Study On Mechanism And Application Of ZIFs-derived Carbon-based Materials Activating Persulfate To Degrade BPA In Water

In recent years,advanced oxidation techniques based on hydrogen persulfate have attracted much attention due to their high redox potential,strong oxidation capacity and wide p H adaptation range.Among them,persulfate activation by transition metal is the most notable,but transition metal activation has side effects,low efficiency of catalyst and oxidant,metal ion residue and so on.Therefore,it is very important to explore a heterogeneous catalyst with high catalytic efficiency,stable effect,no secondary contamination,economic feasibility and practical application for persulfate.Zeolitic-Imidazolate frameworks（ZIFs）are the most representative Metal-Organic Frameworks（MOFs）with good specific surface area,coordination unsaturated active sites,abundant N content,diverse structure and flexible function.Crucially,ZIFs are ideal potential candidates for heterogeneous catalysts for persulfate with similar structural characteristics,excellent catalytic adsorption and excellent charge transfer performance.Based on these,different types of ZIFs-derived carbon-based materials were prepared from three dimensions of metal-free,mono-metal doping,and bimetallic coupling doping to remove typical organic matter from water to study catalytic properties,and combined with Density Functional Theory（DFT）,radical quenching experiments,and Electron Paramagnetic Resonance（EPR）testing to provide insights into the intrinsic mechanism of degradation of organic pollutants by three different types of catalysts.Finally,the structural dimensions of three powdered ZIFs-derived carbon-based catalysts were expanded from nanometres to macroscopic volumes.The dynamic removal of pollutants from the activated persulfate catalysts by homemade catalytic columns was investigated in this paper.The specific contents and main findings of the study are as follows:（1）NC-900,a metal-free carbon-based material,was prepared by pyrolysis at 900°C with ZIF-8 as the precursor,forcing the transition metal Zn to evaporate.To further improve the activation performance of NC-900 on PMS,the NC-KOH-900 with better catalytic adsorption properties was obtained by activating NC-900 with KOH.The results show that KOH modification can improve the adsorption and catalytic properties of NC-900 materials by improving the specific surface area,pore size,material defects and oxygen-containing functional groups.High temperature pyrolysis and KOH activation significantly increased the ability of NC-KOH-900 activated PMS to remove BPA,achieving 95%removal of BPA within 30 min.Based on the fact that the removal effect of BPA by adding PMS after the adsorption of NC-KOH-x reaches saturation is obviously better than that by adding PMS and NC-KOH-x at the same time,it is proposed that the adsorption state of the catalyst can affect the final removal effect.The NC-KOH-900/PMS system has wide p H range adaptability,satisfactory resistance to complex practical water disturbance,and good reusability.Radical quenching assays,EPR assays,degradation pathway assays,and toxicological assays indicate that the NC-KOH-900/PMS system mainly decomposes BPA into environmentally friendly small molecule substances by direct oxidation andβ-break in the presence of ¹O_2.（2）Fe-ZIFs were prepared by introducing transition metal Fe ions based on ZIF-8 and their derived carbon-based material Fe@NC-KOH-900 was prepared by pyrolysis and KOH activation at 900°C The results showed that the introduction of a small amount of Fe significantly enhanced the ability of Fe@NC-KOH-900 activated PMS to remove BPA and achieve 95%degradation of BPA within 15 min.Linear regression and correlation analysis revealed a relative change in the proportion of ROS in the Fe@NC-KOH-x/PMS system as pyrolysis temperatures increased.Of these:Fe@NC-KOH-700/PMS system(·OH+SO₄^-·=27.6%,high-priced iron oxides(Fe ^IV=O)=43.1%,¹O₂=29.3%),Fe@NC-KOH-800/PMS system(·OH+SO₄^-·=16.5%,(Fe ^IV=O)=53.5%,¹O₂=30.0%)and Fe@NC-KOH-900/PMS system(·OH+SO₄^-·=11.2%,(Fe ^IV=O)=56.4,¹O₂=32.4%).By studying the potential correlation between the main active sites of the catalyst Fe@NC-KOH-x and ROS in each system,it was found that the O-C=O,C-O,pyrolysis N and Fe-N₄in the material Fe@NC-KOH-x may be closely associated with·OH,SO₄^-·;The production of ¹O₂was affected by defects,graphite N,Fe-N₄and C=O;C=O,Fe-N₄and defects were strongly associated with the formation of(Fe ^IV=O).At the same time,DFT and the results of potential association between catalytic active centers and ROS indicate that Fe-N₄is the primary active site of Fe@NC-KOH-x.（3）On the basis of Fe-ZIFs,Cu²⁺was introduced after overcoming the generation of Fe³⁺by introducing Cu powder to react with Fe³⁺(Cu⁰+Fe³⁺→Cu²⁺+Fe²⁺).The results showed that in the absence of KOH,the synergy between Cu and Fe atoms significantly enhanced Fe Cu@NC-900 activation of PMS to remove BPA.Based on electrochemical impedance（EIS）analysis,XPS characterization,EPR analysis,and radical quenching experiments,it was concluded that the doping of Cu atoms facilitates the transfer of charges to PMS on the surface of Fe/Cu@-NC-900 to generate free radicals（·OH,SO₄^-·and O₂^-·）and non-free radical pathways（¹O₂）that co-degrade BPA in water;XPS analysis in combination with DFT theoretical calculations further demonstrated that the introduction of Cu atoms with strong electron transfer capacity can generate Fe Cu N₆sites to chemically and physically adsorb with PMS,receive or release electrons to PMS,and break the-O-O-in PMS.Compared to NC-KOH-900 and Fe@NC-KOH-900,the VSM results show that Fe/Cu@NC-900 not only has excellent catalytic properties,but has also good magnetization,which provides favorable conditions for magnetic recovery of materials.（4）Macro-bulk catalysts（NC-KOH-CA,Fe@NC-KOH-CA,Fe/Cu@NC-CA,B-NC-KOH-900,B-Fe@NC-KOH-900 and B-Fe/Cu@NC-900）were prepared by loading powder catalysts（NC-KOH-900,Fe@NC-KOH-900 and Fe/Cu@NC-900）with aerogels and polyester fiber spheres.Fe@NC-KOH-CA,Fe@NC-KOH-CA,Fe/Cu@NC-CA,B-NC-KOH-900,B-Fe@NC-KOH-900 and B-Fe Cu@NC were investigated for dynamic degradation in column.Through experiments,it was found that the removal rates of BPA by six oxidation systems（NC-KOH-CA/PMS,Fe@NC-KOH-CA/PMS,Fe/Cu@NC-CA/PMS,B-NC-KOH-900/PMS,B-Fe@NC-KOH-900/PMS and B-Fe/Cu@NC-900/PMS）in the self-made catalytic column under certain reaction conditions were 80%,89%,91%,92%,95%and 97%,respectively.More importantly,the concentration of metal ions leaching in each oxidation system is significantly lower than the environmental limit.Furthermore,three-dimensional fluorescence was used to analyze the water quality of the leachate MBR effluent before and after treatment,and it was found that the humus substances in the water samples were degraded in large quantities.