Degradation Of Emerging Halogenated Organic Pollutants In Water Using Advanced Oxidation Processes

With growing usage of emerging halogenated organic pollutants in human consumption,these chemicals have recently received extensive concern from scientific community due to the increasingly worse environmental problem from their direct discharge.Natural attenuation and conventional treatment processes are not capable of removing most of these chemicals from wastewater,thus developing cost-effective and environmentally friendly remediation technologies is in urgent demand to address the risks of emerging halogenated organic pollutants.This study aimed to create a sequential environmentally friendly treatment technology with high energy efficiency by improving two kinds of advanced oxidation processes,photochemical reactions and electrooxidation,which were driven by the external energy.Dichlorophen（DDM）,atorvastatin（ATV）and per-and polyfluoroalkyl substances（PFASs）were selected as the model substrates because of their typical structure in emerging halogenated organic pollutants.Firstly,the photolysis of DDM,ATV and perfluorooctanesulfonate（PFOS）was investigated under simulated sunlight irradiation.The destruction of PFASs for treatment purpose is challenging for photochemical reaction that stem from the high energy carbon-fluorine bonds and aliphatic chains,which retard complete degradation of emerging halogenated organic pollutants in wastewater.Therefore,electrooxidation based on the conversion of electric energy was selected to remove PFASs,involving reactive electrochemical membrane（REM）and electrocoagulation.This study could provide an environmentally friendly treatment technology with high energy efficiency for the complete removal of emerging halogenated organic pollutants in wastewater.The main results on the above experiments are as follows:（1）Firstly,the photodegradation of three emerging halogenated organic pollutants mentioned above was investigated.It was found that DDM could be effectively degraded under simulated sunlight irradiation,however,only 16.3%removal of ATV was achieved within a 5h irradiation time,and no degradation of PFOS happened even if UV irradiation was used.The photolysis process of DDM is highly p H-dependent,mainly because of the change in electronic density distribution on the aromatic ring of DDM in its varied ionic speciation under different p H.The significant roles of singlet oxygen（¹O2）and ³DDM*in DDM photolysis were verified by experimental measurements and density functional theory（DFT）calculation.It was also found that photolysis can effectively remove the toxicity of DDM..（2）The photocatalytic activity of rice husk biochar to the degradation of ATV was systematically investigated.Compared with the biochars with larger particle size and higher pyrolysis temperatures,superior photocatalytic activity was observed for the biochars with lower particle size and pyrolysis temperatures in this study,that is,dissolved state biochar with300℃ pyrolysis temperatures（DSB300）facilitated the photodegradation of ATV to a greater extent(327.4%based on kobs,50 mg L^-1).The noteworthy photocatalytic activity was attributed to the dual role of DSB300 in the process of generating reactive species as heterogeneous photocatalyst and photosensitizer.The mineral components are responsible for the heterogeneous photocatalytic activity of DSB300.Organic carbon components could synergistically enhance the heterogeneous photocatalytic activity by enhancement of electron-hole separation,and contribute to the formation of ¹O2 and triplet-excited state（³DSB*）as well.The identification of intermediate products and X-ray photoelectron spectroscopy（XPS）analysis of irradiated DSB revealed that cross-coupling reaction between ATV and DSB existed in the photodegradation process of ATV.Meanwhile,the modification of DSB300 under irradiation could be evidently attenuated with ATV in the reaction system as shown by multiple characterizations,which helped to keep the stability of DSB300 in photochemical reaction process.（3）The effective utilization of electric energy is important to remove emerging halogenated organic pollutants in wastewater,especially for PFASs that could not be degraded under simulated sunlight/UV irradiation.This study investigated the degradation of PFOS in a Magneli phase titanium suboxide REM.Near complete removal（98.30±0.51%）of PFOS was achieved under a cross-flow filtration mode at the anodic potential of 3.15 V vs.standard hydrogen electrode（SHE）.PFOS removal efficiency during the REM operation is much greater than that of the batch operation mode under the same anodic potential.A systematic reaction rate analysis in combination with electrochemical characterizations quantitatively elucidated the enhancement of PFOS removal in REM operation in relation to the increased electroactive surface area and improved interphase mass transfer.PFOS appeared to undergo rapid mineralization to CO2 and F^-,with only trace levels of short-chain perfluorocarboxylic acids（PFCAs,C4-C8）identified as intermediate products.Density functional theory simulations and experiments involving free radical scavengers indicated that PFOS degradation was initiated by DET on anode to yield PFOS free radicals（PFOS^·）,which further react with^·OH that were generated by water oxidation and adsorbed on the anode surface（^·OHads）.The attack of^·OHads is essential to PFOS degradation,because,otherwise,PFOS^·may react with water and revert to PFOS.（4）Enhancement of targeted pollutants concentrations is an efficient way to improve their electrooxidation degradation efficiency.An electrocoagulation and electrooxidation treatment train to degrade perfluoroalkyl substances was studied.Electrocoagulation（Zinc sheet as anode）could effectively remove PFASs from water,and long-chain PFASs（C7-C10）compounds tended to have a higher removal rate.In this study,electrocoagulation-derived foam was generated when a relatively high current density(>1 m A cm^-2)was applied to a relatively high PFASs concentration（>0.1μM）during electrocoagulation.The electrocoagulation treatment efficiency and recovery rates in concentrated solution were evaluated in two different spiked water system,namely high current density(5.0 m A cm^-2)/high concentration（each 0.5μM）and low current density(0.3 m A cm^-2)/low concentration（each 0.005μM）.The floc could be completely dissolved by a small amount of H2SO4.These results suggested that the recovery rates of individual PFASs ranged from 88.0%to 111.0%under high current density,and long-chain PFASs tended to be fractionated into foam.The recovery rates of individual PFASs were87.6-131.6%under low current density system.It was noted that no supplement of electrolyte addition was needed for the floc dissolved solution,but it was needed for foam solution.We also found that PFASs in electrocoagulation-derived concentrate solution were removed efficiently during electrooxidation treatment,that is,the coupling electrocoagulation and electrooxidation is doable to effectively break down PFASs.