MOFs Derived Fe-based Hybrid Micro-/nano-materials For Photoelectro-Fenton Processes To Degrade Antibiotics In Wastewater

Among the global water environment problems,the pollution of antibiotics has become a hot issue of great concern.Sulfamethazine（SMT）is a typical sulfa antibiotic that generally used as an antibacterial drug in aquaculture and a growth promoter in animal husbandry.Due to the poor biodegradability of SMT,it is difficult to completely remove it in traditional treatment processes,therefore it is widely detected in surface water and other water bodies.The heterogeneous electro-Fenton（Hetero-EF）and photo-electro-Fenton（Hetero-PEF）processes have been widely used for degradation of antibiotics due to their in-situ production of H₂O₂,high efficiency and environmental friendliness.However,these processes are faced with problems such as large amount addition of catalyst,low decomposition efficiency of H₂O₂ and insufficient activity at neutral p H value.In this work,metal-organic frameworks（MOFs）were used as precursors to prepare hybrid micro-/nano-materials,which were applied to Hetero-EF and Hetero-PEF processes to remove SMT.It improved the efficiency of Hetero-EF and Hetero-PEF processes through different catalyst preparations and process couplings:Fe²⁺was rapidly formed on Fe⁰ surface by micro-electrolysis of Fe⁰ and carbon;Fe²⁺/Fe³⁺circulation was promoted by Cu⁺or Mo⁴⁺;the application range of p H was widened by sulfur modification;mineralization efficiency was improved by coupling UV technology.The possible catalytic mechanism such as micro-electrolysis was proposed through the analysis of experiment,characterization and density functional theory（DFT）calculation.The main research content and conclusions of this work are as follows:（1）In order to reduce the amount of catalyst and improve process performance,a new Hetero-EF catalyst（Fe/Fe₃C@PC）was prepared through one-pot pyrolysis using MIL-101（Fe）as the sacrificial template,which contained a Fe-based nanoparticle core（Fe/Fe₃C）and a graphitized porous carbon（PC）shell.Fe/Fe₃C@PC outperformed common heterogeneous catalysts such as Fe⁰,Fe₃O₄ and Fe₂O₃ due to the synergistic effect between Fe/Fe₃C nanoparticles and PC.The rate constant for SMT degradation in the Hetero-EF process was 7.17-folds of that in the heterogeneous Fenton（Hetero-F）process,while the catalyst dosage in this chapter was about one tenth of that reported in literature.The plausible catalytic mechanism and degradation pathway of SMT by this Hetero-EF process were proposed,confirming the main role of surface-bound hydroxyl radical(^·OH_ads)oxidation.The micro-electrolysis of Fe⁰ and PC was the key factor for the high activity of the catalyst.Fe/Fe₃C@PC could be easily recycled due to magnetism and well recovered via a thermal treatment in H₂ atmosphere.（2）In order to further improve the decomposition efficiency of H₂O₂,Fe⁰/Cu⁰-based bimetallic sub-micro-particle core with a PC shell hybrid rods,i.e.,core-shell structured Fe Cu@PC-800/6,was successfully fabricated through pyrolysis of the mixture of MIL-88B（Fe）and Cu-BDC.A trace dose catalyst(25.0 mg L^-1)could achieve efficient catalytic activity(turnover frequency（TOF）,3.92 L g^-1 min^-1)due to the synergy of catalytic reactivity among iron,copper and PC,possessing the pseudo-first order rate for SMT removal that was 3.27-folds and 4.08-folds of the Fe@PC-800/6/Hetero-EF and Cu@PC-800/6/Hetero-EF process,respectively.According to experiments and DFT calculations,Hetero-EF reaction mechanisms enhanced by the micro-electrolysis were proposed,in which Cu⁰ promoted the electrons transfer of Fe⁰to PC while the introduced Cu⁺could promote the decomposition of H₂O₂ and the cycle of Fe³⁺/Fe²⁺.When this process removed SMT in the secondary wastewater effluent,the removal efficiency reached 94.4%,indicating that it had the potential for practical application.（3）In order to promote the cycle of Fe³⁺/Fe²⁺,an ultra-efficient Hetero-EF catalyst,which had a core-shell structure of PC-coated Fe/Mo metal（Fe Mo@PC）,was prepared by calcining MIL-53（Fe）@Mo O₃.The removal rate of the Fe Mo@PC-2/Hetero-EF process was 4.58 times that of the Fe@PC/Hetero-EF.Fe Mo@PC-2 exhibited high catalytic performance with TOF as high as 7.89 L g^-1 min^-1 for SMT degradation,which was about 40 times more than that reported in the literature.The influence of initial p H value,current,catalyst amount and SMT concentration on the degradation efficiency of the Fe Mo@PC-2/Hetero-EF process was explored.The micro-electrolysis and co-catalysis improved the performance of the catalyst to activate H₂O₂.The co-catalysis was the rapid reduction of Fe³⁺by Mo⁴⁺.Similarly,the activity of the catalyst could be recovered by H₂ regeneration.（4）In order to improve the performance of the Hetero-EF process at neutral p H,the reduced S-modified MIL-53（Fe）was prepared by sulfurizing MIL-53（Fe）.The influence of calcination temperature and MIL-53（Fe）/S ratio on structure and performance was explored.The best temperature and MIL-53（Fe）/S ratio were 350℃and 1:2.After sulfurization,S₂^2-replaced OH^-to coordinate with Fe²⁺,and the Fe²⁺content increased.At p H=7.0,the TOF value of MIL-53（Fe）/S（1:2）-350 was 0.48 L g^-1 min^-1,which was 12 times that of commercial Fe S₂.When p H=9.0,the removal efficiency of SMT still reached 91.6%.The S₂^2-of MIL-53（Fe）/S（1:2）-350 could efficiently reduce the p H value,and a large amount of Fe²⁺reacted with H₂O₂ to produce^·OH.The catalyst was calcined again by sulfurization method,and the removal efficiency of SMT was over 75%after 5 cycles.（5）In order to improve the mineralization efficiency,UV process and Hetero-EF process were coupled.Compared with Fe/Fe₃C@PC/Hetero-EF and Fe Mo@PC-2/Hetero-EF process,Fe Cu@PC-800/6/Hetero-EF process and UV process had the strongest synergy.At 60 min,the TOC removal efficiency and mineralization current efficiency（MCE）of the Hetero-PEF process was 73.3%and 58.7%,which were both2.23 times that of the Hetero-EF process.Moreover,the Hetero-PEF process had wider p H applicability（3.0-7.0）than the Hetero-EF process.This process could also efficiently remove phenol,carbamazepine and 2,4-D.There were three reasons for the performance improvement of the coupling process:（i）UV activated H₂O₂ to produce^·OH(k _UV/AO-H2O2/k _UV/AO=4.36);（ii）UV excited the catalyst to generate photo-generated electrons and holes,so that the photo-generated electrons activated H₂O₂ to generate^·OH(k _PEF/k _EF=2.32);（iii）UV excited Fe（OH）²⁺and Fe（OOCR）²⁺to promote Fe²⁺regeneration.In the five cycles of experiments,the removal efficiency of SMT reached 97.0%.