Removal Of Cr???from Wastewater By Mn Fe₂O₄@Si_O2-CTAB And MnFe₂O₄/Fe???: Efficiency And Mechanism

Chromium exists in two oxidation states,Cr???and Cr???.The toxicity of Cr???is almost 100 times greater than that of Cr???.Cr???is easy to migrate in water,which has serious harm to public health and ecological environment.In recent years,MnFe₂O₄ nanoparticles have attracted increasing attention.As is expected,MnFe₂O₄ nanoparticles show great advantages in fine particle size,relatively high surface area,high catalytic performance,and high saturation magnetization.Cr???is the goal pollutant in this research.MnFe₂O₄ nanoparticles,prepared by hydrothermal method,were modified by silica and cetyltrimethylammonium bromide?CTAB?,which ease the aggregation of MnFe₂O₄ nanoparticles.Then the MnFe₂O₄@SiO₂-CTAB composites were applied to remove Cr???from wastewater.MnFe₂O₄ nanoparticles,prepared by chemical coprecipitation method,were incorporated with Fe???to remove Cr???,which can overcome the drawbacks of MnFe₂O₄ and Fe???on Cr???removal.The efficiency of MnFe₂O₄@SiO₂-CTAB composites and MnFe₂O₄/Fe???system on Cr???removal were tested in batch experiments,respectively.X-ray diffraction?XRD?,BET specific surface area?BET?,vibrating sample magnetometer?VSM?,scanning electron microscopy?SEM?,transmission electron microscopy?TEM?,Fourier transform infrared spectroscopy?FT-IR?,and X-ray photoelectron spectroscopy?XPS?were used to characterize the crystalline structure,morphology,specific surface area,pore structure,magnetism,functional groups,element content,and valence state of the samples before and after reaction,respectively.At last,the Cr???removal mechanism by MnFe₂O₄@SiO₂-CTAB composites and MnFe₂O₄/Fe???system were proposed respectively.The main conclusions of the study are as follows:?1?MnFe₂O₄@SiO₂-CTAB was a magnetic mesoporous composite which displayed a core-shell structure.With the increase of Mn Fe2O4@SiO₂-CTAB dosage,the removal efficiency of Cr???increased under acidic condition.When the dosage increased to 2.8 g/L,at pH0 3.0,more than 90% of Cr???in wastewater was removed in 10 min and more than 99% of Cr???was removed in 3 h.The removal efficiency of Cr???by MnFe₂O₄@SiO₂-CTAB composites was high in sixth cycle.The adsorption of Cr???onto MnFe₂O₄@SiO₂-CTAB followed the Freundlich isotherm model and pseudo-second-order model.Chemisorption was the rate-controlling step of the adsorption process of Cr???.?2?MnFe₂O₄@SiO₂-CTAB composites can remove Cr???efficiently and quickly from wastewater.There were two reasons for this.First,MnFe₂O₄@SiO₂-CTAB composites with abundant CTA+ could adsorb Cr???from aqueous solution as the HCrO4-anions can combine CTA+ cations and form ion-pairs.Second,with the assistance of H+,the electrons transferred from ?Mn?II?species of Mn Fe2O4 to Cr???,and thus Cr???was reduced to Cr???.The generated Cr???was immobilized on the surface of MnFe₂O₄@SiO₂-CTAB by surface complexation and precipitation.?3?MnFe₂O₄,prepared by chemical coprecipitation method,was a magnetic mesoporous material.The specific surface area of MnFe₂O₄ is 174 m2/g.In MnFe₂O₄/Fe???system,with the increase of Fe???concentration,the removal efficiency of Cr???increased obviously.At pH 3.0,5.0,7.0,and 9.0,when Fe???concentration increased to 0.5 mM,the removal efficiency of Cr???could reach 98%¹00%.MnFe₂O₄/Fe???system achieved an efficient removal of Cr???over a wide pH range.The efficiency of MnFe₂O₄/Fe???system on Cr???removal was better than that of MnFe₂O₄ system and Fe???system.The inhibiting effect of phosphate on Cr???removal was much bigger than that of sulfate.?4?The mechanism of Cr???removal by Mn Fe2O4/Fe???system included two aspects of reduction and adsorption.First,Cr???was reduced to Cr???by ?Mn?II?on MnFe₂O₄ and Fe???,and the generated Cr???was deposited on the surface of MnFe₂O₄ by precipitation of Cr???–Fe?III?hydroxides/oxyhydroxides.Second,H+ ions were produced from the hydrolysis of Fe?III?which generated in the reaction.Then,Mn Fe2O4 surface was protonated so that the surface charge of MnFe₂O₄ was positive.The negatively charged oxyanionic Cr???was adsorbed by MnFe₂O₄ via electrostatic attraction,which further facilitated the removal of Cr???.