Study On The Removal Performance And Mechanisms Of Tetracycline Antibiotics With Zr-MOFs

The persistent use,improper disposal and low metabolism of antibiotics lead to significant water pollution and pose risks to human health and ecological environment.Therefore,it is an urgent need to find green,cost-effective,and efficient antibiotic pollution control technologies and methods.Based on the favorable characteristics of metal-organic framework materials（MOFs）such as high specific surface area,diverse structure,tunable pore size,rich physicochemical properties,and strong functional orientation,the Zr-based MOFs are synthesized to investigate their capabilities for adsorption removal and oxidation degradation of tetracycline antibiotics（TCs）in the present study,and the adsorption and degradation mechanisms are further analyzed.The main results are as follows:（1）Four types of Zr-MOF materials,including MOF-525（Co）,MOF-525（Zn）,MOF-545,and NZVI@MOF-545,are synthesized using solvothermal method and secondary growth method,and the adsorption removal and oxidation degradation of TCS are evaluated by Zr-MOFs.MOF-525（Co）shows superior adsorption ability,while NZVI@MOF-545 displays excellent catalytic performance.The structures and properties of MOF-525（Co）and NZVI@MOF-545 are characterized by Scanning Electron Microscopy-Energy Dispersive Spectroscopy（SEM-EDS）,X-ray diffraction（XRD）,Fourier transform infrared spectroscopy（FT-IR）,and BET analysis.SEM-EDS analysis reveals that the two materials exhibit cubic and hexagonal needle-like structures,respectively,with uniform distribution of elements and no observable enrichment phenomena.XRD results further confirm that MOF-525（Co）and NZVI@MOF-545possess identical crystal structures with MOF-525（Zr）and MOF-545（Zr）,respectively.Additionally,FT-IR spectroscopy detects multiple characteristic peaks attributed to the central ligand of TCPP,and shows the formation of Co-N bonds and various iron oxides.Lastly,Langmuir data demonstrates that MOF-525（Co）has a specific surface area of 1861m²·g^-1,a total pore volume of 0.67 cm³·g^-1,and an average pore size of 1.69 nm;while BET data illustrates that NZVI@MOF-545 exhibits a specific surface area of 2503 m²·g^-1,a total pore volume of 1.84 cm³·g^-1,and an average pore size of 2.94 nm.（2）The adsorption performance,kinetics,isotherm,thermodynamics,effects of p H and ionic strength of Tetracycline Hydrochloride（TCH）and Oxytetracycline Hydrochloride（OTC）by MOF-525（Co）are investigated,and the mechanisms and processes of adsorption between the adsorbent and the pollutants are explored with FT-IR and X-ray photoelectron spectroscopy（XPS）analyses.When the initial concentration of TCH and OTC are 100 mg·L^-1,exhibited rapid the removal efficiencies of MOF-525（Co）for TCH and OTC are 89.4%and 84.2%within 15 min,respectively.Additionally,100%are obtained after 60 min for both antibiotics.The optimal p H for MOF-525（Co）to absorb TCH and OTC are 9.0 and 5.0,respectively.It is found that the pseudo-second-order kinetic model fitting is more appropriate to depict both the TCH and OTC adsorption processes;Langmuir modeling is more suitable to elucidate the adsorption process of TCH,while Freundlich provides a better fit for OTC.Furthermore,based on the calculated thermodynamic parameters,the adsorption processes of TCH and OTC are disclosed as spontaneous,endothermic,and positive entropy.XPS and FT-IR results demonstrate that electrostatic interaction,π-πinteraction,and hydrogen bonding are involved in the adsorption mechanisms of MOF-525（Co）on the two antibiotics.（3）The oxidation degradation kinetics of TCH via Potassium persulfate（KPS）catalyzed by NZVI@MOF-545 is analyzed.It is found that the reaction process is mainly an advanced oxidation system.Response surface methodology（RSM）is adopted to optimize the removal efficiency of TCH,and the optimal degradation conditions are as follows:initial p H of 9.79,temperature of 44.86℃,agitation speed of 266 rpm,KPS dosage of 1.03 g·L^-1,and NZVI@MOF-545 dosage of 0.84 g·L^-1.The ANOVA analysis indicates the model’s high significance with a P-value of less than 0.0001,along with a determination coefficient R²=0.9119.Additionally,the difference between Adjusted R²=0.9119 and Predicted R²=0.8206 is less than 0.2.Under the optimized conditions,the degradation rate of TCH at an initial concentration of 800 mg·L^-1 reaches 90.34±0.66%after six hours of reaction,which is consistent with the predicted value（90%）,indicating that the model has a good agreement.Furthermore,TCH degradation rate is still above85%after three rounds of reuse,indicating NZVI@MOF-545 exhibits favorable reusability.（4）The preliminary reaction mechanism underlying TCH degradation by NZVI@MOF-545/KPS system is established through the analyses of electron transfer mechanisms,generated reactive oxygen specie types,changes in elemental content and chemical states,possible intermediate products,and potential degradation pathways.Linear sweep voltammetry（LSV）demonstrates that a rapid electron shuttle system is existed between NZVI@MOF-545,KPS,and TCH to promote the generation of ROS and degradation of pollutants.Furthermore,XPS and electron paramagnetic resonance spectroscopy（EPR）analyses demonstrate that NZVI@MOF-545 can efficiently catalyze KPS to produce ¹O₂,HO·,O₂^·-and SO₄^·-through its own carbon-functional groups and metal groups,and the relative contribution of different ROS toward TCH degradation is¹O₂>SO₄^·->HO·>O₂^·-.High-performance liquid chromatography/mass spectrometry（HPLC/MS）analysis reveals that TCH can be degraded into 15 intermediates and there are three potential degradation pathways.T.E.S.T toxicity evaluation demonstrates a mitigation in LC₅₀-96h toxicity,low bioaccumulation level,significant reductions in developmental toxicity and mutagenicity for most intermediate products.Based on above findings,the potential degradation mechanism of TCH by NZVI@MOF-545/KPS system can be inferred as follows:NZVI@MOF-545 catalyzes KPS to produce ¹O₂,HO·,O₂^·-and SO₄^·-to attack TCH,which is gradually degraded into small organic molecules through dehydroxylation,deamination,methylation,and ring-opening reactions,and finally completely be mineralized into CO₂ and H₂O.