Application Of Core-shell Particles In Limited Domain Catalytic Degradation Of Emerging Pollutants By Single Electron Electro-fenton System

As the world’s population growth and urban sewage discharge,the emerging contaminants（ECs）in the environment were frequently detected,including drug,endocrine disruptors and personal care products,et al.,which had strong polarity,volatile,solubility in water.The heavy/continuous use and discharge of ECs will cause the pseudo-persistent presence of the environment,which could become the largest potential risk to human health and ecological environment,and lead to the extinction of some ecosystems.Therefore,it was necessery to developed advanced technologies to remove emerging pollutants from the environment.Many technologies had been developed for the removal of ECs.Advanced Oxidation Processes（AOPs）was an effective wastewater treatment method to complete mineralization or decomposition of organic pollutants by the generation of reactive oxidants with super oxidation capacity（such as·OH）,including photocatalysis,UV photodegradation,ozone oxidation and Fenton oxidation.However,the large-scale industrial applications were focused on Fenton and ozonation.The application of Fenton reaction was limited due to p H,iron sludge and the stability of catalyst.The electro-Fenton technology was an effective wastewater treatment method for the organic removal due to its in-site generation of H₂O₂through the two electrons reduction reaction,and direct generation of·OH,which would greatly improve the production of·OH and reduce sludge production without the addition of ferrous ions.However,the low production rate of Fe（Ⅱ）was the limited step of oxygen reduction reaction.The cathode material could accelerate the Fe（Ⅱ）generation by two electron ORR pathways,and improve the production of H₂O₂.How to improve the selectivity of hydrogen peroxide had become a research topic.Thus,most studies in recent years focus on improving the utilization rate of H₂O₂ due to the low decomposition efficiency,but ignore the more effective processing efficiency of one-electron transfer pathway.Moreover,the efficient degradation and mineralization of pollutants were stay on the surface of nano catalysts,and the degradation efficiency of pollutants in water was lower than that of material surface.Herein,we reported a variety of metal doped core-shell nano materials by reasonable form design,including mononuclear@single-shell,bimetallic doped dual-core@single-shell and hollow single atomic shell catalyst.The limited domain inside and outside the core-shell could increase the specific surface area of the catalyst,increase the number of catalytic active sites and the contact surface with pollutants.In addition,the electrons ccould be absorbed between layers to improve the generation of charge.The catalytic activity and selectivity were realized by the synergistic effect of charge generation and surface reaction.And the free radical became the dominant reactive oxygen group by one-electron transfer pathways,which could avoid electron waste by ineffective decomposition of H₂O₂.In addition,the electron mass transfer distance between free radicals and pollutants could be declined and further efficient degradation and mineralization of ECs by coupling electrocatalytic oxidation technology.The main research results were as follows:（1）The metal nano-particle Fe₃O₄ was used as the core,and N,P,S heteroatoms were coated on the surface of the core by polycondensation reaction,and furtherly,the cobalt atoms anchored on porous N,P,S-doping mononuclear@single-shell catalyst（denoted as Co/Fe₃O₄@PZS）,In this highly ordered structure,the electron and mass transfer distance between the active free radical and the pollutant was greatly shortened,which could capture/activate the target substrate,generate reactive free radicals at the catalytic active site and degrade the target pollutant at the adsorption site.In addition to the two-electron reduction（₂→₂₂→·）,O₂ could also be directly electro-activated to ¹O₂via one-electron reduction（₂→·₂^-/·₂^-→¹₂）.The coupling of free radicals and non-free radicals reaction could accelerates the degradation and mineralization of pollutants.Additionally,the regenerating/reducing of≡Fe（Ⅲ）/Co（Ⅲ）could get the electron directly from the cathode without excess consumption of H₂O₂,which avoids the storage and transportation of H₂O₂.The generated non-free radicals had more efficient removal efficiency due to their long survival time.However,Co atoms modified on the surface of the shell were easy to leach in the reaction process,and the active sites in the reaction were shielded under neutral or alkaline conditions,resulting in a decrease in degradation efficiency.（2）For the degradation efficiency decreases under neutral and alkaline conditions and the lenching of Co ion,furtherly the bimetallic doped dual-core@single-shell material（Mn Fe₃O₄@PZS）was synthesized by the low toxic and cheap element Mn coupling with Fe,and used as the cathode to cooperate with electron-catalysis to activate persulfate(S₂O₈^2-)（E-Mn Fe₃O₄@PZS-PDS）for tetracycline（TTC）degradation.Radical scavenger studies demonstrated that non-radicals including atomic H*and singlet oxygen（¹O₂）rather than sulfate and hydroxyl radicals were the crucial reactive oxygen species（ROS）.Electrochemical analysis indicated that Mn doping could promote electro-catalytic process via diverting pathway from four/two-electron to one-electron to generate non-radical H^*/¹O₂ at the cathode,including one-electron oxygen reduction reaction（1e-ORR）（₂→¹₂）,and one-electron hydrogen reduction reaction（1e-HRR）（₂+0）^-→^*),as evidenced by the lowest onset potential（0.072 V）together with electron transfer number（n=1.65）.Besides,the regeneration/reduction of Fe（Ⅱ/Ⅲ）/Mn（Ⅱ/Ⅲ/Ⅳ）and persulfate will not cause excessive consumption of electrons and chemicals due to they could directly get the electron individually from the cathode and anode,and finally,TTC could be completely degraded with low energy consumption(0.655 k W h m^-3).The activity of non-radical was less affected by p H under all test conditions,which guaranteed the R_e of TTC（>87.1%60 min）under a wide p H range（2～10）in the E-Mn Fe₃O₄@PZS-PDS system.（3）The above two core-shell catalysts both involve single-electron transfer pathway,but the selectivity of single-electron only reaches 35%.Single-atom catalysts were expected to achieve the maximum release of atomic activity.Thus,we will further modify the core-shell materials.We designed a route to anchor single Mn metal atoms on porous N,P,S-co-doped carbon framework to synthesize an efficient hollow single atomic shell catalyst（denoted as Mn-SAC@PZS）.The single atomic Mn exhibited super mass activity(11.58 m² g^-1)and kinetic current（1.122 10³μA）with a much lower Tafel slope(4.25 m V dec^-1)at 0.792 V（vs.SCE）.XANES and EXAFS revealed that the mononuclear Mn were inclined to coordinate with N and S rather than P to form the R space of Mn,in which the first coordination shells backscattered with Mn-N and Mn-S.RRDE revealed that one-electron ORR pathway（72～100%）dominated at the potential of 0.5～0.7 V,oxygen molecule was absorbed/activated on site Mn*to from O*intermediate,then further activated to ¹O₂ via one-electron ORR pathway,while H*was electro activated by non-metallic active sites（i.e.pyri-N,sp-N,-P=N and S=O）.In addition,the Mn-SAC@PZS was capable of highly selectively capturing and effectively degrading CIP in the presence of HA.These findings provided new insights into the maximum release of the atomic activity of the catalyst,and provides a possible way to selectively remove aromatics from multiple pollutants in complex water systems.（4）Application and promotion of core-shell coupling electrocatalytic oxidation process.The series of core-shell catalysts were synthesized combined with heterogeneous electron-Fenton system for the degradation of typical antibiotic wastewater.The advanced treatment of aureomycin wastewater produced by Chia Tai Biochemical Co.,Ltd,the operating cost of each ton of water was about 2.196 yuan,and the effluent could achieve the secondary discharge standard in the Discharge Standard for Water Pollutants from Fermentation Pharmaceutical Industry（GB18918-2002）,the effluent COD was less than 60mg/L.The removal rate of COD and chlorotetracycline reached 88%and 99%,respectively.Electrocatalytic oxidation technology treatment technology solves the problem of high carbon load and high treatment cost of wastewater about Chia Tai Biochemical Co.,Ltd.Compared with the traditional Fenton method,the electron-Fenton technology could achieve sewage discharged standards without adding a large number of pharmaceutical,the whole process without adjusting p H and less sludge production.It could reduce 355.73 tons of COD discharge and save 3.4818-million-yuan water treatment cast for companies every year.Therefore,the successful implementation of this project could not only effectively protect the economic benefits of the companies,and also take into account the social and environmental benefits,which were expected to carry out large-scale industrial promotion and application.It was more in line with the strategic goal of sustainable development of socialism with Chinese characteristics to achieve win-win benefits for companies and society,and to realize the coordination of economic development and ecological environment protection.