Degradation Of Sulfanilamide Antibiotics In Water By Reactive Oxygen Systems Catalyzed By Manganese Oxides

Sulfonamides(SNs)were the first used drugs to treat protozoan and bacterial infections.Nowadays,it is commonly employed in livestock and aquaculture as growth promoters,which far more than their medical use.China is the largest producer and consumer of antibiotics all over the world.In 2019,a total usage of 30,903.66 tons of antimicrobial drugs were counted in our country.Among them,SNs were discovered the maximum of preparation.However,SNs are discharged into the environment as the parent compounds or metabolites due to its weakly absorbed or incomplete metabolism.Because of strong chemical stability and water solubility,SNs are often detected in natural environment.The content of sulfamethoxazole(SMX)detected in wastewater was as high as 100?2500 ng·L^-1 and in drinking water was upto 12 ng·L^-1.Studies have found that the environment with low concentration levels of SNs could induce genetic mutations in bacteria,resulting in the formation of SN resistant bacteria.In addition,SNs residues in the environment pose potential risk to the environment and human health,inhibit the growth and survival of microorganisms,as well as accumulate in plants due to their high biological activity.Therefore,it is of great significance to conduct a comprehensive study to developnovel,efficient and sustainable technologies for removing SNs and their derivatives.Advanced oxidation processes(AOPs)are the preferred method for removing refractory organic pollutants in sewage treatment and environmental remediation.Pollutants could be converted into simple and nontoxic molecules by generating strong oxidizing active oxygen species such as hydroxyl radicals(^·OH),sulfate radicals(SO₄^·-),superoxide radicals(O₂^·-)and singlet oxygen(¹O₂).Manganese oxides are widely present in the natural environment,with a multivalent state and crystal structure,which have been proven to quickly cause the activation of H₂O₂,persulfate,etc.Nevertheless,the activation mechanism of manganese oxides with different valences and crystalline forms,produced active oxygen species and the degradation pathway of pollutants need to be further studied.Herein,manganese oxides with different valence states and crystal forms were prepared by changing the experimental conditions.By coupling sodium periodate(NaIO₄)and persulfate(PMS and PDS),a series of advanced oxidation systems were constructed to degrade SN and SMX,the representative drugs of SNs.The degradation characteristics of SNs drugs were comparatively analyzed of different oxidation systems.The catalytic activation mechanism of NaIO₄/persulfates by manganese oxides and degradation pathways of SNs drugs were in depth investigated.A feasible way and novel idea in our study for the treatment of SNs and contaminants with similar physical and chemical properties in actual water bodies.The specific modules of this research were as follows:(1)Manganese oxides with different valences(i.e.amorphous?-MnO₂,Mn₂O₃ and Mn₃O₄)were employed to activate periodate(PID)to degrade SN.By comparing the degradation efficiency of SN,the best PID catalyst was screened out.The amorphous?-MnO₂ could completely remove 10 mg·L^-1 of SN under the condition that the dosage of?-MnO₂ was 0.2g·L^-1 and the PID concentration was 1.0 mmol·L^-1.After three cycles,there is no efficiency loss.With the increase of the dosage of?-MnO₂,the SN degradation efficiency and PID reduction rate linearly increased.Unlike the traditional AOPs systems,the typical water interference factors(anions,HA,etc.)slightly inhabited the removal of SN.On the basis of free radical quenching experiments,EPR characterization and hypoxia tests,it was inferred that ¹O₂ and surface-bound IO₃ in the oxidation system were prominent active oxidants,rather than^·OH and SO₄^·-.The Results showed that the mechanism of?-MnO₂ catalyzing PID involved the formation of metastable Mn(IV)-O-IO₃.The generated ¹O₂ and IO₃ in further activation of Mn(IV)-O-IO₃ ultimately lead to the oxidative degradation of SN.(2)Different crystal structures of MnO₂(?-,?-,?-and?-MnO₂)were prepared by changing the hydrothermal conditions and the molar ratio of KMnO₄/MnSO₄·H₂O.The?-MnO₂ exhibited the best SMX degradation performance in the heterogeneous system by activating PID.It might be attributed to its unique 2*2 tunnel structure and higher specific surface area.Simultaneously,?-MnO₂/PID oxidation system has a removal efficiency of more than 90%for other refractory organics(acid orange,rhodamine B,ciprofloxacin,bisphenol A and sulfamethazine).In the cycle tests,?-MnO₂ showed a fine stability and acceptable reusability.In addition,the effects of PID and catalyst dosage,pH values on SMX removal were investigated.Combining with XRD,XPS,EPR tests and free radical quenching experiments,the conclusion has drawn that the main active oxygen species was ¹O₂ in the system,and that ¹O₂ and Mn(IV)occupied leading parts in the degradation of SMX.(3)Manganese oxides with different valences(?-MnO,MnO₂,Mn₂O₃,Mn₃O₄)synthesized in(1)were coupled with peroxymonosulfate(PMS)to oxidize and degrade SMX in water.Due to the smaller particle size and higher valence of manganese,?-MnO₂exhibited the best PMS catalytic activity.When the pH value was 7.0,all SMX could be degraded within 120 min in?-MnO₂/PMS system.The experimental results showed that the?-MnO₂ catalyst possessed an excellent stability,and the MnO₂/PMS system had a universally applicability for organics removal.In the free radical identification tests,¹O₂was proved to be the main reactive oxygen species responsible for the degradation of SMX in the system.Additionally,small amounts of SO₄^·-and^·OH were also produced in the?-MnO₂/PMS system.XPS measurement was conducted to analyze the valence state changes of MnO₂ before and after use.It was found that Mn-OH formed between Mnand-OH and Mn(?)/Mn(?)redox pairs play key roles in SMX degradation.(4)Different crystal structures of MnO₂(?-,?-,?-and?-MnO₂)prepared in(2)were employed in catalyzing PDS at 50?.The?-MnO₂ was screened out as the catalyst with a best PDS activation performance coupling with 50?thermal and used in subsequent experiments.It was discovered that the 50?thermal coupling of 1 g·L^-1?-MnO₂ could effectively activate PDS and totally oxidized 10 mg·L^-1 of SMX within 180min.Similarly,the constructed system possessed excellent recycling features and removal performance for other types of refractory pollutants.As the dosages of PDS and?-MnO₂increased,the removal efficiency of SMX also increased.And the optimal pH value for oxidative degradation of SMX was 6.0.SO₄^·-was confirmed to be the significant active oxygen species that degraded SMX in the?-MnO₂/50?thermal/PDS system.The main active sites on the surface of?-MnO₂ involved the Mn(IV)/Mn(III)redox pairs.Secondly,the heat provided?-MnO₂ with lower activation energy required for catalytic decomposition of PDS,where[?Mn(IV)-OH-O-SO₃]^II could directly oxidize SMX.