Enhancement And Mechanism Of Phenanthrene Degradation By Persulfate Activation Based On Electron Regulation Of Catalysts

Oil pollution is becoming more and more serious in China.Polycyclic aromatic hydrocarbons（PAHs）are abundant in petroleum hydrocarbons.As a kind of PAHs,phenanthrene（PHE）,is difficult to degrade,which is teratogenic,carcinogenic and mutagenic and belongs to persistent organic pollutants with a wide range of threats.Its symmetrical structure and conjugation determine that its electrostatic potential is relatively uniform,so its chemical properties are stable and it is difficult to remove.In general,advanced oxidation are used to oxidize phenanthrene by generating active species such as free radicals to achieve efficient degradation of phenanthrene.Catalyst-activated persulfate advanced oxidation system has been widely studied for its high degradation efficiency,strong catalytic capacity,simple operation and environmental friendliness.However,the targeted regulation of atomic structure and electron density based on the active site on the catalyst surface and the enhancement of efficiency have not been thoroughly studied.In this thesis,based on the prediction by molecular dynamics and quantum chemistry calculation,four chelating agents with different molecular structures including 1,10-phenanthroline（LFLL）,1,8-naphthyridine（EDN）,acridine（YD）and pyridine（BD）were used to chelated with Fe（the mole ratio of 2:1）and doped in the C/N materials,so that green and stable metal-supported catalysts were synthesized.This method can realize the regulation of the atomic distribution and coordination number of Fe-N active site,and then strengthen the catalytic activation of PMS to degrade phenylene.Then the electron transfer mechanism and spatial limiting effect are analyzed by molecular dynamics and DFT calculation.Based on the above studies,combined with the regulation of active site load（YD-Fe 0.5%-C/N,YD-Fe 1%-C/N,YD-Fe 3%-C/N,YD-Fe 5%-C/N,YD-Fe 10%-C/N）and multi-metal atoms coupling regulation（Fe Co Ni-C/N,Fe Ni Cu-C/N,Fe Co Cu-C/N,Co Ni Cu-C/N,Fe Co Ni Cu-C/N）,the efficiency of C/N catalyst activated PMS to degrade phenanthrene was further enhanced,and the electron transfer mechanism was explored.The main conclusions are as follows:（1）YD-Fe-C/N has high degradation efficiency,and the degradation rate of YD-Fe-C/N reaches 96.65%in 2 h（dose:100 mg/L,p H:7,temperature:35℃,PMS concentration:5 m M,PHE concentration:1 mg/L）.Considering the catalytic performance and economy,the degradation rate of YD-Fe-C/N system reaches 81.65%in 2 h at 35℃.The performance and mineralization rates of the catalyst were stable and gradually improved.In addition,the catalyst was environmentally friendly,and the Fe leaching of four repeated experiments were far lower than the emission standard.In addition,the quenching experiment found that^·OH was the main active species produced by this system,and ¹O₂ and O₂^·-were co-involved,but the ESR results indicated the direct electron transfer and ¹O₂ path.It was speculated that^·OH did not separate from YD-Fe-C/N surface after generation,but partially decomposed to^·O on the surface and further generated ¹O₂.The oxidative degradation of phenanthrene was achieved partly through direct electron transfer on the surface.It was also found that Fe with 0,+2 and+3 valence was the main active component involved in the electron-donating process,and Fe forms with valence between+2 and+3 and higher than+3were formed,which increased the range of valence and ensured the ability of electron donation and electron mediating.（2）Based on the principle of minimum energy,four low-energy state models of LFLL-Fe-C/N,EDN-Fe-C/N,YD-Fe-C/N and BD-Fe-C/N prediction models were built as prediction models with different Fe/N atomic distribution and coordination number.Through the calculation and analysis of molecular dynamics and quantum chemistry,it is found that the asymmetric distribution of N around Fe（EDN-Fe）is stronger than the symmetric distribution（LFLL-Fe）,and the 4-coordination is stronger than the 2-coordination,which has stronger electron-donating ability and generates higher local high-potential sites,which is consistent with the experimental results.In addition,it is found that the electron cloud void is formed on the surface of YD-Fe-C/N where the 2-coordination of Fe-N atoms is asymmetrically distributed,which is different from the common horizontal adsorption configuration.It promotes HSO₅^-to be adsorbed in vertical form and enter the surface of the catalyst.Under the coupling effect of space limiting effect and electron effect,the O-H bond,O-O bond or S-O bond is broken.Further ¹O₂ is generated,which is consistent with the ¹O₂ generation path predicted by the quenching experiment and ESR results.（3）After load optimization,it was found that the degradation rate of YD-Fe 3%-C/N in high concentration of PHE（5 mg/L）in 2 h was as high as 61.56%,and the degradation rate in 2 h was as high as 54.25%when considering degradation efficiency and economy（dose:100 mg/L,p H:5,temperature:35℃,PMS concentration:5 m M,PHE concentration:5 mg/L）.Compared with the low concentration of phenanthrene under the same conditions,the degradation amount of phenanthrene increased by 3.32times.The performance of the catalyst increased with the increase of times of use.The degradation rate of the catalyst was 54.25%,56.11%,76.28%,86.36%,and the mineralization rate was 8.95%,10.46%,27.12%,45.71%for 2 h of recycling.The catalyst is environmentally friendly and safe,and the amount of Fe leaching is 15.23,1.04,14.30,71.65μg/L,which is far lower than the emission standard.In addition,it was found that Fe with higher valence than+3 was generated during the reaction,and the valence state of Fe was higher than that of high valence Fe with low concentration of Fe.Therefore,the range of changing valence was further widened and the ability of electron giving was improved,indicating that the catalyst had adaptive and self-regulating functions in the pollutant system with different concentration.（4）It was found that FeCoNi-C/N catalyst had the fastest degradation efficiency,reaching 92.05%within 5 min and 97.35%within 30 min.The catalyst was economical,and the degradation rate reached 80.9%in 1 h（dose:100 mg/L,p H:7,temperature:35℃,PHE concentration:1 mg/L）at low PMS concentration（0.25 m M）.In addition,the catalyst has low p H requirements for the reaction system,and can achieve high efficiency and rapid degradation of phenanthrene in the range of 3-11.After four times of recycling,the primary degradation rate remained at 81.95%and the performance was stable.The leaching amount of Fe meets the emission standard,and the second use of Co and Ni meets the emission standard,indicating that the catalyst can ensure the green safety after effective pretreatment.The results of quenching experiment and ESR showed that^·OH,SO₄^·-,¹O₂ and O₂^·-were synergically involved in the oxidative degradation of phenanthrene.In addition,it was found that the content of Fe and Co in the intermediate state increased before and after the reaction,a small amount of Ni was transformed from the low state to the high state,and no large amount of Fe,Co and Ni were oxidized,indicating that the active site in the form of metal sulfide crystals has strong stability,and the content of the low state metal is high and stable,which can guarantee its continuous strong electron donating ability.In summary,this study provides strategies for the regulation and design of the surface active site atomic and electron level structures of catalysts,reveals the control mechanism of spatial domain effect and electron effect on catalysis,and provides theoretical guidance for the directional regulation and efficiency enhancement of catalysts.