Competitive Adsorption Of Cd(?) And Pb(?) On Goethite And Cd-music Modeling

With the rapid economic development,soil heavy metal pollutions are increasingly serious,particularly the pollutions of Cd and Pb,and they are often presence simultaneously in natural.Goethite is a common crystalline iron oxide in soil.It deeply affected the adsorption,mobility,bioavailability and toxicity of Cd and Pb in soil with the stability structure and many types'surface active sites.Previous studies have identified the adsorption behavior and mechanism of Cd and Pb on goethite,but little is known about the competitive behavior and mechanism.The Charge Distribution-Multi Site Ion Complexation?CD-MUSIC?model can accurately simulate and predict the interaction of Cd and Pb on goethite in single system,and thus reveals its species distribution characteristics.In this study,we take goethite as the research object;its basic properties are characterized by XRD,TEM and potentiometric titration.By studying the interaction of Cd or Pb on goethite in single system,the adsorption of Cd or Pb on goethite of CD-MUSIC model is constructed.The model is used to analyze the competition adsorption behaviors and species distribution characteristics of Cd and Pb in the competition system,and further to expand the application of CD-MUSIC model.The main conclusions obtained are as follows:1.Adsorption results of Cd,Pb on goethiteAs the pH and the concentration of Cd/Pb increased,the adsorption amount of Cd/Pb increased.The surface sites of goethite gradually proton dissociated with pH increasing,and caused the negative charge and the electrostatic attraction increased which resulted of the adsorption of Cd/Pb increased.The addition order of Cd and Pb does not affect its adsorption behavior on goethite.When Cd was first added and then Pb,the adsorbed Cd could be replaced by Pb;When Pb was first added and then Cd,the adsorbed Pb could not be replace by Cd;when Pb and Cd was added simultaneously,Pb would be adsorbed preferentially by the surface sites of goethite.2.The surface charge characteristics of goethiteThe MUSIC model was fit well of the surface charge of goethite,the optimized affinity constants for the electrolyte ions was LogK_K+=-1.61,LogK_NO3-=-0.68,and the Stern layer capacitance C was 1.80 F/m².3.Fitting of Cd and Pb adsorption by CD-MUSIC ModelSingle system:The obtained LogK of each adsorption species were obtained by the optimized charge distribution f,the values were as follows:f of[?FeOH?₂Cd]⁺was1.05/0.95,LogK was 7.3;f of[?FeOH?₂CdOH]⁰ was 1.05/-0.05,LogK was 12.8;f of[?FeOH?₂Pb]⁺was 1/1,LogK was 8.6;f of[?FeOH?₂PbOH]⁰ was 1/0,LogK was 15.0;f of[?FeOH?₂Pb?Fe₃O?]^0.5+was 1.1/0.9,LogK was 11.2;f of[?FeOH?₂PbOH?Fe₃O?]^0.5-was1.1/-0.1,LogK was 16.8.Competition system:The CD-MUSIC model can accurately predict the adsorption of Cd and Pb in the competition system at pH<6.3 or the 0.1 mmo/L Cd/Pb;however,at pH>6.3 and 0.2 mmo/L Cd/Pb,the model predicted values did not match the experimental values.4.Species distribution characteristicsSingle system:[FeOH)₂Cd]⁺is the main species of Cd adsorbed on goethite at acidic conditions.At pH<4.0,the mainly adsorbed species of Pb was[?FeOH?₂Pb]⁺;at pH ranged from 4.0 to 6.0,[?FeOH?₂Pb?Fe₃O?]^0.5+was the mainly adsorbed species.Competing system:almost 100%Cd adsorbed by[?FeOH?₂Cd]⁺species at low pH;and mainly was[?FeOH?₂CdOH]⁰ at high pH.The concentration of Cd/Pb has no effect on the Cd adsored species in the competition system.The adsorbed Pb was mainly formed[?FeOH?₂Pb]⁺at pH<4.0,and[?FeOH?₂Pb?Fe₃O?]^0.5+at pH 4.0-6.0.With pH increased continues,the species of Pb adsorbed on goethite were influenced by the concentration of Pb:at 0.1 mmol/L Pb and pH 6.0-7.0,[?FeOH?₂Pb?Fe₃O?]^0.5+was the main adsorbed species;at pH>7.0,the dominant adsorption species was[?FeOH?₂PbOH?Fe₃O?]^0.5-.At0.2 mmol/L Pb and pH 6.0-7.0,the mainly adsorbed species of Pb was[?FeOH?₂Pb?Fe₃O?]^0.5+,and the[?FeOH?₂PbOH?Fe₃O?]^0.5-and[?FeOH?₂PbOH]⁰ increased;when pH>7.0,[?FeOH?₂PbOH?Fe₃O?]^0.5-and[?FeOH?₂PbOH]⁰ were the main adsorbed species on goethite.