Preparation Of ?-FeOOH/Fe₃O₄/biochar Photo-induced Composite Material And Mechanism Investigation Of Removal Of Azo Dye

Photocatalysis and Fenton-like processes are two promising advanced oxidation technologies for the degradation of organic pollutants.The combination of these two technologies forms a new Fenton-like photocatalytic system with huge application potential.In photo-induced Fenton-like technology,light excitation of active materials produces superoxide and hydroxyl radicals,which can non-selectively crack the structure of the most challenging organic pollutants in wastewater to achieve degradation.Therefore,preparing photo-induced Fenton-like functional materials and composite photocatalysts to treat azo dye wastewater efficiently,green,and environmentally friendly is one of the common fundamental water treatment problems.In this paper,corncob biomass was used as the raw material.After pyrolysis,activation,co-depositing with polymorphic iron elements,a magnetic nano-?-FeOOH/Fe₃O₄/biochar photo-induced Fenton-like composite material was obtained.The Fenton-like catalytic degradation mechanism of this composite material was investigated.This work is aimed to develop a new material that can degrade azo dyes efficiently and explore a new method for water resource recycling.In this paper,the corncob biochar rich in graphene-like structures and polyphenolic hydroxyl groups was prepared by pyrolysis.The activated biochar was obtained through KOH activation.A new type of?-FeOOH/Fe₃O₄/biochar photo-induced composite material was then synthesized by a one-step co-precipitation method.The azo dye methyl orange(MO)was selected as a model pollutant to study the photocatalytic activity of the composite material under visible light.The graphene-like structure of activated biochar under visible light,the synergistic effect of phenolic hydroxyl,?-FeOOH,and Fe₃O₄ on MO degradation,and the inhibiting effect on the destruction of hydroxyl radicals were explored.The photocatalysis and magnetic separation performance under visible light was also investigated.Besides,the degradation mechanism of azo dyes by reactive species(such as hydroxyl radicals and hydrogen peroxide)produced from the excitation water and oxygen was discussed.The main conclusions are as follows:(1)The specific surface area of biochar prepared with corncob is 468.59 m²/g which is 3.8 times that of corncob biomass,and the maximum adsorption capacity for MO is90.02 mg/g.Since corncob biochar is rich in graphene-like structure and phenolic hydroxyl functional groups,it has electrostatic interaction,synergistic electron sharing,and electron exchangeability.The electron sharing and transfer processes could be promoted under visible light irradiation,leading to the generation of high activity hydroxyl radicals by graphene-like structure and oxygen,which promoted MO degradation.(2)Corncob biochar activated with KOH has a good pore structure(dominated by micro and mesopores),which is continuous and complete.The activated biochar is characterized with abundant graphene-like structure and rich phenolic hydroxyl groups,resulting in good adsorption performance and loading features.The specific surface area of activated biochar is 1424.82 m²/g,which is 3.04 times that of corncob biochar.The photocatalytic degradation rate of activated biochar for MO is 1.32 times of that of corncob biochar.(3)A new type of?-FeOOH/Fe₃O₄/biochar photo-induced Fenton-like composite material with photocatalytic properties and superparamagnetism was developed by a one-step co-precipitation method.XPS spectroscopy confirmed the presence of C–O–Fe bond between?-FeOOH/Fe₃O₄ and activated biochar,which can accelerate the transmission of photo-generated electrons.UV-vis spectroscopy showed that there are electron-hole pairs between?-FeOOH and activated biochar,promoting the transfer of photo-generated electrons from?-FeOOH to the interface of activated biochar and accelerating the production of reactive species under visible light.Electron spin resonance analysis and free radical quenching experiments showed that hydroxyl radicals are the main reactive species in the photodegradation of methyl orange by composite materials.Therefore,in the synergistic photocatalytic system,composite materials show excellent degradation of MO catalytic performance.The photocatalytic efficiency is 2.03 times that of activated biochar.The composite material still maintains a stable structure and catalytic activity after multiple cycles of use and is easy to recover and separate due to its superparamagnetic properties.(4)The photo-induced Fenton-like composite material was used to catalyze the degradation of MO.It was found that the adsorption performance of the composite material follows the Freundlich adsorption model,and its adsorption capacity is insensitive to p H.In the photo-induced Fenton-like heterogeneous catalytic system,the adsorption-catalysis synergy between?-FeOOH/Fe₃O₄ and the carrier activated biochar significantly improves the degradation of MO.The p H value of the catalytic system has a specific impact on the degradation of MO.The generation rate of·OH and degradation of MO increased with decreasing p H in the range of 3.0?8.0.The degradation process conforms to the Langmuir–Hinshelwood process.Furthermore,the heterogeneous catalytic reaction caused by the loss of Fe²⁺in the composite material has little effect on the degradation of MO.The composite material exhibited excellent stability and applicability.(5)The degradation mechanism of MO in the photo-induced Fenton-like catalytic system of?-FeOOH/Fe₃O₄/biochar is studied.?-FeOOH forms a stable C–O–Fe bond with biochar through phenolic hydroxyl group,which could generate electron–hole pairs,promoting photo-generated electron transfer from?-FeOOH to activated biochar interface and thus accelerating the production of·OH and H₂O₂.Fe₃O₄-based Fenton-like catalytic mechanism promotes the decomposition of H₂O₂ into·OH and impedes photo-generated·OH.The synergy of photocatalysis and Fenton-like can break the structure of MO,leading to its degradation.·OH firstly attacks the dimethylamino and sulfonic acid groups of the MO molecule,then breaks the azo bond,leading to ring open and generation of small molecular products.In addition,FTIR,XPS,and LC-MS confirm the synergy of the degradation process and intermediate product production.