Preparation Of G-C₃N₄ Based Composites And Their Application For Photocatalysis Coupled With Persulfate Oxidation Technology

Photocatalytic oxidation technology is considered an effective pretreatment method for antibiotic wastewater because of its advantages such as mild reaction conditions,green economy,environmental friendliness,and wide application range.Among them,photocatalytic fuel cell（PFC）technology can convert chemical energy contained in organic water into electrical energy by using solar energy,thus realizing synchronous pollution removal and electricity generation.Researchers are paying more and more attention to the technology.As a typical representative of nonmetallic semiconductors,g-C₃N₄ has the advantages of stability,innocuity,easy control of energy band structure,low cost,and easy availability.However,its application is seriously restricted due to the defects of high electron-hole recombination rate,poor response ability to visible light absorption,and weak redox ability.And it should be noted that the removal of the parent pollutant using photocatalytic oxidation technology alone is good,but its mineralization efficiency is very low.Persulfate（PS）oxidation is another typical advanced oxidation technology.Persulfate can generate·SO₄^-with strong oxidation ability through self-decomposition,which can degrade most organic matter in wastewater.However,its application is limited due to the improper selection of cocatalysts or improper disposal of residual SO₄^2-,which easily leads to high energy consumption,secondary pollution,and other risks.Based on the above problems,g-C₃N₄ is modified by morphology adjustment,metal/non-metal element doping,and homo/heterojunction construction.In addition,PS coupled with the PFC with the triple Z-scheme system is used to achieve efficient removal of pollutants and synchronous power generation.The research results are as follows:（1）O-C₃N₄/C₃N₄qds composite was prepared by classical chemical oxidation and hydrothermal methods.The morphology of the photocatalyst was characterized by SEM and TEM,and the average particle size of C₃N₄qds was about 5 nm.DRS and Mott-Schottky analysis results indicated that the band gap changes make O-C₃N₄ and C₃N₄qds form the homojunction system.Surface voltage and surface charge density analysis results indicated that the intensity value of the built-in electric field between the junction of O-C₃N₄/C₃N₄qds was optimal.And PL,EIS and photo-current analysis results indicated that the composites had excellent photochemical properties.Under visible light irradiation for 0.5 h,the degradation efficiency of 100 m L tetracycline（TC）with the concentration of 10 mg·L^-1 by 50 mg catalyst was about 98.2%.And the corresponding first-order kinetic constant was 2.43,3.06 and 4.29times that of C₃N₄/C₃N₄qds,C₃N₄/C₃N₄non-qds and g-C₃N₄,respectively.The O doping and the quantum size effect of C₃N₄qds were the key factors for the energy band engineering to construct the homojunction.This unique structure facilitated to improve the inter-crystal compatibility and enhance the IEF intensity,thus facilitating electron transfer at the interface.Consequently,the separation of photoexcited charge carriers was promoted,and the photocatalytic performance was improved.（2）N-C₃N₄/Cu/Cu₂O composite was prepared by simple calcination and glucose reduction method.SEM and TEM analysis results indicated that the nanosphere structure of Cu/Cu₂O existed on the surface of N-C₃N₄.Under visible light irradiation for 0.5 h,the degradation efficiency of 100 m L TC with the concentration of 10 mg·L^-1 by 50 mg catalyst was about 99%.And the corresponding first-order kinetic constant was 2.32,5.10,and 15.45 times that of C₃N₄/Cu/Cu₂O,C₃N₄,and Cu₂O,respectively.The results of the fourth cycle experiment showed that the composite still had about 90%degradation efficiency of TC.The N doping is a key factor in optimizing the band structure of the system.By considering the band structures of different materials and the effect of Schottky barrier,a Z-scheme heterojunction between N-C₃N₄ and Cu₂O was established with Cu as the electron conductor.With the help of this Z-scheme heterojunction,Cu₂O photocorrosion was alleviated,and moreover,the photocatalytic activity of the composite N-C₃N₄/Cu/Cu₂O was enhanced greatly.Although the generation of Cu O seemed inevitable,N-C₃N₄could still form another heterojunction with Cu O,thus maintaining the overall photocatalytic activity of the composite catalyst.（3）Fe-C₃N₄/Bi₂Sn₂O₇ composite was prepared by simple calcination method.SEM analysis results indicated that the cubic polyhedral structure of Bi2Sn2O7 existed on the surface of Fe-C₃N₄.Under visible light irradiation for 0.5 h,the degradation efficiency of 100 m L TC with the concentration of 20 mg·L^-1 by 50 mg catalyst and 2 m M PS was about 92.2%.And the corresponding first-order kinetic constant was 6.04 and 16.13 times that of Fe-C₃N₄/Bi₂Sn₂O₇and PS,respectively.With the help of the Z-scheme system constructed between Fe-C₃N₄ and Bi₂Sn₂O₇,the separation of carriers was more efficient,and more electrons in the negative conduction band were promoted to participate in the Fe^2+/3+cycle reaction,thus improving the activation efficiency of PS.Meanwhile,the holes at the more positive valence band positions are reserved,thereby ensuring the high catalytic oxidation performance of the coupling system.（4）Bi₂O₄/Bi₂WO₆/C₃N₄qds composite was prepared by simple hydrothermal methods.SEM and TEM analysis results indicated that Bi₂O₄ and Bi₂WO₆ were nanorods and nanospheres,respectively.Under visible light irradiation for 0.75 h,the degradation efficiency of 100 m L TC with the concentration of 10 mg·L^-1 by the double photoelectrode PFC（Bi₂O₄/Bi₂WO₆/C₃N₄qds-Cu Fe₂O₄）coupled PS system,and its maximum power density was about 37.0μW·cm^-2,which was 1.89 times that of the PFC system.With the help of the ternary Z-scheme system constructed among Bi₂O₄,Bi₂WO₆,and C₃N₄qds,the separation of carriers was more efficient.Meanwhile,the electrons were transferred to the photocathode through an external circuit more efficiently,prompting more electrons generated by photoexcitation at the photocathode to participate in the Fe^2+/3+and Cu^1+/2+/3+cycle reactions,further improving the activation efficiency of PS,thus providing guarantee for TC degradation and chemical energy recovery.In addition,the removal effects of·O₂-,h⁺and·OH（·SO₄^-）on TC were investigated by active species capture experiments.Meanwhile,the degradation pathway of TC was inferred by HPLC-MS test,and the toxicity of intermediates involved in the degradation process of TC was analyzed by ECOSAR program and Q67 toxicity test.In summary,g-C₃N₄based semiconductor composites with Z-scheme homo/heterojunction system were prepared to improve the photocatalytic performance,and the PFC coupled PS system was used to achieve efficient removal of pollutants and simultaneous recovery of chemical energy.This paper provided a novel idea for the treatment and recycling of organic wastewater.