Mechanism Of Lead Adsorption On Goethite-birnessite Binary System And Modeling

Iron(hydr)oxides and manganese oxides are the active components in soils,with the large specific surface and variable surface charge properties,which can adsorb and fix heavy metal ions in soils effectively.Iron oxides and manganese oxides have close contact in soils.The concomitant body of them in soils such as iron manganese film,iron-manganese nodules can also enrich a lot of heavy metal ions.However,the interaction mechanism between iron(hydr)oxide and manganese oxide and its influence on the heavy metals adsorption are still ambiguous.Goethite is the most thermodynamically stable and abundant iron oxyhydroxide and birnessite is the typical manganese oxide in soils.In this study,goethite and birnessite were prepared in laboratory as the research material.X-ray diffraction(XRD),automatic specific surface tester,scanning electron microscope(SEM)and potentiometric titration methods were used to characterize their basic properties.On the basis of Fe and Mn content in soils,the binary systems were prepared by mixing goethite and birnessite in different ratios.The binary system with the Fe/Mn mole ratio 1,10,20,50,80 are defined as GB1,GB10,GB20,GB50,GB80 respectively.The interaction mechanism between those two minerals in the binary system and the binding characteristics of Pb2+ to goethite,birnessite and the binary systems were investigated by using a combination of batch adsorption experiment,X-ray absorption fine structure spectroscopy(XAFS)and charge distribution and multisite surface complexation model(CD-MUSIC).Main results are as follows:1.In the GB20,GB50,GB80 binary system,goethite wraped around the surface of birnessite,which can mask the edge surface sites on birnessite.The points of zero charge(PZC)of synthetic goethite and birnessite are 9.1 and 2.7,respectively.These two minerals have opposite charges on the surface at pH5.Goethite wraped around the birnessite tightly in the GB20,GB50,GB80 binary system because of the electrostatic effects,which can make birnessite edge surface sites inactive.2.The density of MnOH1/3-,Mn2O?2/3 and Mn20?2/3-active surface sites on birnessite is much higher than that of goethite.The specific surface area of goethite is 82 m2/g,and the external surface area and the basal plane area of stratiform birnessite are 90 and 824 m2/g,respectively.The fitting results of the MUSIC model show that the density of surface site FeOH0.5-and Fe3O0.5-on goethite,respectively,are 0.47 and 0.37 mmol/g,while the density of surface site MnOH1/3-,Mn2O?2/3 and Mn20?2/3-on birnessite are 2.1,2.1 and 4.1 mmol/g,respectively.3.The overlap effect of diffusion double layer between birnessite particles in the high concentration birnessite suspension,will decrease the adsorption of protons.At pH 4,pH 5,pH 6,after adding 10 mL 10 g/L birnessite suspension to 50 mL 2 g/L goethite suspension,the amounts of proton adsorption are 13.74,1 1.51,and 6.0 ?mol,respectively.However,when 10 mL 10 g/L goethite suspension was added to 50 mL 2 g/L birnessite suspension,the amounts of proton adsorption are only 1.9,2.46,and 2.5 ?mol,respectively.There is a higher adsorption capacity of protons at lower concentration birnessite in the system.4.In acidic condition,the Pb2+ adsorption ability of pure birnessite outclasses that of the goethite.There is differences between the adsorption amount of Pb2+ on binary system and the linear superposition of that on pure system.At pH 5,when the Pb2+ equilibrium concentration is 100 mg/L,the adsorption amount of Pb2+ on birnessite is 268.33 mg/g,while the adsorption amount of Pb2+ on goethite is only 14.23 mg/g.The adsorption amount of Pb2+ on birnessite is about 19 times larger than that of Pb2+ on goethite.As the birnessite proportion increases,the adsorption amount of Pb2+ on the binary system enhanced,and the adsorption amount of Pb2+ increases with increasing pH.The adsorption amount of Pb2+ on both GB1 and GB10 binary system are higher than the linear superposition Pb2+ adsorption quantity of pure goethite and birnessite,while the Pb2 adsorption amount of GB20,GB50 and GB80 binary system are all higher than the linear superposition Pb2+ adsorption amount of pure goethite and birnessite.5.CD-MUSIC model can describe the adsorption behavior of Pb2+ on goethite,birnessite and goethite-birnessite binary system well.In the pure system,tridentate corner-sharing complexes[(FeOH)2Pb(Fe30)]0 5+and[(FeOH)2PbOH(Fe30)]0.5-are the main complexes of Pbo+adsorption on the surface of goethite.On the birnessite surface,Pb2+ is mainly adsorbed to the birnessite basal plane above vacancy sites as a tridentate corner-sharing complexes[(Mn2O?)3Pb]0.These fitting results are consistent with the XAFS results.In the binary system GB1 and GB10,Pb2+is mainly adsorbed to the birnessite surface.[(Mn2O?)3Pb]0 and[(MnOH)2PbOH]1/3+are the two main complexes species.In the binary system GB20,GB50 and GB80,goethite can mask birnessite edge surface sites.Pb2+is mainly adsorbed to the binary system as[(Mn2O?)3Pb]0 and[(FeOH)2Pb]+.With decreasing amount of birnessite,[(Mn2O?)3Pb]0 complex species decrease,and[(FeOH)2Pb]+and[(FeOH)2Pb(Fe3O)]0.5+complex species increase.