Preparation Of Fe₃O₄ Loaded Sulfur-doped Graphene-like Carbon/Layered Double Oxides For Removal Of Methyl Orange And Pb²⁺

With the rapid development of industrialization in recent years,such as textile and dyes manufacturing,the pollution caused by organic wastewater poses an increasingly serious threat to the safety of ecological environment and human health.Layered adsorbents are most widely used in the treatment of organic wastewater due to its special layered structure,high surface area and low-cost.Nevertheless,the generated solid products adsorbed organic pollutants,which were defined as organic layered wastes,are considered as hazardous materials,which easily cause secondary pollution to environment.Therefore,in order to further improve the application of layered materials in organic wastewater treatment,it is imperative to dispose organic layered wastes.To date,the tradition methods for disposal of organic layered wastes such as direct combustion,are simple to operate.However,aerobic calcination not only releases greenhouse gases（e.g.,CO₂,SO₂and NO_x）,causing pollution to the atmosphere,but also waste valuable carbon,sulfur and nitrogen in organic pollutants.Therefore,it is urgent to find a suitable method to realize the high value-added resource disposal of organic layered wastes by a green environmental protection way.In this study,we attempted to address this dilemma by an intriguing“precursor-calcinaiton”strategy,i.e.,selecting the sodium dodecyl sulfate-supported Mg-Al layered double hydroxide（O3D-SDS）as the representative to synthesize Fe₃O₄ loaded sulfur-doped graphene-like carbon/layered double oxides（MG/S-LDO）via calcination.In detail,we introduced magnetic Fe₃O₄ into the O3D-SDS precursor via a solvothermal method,followed by pyrolysis at high-temperature under argon（Ar）atmosphere protection,to obtain the functional MG/S-LDO sample.The effect of SDS concentrations on the organic loading amount of O3D-SDS was investigated.Also,the influence of introduced Fe³⁺concentrations on the MG/S-LDO was explored.Based on these discussions,the optimum parameters of the added SDS concentration and the introduced Fe³⁺concentrations were determined.Besides,the composition and morphology of the as-prepared MG/S-LDO were characterized by X-ray diffraction（XRD）,Vibrating sample magnetometer（VSM）,Scanning electron microscopy（SEM）,Transmission electron microscopic（TEM）,Energy-dispersive X-ray spectroscopy mapping（EDS-mapping）,Raman spectra（Raman spectra）,Fourier transform infrared spectroscopy（FT-IR spectra）,and X-ray photoelectron spectroscopy（XPS）.Meanwhile,in view of the underlying risk of dyes or heavy metal wastewater on the ecological environment and human health,MG/S-LDO was employed to treat methyl orange（MO,representative of dyes）simulated wastewater and lead(Pb²⁺,representative of heavy metals)simulated wastewater,respectively,to explore its adsorption performance.The adsorption performance was evaluated via the investigation of various influence factors（e.g.,system p H,reaction time,solution initial concentration and reaction temperature）.The adsorption mechanisms of MG/S-LDO on the MO and Pb²⁺were determined by combining with the fitting results of kinetic,adsorption isotherm model,and thermodynamic model,as well as characterizations（XRD,FT-IR and XPS）of the solid products before and after the reaction.The detailed results are as follows:（1）The optimum parameters of the added SDS concentration and the introduced Fe³⁺concentration were 0.1 mol/L and 0.03 mol/L,respectively.The characterization results of MG/S-LDO_0.03 prepared with the optimal conditions illustrated:O3D-SDS₁was successfully converted into MG/S-LDO_0.03 after solvothermal reaction under the high-temperature inert atmosphere pyrolysis.That is,Mg Al-LDH component was converted to Mg Al-LDO,SDS components were converted into a sulfur-doped graphene-based carbon material and the magnetic Fe₃O₄ was successfully introduced.The saturation magnetization of MG/S-LDO_0.03 reaches 11.0 emu/g,which can achieve the purpose of solid-liquid separation.The SEM and TEM results of MG/S-LDO_0.03showed that its morphology was 3D flower-like structure consisting of numerous thin LDOs nanosheets,and the surface was loaded with the Fe₃O₄ microspheres with an average size of about 62.3 nm.Zeta potential results displayed that the MG/S-LDO_0.03has a negative charge on the surface of aqueous solution.（2）The removal of MO by MG/S-LDO_0.03 showed that:MG/S-LDO_0.03possessed a well anti-p H-interference ability due to the surface rich groups（such as-COOH,-C-S and-OH）.MG/S-LDO_0.03 adsorption of MO reached equilibrium time at 6 h,and the removal efficiency was 100%.When the reaction temperature was 25 ^oC,the initial concentration of MO was 2000 mg/L and the dosage of MG/S-LDO_0.03was 1 g/L,the theoretical maximum adsorption capacity of MG/S-LDO_0.03 for MO was 1472.58 mg/g.The thermodynamic fitting results showed that the removal reaction was spontaneous and endothermic.Increasing the reaction temperature was conducive to the progress of the adsorption reaction.（3）The removal behavior of MO on MG/S-LDO_0.03 followed the pseudo-second-order kinetic model（R²=0.9998）and Langmuir adsorption isotherm model（R²=0.971）,indicated that the adsorption was monolayer and chemisorption.The XRD,FT-IR and XPS results showed that part of LDOs recovered LDH structure by intercalating MO anions based on“memory effect”.Meanwhile,the metal-oxygen bond（eg,Mg-O and Al-O）of LDOs was complexed with the functional group of MO,further promoting the removal of MO.In addition,the O-C=O and C-S groups on the surface of the carbon components haveπ-πelectron donor–acceptor（EDA）effect with the N（CH₃）₂ groups on the MO.The removal efficiency of MO was still maintained above 85%after five cycles,indicated that MG/S-LDO_0.03 was an excellent adsorbent for purifying dye wastewater.（4）The removal of Pb²⁺by MG/S-LDO_0.03 showed that:MG/S-LDO_0.03adsorption of Pb²⁺was less affected by solution p H.When the p H was 3-7,the removal rate of Pb²⁺with initial concentration of 100 mg/L can reach more than 95%.MG/S-LDO_0.03adsorption of Pb²⁺reached equilibrium time at 4 h.When the reaction temperature was25 ^oC,the initial concentration of Pb²⁺was 2000 mg/L and the dosage of MG/S-LDO_0.03was 1 g/L,the theoretical maximum adsorption capacity of MG/S-LDO_0.03for Pb²⁺was655.92 mg/g.The thermodynamic fitting results showed that the removal reaction was spontaneous and endothermic.Increasing the reaction temperature was conducive to the progress of the adsorption reaction.（5）The removal behavior of Pb²⁺on MG/S-LDO_0.03 followed the pseudo-second-order kinetic model（R²=0.9998）and Freundlich adsorption isotherm model（R²=0.974）,indicated that the adsorption was multilayer and chemisorption.After the adsorption of Pb²⁺,the characterization results of solid products indicated that the OH^-released by the hydration reaction of LDOs component was precipitated with Pb²⁺,and the C-S group on the surface of the carbon components was formed a strong covalent bond with Pb²⁺,thereby promoting the removal of Pb²⁺.In addition,the removal efficiency of Pb²⁺was still maintained above 85%after five cycles,indicated that MG/S-LDO_0.03was an excellent adsorbent for purifying dye wastewater.