Surface Electronic Structure Reconstruction And Activation Of Carbon Nitride And Its Derived Materials For Oxidation Of Organic Pollutants In Water By Persulfate

Water environment protection is an issue of wide concern for human society at present.However,since the statistics,China's total sewage discharge has reached 13,242,062,626,000m³,while the direct discharge of sewage has 6,120,084,996,000 m³,which is close to one-half and one-quarter of the total water resources in China respectively,enough to see the seriousness of water pollution in China.The massive production,use and emission of various organic compounds prompted by the chemical synthesis developed after the industrial revolution further aggravated the ecological pollution.These compounds are easy to accumulate and resistant to removal,which seriously threaten the earth's water ecology and biosecurity.Due to the limitations of traditional physical and biological wastewater treatment methods,some industrial wastewaters that fulfill the standard discharge such as pharmaceuticals,consumer products and industrial chemicals still have considerable concentrations of organic pollutants,and the discharge of these wastewaters will allow these refractory organic compounds to continue to be introduced into the water system.Advanced oxidation processes(AOPs)are promising as capable of completely mineralizing most of the organic matters or improving their biochemical properties through oxidative decomposition of macromolecules.AOPs are a series of chemical treatment methods aiming to generate highly reactive and oxidizing reactive oxygen species(ROS),including hydroxyl radicals(^·OH),and to use them to oxidize and remove organic(and sometimes inorganic)substances from water and wastewater.Traditional AOPs are represented by Fenton reaction,yet the Fenton reaction has a series of problems such as relatively high requirements for reaction conditions(p H=2-3),generation of iron sludge,and low utilization of hydrogen peroxide(H₂O₂).Among the AOPs that have been developed rapidly in recent years,AOPs based on persulfates have significant advantages due to their ability to generate sulfate radicals(SO₄^·-)with high redox potential(2.60-3.10 V,varying with p H)and long half-life(30-45?s)to eliminate recalcitrant organic contaminants.Persulfates,currently distinguished by chemical structure,can be divided into two species,peroxymonosulfate(PMS)and peroxydisulfate(PDS).Persulfates can be activated by various methods,such as photo activation,ultrasonic activation,thermal activation,transition metal activation,microwave activation,and so on.With the advantages of good stability,wide applicability and high solubility,most of the results showed that the degradation efficiency of AOPs based on persulfates would be significantly higher than that of Fenton reaction at the same degradation conditions and targets,which demonstrated the absolute advantage of persulfates as an emerging type of AOPs.As with AOPs based on hydroxyl radicals,although homogeneous activation of persulfates is highly effective in degrading and mineralizing pollutants in a short period of time,the difficulty of catalyst recovery and the excessively high cost of inputs have limited their further practical engineering applications.The research of non-homogeneous catalytic oxidation has solved this problem to some extent.The research of heterogeneous catalytic oxidation has somewhat solved this problem.However,at present,there are still several problems in the development of persulfate heterogeneous catalysts:firstly,the leaching of metal ions from transition metal catalysts should not be neglected.Secondly,the catalytic activity of non-metallic materials is still limited.Thirdly,the mechanism of non-metallic materials catalytic activation of persulfates for degradation of organic pollutants remains unclear.Therefore,based on the three key issues listed above,this thesis proposes to develop non-metallic catalysts with good performance for the heterogeneous catalytic oxidation of persulfates and to reveal the mechanism of its activation reaction in depth.Since graphitic carbon nitride(g-C₃N₄)has unique two-dimensional?-electrons conjugated planar polymeric framework,besides,its non-metallic modification is easy to manipulate and regulate,as well as it can change its surface electron distribution while ensuring its non-metallic properties.Based on the previous research results of non-metallic doped g-C₃N₄,further modification of g-C₃N₄ was carried out to make the modified g-C₃N₄ capable of activation of persulfates by accelerating the electron transfer under the condition without visible light radiation,thus achieving the degradation of pollutants.The specific research is divided into two parts as follows.A successful construction of g-C₃N₄-based in-plane heterostructure by intimately connecting g-C₃N₄ framework with carbon ring via strong?-conjugated bonds was demonstrated,which regulates the electron density distribution of the g-C₃N₄ composite.The optimized carbon ring conjugated carbon nitride(CCN_0.1)with porous architecture and modulated electronic structure delivers admirable catalytic performance in peroxymonosulfate(PMS)activation without the need of light,with its specific activity being significantly superior to nonmetal-doped g-C₃N₄ and comparable to part of active nitrogen-doped carbon materials.Experimental data and theoretical results revealed the two domains of g-C₃N₄ with higher electron density and the conjugated carbon ring with lower electron density in CCN_0.1 involve in PMS conversion accounting for PMS reduction and oxidation,respectively,which simultaneously facilitates dissolved oxygen reduction over the g-C₃N₄ field.Moreover,CCN_0.1shows robust stability in PMS activation,and meanwhile,the CCN_0.1/PMS system exhibits potential effectiveness in actual industrial wastewater remediation.Nitrogen-doped carbon nanosheets(NCN-900)derived from graphitic carbon nitride were developed for activation of PMS and elucidation of ¹O₂ production.With a large specific surface area(1218.7 m² g^-1)and high nitrogen content(14.5 at%),NCN-900 exhibits superior catalytic activity in PMS activation,as evidenced by complete degradation of bisphenol A within 2 min using 0.1 g L^-1 NCN-900 and 2 m M PMS.Moreover,the reaction rate constant fitted by pseudo-first-order kinetics for NCN-900 reaches an impressive value of 3.1 min^-1.Electron paramagnetic resonance measurements and quenching tests verified ¹O₂ as the primary ROS in the NCN-900/PMS system.Based on X-ray photoelectron spectroscopy analysis and theoretical calculations,an unexpected generation pathway of ¹O₂ involving PMS oxidation over the electron-deficient carbon atoms neighboring graphitic N in NCN-900 was unraveled.Besides,the NCN-900/PMS system is also applicable for remediation of actual industrial wastewater.