Photochemical Transformation Of Several Halogenated Organic Pollutants In Liquid/Solid And Gas/Solid Heterogeneous Systems

The widespread use of halogenated organics in industrial production has led to their widespread detection in the environment.Most halogenated organic pollutants have the characteristics of persistent organic pollutants,which can transport over long distances,accumulate in living organisms,and exist in the environment for a long time.Due to the biological toxicity and ecological risks,halogenated organic pollutants have attracted widespread attention from researchers.In the natural environment,halogenated organic pollutants undergo various environmental processes leading to their own migration and transformation.Photochemical transformation is an important conversion process for halogenated organic pollutants in the natural environment.The photodegradation of halogenated organics can change their structure,environmental persistence and bioavailability.These changes may be beneficial or harmful.Therefore,it is necessary to systematic study on different kinds of halogenated organics.In addition,the natural environment is a complex heterogeneous system,rather than a homogeneous system.Therefore,it should be considerd the real environment system when studying the photoconversion behavior of halogenated organic pollutants.In this study,typical halogenated organic pollutants such as brominated flame retardants,polychlorinated biphenyls,and halogenated phenols were selected as research objects.The simulated sunlight or sunlight was used to study halogenated organic compounds in particulate-water and particulate-air systems.Photodegradation kinetics,reaction path and mechanism were studied in detail.Polybrominated diphenyl ethers?PBDEs?are brominated flame retardants which have received considerable attention due to their global distribution,bioaccumulation potential,environmental persistence,and possible toxic effects.In this work,the photodegradation of decabromodiphenyl ether?BDE-209?in aqueous system was investigated by preloading it on the surface of various solid matrices.After 6 h of Xe lamp irradiation,almost complete degradation of BDE-209 was observed on silica gel?SG?,with much slower degradation occurring in other adsorbents.The degradation of BDE-209 on SG sample followed pseudo-first-order kinetics,and the observed reaction rate constant was decreased by lowering p H,adding humic acid and increasing the initial BDE-209 concentration.In addition to direct photolysis,BDE-209 could be oxidized by hydroxyl radicals generated from SG,as confirmed by the electron paramagnetic resonance?EPR?technology.Product analysis showed that BDE-209 was mainly decomposed into lower brominated PBDEs,polybrominated dibenzofurans?PBDFs?,hydroxylated PBDEs?OH-PBDEs?,hydroxylated PBDFs?OH-PBDFs?,bromophenols and bromide ions.Thus,consecutive debromination,intramolecular elimination of HBr,hydroxyl addition and the cleavage of ether bond were proposed as the degradation pathways.In this part,it is found that BDE-209 can degrade rapidly in silica gel-water system under simulated sunlight.Therefore,the next part explores the photodegradation behavior of DBDPE in silica gel-water system.Based on the above findings,this part studies the photodegradation process of DBDPE,a substitute of BDE-209,in silica gel water system.Decabromodiphenyl ethane?DBDPE?is a kind of brominated flame retardant and an alternative to decabromodiphenyl ether?BDE-209?.Significant photodegradation of DBDPE was found on the surface of silica gel?SG?in aqueous solution under simulated solar irradiation.The removal of DBDPE was enhanced under high p H conditions.The DBDPE degradation rate decreased with the addition of humic acid and an increase in the amount of DBDPE.The favorable stability and reusability of SG were presented for the promotion of DBDPE degradation.According to EPR analysis,quenching experiments and theoretical calculations,·OH can be generated from SG under simulated solar irradiation and was the main radicals for the DBDPE decomposition.Low brominated and hydroxylated DBDPEs?OH-DBDPEs?were identified by liquid chromatography time-of-flight mass spectrometry?LC-TOF-MS?and gas chromatography-mass spectrometry?GC-MS?,from which hydrodebromination and hydroxyl substitution were proposed as the major reaction pathways.The photochemical transformation of organic compounds is ubiquitous on a solid surface in the natural environment,and thus,low brominated DBDPEs and OH-DBDPEs can be generated from the DBDPE parent compound.This study can provide reference information for the photochemical transformation of DBDPE adsorbed on a solid surface in natural waters.Meanwhile,according to the above results,the photochemical transformation of decachlorobiphenyl?PCB-209?on the surface of several solid particles were systematically evaluated under simulated solar irradiation.The degradation kinetics of PCB-209 were first investigated using silica as a model aerosol particulate.It was found that PCB-209 photodegradation was enhanced at small silica particle size,low surface coverage and low humidity.EPR analysis and radicals quenching experiments demonstrated that hydroxyl radicals contributed to PCB-209 degradation.Step wise hydrodechlorination,hydroxyl addition and cleavage of the C-C bridge bond were mainly observed in the reaction process,leading to the formation of lower chlorinated PCBs,hydroxylated PCBs?OH-PCBs?and chlorophenols.Based on density functional theory?DFT?calculation,the dissociation energy of the C-Cl bond requires354.81?359.79 k J/mol energy that corresponds to a wavelength of less than 322 nm.And the minimum activation energy of·OH attack on PCB-209 is only 18.12 k J/mol.Photochemical transformation of PCB-209 can also occur on the surface of natural particles,but the rates were inhibited as compared to silica.The hydroxylation and hydrodechlorination products of PCB-209 were detected in all-natural particles.This study would give significant contribution to understanding the fate of PCBs in solids/air interface.Based on the fact that organic pollutants can degrade rapidly on the surface of silica gel under simulated solar irradiation,the silica gel was characterized in detail,including morphology,electronic structure and photoelectrochemical properties.It is found that silica gel has porous structure with the pore size 3?15 nm and the average pore size of 8.36 nm,which is mesoporous material.The BET specific surface area of silica gel is 337.9 m²/g,and silica gel can also generate stable transient current under light irradiation.In addition,BDE-209 and halogenated phenol?C₆X₅OH,X=F,Cl,Br?can all undergo photodegradation in silica gel/air systems,but the photodegradation rates are different.Product analysis shows that they are converted into various hydroxylated products in silica gel/air system,and hydroxyl radical was determined as the major reactive species.According to density functional theory?DFT?calculations,the reaction of physically adsorbed water with reactive silanone sites?>Si=O?on silica was indispensable for the generation of·OH,where the required energy matches well with the irradiation energy of visible light.Then,the BDE-209was selected as a representative compound to evaluate the photocatalytic performance of SG under different conditions.The SG showed good stability in the photodegradation process,and was able to effectively eliminate BDE-209 under natural sunlight.These findings provide new insights into the potential application of SG as a solid surface photocatalyst for contaminants removal.In summary,the photochemical transformation of BDE-209,DBDPE,PCB-209and halogenated phenols in liquid/solid and gas/solid systems were studied.It was found that several of the halogenated organic compounds studied could undergo photodegradation,and the type of particulate matter had a greater effect on the degradation rate of organic matter.Under simulated solar irradiation,silica gel materials can induce the production of·OH.The photodegradation process of halogenated organic pollutants includes direct photolysis and indirect photolysis.Direct photolysis is mainly based on hydrodehalogenation,and indirect photolysis is mainly caused by·OH oxidation.This study simulates the real environmental system as much as possible,and investigates the photodegradation process of halogenated organic pollutants in detail.The results not only have high scientific value and environmental significance,but also show practical value.