develop treatment systems focompounds (VOCs) especially polyaromaticpolychlorinated dibenzo-p-dioxins (PCDDs) or poly-hydrocarbons (PAHs), PCBs and PCDD/Fs. VOCs inchlorinated dibenzo-furans (PCDFs) and poly-particular have gained more attention due to the factchlorinated biphenyls (PCBs) from sediments. Thesethat some are carcinogenic (Malkow, 2004; Ferreira etmethods include mechanical, physical, biological,al, 2003; Benfenati et al, 1991; Grifn, 1986;chemical and thermal processes (Kenna el al, 1996;Morselli et al, 1992). As the legislation for dioxinsRausa et al., 1999).emissions from MSW is now becoming strict in manyThermal technologies such as incineration ancountries, the development of new environmentallypyrolysis (Malkow, 2004; Calaminus et al, 1997),sound technology for treatment of dioxin-conta-which volatilize or destroy the organic contaminants,minated sediments is necessary.are the preferred methods for municipal solid wasteIn recent years, pyrolysis had been considered as(MSW) treatment and disposal. Among these,In. waste incinerationincineration is the most common method applied totechr中国煤化工”recig wastedeal with the increasing production of MSW(Yos:YHCNMHGuandYang,2004).throughout the world (Ferreira et al., 2003). InHowever, the competitiveness oI pyrolysis with wasteconventional incineration, partial decomposition 01incineration has to be proven (Weber and Sakurai,complete destruction of the pollutants in the presence2001). MSW pyrolysis is different from incinerationFoundation item: The New Energy and Industrial Technology Development Organization of Japan; *Corresponding author990HU Zhan-bo et al.Vol.18due to the absence ofO2 (Khiari el al., 2004). Hence,were then passed through a XAD-2 resin columnit is very effective in preventing PCDD/Fs formationwhere the dioxin compounds were absorbed. A final(Malkow, 2004; Calaminus et al., 1997; Ferreira etcolumn containing activated carbon was installed foral., 2003).the cleanup of remaining exit gas prior to its release toDespite the great deal of research andthe atmosphere.investigation about the occurrence of dioxins inThe quartz tube reactor was heated at a constantincineration of MSW, less information is availableheating rate of 109C /min to a desired pyrolysisregarding the dioxins formation during pyrolysistemperature and maintained at this temperature forprocesses. Especially， there is limited literaturevarious retention times of 30， 60 and 90 min.concerning the behavior of dioxins during coolingThermocouple readings showed that the sampledown period of pyrolysis treatment for contaminatedreached a desired temperature according to thedredged sediment. Now we have developed a pilotprogrammed value. After each run, the furmace wasscale system of oxygen free pyrolyzer for treating theturned off and the reactor was allowed to cool down atdioxins contaminated sediment. This paper presentsN2 gas or air atmosphere to room temperature. Thethe results of preliminary studies on a lab scalecooled samples were collected, weighed and subjectedpyrolysis of sediments. The main objective of theto chemical analysis.paper was to obtain detailed information on the fate of1.2 Thermal analysisdioxins during such thermal treatment and to findThermal analysis (TG-DTG) experiments weresignificant information that can be applied in the largeperformed with a TG/DTA-6300 analyzer (Seikoscale operation of a pilot scale pyrolizer.Instruments Inc., USA). Approximately 20 mg ofsample was placed in an open platinum holder. Each1 Materials and methodsTG-DTG run was carried out up to 900C at a heating1.1 Sediment samples and pyrolysis processrate of 10C/min in inert atmosphere (400 ml/min ofDioxin-contaminated sediment was obtained fromAr). The thermogravimetric weight loss curve (TG, wtTagonoura Harbor, Japan. All the samples were air%) and the weight loss derivative curve (Derivativedried and then pulverized to achieve homogenizedThermo-Gravimetry (DTG), μg/min) were recorded assize distribution prior to use.a function of temperature.A special horizontal lab-scalc pyrolyzer was1.3 Dioxin analysis methodsdesigned and used for pyrolysis experiments (Fig.1) .Quantitative analysis of PCDD/Fs and PCBsThe pyrolysis chamber is made up of quartz tube (i.d.were performed for solid residues at each run.internal diameter = 15 cm, length = 50 cm). TheReference standards, which were "C1-2,3,7,8-TCDD-quartz tubewas housed within athree-zone0CDD,9Cr-2,3,7,8-TCDF-OCDF，"Cr-TCB-HCB,electrically heated furace with a programmable"C7r-2,3",4",5-TeCB,"Cn-1,2,7,8-TeCDF and "Cr-1,temperature controller. Temperature profiles were2,3,4,6,8,9-HpCDF congener standards solution, wereobtained using a thermocouple positioned at the centerobtained from Wellington Laboratories (Ontario,of reactor.Canada).Furoace isolationAnalysis of PCDD/Fs and PCBs were performedQuartz tube reactorQuartz woolFlow meterusing an HRGC (TRACE GC 2000, ThermoQuestSample ThermocoupleActive carbon column ValveCo, USA)/HRMS (Finnigan MAT 9SXL, ThermoExhaust gas -Quest Co, USA) with selected ion monitoring (SIM)wwwwwwwww.mode. HRMS was operated in clectron impactionization mode at a resolution of R≥10000 (10%“FuraceVacuum pumpvalley).Cold Tpr cleerAir compressor.NitrogeoA gas chromatograph equipped with a capillaryXAD-2 resin columnTemperature controllercolumn RH-12ms (60 m X 0.25 mm, Inventx Co.,Fig.1 Schematic diagram of the pyrolysis systemUSA) and a mass spectrometric detector was used.Nitrogen gas was allowed to flow into the furmaceThe conditions were as follows: injector temperaturetube to maintain non-oxidative conditions during theat 280C, interface temperature at 280C，injectionentire pyrolysis process. The flow rate of nitrogen gasvolume: I μl. Helium at a column head pressure ofwas fixed at 1000 ml/min. This flow rate was175 kPa was used as the carrier gas. The temperatureconsidered sufficient to prevent the accumulation aprogram for the GC oven was as follows:中国煤化工s containing 4- -8pyrolysis gas generated in the tube fumace and at thesame time not to affect the temperature of the surfacechloit 130C for 1.0of sediment sample. The gases generated by themin.lYHI CNMH. C ther 10 310Ca .pyrolysis reaction were allowed to pass through a cold3C/min, then to 320C at 59C/min with a final holdingtrap to collect tar condensates. The uncondensed gasestime of 3 min.No.5Removal of PCDD/Fs and PCBs from sediment by oxygen free pyrolysis91For analysis of2, 3, 4, 7, 8-PeCDF, 2,3, 4, 6, 7,00 r1408-HxCDF, 1, 2, 3, 7, 8, 9-HxCDF and PeCDFs:95 t一-DTG”TCl 120injector temperature at 280%C, interface temperature at| 100.52809C, initial oven temperature at 130C for 1.0 min,heating to 2109 at 15C/min, then to 310C at 39C。85-)/min, then to 3209C at 59C/min with a final holding30 t400g20time of 12 min.For analysis coplanar-PCB: initial oven tempera-010020030040500600700800900ture at 130C for 1.0 min, heating to 2109C at I5C/min, then to 310C at 39C/min.The mass spectrometric detector conditions wereFig.2 TG-DTG curves of Tagonoura Harbor sediment in Ar gas at I0Cas follows: ion source: EI, positive; resolution: R≥10000 (10% valley); ion source temperature : 260C.solid products.For total PCDDs and PCDFs, therefore, almost2 Results and discussioncomplete removal has been achieved. Only at the 602.1 TG-DTG analysis of sedimentsmin retention time where some PCDDs were detectedTo evaluate the temperature suitable for sedimentyet these TEQ concentration were very low.pyrolysis, thermo-gravimetry (TG) and derivativeFor the dioxin-like PCBs, the highest removalthermo-gravimetry (DTG) experiments were initiallyefficiency of 9.9994% has been obtained at theconducted. Data from TG analysis is essential inshortest retention time of 30 min. At this condition,evaluatingnass reductions at a wide range ofthe lowest TEQ concentrations of 0.000085 pg-TEQ/gtemperature so that optimum temperature fowas achieved. The content of PCBs in solid residuesmaximum removal of sediment components may beincreased to 0.0005 pg-TEQ/g and 0.0066 pg-TEQ/gestablished.when the retention time was extended to 60 and 90TG-DTG curves for Tagonoura Harbor sedimentmin, respectively. This result demonstrated that traceare shown in Fig.2. Main and minor peaks in the DTGamount PCBs could possibly be formed at longercurve (reaction rates) were observed at around 250retention times at 800 during oxygen free pyrolysis.600C and 750- -780，respectively. Such peaks,This might be due to the fact that the formation ofwhich are related to mass reductions, may indicatePCBs molecules does not require oxygen (Fig.3).volatilization as well as decomposition reactions ofFurther experiments are needed to understand theinorganic and organic compounds presented in thepossible mechanism for formation of PCBs in thissediment. Also, from a previous study on the thermalrespect.treatment for contaminated soil, temperatures rangingTo summarize, the above data showed that afrom 500- -600C were found sufficient to obtainretention time of 30 min was already suficient toremoval efficiencies of more than 99% for organicachieve maximum removal of all dioxins andcontaminants (Rienks, 1998). Although the Tagonouradioxin-like PCBs. Furthermore, removal efficienciesHarbor sediment also displayed more or less similarslightly decreased when the retention time is increasedtemperature range for maximum mass reductions,to 60 and 90 min due mainly to some remainingsubsequent pyrolysis experiments were conducted at aPCBs. At a retention time of 30 min some PCBstemperature of 800C to assure the complete removalparticularly 2, 3, 4, 4, 5-PeCB (#123) was alsoof all the organics.detected but its concentration was almost negligible at2.2 Fate of dioxins in sediment before and after0.000085 pg-TEQ/g. Therefore, from the view ofoxygen free pyrolysiseconomics point of reducing energy requirements, theThe removal efficiencies of PCDDs, PCDFs andlowest retention time of 30 min at 800C may bedioxin-like PCBs in oxygen-free pyrolysis experi-sufficient for actual pyrolysis operations.ments were compared over the retention time range of2.3 Mechanism of removal dioxins during oxygen30, 60 and 90 min at 800C (Table 1). Results showedfree pyrolysis processthat almost all PCDFs molecules were removed at allIn any thermal treatment systems, the resultingapplied temperatures and retention times. Secifically,dioxins concentration in the solid product may bethe toxic equivalents (TEQ) concentration of PCDFsrelated to the mass balance of formation and thermaldecreased from 4.805 pg TEQ /g to zero values duringdest中国煤化工rganic materials canpyrolysis. The removal of PCDDs also reached 100%bement in an inertat the retention times of 30 and 90 min, respectively.atmdTYHC N M H Gerial to change intoMore than 99.9990% of PCDDs was removed at athree phases (iquid, gas and carbon) (Khiari et al.,retention time of 60 min, with only trace amounts of2004). Previous pyrolysis tests on PCDD/Fs suggestedOCDD (0.0005 pg-TEQ/g) of PCDDs detected in thethat they could be toiully destroyed at temperatures992HU Zhan-bo et al.Vol.18Table 1 TEQ concentrations of PCDD/Fs and PCBs inabove 700C (Hatanaka et al., 2001). Furthermore, theTagonoura H arbor sediment before and after pyrolysis*previous works have also reported that theRaw Solid residues at 800C and differentvolatilization behavior of dioxin from fly ash wasediment,_ retention time .Pg-TEQ/g dwCongenerpg-TEQ/g30 min 60 min0 minshown at 400C (Shiomitsu et al, 2001). Now, to_dvdetermine the mechanism of dioxins removal, that is,PCDFsto evaluate the extent of volatilization and2,3,7,8-TeCDF1.10decomposition (if any) reactions, the dioxins1.2.3,7,8-PeCDF0.09composition of the offgas during pyrolysis at 30 min2.3.4.7.8-PeCDF1.65retention time was also monitored. For this purpose,1.23.4.7.8-HxCDF.59dioxins in the tar of cold trap as well as in the gas1.2.3.6,7,8-HxCDF.39adsorbent (XAD-2 resin) were analyzed,1.2.3,7,8,9-HxCDFMass balance data showed that almost equal2.3.4.6.7.8-HxCDFamounts of total dioxins were present in the raw1.23,4.6.7.8-HpCDF0.41material as well as in the gas phase (Fig.4).1,2.3.4.7,8,9-HpCDF0.061Speifically, around 99.45% of the total dioxins thatOCDF0.014was originally present in the raw sediment wereTotal PCDFs805detected in the gas phase, which is distributed in theRemoval of PCDFs, %100tar condensate and gas adsorbent. The individualPCDDcontents of total dioxins in tar condensate and gas2,3,7,8-TeCDDadsorbent were 53.72% and 45.73% ，respectively.1.23.7.8-PeCDD3.1From these results, it appeared that volatilization is the1,2,3,4,7,8-HxCDDmain mechanism for its removal from sediment under1,23.67,8-HxCDD3.8experimental conditions. The dioxins had been1.2.3.78,9-HxCDD.9escaped prior to their destruction at higher1,2.3.4.6,7,8-HpCDD12temperature of 8009C.0CDD0.770.0005Extensive research has been done to study theTotal PCDDs52.44formation of PCDD/Fs in thermal processes, theRemoval of PCDDs, %99.999010precise mechanisms have not been clarified yet for thecomplexity of the processes (Dickson et al, 1992;Stanmore, 2004). In general, two mechanisms' have3.4.4,S-TeCB(#81)0.016been put forward primarily to explain for dioxin3,3,4,4-TeCB(#77)0.370.00024 0.0022synthesis - the precursor mechanism and the De3,3,4,4,5-PeCB(#126) 1Novo synthesis (Lundin and Marklund, 2005; Chang3,44,5,5"-HxCB0.047and Huang， 2000; Dickson et al., 1992). In the(#169)precursor mechanism, chloroaromatic precursors are2134,4,5-PeCB(#123)0.0230.000085necessary for dioxin formation. In most cases, these23,4,4,5-PeCB0118) 0.630.00026 0.00421compounds might be already present in the fuel, but2.3,4,4-PeCB(#105) 0.16they could also be formed at higher temperature by2.3.4.4,5-PeCB(#114) 0.085multistep reaction, including aromatization of2344.5,5'-HxCB0.00560.00012aliphatic compounds and chlorination. Aromatic rings(#167)are chlorinated by Ch2 from HCI with O2 via the2.3.4.4,5.-HxCB0.55Deacon reaction (Eq.(1); Vogg et al, 1987; Griffin,(#156)1986; Gullett et al, 1990; Dickson et al, 1992), and2.3.3.4.4,5-HxCB0.13formation of dual ring structures by metal-catalyzed(#I57)reaction (Eq.(2); Gullett et al, 1992). In the formation2344.5,5"HpCB0.022of PCDD/Fs via De Novo synthesis, on the other hand,(#189)Total PCBs14.0386 000055 0005 0.006622carbon can be oxidized to form co or CO2 by oxygen,Eq. (3) (Addink et al, 1995) and Eq. (4) (TuppurainenRemoval ofPCBs, %9.9994 9.9964 99.9528et al., 1998). Gaseous oxygen also plays a crucial roleTotal dioxins"710000850.0010.0066for the De Novo theory (Huang and Buekens, 1995,Removal of totalra。 0009.9999 99.9986 9.9907dioxins, %中国煤化工,0+Ch(1)Notes: *The TEQ value is calculated using the toxicity equivalentTHC N M H Ged aromaticsfactor (TEF) according to the World Health Organizaion (WHO)(1998); the limit of determination (LOD): for PCDD/Fs: 0.1 pg/g dw;(e.g. PCDD/Fs)(2)for PCBs: 0.2 pg/g dw; in the calculations of TEQ, results below theC+O2-CO2(3)LOD were considered zero; * total dioxins: PCDFs+PCDDs+PCBs2C +O- 2CO(4)No.5Removal of PCDD/Fs and PCBs from scdiment by oxygen free pyrolysis993i‘ClrFig.3 Structual formulate of PCDDs, PCDFs and PCBs0 PCDFs曰 PCDDs口 PCBsto enter the system. As a result, dioxin concentrationsin solid residues dramatically increased to 130.99times that of its original content in the raw sediment..8783.1213.34Following the same process, the concentrations oftotal dioxins in solid residues were also increased by197.18 and 154.93 times at 600C and 500C at夏”10.3371.4818.00retention time of 60 min, respectively. It can beconcluded that the formation of dioxins was冒着=remarkably enhanced by the presence of oxygen even“6.773.8619.77during the cooling period. Furthermore, there wasincreasing of 1732.30 to 2455.15 times in the TEQconcentration of PCDFs. Nevertheless, for PCDDs and0406080PCBs, the TEQ increase was range from 11.09 toPencentage of PCDDs, PCDFs and PCBs, %81.18 times only. The above results demonstrated thatFig.4 Fate of dioxins in the raw sediment, solid residues and volatilePCDFs, can be synthesized more easily than PCDDsphase during pyrolysisand PCBs under these conditions. Finally, it has beenFor both mechanisms mentioned above, thementioned that dioxins can form at the temperatureformation of dioxins is much dependent on thwindow of 200- -450C (Vogg et al, 1987). In oneconcentration of oxygen so that even at oxygenexperiment when the nitrogen atmosphere wasconcentrations as low as 1%, increases in PCDD/Fscontinuously supplied for cooling down the systemfrom 800C to 400C prior to the entry of air beloware observed (Addink and Olie, 1995). This was also400C until room temperature is reached, very fewconfirmed in this work where the role of oxygen fordioxins were generated and none of the PCDDs wasformation of dioxins was evaluated during the coolingdetected. These data further indicate that the range ofstage (Table 2). In these runs, pyrolysis was initiallytemperature for dioxins formation particularly for thisallowed to occur at 800C and 30 min retention time.pyrolysis sediment system may be higher than 400C.However, during the cooling period, air was allowedTable 2 Increasing times of dioxin concentration in solid residues under diferent atmosphere during Cooling down periodIncreasing times of dioxin concentration'Pyrolysis temp.,. C Retention time, min Cooling atmospherePCDFsPCDDs .PCBsTotal dioxins80030N1732.3011.0929.02 .130.9950N(air)"0.02390.01490.0046s0Air2455.1525.3981.18197.18Ss001926.7822.7938.19154.93Notes: * TEQ concentration of dioxin in solid residues 1 TEQ concentration of dioxin in raw sediment; ** 800- 400C: N atmosphere, 4009- roomtemperature: air atmosphereGoing back to the pyrolysis result, theWith these information, it can be stated that the fullnon-formation of dioxins in the solid residues may bescale pyrolysis must be conducted at an oxygen freeclearly attributed to the absence of oxygen in theatmosphere during the entire operation to preventsystem. If any increase in the concentration of afurther dioxin formation. Concerning the dioxins thatcertain dioxin species or any new species has beenare initially present in the sediment, since the maindetected in the gas phase, these can only be attributedmecl中国煤化工-volatilization ratherto transformation reactions since no oxygen is presentthanff-gases from the) initiate formation of new species. Although thepyrolTYHC N M H G secondary burmer,formation of dioxin-like PCBs may occur since itswhich operates at higher temperatures (> 1000C) toreaction does not require oxygen， experimentsaccomplish their complete destruction. These arerevealed that this occurrence was rather minimal.important steps that must be undertaken in the design