Study On The Formation Mechanism Of Dioxins And The Nucleation Mechanism Of Organic Compounds

The dioxins in the environment,as one kind of the persistent organic pollutants(POPs),are famous for its strong toxicity.They are extraordinary harmful to human beings as their carcinogenicity,teratogenicity and mutagenicity,especially for halogenated dioxins.The sources of dioxins include natural and anthropogenic sources,where human sources occupy the main part.But,the dioxins from anthropogenic sources are unintentionally generated.The study for the formation of dioxin has been going for decades,and it is difficult to detect their intermediates in the process of formation.Most of dioxins were produced in high temperature process,which increases the difficulty of the research for them.Hence,many of their specific formation mechanism are still fuzziness.Chlorophenols(CPs)are generally considered as the predominant precursors of polychlorinated dibenzo-p-dioxin/dibenzofuran(PCDD/Fs)and implicated as key intermediates in the de novode novo synthesis.And bromophenols(BPs)are structurally similar to CPs and have also been demonstrated to be the predominant precursors of PBDD/Fs.PBCDD/Fs can be formed from the combustion or high-temperature pyrolysis of the CP/BP mixture.These three analogues are known as halogenated dioxins(PHDD/Fs).Previous studies mainly focused the formation of PCDD/Fs from CPs or the formation of PBDD/Fs from BPs,the reaction mechanism of forming PBCDD/Fs was ignored.So,we need to take a research on this blank.Water which has strong ability to form hydrogen bonds is ubiquitous in PHDD/Fs formation,and its effect on dioxins formation is not yet clear.In the experiment,there is a controversy about whether water exists to inhibit or promote dioxins formation,and the corresponding phenomenas observed in the experiment are inexplicabl.Therefore,the research about the PBCDD/Fs formation from cross-condensation of CPs with BPs and the influence of water on PHDD/Fs formation mechanism becomes very meaningful.Aerosols,small particles suspended in air,can be formed in the atmosphere via nucleation of gaseous phase species or be directly discharged into the atmosphere through primary sources.The aerosol nucleation events produce a large amount of atmospheric aerosols.New particle formation happens in two stages,including the critical nucleus formation and growth.And the subsequent growth of the critical nucleus to a larger size is competitive with pre-existing aerosols that can capture and remove the freshly nucleated nanoparticles.Nucleation processes are characterized by reducing in both enthalpy and entropy of the system.According to the first law of thermodynamics,nucleation is thermodynamically favorable,but it is hindered in entropy according to the second law of thermodynamics.As a result,the Gibbs free energy of formation(?G)is usually used to describe the ease of nucleation.The equilibrium vapor pressure above small cluster and nanoparticle is another influencing factor in the nucleation and growth of atmospheric nanoparticles.Atmospheric aerosols can affect radiation balance of earth,climate,weather,and human health and so on.Until now,China has been plagued by smog.It is urgent to elevate an excellent air quality.The involvement of organics in nucleation has been a hot topic in recent years.It has been perplexing people for their roles in the formation of new particles and the specific organics involved in nucleation.Atmospheric aldehydes,ketones,alcohols and organic acids in the oxidation process are extremely complex.The effective experimental methods can only detect the scale of the 1-2 nm for organic clusters,and it is difficult to analyze the specific compositions of nanoparticles.Using quantum chemistry method in data of thermodynamics and kinetics of formation of clusters is effective and feasible method to solve the difficult or controversial problems in the current experiment.Therefore,this paper used quantum chemistry methods to study the dioxins formation from cross-condensation of chlorophenoxy with bromobenzene and the effect of water on dioxins formation.At the same time,the process of nucleation of aldehydes,pinonic acid and their gas products with sulfuric acid were also analyzed at the molecular level.The related research topics can be divided into the following four parts.1.PBCDD/F formation from radical/radical cross-condensation of chlorophenoxy with bromobenzeneUsing Density Functional Theory(DFT),the mechanism of mixed polybrominated/chlorinated dibenzo-p-dioxins/benzofurans(PBCDD/Fs)formation from the cross-condensation of 2-chlorophenoxy radical(2-CPR)with 2-bromophenoxy radical(2-BPR),2,4-dichlorophenoxy radical(2,4-DCPR)with 2,4-dibromophenoxy radical(2,4-DBPR),and 2,4,6-trichlorophenoxy radical(2,4,6-TCPR)with 2,4,6-tribromophenoxy radical(2,4,6-TBPR)was studied in this artical.The geometrical parameters and vibrational frequencies of intermediates,transition states and products were performed at the MPWB1K/6-31+G(d,p)level,and corredponging single-point energy calculations were calculated at the MPWB1K/6-311+G(3df,2p)level.By the canonical variational transition-state(CVT)theory with the small curvature tunneling(SCT)correction,the rate constants of the elementary reactions were evaluated over a temperature range of 600-1200 K.Our studies show that the substitution pattern of halogenated phenols can determine both the substitution pattern of the resulting PBCDD/Fs and the formation mechanism of PBCDD/Fs,especially on the coupling of the halogenated phenoxy radicals.2.The effect of water on the mechanism of halogenated dioxins from chlorinated/brominated phenols as precursorsWater has many important roles because of its strong ability to form hydrogen bonds.Due to the persistence,bioaccumulation and high toxicity,polyhalogenated dibenzo-p-dioxin/benzofurans(PHDD/Fs)are notorious.It is necessary to understang the formation mechanism of PHDD/Fs in the presence of water.This work investigated the influence of one water molecule on the gas-phase formation of PHDD/Fs from halogenated phenols(HPs)as precursors via using quantum chemical methods under the MPWB1K/6-311+G(3df,2p)//MPWB1K/6-31+G(d,p)level of theory.Our study reveals that water molecules promote the formation of halogenated phenoxy radicals(HPRs)from the H abstraction reactions of HPs with atomic H and OH radicals by means of opening new low-energy pathways and lowering the reaction energy barriers.And a water molecule can produce a catalytic effect on the H-shift step involved in the formation of PHDFs and thus their formation potential is enhanced.3.Aldehydes and their atmospheric products participate in nucleationAldehydes have been speculated to play an important role in the new particles formation.In the current work,quantum chemical calculations were performed to investigate the hydrogen bonding interaction and the Gibbs free energy of formation(?G)for clusters consisting of sulfuric acid and aldehydes as well as their atmospheric reaction products.Calculations were conducted at 298 K and 1 atm under the M06-2X/6-311+G(3df,3pd)level.The results reveal that the addition of small molecule aldehydes to sulfuric acid molecules is unfavorable in thermodynamics.Nevertheless,their products from aldol condensation,hydration,and polymerization reactions can promote new particle formation and growth by stabilizing sulfuric acid molecules.Moreover,the favorability of the interaction in absence of water between sulfuric acid and the addition products are as follows:the hydration products>aldol condensation>aldehydes,but this order may be change with the introduction of water.In particular,our calculated ?G values imply that monohydrate of glyoxal is more likely to nucleate with sulfuric acid compared with ammonia in the presence or absence of water.4.Pinone acid and its hydrolysate are involved in nucleationPinonic acid(CPA),one of the major photooxidation products of a-pinene,is believed to be related to the formation of aerosols formed over forested areas.In this article,we used the method of quantum chemical calculation to investigate the interaction between sulfuric acid(SA)and CPA as well as the hydrolysate of CPA(HCPA)in presence water or ammonia in the atmosphere.The lowest free energy configurations,reactants,transition states,intermediates,and products were optimized at 298/278 K and 1 atm under the M06-2X/6-311+G(3df,3pd)level.Our results show that one CPA molecule may nucleate with sulfuric acid molecules in the beginning.However,the subsequent nucleation process of CPA is more likely to participate in the new particle formation and growth in the form of HCPA.Meanwhile,more than one HCPA molecules may be involved in the critical nuclei.H2SO4 and HNO3 can effectively catalyze the hydrolysis reaction of CPA in presence of water,which greatly increases the content of HCPA molecules in the atmosphere.Hence,sulfuric acid can not only directly promote nucleation,but also can play a catalyst for the hydrolysis of carbonyl organics.