Experimental And Theoretical Study On The Environmental Transformation Processes Of Polyfluorinated Dibenzo-p-dioxins And Dibenzofurans (PFDD/Fs)

Polyfluorinated dibenzo-p-dioxins/dibenzofurans(PFDD/Fs)are typical fluorine-containing compounds whose structures are similar to those of polychlorinatd dibenzo-p-dioxins/dibenzofurans(PCDD/Fs).Therefore,they are called fluorinated dioxins for short.Since PFDD/Fs contain fluorine atoms with high electronegativity,they may be ascribed to the fluorinated organic compounds.Therefore,PFDD/Fs share some properties with both PCDD/Fs and fluorinated organic compounds such as toxicity and high stability.They have wide applications in many fields such as chemical industry,materials and pharmaceuticals.Moreover,PFDD/Fs have been detected in fly ashes from waste incineration,paper mill wastewater and aluminum smelting process.Up to now,however,very few researches have been reported on environmental transformation behaviors of PFDD/Fs.Thus,based on the conditions mentioned above,some related research works were conducted on PFDD/Fs in this study.Detailed research contents and related conclusions are summarized as follows:(1)Eighteen polyfluorinated dibenzo-p-dioxins(PFDDs)were synthesized by pyrolysis of fluorophenols.Using a 500 W Xe lamp as the light source,the PFDDs photodegradation kinetics in n-hexane were investigated.The photolysis reactions obeyed the pseudo-first-order rate equation,and higher fluorinated PFDDs tended to photolyze more slowly.Theoretically calculated parameters reflecting the molecular structural properties were used to develop a new model of PFDDs photolysis rates.The results indicated that the substitution pattern for fluorine atoms and the C-O bond length were major factors in determining the photolysis rates of PFDDs.We selected octafluorinated dibenzo-p-dioxin(OFDD)as a representative PFDDs to explore the influence of solvent on the photolysis rate of OFDD,and the results indicated that neither the polarity nor hydrogen-donating ability of organic solvents are independent influencing factors.Mechanistic pathways for the photolysis of OFDD in n-hexane were first studied.The results indicated that photodegradation of OFDD produces octafluorinated dihydroxybiphenyls,octafluorinated phenoxyphenols,and fluorinated phenols.The major pathway for photodegradation of OFDD was C-O bond cleavage.Defluorination reactions did not occur during the photolysis process.(2)The photodegradation of octafluorodibenzo-p-dioxin(OFDD)in a water/acetonitrile(v/v = 4:1)mixed solvent was here investigated for the first time in the presence of Fe(?)and potassium monopersulfate(PMS)under UV(? = 365 nm)irradiation.The degradation efficiency was found to depend on the presence of UV irradiation,and on the initial concentrations of PMS and Fe(?).OFDD with an initial dosage of 1 mg·L-1 was successfully removed by UV/Fe(?)/PMS with OFDD:PMS:Fe(?)= 130:5 molar ratio within 10 min irradiation.The effects of humic acid,bicarbonate,chloride and solution pH were also evaluated.The degradation process of OFDD by UV/Fe(?)/PMS can be initiated by an oxidative attack of hydroxy radical on carbon atoms in the aromatic ring,the electron transfer reaction of sulfate radical with OFDD and a direct C-O bond homo lysis of OFDD.The intermediates and products of OFDD photodegradation were analyzed by using LC/MS technique and the associated photochemical pathway was proposed.The combined process with Fe(?),PMS and UV could be a useful technology for the treatment of wastewater containing OFDD.(3)Density functional theory(DFT)calculations at the B3LYP/6-311++G(2df,p)level of theory have been carried out to investigate the atmospheric oxidation mechanisms of some PFDDs,initiated by OH radical.The effects of substitution number and position of F atom on the total rate constants for OH addition reactions of PFDDs were analyzed.The computed results show that all OH addition reactions of PFDDs are thermodynamically spontaneous processes and the branch ratio of the PFDD-OH adducts is determined by the magnitude of the Gibbs free energies of activation(?rG?)and hence rate constants(k)for addition reactions.The OH reactions with all studied PFDDs are dominated by C?-addition and the total rate constants for OH addition decrease with increasing the number of fluorine atom substituting at ? position.Under the atmospheric conditions,the subsequent O2 addition reactions of PFDD-OH adducts occur hardly thermodynamically and are slow kinetically.For PFDD-?(?)-OH adducts without F atom at same positions the main reaction pathway is H abstraction by O2,while PFDD-?-OH adducts will undergo fused-ring C-O bond cleavage,affording the substituted phenoxy radicals.(4)The atmospheric chemical reactions of some PFDF and PCDF compounds,initiated by OH radical,were studied by performing theoretical calculations using density functional theory(DFT)and B3LYP/6-311++G(2df,p)method.The influences of sub stitution number and position of F(Cl)atom on the total rate constants for OH addition reactions of PF(C)DFs were investigated,and the differences between PFDFs and PCDFs in the OH addition reactions were compared.The obtained results indicate that OH addition reactions of PFDFs and PCDFs occurring at C1-4 and CA sites are thermodynamic spontaneous changes and the branching ratio of the PF(C)DF-OH adducts is decided primarily by kinetic factor.The OH addition reactions of PFDFs taking place at fluorinated C1?4 positions are kinetically comparable with those occurring at nonfluorinatedC1?4 positions,while OH addition reactions of PCDFs occurring at chlorinated C1?4 sites are negligible.The total rate constants of the addition reactions of PFDFs or PCDFs become smaller with consecutive fluorination or chlorination,and substituting at C1 position has more adverse effects than substitution at other sites.The succedent O2 addition reactions of PF(C)DF-OH adducts are thermodynamic nonspontaneous processes under the atmospheric conditions,and have high Gibbs free energies of activation(?rG?).The substituted dibenzofuranols are the primary oxidation products for PCDFs under the atmospheric conditions.However,other oxidative products may also be available for PFDFs besides substituted dibenzofuranols.