Study On The Efficiency And Mechanism Of Removing Organic Pollutants By Iron Manganese Oxides Activated Bisulfite

Sulfate radical-based advanced oxidation processes（SR-AOPs）has great advantages in the removal of refractory organic pollutants.The generation of sulfate radical(SO₄^·—)is mainly realized by using persulfate as precursor.However,the high cost and biotoxicity of residual materials limit the practical application of persulfate.In recent years,bisulfite（BS）or sulfite,also known as S（IV）,which is low toxicity,cheap and easy to prepare,has become an emerging direction in SR-AOPs as an economical and green persulfate substitute,and the search for efficient,low-cost and environmental friendly S（IV）activation technology has also become the focus of scholars.Among many activation technologies,the transition metal heterogeneous activation method has obvious advantages of low energy consumption and recyclability.Iron manganese oxides raw materials are cheap,easy to obtain and environment-friendly.Using iron manganese oxides as catalyst can not only save the preparation cost,but also realize the double improvement of material stability and catalytic activity by using the synergistic effect between iron and manganese bimetal.It is a heterogeneous activation material with great development potential.However,there are few reports on the activation of S（IV）by iron manganese oxides to degrade organic pollutants at this stage,and the related activation mechanism between iron manganese oxides and S（IV）is still lack of in-depth research.In this study,based on the research status of S（IV）activation technology and iron manganese oxides heterogeneous activation materials,a new advanced oxidation system of iron manganese oxides/BS was constructed.Rhodamine B（RhB）,a typical refractory organic pollutant in wastewater,was used as the target pollutant,and the degradation efficiency and reaction mechanism of RhB were systematically explored.The main research contents and conclusions are as follows:（1）The catalysts with different Fe/Mn molar ratios were prepared by sol-gel method and characterized by X-ray diffraction（XRD）,X-ray photoelectron spectroscopy（XPS）and scanning electron microscopy（SEM）.The results show that the catalysts with different Fe/Mn molar ratios have different crystal structure and morphological characteristics.The crystal form of Fe1Mn5 catalyst used in subsequent studies is mainly Mn₃O₄ with loose lamellar structure with micropores on the surface.And Fe and Mn in Fe1Mn5 exist in the form of surface Fe（II）,surface Fe（III）,surface Mn（II）and surface Mn（III）,respectively.（2）The iron manganese oxides/BS system was constructed,and the effects of different reaction systems（different system conditions,catalysts with different Fe/Mn molar ratios and different reaction modes）and typical factors（initial p H value of solution,catalyst dosage,BS dosage,BS dosage mode and coexisting substances）on the degradation efficiency of RhB were investigated.The results show that dissolved oxygen（DO）plays an important role in the activation system,and the catalyst has the highest catalytic activity for BS when the Fe/Mn molar ratios is 1:5.Under co-mixing mode,when Fe1Mn5=25 mg/L,BS=10 mm,p H=4,gas flow rate=0.6 L/min,T=25±1℃,the degradation efficiency of 10 mg/L RhB can reach92%within 30 min in Fe1Mn5/BS/aeration system.At the same time,the effective p H range of the reaction system is wide,and the degradation process of RhB is consistent with the quasi-first-order kinetic model.In addition,multiple low-dose and equally divided BS dosages can not only reduce the cost of the reaction system,but also achieve the best degradation efficiency of RhB.The presence of inorganic anions（Cl^—,HCO₃^—and H₂PO₄^—）in the aqueous matrix had little interference with the degradation of RhB in Fe1Mn5/BS/aeration system,while the presence of natural organic matter（NOM）represented by humic acid（HA）in higher concentration significantly inhibited the degradation of RhB.（3）The results of three cycle experiment and XRD characterization before and after use of Fe1Mn5 show that Fe1Mn5 has excellent reusability and stability.After recycling,the leaching amount of Fe and Mn ions in the reaction solution can meet the requirements of China’s sanitary standard for drinking water,indicating that Fe1Mn5 is environmentally friendly.The good removal effect of Fe1Mn5/BS/aeration system on gold orange II,methylene blue（MB）,tetracycline hydrochloride（TCH）,bisphenol A（BPA）and other organic pollutants shows that the system has excellent wide applicability and broad practical application prospects.（4）The degradation mechanism of RhB in Fe1Mn5/BS/aeration system was deeply analyzed and studied.The results showed that the reaction system followed the heterogeneous activation mode,and the generation of active species and degradation of RhB occurred on the surface of Fe1Mn5.The degradation of RhB is mainly caused by the attack of active species on aromatic ring structure and conjugated chromophore,and finally decomposed into CO₂,H₂O and other inorganic ions.In Fe1Mn5/BS/aeration system,the main reactive oxygen species（ROS）leading to the degradation of RhB is not the traditional SO₄^·—,hydroxyl radical（HO^·）or strong oxidizing Mn（III）species,but the superoxide radical(O₂^·—)which degraded RhB efficiently.In the process of BS activation,trace Fe in Fe1Mn5 improves the catalytic activity of the material by promoting the synergistic mode of electron transfer between Mn（II）/Mn（III）,thus accelerating the generation of O₂^·—and achieving efficient degradation of RhB.In this study,a new advanced oxidation system of iron manganese oxides/BS with high efficiency,economy and green was successfully constructed,and a new activation mechanism between iron manganese oxides and S（IV）was proposed,which provided data reference and theoretical basis for the application of iron manganese oxides activated S（IV）system in practical organic wastewater purification and related mechanism research.