Mechanisms Study On Actuating Advanced Oxidation Processes For Highly Efficient Tetracycline Degradation By Polyvalent Metal Modified Iron-based Catalysts

Tetracycline antibiotics（TCs）are widely used in livestock breeding,aquaculture and medical care industry for their low price and broad-spectrum antibacterial properties.However,they can not be completely detoxicated from wasterwater even after technical treatment,which leaded to their accumulation in environment and further induced the production of resistant gene.This problem poses a potential threat to both ecological environment and human health.Hence,it demands prompt solutions to control and reduce the pollution of TCs at the source.Recently,advanced oxidation processes（AOPs）that can in-situ generate various reactive oxygen species（ROS）displayed immense potential for the treatment of stubborn contaminants.Meanwhile,traditional AOPs have disadvantages with low efficiencies and secondary pollutant production.The development of green catalysts in AOPs aroused tremendous interest recently,thus the researches of fabricating new heterogeneous catalysts with high-efficiency for techniques optimization are of great significances.In this study,to solve the problems of tough activiation of oxidants in Fenton and Persulfate（PS）systems,we coupled cheap iron-based materials and polyvalent metals（Ce and Co）to synthesize a series of iron-based bimetallic nano-catalysts.We further applied these new catalysts into special AOPs for tetracycline（TC）degradation and clarified the relevant activation mechanism.The primary research results are as follows:（1）To resolve the inefficient Fe²⁺regeneration in natural ferrihydrite（Fh）in Fenton system,a novel heterogeneous photo-Fenton catalyst（yCeO₂/Fh）with excellent TC degradation activity was developed by decorating bio-template synthesized ceria（yCeO₂）on Fh.The characterizations verified the yCeO₂ with excellent semiconductive property uniformly deposited on Fh surface.Different influential reaction parameters were optimized and the 15-yCeO₂/Fh composite with 15%mass ratio of yCeO₂ showed excellent degradation rate（93.6%）and mineralization rate（70.1%）of TC in 60 min.The corresponding kinetic rate was 0.01025 min^-1,which was 11.26 times than that of pure Fh.Based on the emerging intermediates identified by LC-MS,TC degradation pathways were proposed including hydroxylation and demethylation processes.The co-existence of anions decreased TC removal efficiency in the simulated actual wastewater.Radicals trapping experiments and electron spin resonance measurement proved that·OH played a critical role in organics degradation.Mechanism analyses confirmed that the appropriate yCeO₂introduction accelerated Fe²⁺and Ce³⁺regeneration with the assistants of photo-e^-and Ce⁴⁺/Ce³⁺circulation,which further facilitated H₂O₂ consumption and·OH production,then resulted in higher TC degradation efficiency.（2）Fh is available and widespread in the environment.To settle the poor visible-light（Vis）sensitivity of Fh and develop its catalytic abilities in SO₄^·-dominated AOPs,we proposed the idea of preparing different iron-cabalt composites（i.e.Fh-c400,CoFe₂O₄ and CoO/CoFe₂O₄）via calcining Co²⁺doped Fh,which was controlled by changing the amount of Co²⁺.Then,these composites were applied to Vis assisted PS system for TC degradation.The CoO/CoFe₂O₄ heterojunction showed the great catalytic activity for PS.With the addition of 0.3 g·L^-1 5%CoO/CoFe₂O₄ and the concentration of 2.0 mM PS,96.1%TC were degraded after 60 min of catalytic reaction under neutral condition,which was much higher than that of Fh-c400 and CoFe₂O₄.The mineralization rate of TC by 5%CoO/CoFe₂O₄ was 69.3%.After 3^th consecutive recycling utilization,the TC degradation efficiency decreased to 88.2%,which was primarily attributed to the corrosive effect of CoO during catalytic procedure.CoO/CoFe₂O₄ composite exhibited superior Vis response and electrical conductivity compared with CoFe₂O₄ or CoO.The enhanced TC catalytic performance of CoO/CoFe₂O₄ could be ascribed to the following reasons:（a）the interfacial interaction between semiconductor CoO and CoFe₂O₄ accelerated the separation and migration of h⁺-e^-pairs in CoO/CoFe₂O₄ heterojunction under Vis illumination,and promoted the e^-acquisition of PS and the reduction of Fe³⁺,thus increasing the amount of SO₄^·-production;（b）the capture of e^-by PS can reversely accelerate the generation of charge carriers.（3）For further improving the activation performance of PS and avoiding the problem of high ion leaching rate of CoO in PS system,the Co₃O₄ with stable structure was used,and novel nZVI anchored bio-matrix supported Co₃O₄（nZVI/yCo₃O₄）composites were fabricated for TC efficient degradation in wide pH ranges by activating PS.The systematical characterizations verified that the nZVI/yCo₃O₄ composites with magnetism have higher surface area than pure Co₃O₄ and yCo₃O₄,contributing to more accessible active sites.Results from the optimization of experimental parameters indicated that TC removal efficiencies were 97.6%,93.4% and 77.3%within 15 min at initial pH 3.0,6.0 and9.0,respectively,when the 10%nZVI/yCo₃O₄ dosage and PS concentration were separately0.1 g·L^-1 and 0.9 mM.Based on four successive degradation runs,the excellent reusability and mineralization rate of nZVI/yCo₃O₄ composites were mainly benefited from the suppressed metals leaching.The PS activated mechanisms by nZVI/yCo₃O₄ were proposed as the novel environment pH-switch ROS transformation paths:non-radicals（¹O₂）dominated pattern at acidic conditions and radicals(SO₄^·-/·OH)predominant pattern at alkaline environment,which may be highly related to the electron donating capacity of nZVI at different pH and the M^（n+1）+/Mⁿ⁺redox cycling between Fe or Cometal.LC-MS analysis presented two plausible TC degradation routes in nZVI/yCo₃O₄/PS system.Finally,TC and other intermediates can be degraded into small molecules and even mineralized into H₂O and CO₂.In conclusion,we developed cheap and efficient iron-based bimetallic nano-catalysts,overcame the defects of narrow application ranges,and poor activation performances and stabilities of conventional iron-based materials.Moreover,this study not only offered new strategies for antibiotics wastewater treatment,but also provided theoretical bases for broadening the application of excellent iron-based composites in AOPs.