Synthesis Of Schwertmannite-based Composite Catalyst And Its Catalytic Performance For Heterogeneous Fenton

Advanced oxidation processes（AOPs）are considered as a promising approach for wastewater treatment.AOPs are usually involved in the generation of a series of reactive free radicals,which oxidize refractory and non-biodegradable organic compounds into intermediates with low molecular weight,and finally converse CO₂.Among various oxidants,H₂O₂widely used in AOPs has a strong oxidation potential（2.8 e V）and no secondary pollution in the environment.In Fenton reaction,H₂O₂can produce hydroxyl radical（·OH）with stronger oxidation activity,which is especially conducive to the efficient decomposition of organic pollutants in wastewater.However,Fenton has some inevitable disadvantages,such as a narrow reaction pH range（2-3.5）,high consumption of H₂O₂,difficulty in recycling,and the production of iron-containing sludge.Therefore,heterogeneous Fenton,as a kind of AOPs technology,has become a promising and environmental friendly water treatment method.Insoluble iron oxide solid instead of solution Fe²⁺has a potential application due to expanding the work range of the pH,preventing the generation of a large number of iron sludge and good reusability.However,there are some drawbacks in the traditional heterogeneous Fenton:（1）the activation of H₂O₂to produce large quantities of free radicals（e.g.,·O₂^-,·OH）is often limited because of the finite≡Fe²⁺(or Fe²⁺)species.（2）The Fenton reaction conditions require an acidic environment.（3）Fenton system needs to add a large amount of H₂O₂,which would cause security risks and high costs.（4）Low surface adsorption rate of H₂O₂occurs in heterogeneous Fenton.Therefore,the development of a new iron-bearing solid catalyst can generate a large of reactive radicals in heterogeneous Fenton system and the in-situ production of H₂O₂is of great significance in practical wastewater treatment.Secondary Fe hydroxysulfate and other Fe（hydr）oxides are often produced in acid mine drainage（AMD）environment,mainly oxyhydroxy-sulfate minerals,including schwertmannite（Sch）and jarosite,which could reduce the toxic elements in AMD by co-precipitation and adsorption.Sch as a Fe（III）-oxyhydroxy-sulfate mineral commonly found in acidic sulfate-rich environments.It is prepared by H₂O₂oxidation of FeSO₄solution or biological method(the oxidation of Fe²⁺by Acidithiobacillus ferrooxidans).Due to its unique structure of SO₄^2-and Fe species,Sch as a heterogeneous catalyst has been used in the degradation of organic pollutants in water for several years.However,the Sch mainly contained Fe³⁺species,the slow transformation of Fe³⁺to Fe²⁺led to the existence of induction period during the Fenton reaction.Fe₃O₄nanoparticles have excellent electrical conductivity,strong magnetism,large specific surface area,low toxicity,good biological compatibility and their unique anti-spinel structure.Fe₃O₄were also widely applied in the degradation of organic pollutants.However,Fe₃O₄tended to agglomerate,resulting in poor catalytic activity.Therefore,organic or inorganic materials were used as supports for Fe₃O₄to improve its catalytic activity.In this paper,Fe₃O₄nanoparticles were added into the formation of chemical or biological Sch to prepare new composite catalysts to solve the problem of traditional heterogeneous catalysts.The new composite catalysts were characterized by a series of characterization methods to analyze its morphology structural and properties,and then tested its catalytic activity through the degradation of organic pollutants.The catalytic degradation mechanisms of Sch-based catalysts were explored.In addition,the in situ production of H₂O₂and its application in heterogeneous Fenton were also studied.Based on the above researches,the main conclusions of this paper are as follows:（1）Fe₃O₄nanoparticles as heterogeneous nuclei were added into the the synthesis of chemosynthetic Sch to prepare a heterogeneous Fenton catalyst（Fe₃O₄/Sch）.Fe₃O₄induced Fe³⁺was more easily reduced to Fe²⁺during chemo-synthesis.A smaller Fe₃O₄/Sch particle tended to adsorb more solution Fe²⁺ions during preparation process.Thus,Fe₃O₄was loaded on Sch,resulting in more≡Fe²⁺and adsorption state Fe²⁺on the composite Fe₃O₄/Sch,which would accelerate the conversion of≡Fe³⁺to≡Fe²⁺(or Fe³⁺/Fe²⁺)and fuehrer promote the utilization rate of H₂O₂.Fe₃O₄/Sch showed excellent catalytic performance（97%）and stability（cycle 4 times）in the degradation of ciprofloxacin（CIP）.Through experimental data and theoretical calculation,the·OH,O₂^·-and SO₄^·-generation pathway and the degradation pathway of CIP were deduced.When Fe₃O₄/Sch was applied in Fenton system,≡Fe²⁺on composite catalyst could effectively activate H₂O₂to produce a large amount of·OH,and then SO₄^2-in the Sch structure could react with·OH to produce SO₄^·-,while≡Fe²⁺activated O₂to produce O₂^·-.Therefore,Fe₃O₄/Sch-actived Fenton reaction could produce a variety of oxidizing species to greatly improve the degradation rate of organic pollutants in water.（2）Fe₃O₄modified Sch as photo-Fenton-like catalyst was prepared by chemical method.Compared with single Sch and Fe₃O₄catalysts,Fe₃O₄loaded Sch had the high utilization rate of H₂O₂（98.5%）and high photocatalytic activity（98%）in degradation of phenol,which could be attributed to the photo-generated electrons captured by Sch with sulfate group.The composite catalyst promoted the separation efficiency of photo-generated electron-hole pairs and accelerated the conversion between≡Fe³⁺and≡Fe²⁺.On the other hand,the formation of Z-scheme system will efficiently improve the separation of photo-generated electrons and holes pairs to enhance the photocatalytic performance.Due to the unique structure of Sch,the composite catalyst can effectively degrade phenol at a wide pH（3-9）.In addition,the composite catalyst with magnetic properties can be easily separated from treated water.（3）Fe₃O₄nanoparticles were added into the formation of biosynthetic Sch to prepare Fe₃O₄/Sch containing organic carbon（Fe₃O₄/Sch/C）.On the one hand,the ingestion of iron during the growth of Acidithiobacillus ferrooxidans（A.ferrooxidans）could form the magnetosomes in cell.Introducing strong magnetic Fe₃O₄in the biosynthesis process of Sch would bring about more A.ferrooxidans cells to participate in the formation of Sch,and hence increased the organic carbon content（e.g.,A.ferrooxidans,exopolymeric substances）in Sch-based catalyst.The presence of organic carbon accelerated the reduction of≡Fe³⁺to≡Fe²⁺and protected the oxidation of Fe²⁺.On the other hand,more≡Fe²⁺sites was exposed on the surface of Fe₃O₄/Sch/C,which also increased the reaction rate in the early stage of catalysis,resulting in eliminating the induction period of Sch.By comparing the catalytic degradation of quinolones with different catalysts（Fe₃O₄,Sch,Fe₃O₄/Sch）,it showed that Fe₃O₄/Sch/C had the highest catalytic activity（95%）.It was also found that piperazine ring or quinolone ring were the most vulnerable sites of reactive free radicals.（4）The synthesis of Fe₃O₄/Sch/C composite catalyst by biological method could produce H₂O₂in situ.There were oxygen-containing functional groups（e.g.,-COOH,C=O,C-OH）on the surface of Fe₃O₄/Sch/C,which was beneficial to the adsorption of O₂and solved the problem of poor mass transfer of O₂from water to the surface of the catalyst,at the same time,organic carbon and≡Fe²⁺provided enough electrons for O₂reduction.In the Fe₃O₄/Sch/C-H₂O system,it was found that O₂was mainly reduced by one-step two-electron rote to produce H₂O₂（O₂+2H⁺+2e^-→H₂O₂）,which greatly improved the selectivity of H₂O₂.Meanwhile,≡Fe²⁺or Fe²⁺effectively activated H₂O₂to produce·OH for degrading organic pollutants.The identification of intermediate products of pollutants showed that degradation reaction occurred in the Fe₃O₄/Sch/C system,further indicating that the in-situ production of H₂O₂by Fe₃O₄/Sch/C could be also effectively activated to produce the free radicals for degrading organic pollutants.Sch-based catalysts with SO₄^2-could release a large amount of H⁺when pH exceeded 4.The generation of H⁺from the hydrolysis of the Fe₃O₄/Sch/C would meet the needs of activating O₂.Thus,Fe₃O₄/Sch/C could produce H₂O₂at a wide pH（3-9）.Therefore,adding Fe₃O₄nanoparticles in the process of chemical or biological synthesis of Sch,the Fe₃O₄/Sch and Fe₃O₄/Sch/C composite catalysts were prepared to improve the heterogeneous catalytic activity by eliminating the induction period of Sch and solving Fe₃O₄agglomeration.In addition,it was found that organic carbon modified composite catalyst could produce H₂O₂in situ,which solved the problems of poor mass transfer and adding H₂O₂in traditional Fenton.These findings not only provide an environmentally friendly,economical and efficient catalyst,but also give a new perspective for the application of iron oxyhydroxysulfate minerals in environmental remediation.