Inorganic Pollutants Removal By Expanded Graphite Loaded With Zero-valent Iron: Performance And Mechanism

With the rapid development of modern industry and urbanization,the pollution of water has become more and more serious.Main pollutants in wastewater include the heavy metal,nitrate and other typical inorganic pollutants.These pollutants can make the organism mutations,and even lead to cancer deformity,which could cause great harm to human health.Removal of inorganic pollutants from wastewater can be achieved by various methods including electrochemical treatment,adsorption,ion exchange,and so on.However,many of these methods are not practical because of drawbacks such as low removal rates,secondary pollution,high cost,or long operating times.Zero-valent iron(ZVI)has been proven to be an effective reagent for the removal of pollutants from wastewater because of its high activity and good availability.However,the undesirable precipitation of ferrous hydroxide on the surface of ZVI,which impedes the contact of the pollutants and ZVI by blocking the reactive ZVI sites,results in a low efficiency for ZVI,especially at neutral and alkaline pH.In this research,we developed ZVI-modified expanded graphite(EG)using natural flake graphite as a precursor and evaluated its ability to remove inorganic pollutants from aqueous solutions.We characterized the EG-ZVI composite before and after reaction with inorganic pollutants by X-ray diffraction(XRD),Brunauer-Emmett-Teller(BET)analysis,Fourier-transform infrared(FTIR)spectroscopy,scanning electron microscopy(SEM),and X-ray photoelectron spectroscopy(XPS)to illustrate the structure of EG-ZVI and the reaction mechanism,with the aim of developing a better understanding of the properties of the EG-ZVI composite.The main results of this study are as follows:1,The EG-ZVI had a high specific surface area and consisted of sub-micron sized particles of zero-valent iron.However,a small amount of zero-valent iron is oxidized,which is attributed to the long exposure to the air before XRD characterization.2,The results showed that the Cr(?)removal rate was 98.80%for EG-ZVI,which was higher than that for both EG(10.00%)and ZVI(29.80%).Furthermore,the removal rate of Cr(?)by EG-ZVI showed little dependence on solution pH within a pH range of 1-9.Even at pH11,a Cr(?)removal efficiency of 62.44%was obtained after reaction for 1 h.The galvanic cell effect and the high specific surface area could enhance the removal of Cr(?)by EG-ZVI with chemical reduction and physical adsorption,respectively.3,The results showed that the removal rate of Pb(?)was 1.96 times higher than that for EG.Galvanic cell effect and high specific surface area are the main reasons for the enhanced removal of Pb(?)by EG-ZVI with chemical reduction and physical adsorption.And first-order kinetic model is well fitted.XPS were used to analyze the mechanisms of Pb(?)removal,which indicated that ZVI loaded on the surface was oxidized,and the removed Pb(?)was reduced to Pb and then generated its oxides.In addition,EG-ZVI could overcome the shortcomings of ZVI which formed undesirable precipitation of ferrous hydroxide on the surface during reaction frequently.High removal capability,excellent stability and broad applicability make EG-ZVI ideal candidates for Pb(?)removal in practical application.4,The results showed that the removal rate of nitrate was 2.3 times higher than that for EG.Furthermore,the removal of nitrate by EG-ZVI shows little dependence on solution pH.Even at pH 9,the nitrate removal efficiency could attain 65%after reaction for 90 min.Galvanic cell effect and high specific surface area are the main reasons for the enhanced removal of nitrate by EG-ZVI with chemical reduction and physical adsorption.And third-order kinetic model is well fitted.XPS were used to analyze the mechanisms of nitrate removal,which indicated that ZVI loaded on the surface was oxidized,and the removed nitrate was reduced to NH4+-N and NO2--N.In addition,EG-ZVI could overcome the shortcomings of ZVI which formed undesirable precipitation of ferrous hydroxide on the surface during reaction frequently.High removal capability,excellent stability and broad applicability make EG-ZVI ideal candidates for nitrate removal in practical application.