Study On Degradation Of Tetracycline Wastewater By Persulfate Assisted Modification Of G-C₃N₄-Based Photocatalyst

Tetracycline（TC）has strong antibacterial power and is useful for treating respiratory tract infections and intestinal dysfunction,and has been widely used in medicine,veterinary medicine and animal husbandry,but its long-term use in large quantities can lead to the generation of drug-resistant bacteria and aggravate environmental pollution;therefore,the removal of TC in water,which is difficult to biodegrade,has become an urgent problem.Photocatalytic synergistic persulfate advanced oxidation（PS-AOPs）has the advantages of strong oxidation performance,mild reaction conditions,no secondary pollution and low energy consumption,and has good prospects for application in the field of water treatment.However,traditional photocatalysts（Zn O,Ti O2）use UV light as the light source,which poses a great challenge to the practical application of photocatalytic technology.Under the current trend of energy saving and environmental protection,non-metallic graphitic phase carbon nitride（g-C₃N₄）has attracted much attention because of its good thermal stability,acid and alkali corrosion resistance and visible light catalysis.In this study,on the one hand,carbon dots（CDs）loading and heterojunction construction of g-C₃N₄were carried out to enhance its visible light response range and photogenerated carrier transport rate;on the other hand,the effects of various factors on TC degradation by g-C₃N₄-based photocatalysts with peroxydisulfates（PDS）were investigated,based on which the generation of active species,degradation pathways and mechanisms in the degradation process were dissected to provide theoretical support for the treatment of This paper provides theoretical support for the treatment of actual antibiotic wastewater.The main findings of this paper are as follows:（1）Pure g-C₃N₄（CN）was prepared by thermal polymerization（550°C,5°C/min）using melamine as the precursor,and the UV-vis diffuse reflectance spectroscopy（UV-vis DRS）and photoluminescence（PL）analysis showed that g-C₃N₄has low visible light utilization（460～470nm）and the photogenerated electron-hole pairs are easily recombined.Meanwhile,carbon dots（CDs）with up conversion effect and electron-conducting ability were prepared by a simple hydrothermal reaction and successfully loaded onto the surface of g-C₃N₄（CNC）by secondary calcination.the visible light response boundary line of CNC was raised to 517 nm,and the modification of CDs allowed the photocatalyst to absorb longer wavelengths of visible light and excite photocatalytic reactions in a broader range.In addition,the overall forbidden band gap of CNC is 0.32 e V narrower than that of g-C₃N₄,which means that the distance of photogenerated carriers migrating from the valence band（VB）to the guide band（CB）of the photocatalyst is scaled down,also indicating that CDs are able to change the energy band structure.（2）When the initial concentration of TC was 10 mg/L,7.1%of TC was removed by adsorption of CNC in the dark reaction stage,which was more than twice that of g-C3N4.This was because the tightly layered structure of g-C₃N₄was peeled off and delaminated after secondary calcination and loading of CDs,and the specific surface area increased from 6.19 m²/g to 13.65m²/g.In the light reaction stage with the LED lamp as the light source,CNC alone and g-C₃N₄removed 63.9%and 29.6%of TC,respectively.In the photoreaction phase with the LED lamp as the light source,CNC alone and g-C₃N₄removed 63.9%and 29.6%of TC,respectively,and CDs provided a large number of active sites and catalytic centers（oxygen-containing functional groups）for CNC to participate directly in the photocatalytic reaction or act as catalytic centers to promote the CNC carrier reaction.the proposed primary rate constants for the synergistic degradation of TC by CNC（g-C₃N₄）and PDS(K_obs),increased from 0.0124（0.0023）to 0.0299（0.0041）,and PDS contributes to the photocatalytic reaction.ESR detection and active species burst experimental analysis showed that vacancies（h⁺）and superoxide radicals（·O₂^?）were the main active in the CNC/PDS system substances,while sulfate radicals(SO₄^?·)and hydroxyl radicals（·OH）were also detected in the system,proving that PDS was indeed activated by CNC,and PDS mainly acted as an electron trap in the aqueous system to inhibit the electron-hole pair recombination of CNC.In summary,the CNC/PDS reaction system is that CNC plays a major role in the degradation of TC,while PDS acts as an electron acceptor or electron trap to assist photocatalysis,thus achieving a synergistic effect with CNC.（3）To further analyze the electron-conducting properties of CDs,it is necessary to choose a semiconductor that becomes very good in visible light response after compounding with g-C₃N₄into a heterojunction,which can avoid the interference of conversion effects on CDs.Since the visible light effect of Bi OI is very broad and the boundary line reaches near 627 nm,the absorbing performance of Bi OI/g-C₃N₄is close to that of Bi OI,which shows a clearer red shift and stronger visible light response compared to g-C₃N₄.Therefore,the transport rate of photogenerated carriers at the heterojunction coupling interface was analyzed after loading CDs as charge mediators into Bi OI/g-C₃N₄（BCN）heterojunctions.Further deposition of CNC onto Bi OI to form heterojunctions（CDs@BCN）using in situ precipitation method shows a slight improvement in the optical response.transient photocurrent response results of CDs@BCN exhibit excellent photogenerated carrier transport rates and very low recombination rates of electron-hole pairs because CDs,as zero-dimensional carbon nanoparticles,can provide better channels for electrons to move and prolong the duration of charge separation,which in turn effectively improves the efficiency of electron-hole utilization.（4）When the initial concentration of TC was 20 mg/L,CDs@BCN could adsorb and remove26.7%of TC in the dark reaction stage,which was much higher than g-C3N4,Bi OI,Bi OI/g-C3N4and CNC.Due to the load of CDs and the construction of heterojunction,the specific surface area of CDs@BCN（41.374 m²/g）is 6.41 times higher than that of G-C3N4.After 60 min photoreaction,CDs@BCN alone removed 66.3%of TC,5.39,2.49 and 1.37 times that of g-C₃N₄,BIOI and BIOI/g-C₃N₄.CDs@BCN synergistic PDS further increased the TC removal rate（84.3%）.The photoluminescence and electrochemical impedance analysis showed that the optical absorption area and active site of the heterojunction were greatly increased by the composite of two semiconductor substrates,and the electron transport ability and electrical conductivity were significantly enhanced.CDs@BCN has extremely high light absorption capacity,electron conductivity and active site density,can use light energy to promote electron movement and catalyze reactions,showing very excellent photocatalytic performance.CDs,as the electron transport medium at the heterojunction coupling interface,further enhances the transmission rate of photogenerated carriers.（5）PDS may have residues in the process of in situ remediation,and they react with halogens on natural organic matter and organ halogen pollutants to produce reactive halogen substances,which in turn combine with organic matter to transform again into more toxic organ halogen pollutants.By analyzing the residual concentration of PDS after degradation of TC,it was found that CNC（0.25 g/L）could effectively activate 89.3%of PDS when the initial concentration of PDS was 10 mg/L;CDs@BCN（0.4 g/L）could effectively activate 86%of PDS when PDS was 20mg/L.Therefore,the loading of CDs and the construction of heterojunctions could reduce the secondary contamination of PDS potential.LC-MS was used to detect the intermediates during the degradation of TC,and the possible degradation pathways were inferred.Although some relatively stable intermediates were produced during the degradation process,the oxygen-containing functional groups such as hydroxyl and carboxyl groups on CDs as active sites were able to attack the carbon-carbon double bonds on tetracycline A-ring 1 and D-ring 6 with the dissociation energy（118.8 k J/mol）and electronegativity（-0.13）are minimal,which can easily undergo affinity substitution reactions,thus promoting the dealkylation and dihydroxylation of TC and improving the mineralization rate of the CDs@BCN system.