The Interface Reaction Mechanisms Of Enhanced Peroxymonosulfate Activation For Tetracycline Degradation By Ferrous Bimetallic Catalysts

The residues of antibiotics left in the environment can aggravate the contamination by pathogenic microorganisms and cause serious problems in drinking water for human health.Peroxymonosulfate（PMS）based oxidation processes can efficiently degrade persistent and refractory biodegradation organic compounds due to the strong oxidation capacity of generated reactive species.However,the disadvantages of catalyst deactivation,low reaction activity and complicated systems are still difficult to overcome.Therefore,much more attentions have been attracted to finding catalysts with high catalytic activity,high reusability and good stability to overcome these challenges in heterogeneous oxidation processes.Iron-based catalysts have attracted much more attentions due to its great reserves,high electron conductivity,low toxicity,and excellent stability.Ferrous bimetallic catalysts are becoming research hot spots owing to their unique stability and catalytic activity for PMS activation.However,the multiple reactive components in bimetallic iron-based catalysts could make the PMS activation mechanisms more confused,resulting in the difficulty to evaluate the catalytic activities of various catalyst for PMS activation in various reaction systems.Herein,we prepared series of catalysts to investigate the effects of crystals,morphologies and active sites on catalytic activity.And the possible reaction mechanisms among catalysts,oxidants and pollutants were proposed.（1）Series of ferromanganese oxides（FMOs）were synthesized with different Fe and Mn ratio using sol-gel method and showed good activity in the activation of peroxymonosulfate（PMS）for tetracycline（TC）degradation.The FMO-46 structured as Fe Mn O3 with cubic structure can efficiently activate PMS due to the larger surface area and more surface Mn（Ⅲ）sites.While the high catalytic activity of FMO-73 was attributed to the high content of surface Fe（Ⅱ）and Mn（Ⅱ）.The more vicinities of Fe and Mn existed on the surface of FMOs result in a better catalytic activity towards PMS and degradation of TC.The catalytic mechanism was elucidated by detecting the reactive oxidation species with ESR measurements and radical quenching tests.It was found that Mn-oxides with Fe-regulated-surface active sites were expected to enhance the catalytic activity in the generation of SO4^-·and·OH.The experimental results demonstrated that SO4^-·made more contribution to tetracycline degradation in FMO+PMS system than·OH.（2）To further improve the catalytic activities,ruthenium was used as the reactive component to replace low reactive Mn（Ⅲ）.The results showed that the highly dispersed ruthenium played an important role in catalytic activities.About 90%of TC could be oxidized in 5 min which was very fast than that in FMO+PMS systems.Both HO·and SO4^-·were the major reactive oxygen species for TC degradation,and SO4^-·made more contributions to oxidizing TC in Fe Ru/C activated PMS system.Noteworthy,significantly different dynamic behavior from Fe3O4 systems indicated the existence of other activation oxidation mechanisms in Ru-contained reaction systems.Ru（Ⅲ）-TC complex between TC and loaded Ru sites on Fe Ru/C surface could significantly improve the degradation of TC.The complexation between Ru and TC could not only weaken the stability of TC,but also enrich the oxidation by electron transfer,resulting in the acceleration of TC degradation.The synergistic effect between iron and ruthenium favored the redox cycles of Ru（Ⅲ）/Ru（Ⅱ）and Fe（Ⅲ）/Fe（Ⅱ）,enhancing the catalytic activity of composite catalysts.Moreover,the introduced Ru changed the activation mechanism of PMS,resulting in the existence of free radical mechanism and non-free radical mechanism for TC oxidation.（3）To reduce the effects of coexisting inorganic anions on TC degradation,iron-cerium bimetallic composites with surface defect characteristics were used to transfer the activation mechanism from non-radical to radical mechanism for PMS activation.Iron-cerium bimetallic oxides with various morphologies were synthesized and used to activate PMS for TC degradation.The results showed that different morphologies of Ce O2 had various effects on catalytic activities due to the various exposed lattice planes with different oxygen vacancy contents.The iron-cerium bimetallic compounds with nanorods structures had abundant oxygen vacancy on（100）and（110）planes and showed the highest catalytic activities while catalysts with nanoparticle structures had low oxygen vacancy concentration on（111）plane and showed the lowest catalytic activities.And inhibition tests showed that oxygen vacancy made major contribution for PMS activation to generate ¹O2 to degrade TC.Compared with FMO+PMS and Fe Ru/C+PMS systems,NR-Fe+PMS system is less affected by the co-existence of inorganic anions.（4）Though SO₄^-· and OH· with strong electrophilicity mainly attacked electron-rich groups in A ring of TC,the degradation products in Fe Ru/C+PMS and FMO+PMS systems were quite different.The degradation pathways mainly included hydroxyl addition,deamination,demethylation and ring-opening reactions.The difference in degradation products mainly attributed to the complexation between Ru and TC.The complexation of TC at C3-O or C1-O inhibited the intramolecular hydrogen-bond and facilitated the oxidation of TC.The complexation with TC at A ring resulted in the electron transfer from-C=C-to coordinate bonds,then the stability of-C=C-was weakened.The toxicity evaluation results showed that the toxicity of TC oxidation products in the Fe Ru/C+PMS system was significantly reduced.Furthermore,¹O2,as dominated active species in NR-Fe+PMS systems,could attack both electron-rich groups in A ring and electron-deficient groups in BCD ring of TC,resulting in the different degradation products of TC and lower toxicity than that in Fe Ru/C+PMS and FMO+PMS systems.