Synthesis Of Carbon-coated Fe₃O₄ Nanoparticles From Ferrous Sulfate Waste And Its Application For The Degradation Of Methylene Blue

The main solid by-product from the sulfuric acid titanium dioxide industry is ferrous sulfate waste.In2020,nearly 10 million tons of ferrous sulfate waste was produced.At present,the high-quality utilization rate of ferrous sulfate waste is relatively low,which limiting the sustainable development of the sulfuric acid titanium dioxide industry.In view of the poor application effect of the nano-magnetite prepared by the solid-phase method from ferrous sulfate waste,the secondary pollution of iron sludge and stringent p H requirements in the traditional treatment of organic dye wastewater,the process route for synthesizing carbon-coated Fe₃O₄ nanoparticles（carbon-coated MNPs）by in-situ solid-phase method using ferrous sulfate waste,pyrite,and different carbon resources as raw materials was proposed.Furthermore,the carbon-coated Fe₃O₄nanoparticles were applied to the removal of organic dye wastewater.Firstly,the chemically stable carbon black（CB）was selected as the carbon source,and the experimental study on the in-situ solid-phase synthesis of carbon-coated MNPs（CB-MNPs）was carried out,and the most suitable synthesis conditions were discussed.The morphology of the as-synthesized CB-MNPs is spherical,the particle size is 15-100 nm,the thickness of the carbon-coated shell is about 10 nm,and the carbon shell is mainly composed of amorphous carbon.When CB-MNPs were applied to the degradation of methylene blue under the adsorption-Fenton system,the maximum adsorption capacity of CB-MNPs for methylene blue was 23.02 mg/g,showing good adsorption performance.However,because the carbon shell coated on the outer surface of CB-MNPs is thick and in the form of amorphous carbon,although it can efficiently adsorb methylene blue molecules in the solution,the tightly wrapped carbon shell will hinder the exposure of Fe ions on the particle surface,which reduced its Fenton activity,and the removal rate of 100 mg/L methylene blue solution for 180 min in a neutral environment was only 53.15%.Subsequently,in order to enhance the Fenton activity of the particles,activated carbon（AC）composed of graphite crystallites and amorphous carbon was selected as the carbon source,and carbon-coated MNPs（AC-MNPs）synthesized by the in-situ solid phase method were carried out and the optimum experimental conditions were discussed.The morphology of the AC-MNPs is spherical,the particle size is 15-100 nm,the thickness of the carbon-coated shell is about 1.5 nm,and the carbon shell is composed of part of amorphous carbon and graphitized carbon.When AC-MNPs were applied to the degradation of methylene blue under the adsorption-Fenton system,the maximum adsorption capacity of AC-MNPs for methylene blue was 18.52mg/g,which is lower than that of CB-MNPs,but after reacting 180 min in a neutral environment,the removal efficiency of the 100 mg/L methylene blue solution increased to 69.68%,indicating that the reduction in the thickness of the carbon shell and the change in composition would reduce the adsorption efficiency and increase the Fenton activity.Thirdly,in order to further improve the Fenton activity of the particles in a neutral environment,PVP K30 with higher chemical activity was selected as the carbon source,and the carbon-coated MNPs（P-MNPs）synthesized by in-situ solid phase method were carried out and the suitable preparation process was obtained.The morphology of the synthesized P-MNPs is spherical,the particle size is 20～30 nm,the thickness of the carbon coating shell is about 2 nm,and the carbon-shell is composed of part of amorphous carbon and graphitized carbon.The experimental results show that most of the active functional groups in PVP K30would be used as reducing agents to participate in the process of solid-phase reaction to form the core Fe₃O₄,where part of the carbon bonds that did not participate in the solid-phase reaction would form the carbon-shell under nitrogen atmosphere calcination.When P-MNPs were applied to the degradation of methylene blue under the adsorption-Fenton system,the maximum adsorption capacity of P-MNPs for methylene blue was 18.60 mg/g,which was similar to AC-MNPs,but after reacting 180 min in a neutral environment,the removal rate of methylene blue solution at to 100 mg/L was further increased to 97.75%,indicating that PVP K30 can not only form a carbon coating layer on the surface of the particles,but also significantly increase the Fenton activity of the particles.The study of the mechanism of the adsorption-Fenton system found that the adsorption process and the Fenton process have a synergistic effect in the removal of methylene blue.The carbon-coated MNPs will first adsorb the methylene blue molecules in the solution,and then after adding H₂O₂,Fenton reaction occurs at the solid-liquid interface to achieve the purpose of degrading methylene blue.Finally,using ferrous sulfate waste and pyrite instead of analytically pure ferrous sulfate and Fe S₂reagents,using PVP K30 as carbon source additive,the particle size of carbon-coated MNPs synthesized by in-situ solid-phase method is about 30 nm,the particle morphology is spherical and the thickness of carbon-shell is about 2 nm.In the adsorption-Fenton system,when the initial p H of the methylene blue solution is 2,the degradation rate of the 100 mg/L methylene blue solution reaches 99.79%at 120 min.When the initial p H of the methylene blue solution is 6,The degradation rate of 100 mg/L methylene blue solution reached96.97%at 180 min,showing excellent catalytic activity.Compared with the heterogeneous adsorption-Fenton reagent reported in the literature,carbon-coated MNPs synthesized by in-situ solid-phase method using ferrous sulfate waste,pyrite and PVP K30 as raw materials not only have good application value in the field of organic dye wastewater,but also have the characteristics of simple preparation process and easy control of process parameters,which are more industrialized.It not only alleviates the accumulation of industrial solid waste titanium dioxide waste slag ferrous sulfate,but also solves the iron mud pollution caused by the homogeneous Fenton reagent commonly used in the degradation section of the current industrial organic dye wastewater treatment.