Adsorption Behavior And Mechanism Of Arsenic On Mesoporous Silica Modified By Iron-manganese Binary Oxide (FeMnO_x/SBA-15) From Aqueous Systems

Contamination of arsenic has become an urgent problem globally and attracted attention from researchers all over the world.Adsorption method,because of its features such as convenient operation,high effciency and cost effectiveness,was considered to be an important approach for arsenic treatment.Among all types of adsorbents,iron-manganese binary oxide(FeMnOx)has been proved to be highly efficient for arsenic removal from aqueous solutions.However,FeMnOx is easy to agglomerate and hard to separate,making it difficult to be applied in practical engineering projects.In this paper,the mesoporous silica(SBA-15),with huge specific surface area and high stability,was introduced as a supporting matrix to disperse FeMnOx.The synthesized novel adsorbent(FeMnOx/SBA-15)showed advantages of improved adsorption efficiency and less agglomeration effect over bare FeMnOx.Various characterization techniques such as transmission electron microscopy(TEM),Brunauer-Emmett-Teller(BET)method,X-ray diffraction(XRD)and X-ray photoelectron spectroscopy(XPS)were employed as analyzers to explore the physicochemical properties of the lab-prepared adsorbent.Being calcined under 350? with soaking time of 2h,manganese in the adsorbent remained in amorphous state while iron appeared to be transferring into crystalline state.Compared with bare FeMnOx,FeMnOx/SBA-15 was found to have much higher specific surface area(596 m2 g-1),resulting in more room and opportunities for arsenic adsorption.Comparative experiments of varying loading ratio of FeMnOx and Fe:Mn molar ratio were conducted.It was found that active substance FeMnOx possessed brilliant adsorption behavior when the mass ratio of FeMnOx to FeMnOx/SBA-15 was 45%,and the molar ratio between Fe and Mn was 3:1.Adsorption capacities for As(?)and As(?)reached maximum of 61.29 mg As g-1 FeMnO,and 67.68 mg As g-1 FeMnOx respectively(initial concentration of arsenic=7.0 mg L-1).As(III)adsorption was better described by the Freundlich model while As(V)by the Langmuir model.The removal process of As(?)and As(?)both perfectly matched with pseudo second-order kinetic model.Effects of various anions and organic matter(humic acid)on arsenic removal by FeMnOx/SBA-15 were investigated.Different concentrations of coexisted SO42-and HA had hardly little effect on arsenic removal efficiency,while PO43-competed with arsenic and inhibited its removal significantly.Furthermore,the adsorbent also showed great recyclability and regeneration performance through recycling experiments,which make it a promising material in engineering application.Compare the state of iron in FeMnOx/SBA-15 and its corresponding arsenic removal efficiency,conclusion could be drawn that calcination temperature have great impact on arsenic removal efficiency.The higher calcination temperature,the more well-developed crystalline state,and thus resulting in poor remocal efficiency.Both manganese and iron played important role for arsenic removal.On the one hand,MnO2 acted to oxide As(?)into As(?)and iron(?)had high affinity towards As(V),on the other hand,SBA-15 itself had little influence on arsenic removal and mainly function as a fine favorable carrier to disperse iron and manganese oxides.The pHpzc of FeMnOx/SBA-15 before and after adsorption were recorded and further confimed the theory of surface complexation mechanism.Besides,Suface Complexation Model(SCM)was also adopted to describe the formation of As(?)being adsorbed on surfaces for better understanding.