Construction Of Efficient G-C₃N₄-based Photocatalyst And Study On The Degradation Of Tetracycline By Visible Light

Tetracycline antibiotics（TC）are widely used in human treatment,animal husbandry and aquaculture,and their production and use of antibiotics rank second in the world and rank first in China.However,because it is not easy to metabolize and absorb organisms,most of them are discharged into sewage.It is difficult to completely degrade it,resulting in toxic intermediate metabolites,destroying water’s ecological balance,and then threatening human health.Therefore,developing a new high-efficiency and non-secondary pollution technology is urgently needed to degrade tetracycline.As a kind of advanced oxidation technology,photocatalytic oxidation technology has a wide application prospect in tetracycline antibiotic wastewater treatment due to its advantages of simple operation,low cost,low energy consumption,green and high efficiency,and no secondary pollution.Graphitic carbon nitride（g-C₃N₄）is widely used in wastewater treatment as a metal-free semiconductor,easy to obtain and low cost.Still,a single g-C₃N₄has an insufficient response to visible light,the low separation efficiency of photogenerated carriers,and low electronic conductivity.To solve these problems,this paper modified g-C₃N₄by constructing heterojunctions,and performed visible light declassification of tetracycline hydrochloride,a typical tetracycline antibiotic.The composite photocatalysts were characterized by scanning electron microscopy（SEM）,X-ray diffraction（XRD）,Fourier infrared spectroscopy（FT-IR）,UV-visdiffusereflectancespectroscopy（UV-visDRS）.Tetracycline-containing hydrochloride-containing simulated wastewater was selected to explore the photocatalytic degradation effect of photocatalysts,and the photocatalytic degradation mechanism and metabolic pathway of photocatalysts on tetracycline wastewater were explored through electron spin resonance（EPR）and density functional theory.The main findings of this paper are as follows:（1）Construction of g-C₃N₄-based composite photocatalyst.In this study,pure g-C₃N₄was synthesized by high-temperature calcination using urea as the precursor.A series of MgO/g-C₃N₄composite catalysts with different synthesis ratios and temperatures were prepared by combining hydrothermal and calcination,and the sample information of MgO/g-C₃N₄composite catalysts with the best photocatalytic activity was obtained:The microscopic morphology of the composite catalyst was observed by SEM,and the results showed that MgO was successfully loaded onto the g-C₃N₄nanosheet.XRD analyzed the composition of the MgO/g-C₃N₄composite catalyst,and the results showed that it had a typical g-C₃N₄（002）crystal plane and a characteristic peak belonging to MgO.The XPS results show the formation of Mg-N bonds and a synergistic effect between the two materials.FT-IR further demonstrated that MgO was successfully loaded onto the g-C₃N₄nanosheet.UV-vis DRS confirmed that the light absorption edge of MgO/g-C₃N₄showed an obvious redshift,and the bandgap energy value was 2.57 e V,which enhanced the absorption range of visible light.（2）The influence of external conditions on the performance of MgO/g-C₃N₄composite photocatalyst was explored.The MgO/g-C₃N₄composite photocatalyst showed the best TC photocatalytic activity,reaching a degradation rate of 84.98%within 90 min.The photocatalytic degradation of TC by composite photocatalysts conformed to the first-order kinetics,and the k value of MgO/g-C₃N₄was up to 0.0148 mg/L/min,which was 2.43 times that of unmodified g-C₃N₄（k=0.0061 mg/L/min）.The photocatalytic activity of MgO/g-C₃N₄on TC was affected by the substrate concentration and the catalyst dosage,and the activity was strongest when the substrate concentration was 10 mg/L,the dosage was 600 mg/L,and p H=5.The photocatalytic activity decreases with the increase in substrate concentration and dosage,and the photocatalyst also shows excellent degradation activity in the actual water body simulation TC wastewater test,and its k=0.0172 mg/L/min is 2.82 times that of the unmodified g-C₃N₄（k=0.0061 mg/L/min）.In addition,MgO/g-C₃N₄maintained good stability in the four degradation cycles,and the removal rate of TC was higher than 75%,indicating that the composite catalyst had stable and efficient degradation efficiency for TC wastewater.（3）The photocatalytic degradation mechanism of MgO/g-C₃N₄composite photocatalyst on TC was explored.The free radical quenching test results showed·O₂^-and h⁺are the main active species for photocatalytic degradation of TC.Electron spin resonance spectroscopy（EPR）verified free radicals’strength,which showed·OH signal ratio·O₂^-free radicals are strong,and a free radical transformation is possible.The conduction band（CB）and valence band（VB）values of MgO/g-C₃N₄were calculated according to ultraviolet photoelectron spectroscopy（UPS）,and the band structure of MgO/g-C₃N₄was determined.Exploring free radicals through the Fukui index·O₂^-and h⁺easily attack the C7,O20 and N24 TC Takfukui Index（f⁰）atoms,and degrade TC macromolecules into small acids(such as C₁₈H₂₀O₄)through hydroxylation,cyclization,demethylation and ring-opening to achieve mineralization of TC.