Study On The Adsorption Of Hydrated Lead(Ⅱ) On The Surface Of Doped Kaolinite By Density Functional Theory

The adsorption treatment of heavy metal wastewater is a effective and economical method.Kaolinite has a large specific surface area and structural channels,the surface contains active groups and permanent charges,and the materials easy to obtain with a low price.It has become the main adsorption material of wastewater treatment.At present,the research of lead-bearing adsorption of kaolinite is mainly based on experiments,and the calculation methods based on quantum mechanics to study the surface properties and adsorption mechanism of kaolinite are few.In this study,using Materials Studio（MS）software,based on the first-principles method of plane wave pseudopotential density functional theory,the influence of aluminum hydroxyl surface were researched for the single-doped iron（Fe）,magnesium（Mg）,calcium（Ca）and double-doped（FeMg、FeCa、MgCa）surface of kaolinite（001）.Pb（H₂O）₆²⁺model was constructed and analyzed its properties by density functional theory;Pb（H₂O）₆²⁺was adsorbed on the pure surface and single doped and double doped（001）kaolinte aluminum hydroxyl surface.The structure bind with the surface in out-layer and monodentate and bidentate way.It is of great significance to study the adsorption mechanism of lead ions in doped kaolinite wastewater from the microstructure,to master the adsorption properties of kaolinite,to improve its adsorption performance by modification,and to broaden the application and development of kaolinite materials.The main conclusions of the research are as follows:（1）Whether it is single atom（Fe、Mg、Ca）or double atoms（FeMg、FeCa、MgCa）doped（001）kaolinte aluminum hydroxyl surface,the doped atoms form new chemical bonds with the surrounding six O atoms.Only the surface of the iron-doped has no residual charge,the other doping methods introduce negative charge.The population values of the Fe-O bond and the Ca-O bond are both positive values,which show an attractive effect;the population value of the Mg-O bond is a negative value,and the absolute value is the largest,which shows a repelling effect,and the covalentity is the most strong.Both Mg and Ca single doping strengthen the covalentity of the surface,while Fe weakens the covalentity of the surface.After double doping,the covalentity of the Fe-O bond has been enhanced,while the covalentity of the Mg-O bond is significantly weaker,while the covalentity of the Ca-O bond has not changed significantly.（2）In water,there are many forms of lead ions,including ionic structure,hydration structure,hydrolytic complex structure,etc.Pb（H₂O）₆²⁺is the main hydrate of lead,and this study takes it as the adsorption target.The Pb（H₂O）₆²⁺model was constructed and analyzed by density functional theory.The 6s,6p,and 5d orbits of Pb accounted for 1.93,1.9,and 10electrons,respectively,with a total charge of 0.08.The combination of Pb and 6 H₂O is mainly due to the contribution of its 5d orbit.Analysis of Pb-O bond overlapping population value,the Pb-O₂ bond length 2.6526 is the shortest,and its covalentity is the strongest;and the Pb-O₆bond length 3.1679 is the longest,the covalentity is the weakest.（3）The outer layer of Pb（H₂O）₆²⁺on the surface of pure-kaolinite mainly relies on hydrogen bonding.For the monodentate complex adsorption on the pure-surface,the O₆₁position was found to be more conducive to Pb（II）coordination by monodentate complex adsorption.In the bidentate complex adsorption on the pure surface,it was found that the removal of H₁₈H₃₁1 on the surface was more favorable to surface adsorption than the removal of H₁₈H₁₆.Comparing the adsorption energy and the formation of hydrogen bonds,it was found that monodentate complex adsorption is more likely to occur than bidentate complex adsorption.In water,monodentate complex adsorption is more common,followed by bidentate complex adsorption and outer layer adsorption.（4）The adsorption of Pb（H₂O）₆²⁺on the outer layer,monodentate complex and bidentate complexesof the single doped surface of Fe,Mg and Ca.Adsorption on different single-doped surfaces has little difference in adsorption energy.Compared with pure surface adsorption,the adsorption energy increases slightly.After the surface is doped with Mg and Ca,the overall state density of Pb moves to the high-energy direction.The adsorption energy of bidentate complex adsorption is smaller than that of monodentate complex adsorption.The adsorption energy of H₁₈8 H₃₁ on the surface is smaller than that of H₁₈8 H₁₆.The Pb-O population formed by adsorption is smaller,indicating that the covalentity of the bonding is weak.（5）The outer layer,monodentate complex and bidentate complex adsorption of Pb（H₂O）₆²⁺on the double-doped surface of FeMg,FeCa and MgCa.The adsorption energy of monodentate complex adsorption is greater than that of bidentate complex adsorption,and the outer-layer adsorption energy is significantly smaller,indicating that on the double-doped surface,the monodentate complex adsorption is also dominant.Compared with the single-doped surface,after the double-doped surface is adsorbed,the state density of the surface does not change significantly,and the overall Pb state density movement range increases significantly;the population value also changes more significantly.After Ca doping,the surface Price.In the monodentate complex adsorption on the surface doped with FeMg and FeCa,the Pb-O₆₁ bond population is smaller than the pure surface adsorption.