Induced Generation Of Active Iron Species And Its Application In Rapid Oxidation And Detection Of Organic Pollutants

In recent years,with the rapid development of economy and the continuous expansion of human activities,emerging organic contaminants（EOCs,such as persistent organic pollutants,endocrine disruptors,pharmaceuticals and personal care products）have been entering water bodies.Although low in concentration,these pollutants pose a significant threat to the ecological environment and humanity due to the great variety and toxicity.Thus,rapid detection and effective treatment of EOCs in water have attracted great attention to researchers.Advanced oxidation process（AOPs）and chemiluminescence（CL）analysis based on sulfate radical(SO₄^·-)and hydroxyl radical（and HO^·）have been widely used for EOCs removal and detection.However,the complex background matrix can consume a large number of radicals,which not only severely inhibits the pollutant removal efficiency of the SO₄^·-/HO^·-based-AOPs,but also makes it difficult for traditional SO₄^·-/HO^·-based-CL analysi to be directly applied to the rapid detection of EOCs in practical water.Compared with traditional active species such as radicals,intermediate-valent iron-oxo species（reactive iron species,RFeS）have attracted extensive attention,owing to the environmental friendliness,strong oxidation selectivity,high steady-state concentration,long lifetime,and resistance to background matrix,showing great potential in EOCs degradation and detection.However,Fe²⁺and common iron-based materials can produce a variety of active species except RFeS when activating PMS,which is difficult to meet the above requirements.Therefore,based on the axial-coordination-effect,a variety of materials with Fe-N active centers were constructed and RFeS were generated in situ by activating PMS.The efficiency and mechanism of RFeS in the rapid oxidation and detection of EOCs were systematically studied.The main research contents and conclusions are listed as follows:As one of the molecular catalysts,iron phthalocyanine（FePc）is the most commonly available Fe-N material.In the preliminary experiments,we found that the same batch of iron phthalocyanine（FePc）with different storage time exhibited different performance for activating PMS to degrade EOCs.Based on it,two modified FePc materils were prepared（named as FePc-O₂ and FePc-hh）and bisphenol A（BPA）was selected as the probe EOC to evaluate the performance and mechanism of them for activating PMS to degrade EOCs.The FePc-O₂/PMS system showed a much better performance for BPA degradation than FePc-hh/PMS system.Radical quenching experiments and methyl phenyl sulfone（PMSO₂）yield experiments suggested that RFeS was the key active species responsible for BPA degradation.Thus,the FePc-O₂/PMS system was negligibly affected by anions and humic acid,and could achieve an effective oxidation of BPA over a wide pH range（3.0～11）.FeK-edge X-ray absorption fine structure（XAFS）spectra and ⁵⁷FeM?ssbauer spectra demonstrated that the structure and valence state of the Fesite in FePc-O₂ were obviously changed by the oxidation modification,resulting in the theoretical Fe^ⅡN₄ site changed to the O-Fe^ⅢN₄site with an axial coordinated O atom.The axial O coordination could pull Featoms away from the N₄ plane and optimize the electrons and orbitals of Fe,endowing FePc-O₂ with an oxidase-like push-effect mechanism.Therefore,compared with the Fe^ⅡN₄site in FePc-hh,the O-Fe^ⅢN₄ site in FePc-O₂ could activate PMS to generate RFeS at a faster rate and in a a higher proportion.Based on the finding in the previous chapter,we attempted to extend the axial-coordination-effect to the synthesis of Fe-based single atom catalysts（Fe-SACs）.Via a two-step isolation synthesis strategy,we constructed Fe-SACs with a carbon nanotube（CNT）as an axial ligand(named as Fe_SA-N-CNT)to generate RFeS in the presence of PMS.The obtained Fe_SA-N-CNT/PMS system exhibited outstanding and selective reactivity for oxidizing EOCs over a wide pH range（3.0～9.0）.～100%of BPA（50μM）was degraded in the Fe_SA-N-CNT/PMS system within 1.0 min and～40%of total organic carbon（TOC）was removed within 4.0 min.The results of main transformation products showed that the hydroxylation and C-C scission reaction products are gradually transformed to the ring-opening products and eventually mineralized.Thus,the toxicity of BPA was greatly reduced after treatment in the Fe_SA-N-CNT/PMS system.Several lines of evidences suggested that RFeS existing as an FeN₄=O intermediate was the predominant oxidant,while SO₄^·-and HO^·were the secondary oxidants.XAFS analysis demonstrated the interaction between CNT ligands and FeN₄ sites due to the obvious Fe-C bone.Density functional theory calculation results revealed that CNT played a key role in optimizing the distribution of bonding and antibonding states in the Fe3d orbital,resulting in the outstanding ability of Fe_SA-N-CNT for PMS chemical adsorption and activation.Moreover,CNT functionded as the axial ligands could significantly enhance the reactivity of the FeN₄=O intermediate by increasing the overlap of electrons of the Fe3d orbital,O 2p orbital,and bisphenol A near the Fermi level.On the basis of finding that axial-coordination-effect can indeed improve the ability of Fe-SACs for PMS activation and enhance the reactivity of generated RFeS,we attempted to the design and construct Fe-SACs with stronger axial-coordination-effect.Fe-SACs with N-FeN₄ sites were fabricated on CNT,rGO,SiO₂,and C₃N₄ via a simple one-step calcination of mixed powders of dicyandiamide,carrier material（CNT,rGO or SiO₂）and FePc（named as Fe-CNT,Fe-rGO,Fe-SiO₂,and Fe-C₃N₄）.Dicyandiamide could form N-doped sites at the defects of the carrier materils at high temperature.FePc with Fe-N₄ structure could combine with the N-doped site to form the N-FeN₄ site at high temperature.Scanning transmission electron microscopy（STEM）,X-ray photoelectron spectroscopy（XPS）,and XAFS result demonstrated the atomically dispersed N-FeN₄ sites.Temperature-dependent magnetizations obtained by zero field cooling and field cooling procedure（ZFC-FC）and the room-temperature⁵⁷FeM?ssbauer spectra suggested the high spin state Fein N-FeN₄ site,in which the electrons in e_g orbital could transfer with PMS more easily to produce RFeS.Combined with the results of PMSO₂ yeild experiments and the spin state proportion of Fein each Fe-SACs,it was proved that the generation of RFeS in Fe-SACs/PMS system was greatly related to high spin N-FeN₄.Thus,owing to the highest proportion of high spin N-FeN₄ sites,a large amount of PMSO₂ can be detected during the degradation of methyl phenyl sulfoxide（PMSO）in the Fe-CNT/PMS system with the PMSO₂ yield close to 100%.In other words,the oxidizing species in Fe-CNT/PMS system were～100%RFeS.Moreover,the generated RFeS intermediate in Fe-CNT/PMS system was further proved by the in-situ FeK-edge XAFS results.Based on the unique advantages of RFeS in resistance to background matrix,Fe-CNT/PMS/luminol CL system was developed.Compaerd with radicals-based CL systems,Fe-CNT/PMS/luminol system was less easily affected by anions and humic acid,showing better adaptability in practical water.Finally,the RFeS-based Fe-CNT/PMS/luminol CL system for determination of phenol was developed with a good linear relationship ranging from 0.1μM to 100μM.In summary,in view of the environmental requirements of developing new AOPs and CL analysis motheds that are resistance to background matrix,combined with the matched advantages of RFeS,Fe-N materials were constructed to generate RFeS in situ in the presence of PMS to achieve the the selective degradation and rapid detection of EOCs.This study also proposed an axial-coordination-effect for the induced generation of RFeS,shed some light on the electronic-level insight into the activation of PMS by Fe-N materils,and expanded the application scenarios of RFeS-based Fe-N materils/PMS system.It is expected to push the RFeS-based PMS activation technology into practical application faster.