De Novo Formation Of PCDD/F From Model Fly Ash

Polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans(PCDD/F),collectively called ""dioxins" are persistent organic pollutants(POPs)that are formed almost invariably as unwanted by products in all thermal systems with municipal solid waste incinerators(MSWIs)as one of the major sources.In the post combustion zone of incinerator,presence of products of incomplete combustion(PICs),chlorine sources,catalytic metals and organic precursors create PCDD/F via a series of chemical reactions.Two surface catalysed formation mechanisms,de novo and precursor synthesis have been widely investigated and accepted as main formation pathways for PCDD/F formation but relevant importance of each is still debatable.A series of experiments have been conducted to further explore PCDD/F formation from milled and non-milled model fly ash.The main results are summarized as follows:(1)Model fly ash(MFA)samples composed of silica,sodium chloride,and activated carbon,doped with metal chloride(0.2 wt%Cu,Cr,Fe,Zn and Ca),and were subjected to high intensity ball milling at 300 rpm.The milling effect on MFA was studied by thermogravimetric analysis(TGA)and differential scanning calorimetry(DSC)in the temperature range from 25 to 600 ? in oxygen.Surprisingly,TG curves showed gradual weight gain with slight variation within samples after 1 and 8 h,which was also supported by measuring physical weight gain of samples after milling treatment.MFA samples on average showed an exothermic peak at a temperature of 404.4 ?.The Raman spectroscopy analysis revealed that milling has varied effect on graphitic structure of activated carbon in MFA and highest ID/IG ratio was observed for Ca-MFA following Cr-MFA after 8 h milling.XRF analysis revealed that milling introduces supplemental metals(Fe,Ni,Cr,Mn...),following wear of both steel balls and housing,moreover milling reduces the particle size of fly ash and disperses catalytic metal,potentially rising the reactivity of fly ash to form and destroy 'dioxins',i.e.,polychlorinated dibenzo-p-dioxins and dibenzofurans(PCDD + PCDF or PCDD/F).(2)Formation of polycyclic aromatic hydrocarbons(PAH),and of polychlorinated phenols(CP),benzenes(CBz),biphenyls(PCB)and 'dioxins'(PCDD + PCDF)was investigated in Cu-doped MFA at conditions,prevailing in the post-combustion zone of an incinerator,where dioxins are formed and destroyed.These compounds are all characterised by their rate of generation(ng/g MFA)and their signature,i.e.,internal distribution over congeners as a means of gathering mechanistic indications.PAH and CBz total yield did not decrease in MC treated MFA with milling time,while total pentachlorophenol(PeCP),PCB and PCDD/F yield decreased up to 86,94 and 97%respectively.I-TEQ concentration decreased more than 90%,while degree of chlorination varied inconsistently for PCB and PCDD/F,and average congener patterns of PCDD/F do not vary considerably with milling time for both gas and solid phase.(3)De novo tests were further conducted utilizing Cu,Cr,Fe,Zn and Ca-doped MFAs(0.2%).Metal chlorides produce specific and distinct homologue and isomer patterns.Principal component analysis(PCA)applied to the 2,3,7,8-PCDD/F-congeners,indicating CuCl2 as major contributor of I-TEQ values.Surprisingly,for CuCl2·2H2O the total yield of dioxins reduced drastically with milling,compared to other selected metal chlorides.(4)De novo formation of Polychlorinated biphenyls(PCB)and polychlorinated dibenzo-p-dioxins and dibenzo-furans(PCDD/F)in Cu-doped MFA,milled for selected time(0,1,2,4 and 8 h)was studied in simulated conditions to investigate effect of milling treatment time.?PCDD/F,?PCB,?2,3,7,8-PCDD/F,?dl-PCB(dioxin like PCB)and corresponding ?I-TEQ(International Toxic Equivalents)concentrations were greatly reduced after 8 h treatment both in gas and solid phase of MFA.The homologue and isomer signatures of both dioxins and PCB were variable in concentration,while chlorination degree successively decreased with prolonged milling augmentation.MFA still exhibits de novo activity after long 8 h MC treatment and reformation of some congeners was observed,among dl-CB erratic reformation of penta-CB,#81 was observed with 1296.6%increase in concentration in solid phase,compared to 4 h milled sample.(5)PCDD/F formation from 2,4,6-trichlorophenol(TCP)added model fly ash(Cu-MFA)was studied at selected temperatures and atmospheric conditions for a reaction time of 60,20 and 10 min with gas flow rate of 300 mL/min.PCDD/F were produced in high concentration at 300 ? both in oxidising and anoxic atmosphere.PCDD/F and corresponding I-TEQ concentrations increased with selected temperature for 20 and 10 min reactions,while in 60 min reaction time,PCDD/F concentration increased at first from 150 to 300? and then start declining with further increase in temperature.PCDF isomers dominated over PCDD in presence of 10%O2 + 90%N2,yet with some exceptions and PCDD dominate in nitrogen atmosphere.TCP precursor produced high levels of CP-route PCDD isomers(1,3,6,8-and 1,3,7,9-T4CDD,1,2,4,7,9+,1,2,4,6,8-,1,2,3,6,8-,1,2,3,7,9-P5CDD and 1,2,3,4,6,8-H6CDD),while PCDF congeners were similar to CuCl2 catalysed congeners through de novo formation.