Preparation Of Manganese Oxide-based Anode For Electro-chemical Oxidation Of Oxytetracycline:Degradation Pathway And Toxicity Assessment

Due to the ubiquitous existence of tetracyclines in the environment,ecological risks such as the generation of resistance genes have been caused.The removal of oxytetracycline,as the typical tetracycline in the environment,has received extensive attention.Electrochemical oxidation is an environmentally friendly treatment technology that does not require additional chemical reagents and uses electrons as the main cleaning reactant.Manganese oxides are widely distributed,non-toxic,and inexpensive as a common electrode material.However,its low electron transfer rate and stability limit its application in electrochemical oxidation.Therefore,in this dissertation,the electron transfer rate and the stabilization and regeneration performance of electrode materials are accelerated by the modification of polyelectrolytes,noble metal nanoparticles and carbon materials.In addition,the toxicity of electrochemical oxidation intermediates and reaction solutions were also investigated.The content and results of this study are as follows:（1）MnO₂ covered with polyelectrolyte films has been fabricated using layer-by-layer（LBL）assembly to modify the anode for efficient oxytetracycline（OTC）oxidation in water.It is found that over 89.19%of OTC was degraded in 20 min in this EAOPs system with MnO₂@（PSS/PDDA）₂PSS modified anode.Quenching experiments and electron-spin resonance（ESR）spectra have shown that hydroxyl radicals（^·OH）and direct electron transfer play the dominant roles in OTC oxidation.Several possible pathways of OTC degradation were proposed on the basis of the by-products detected by liquid chromatography-mass spectrometry（LC-MS）.Toxicity Estimation Software Tool（T.E.S.T）based on quantitative structure-activity relationships（QSARs）revealed increased toxicity of several transformation products.Moreover,flow cytometry method was applied to quantify the cytotoxic effects of the reaction solution.It may provide an alternative approach to design metal oxide/polymer composites modified anodes for contaminants degradation.It is notable that toxicological assessment of the transformation products is essential especially for newly developed anode materials.（2）MnO₂ modified with polyelectrolytes and Au nanoparticles as an anode were prepared via self-assembly and in-situ growth methods to activate peroxymonosulfate（PMS）for oxytetracycline（OTC）degradation.Electrochemical activation of PMS at MnO₂@（PSS/PDDA）Au anode（MnO₂@（PSS/PDDA）Au anode/EPMS system）removed 92%of OTC in 60 min.The effects of current density,different electrolytes,Na Cl concentration,initial p H,and PMS dosage on OTC degradation in the MnO₂@（PSS/PDDA）Au anode/EPMS system were systematically investigated.Radical quenching tests and electron paramagnetic resonance（EPR）results demonstrated sulfate radical(SO₄^·-),hydroxyl radical（·OH）,and singlet oxygen（¹O₂）were the dominant reaction species in the MnO₂@（PSS/PDDA）Au anode/EPMS system.The degradation intermediates of OTC were identified by LC-MS and several possible pathways for OTC degradation were proposed.Moreover,toxicity estimation software tool（T.E.S.T）calculation based on quantitative structure-activity relationship（QSAR）revealed the toxicity of intermediates.The cytotoxicity of the reaction solution with mixed intermediates and oxidants was also detected by the flow cytometry method.This study may provide an alternative approach to designing metal oxides-based anode for enhanced degradation of organic pollutants in water.（3）The degradation of OTC was investigated using an electrochemical oxidation system with oxygen vacancy-rich MnO@N-doped carbon（ov-MnO@NC）electrode.It is found that nearly 95%of OTC could be degraded within 60 min using the ov-MnO@NC anode.The effects of electrolytes,OTC concentration,ov-MnO@NC dosage,current density and solution p H on OTC degradation were examined.Radical quenching tests and EPR spectra reveal that ¹O₂,^·O₂^-and^·OH all contributed to OTC elimination.A total of 11 transformation products were identified by LC-MS during electrochemical degradation.T.E.S.T calculation was used to assess the toxicity of individual transformation products.The flow cytometry test provided information regarding the cytotoxicity of the reaction solution with mixtures of transformation products and other reactants at different reaction times.Furthermore,the removal efficiency of OTC at ov-MnO@NC anode maintained over 85%after recycling for four times,demonstrating the considerable durability.Overall,this study provides new insight into the development of metal oxide-based anodes with tuned oxygen vacancies,as well as comprehension of the transformation pathway and potential toxicity of the parent pollutants.