Study On The Treatment Of Heavy Metal Wastewater By Fe₃O₄@Ce-Zr Oxide Composites And Ferrihydrite-humic Acid Co-precipitates

In natural water and soil,chromium mainly exists in the form of Cr（Ⅵ）and Cr（Ⅲ）,and antimony mainly exists in the form of Sb（Ⅲ）and Sb（Ⅴ）.The toxicity of Cr（Ⅵ）and Sb（Ⅲ）is much higher than that of Cr（Ⅲ）and Sb（Ⅴ）.Nowadays,chromium and antimony not only cause serious pollution to the environment,but also potentially threaten people’s health.Iron（hydr）oxide are widely found in soil,sediment and aquatic environment.Iron（hydr）oxide has the advantages of large specific surface area,natural availability,low price,eco-friendliness,and more functional groups and active sites on the surface.Both magnetite and ferrihydrite are widely used iron oxides and are often used to treat soil or water contaminated by heavy metals.In this study,cerium-zirconium bimetallic oxide-supported magnetite（Mag@Ce-Zr）and ferrihydrite-humic acid co-precipitates were prepared by chemical precipitation to treat Cr（Ⅵ）and Sb（Ⅲ）wastewater and explore the effects of various experimental factors on the removal of Cr（Ⅵ）and Sb（Ⅲ）by Mag@Ce-Zr.Besides,The redistribution behavior of Cr（Ⅵ）in the aging process of ferrihydrite-humic acid co-precipitates was studied.X-ray diffractometer,surface area analyzer,transmis sion electron microscopy coupled with energy dispersive X-ray spectroscopy,Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy were used to investigate the structure and morphology of the samples before and after the reaction and the removal mechanism of Cr（Ⅵ）and Sb（Ⅲ）.The main conclusions drawn are as follows:（1）The specific surface area of Mag@Ce-Zr is 133.3 m²/g,and the maximum adsorption capacity for Cr（Ⅵ）and Sb（Ⅲ）are 66.7 mg/g and 365.2 mg/g,respectively.The composite material has a saturation magnetization value of 43.8 emu/g,and the removal efficiency of Cr（Ⅵ）and Sb（Ⅲ）can still reach 82.0%and 90.0%after 6 cycles of recycling,reflecting its excellent magnetic separation and recycling performance.Acidic conditions are conducive to the removal of Cr（Ⅵ）by Mag@Ce-Zr,and the presence of SO₄^2-and PO₄^3-will have a negative impact on the adsorption of Cr（Ⅵ）.For Sb（Ⅲ）adsorption,Mag@Ce-Zr shows good Sb（Ⅲ）adsorption performance in a wide p H range and in the presence of coexisting ions.（2）The adsorption of negatively charged Cr（Ⅵ）by Mag@Ce-Zr is mainly through electrostatic interaction and surface complexation,while the adsorption of uncharged H₃Sb O₃is mainly through the formation of Ce/Zr-O-Sb complex.Mag@Ce-Zr can convert highly toxic Cr（Ⅵ）and Sb（Ⅲ）into less toxic Cr（Ⅲ）and Sb（Ⅴ）through redox reaction.（3）Co-precipitated sodium humate will inhibit the transformation of ferrihydrite during the aging process.Co-precipitated humic acid can not only promote the fixation of chromium through complexation,but also reduce the complexed Cr（Ⅵ）to Cr（Ⅲ）through the reducing polysaccharide group.（4）The concentration of ferrous ions is the main factor that affects the removal and immobilization of Cr（Ⅵ）during mineral aging.Ferrous ions induced ferrihydrite to accelerate the conversion to hematite and goethite,and then promoted the incorporation of non-desorbable chromium into secondary minerals.The way for chromium to be incorporated into minerals is as follows:Cr（Ⅵ）adsorbed on the surface of ferrihydrite and ferrihydrite-humic acid co-precipitates enters hematite through structural incorporation during the conversion process,which can be further promoted in the presence of ferrous iron.Under neutral conditions,when the initial concentration of ferrous iron is low（0.2 m M）,the residual Cr（Ⅵ）in the aqueous solution can be incorporated into the Cr（Ⅲ）-Fe（Ⅲ）co-precipitates formed in the secondary minerals.Finally,Cr（Ⅵ）incorporated into the secondary minerals is reduced and complexed by the co-precipitated humic acid to further realize the immobilization of Cr.