Hydroxyl Radical-and Sulfate Radical-based Advanced Oxidation Processes For The Removal Of Organic Pollutants

Since the Environmental Protection Agency of the United States has discovered the organic contaminants in groundwater for the first time in the 1970s,the problem of groundwater organic contamination has been increasingly concerned by domestic and foreign researchers,especially those refractory and/or non-biodegradable pollutants that can hardly be treated by conventional methods,such as nitrobenzene and chlorophenol.Advanced oxidation processes（AOPs）are the promising alternatives for the degradation of organic pollutants via the generation of reactive oxygen species（ROSs）such as hydroxyl radicals（HO·,1.8-2.7 V）and sulfate radicals（SO₄·^-,2.5-3.1 V）.These processes have value for the activation of oxidants such as hydrogen peroxide（HP,1.78 V）,peroxymonosulfate（PMS,1.82 V）,or persulfate（PS,2.01 V）.Compared with·OH-based AOPs,SO₄·^--based AOPs have utility in the destruction of organic compounds due to their higher redox potential of 2.5-3.1 V,selectivity,and longer lifetime.However,the capacity of an oxidant alone in the organic pollutants degradation is limited.Based on the different reaction mechanisms of HO·and SO₄·^-,a dual-oxidant combination technology in this paper was designed to construct a synergistic mechanism for the degradation of organic pollutants.As a common mineral in groundwater,the natural ferromanganese mineral is resourceful and environmental-friendly as a crude activator.Therefore,it is of important scientific and engineering practical value to explore the pathways of active free radicals generated by the combined systems of PS/HP,PS/PMS,and PMS/HP on the activation of natural ferromanganese mineral and the degradation mechanism of organic pollutants.PMS or HP has unique redox properties and can be both oxidized and reduced.Thus,PMS or HP can mediate Fe and Mn species that are amenable to the activation of PS.It can also enhance the degradation efficiency for the refractory organic pollutants as an additional oxidant.The detailed works were shown as the following:（1）FMBOs with different molar ratios of Fe and Mn（Fe:Mn=1:1-8:1）were synthesized via the redox-precipitation route.The structure and chemical properties of the as-prepared FMBOs were thoroughly analyzed by X-ray diffraction（XRD）,field emission scanning electron microscope（SEM）,transmission electron microscopy（TEM）,the Brunauer-Emmett-Teller（BET）method,Fourier-transform infrared spectroscopy（FT-IR）,and Raman spectrum.The results show that all of the characteristic diffraction peaks of FMBOs were identical and overlapped with cubic spinel Fe₃O₄（JCPDS no.19-0629）and cubic Mn₃O₄（JCPDS no.13-0162）.The peaks were suggestive of space groups Fd-3m（227）and I41/amd（141）.SEM images clearly illustrating the plate-like morphology of the as-synthesized samples with diameters of～10 to～250 nm.The absorption band at 1126 cm^-1 and1046 cm^-1 in the fresh F_4.5M₁BO corresponded to Fe-OH bending vibration.The band at 582 cm^-1corresponded to Fe-O stretching vibrations of magnetite.A new band appeared at 475 cm^-1 was attributed to the Mn-O stretching mode of the octahedral sites.The FT-IR spectrum provides concrete evidence for the formation of Fe₃O₄ and Mn₃O₄.This study was to investigate the degradation efficiency and kinetics in the combined FMBOs/PS/HP system.Here,2,4-dinitrotoluene（2,4-DNT）was selected as a representative target refractory organic pollutant due to frequent detection in aquatic environments.The effects of key reaction parameters including initial HP and PS concentrations,catalyst dosage,and initial p H on degradation efficiency and kinetics of 2,4-DNT were investigated.The dominant reactive oxygen species and their contribution to the degradation efficiency and kinetics of 2,4-DNT were identified.Furthermore,a possible activation mechanism in FMBOs/PS/HP system was proposed.In addition,the impacts of several coexisting anions ubiquitously presented in natural waters on the degradation efficiency and kinetics were also systematically investigated.The degradation intermediates detected by the LC-MS analysis and an oxidative degradation pathway in association with the aforementioned oxidative intermediates can be proposed for the removal of 2,4-DNT in the F_4.5M₁BO/PS/HP system.（2）Three natural ferromanganese minerals with different mole ratios of Fe and Mn were selected.ICP-OES、XRF、XRD、SEM、EDS、FT-IR and Raman were used to demonstrate the mole ratios of Fe and Mn,the chemical composition,the structure and chemical properties of the chosen natural ferromanganese minerals.The results indicate that the components were complex and weakly crystallized.The sheet-like morphology was found.The chosen minerals were firstly ground by ball mill,sieved to obtain the 100 mesh fraction and then set aside.The fine powder obtained were transferred into microwave vessels in contact with the HCl and HNO₃ mixtures by microwave-assisted acid solubilisation.After filtered with 0.45μm membrane,the settled solution was analyzed with inductively coupled plasma-optical emission spectrometry（ICP-OES）.Consequently,the chosen natural ferromanganese minerals were abbreviated as NFe_4.5Mn₁O,NFe₁Mn₃O,NFe₂Mn₁O,which were representative of the molar ratio of Fe to Mn as 4.5:1,1:3,2:1.Through batch experiments,optimized combination of NFe_4.5Mn₁O/Na₂S₂O₈/H₂O₂ system,NFe₁Mn₃O/Na₂S₂O₈/KHSO₅ system,and NFe₂Mn₁O/KHSO₅/H₂O₂ system were proposed.（3）In NFe_4.5Mn₁O/Na₂S₂O₈/H₂O₂ system,the effects of key reaction parameters including initial H₂O₂ and Na₂S₂O₈ concentrations,initial p H,NFe_4.5Mn₁O dosage,initial 2,4-DNT concentrations,and co-exciting anions concentrations on degradation efficiency and kinetics of 2,4-DNT were investigated.Based on the results from XPS,FT-IR,electron paramagnetic resonance and radical quenching tests,the possible activation mechanism was proposed.In addition,the reusability of the NFe_4.5Mn₁O was performed.（4）In NFe₁Mn₃O/Na₂S₂O₈/KHSO₅ system,the effects of key reaction parameters including initial KHSO₅ and Na₂S₂O₈ concentrations,initial p H,NFe₁Mn₃O dosage,initial 2,4-DNT concentrations,and co-exciting anions concentrations on degradation efficiency and kinetics of 2,4-DNT were investigated.Based on the results from XPS,FT-IR,electron paramagnetic resonance and radical quenching tests,the possible activation mechanism was proposed.（5）In NFe₂Mn₁O/KHSO₅/H₂O₂ system,the effects of key reaction parameters including initial H₂O₂ and KHSO₅ concentrations,initial p H,NFe₂Mn₁O dosage,initial 2,4-DNT concentrations,and co-exciting anions concentrations on degradation efficiency and kinetics of 2,4-DNT were investigated.Based on the results from XPS,FT-IR,electron paramagnetic resonance and radical quenching tests,the possible activation mechanism was proposed.In addition,the reusability of the NFe₂Mn₁O was performed.