Studies On The Sorption Of Pb(Ⅱ) On Layered Double Hydroxides

The environmental problems are attracting more and more attention all over the world now.The much pollutant may cause serious harm to ecological safety and human health,and the pollution of heavy metals is the focus of environmental science and technology.The adsorption/desorption behavior of heavy metals at the solid liquid interface is the fundamental which determines the geochemical process of heavy metals and environmental ecological effect.The scientific understanding of the basic phenomenon is the basis of understanding the speciation distribution,transport and fate of heavy metal pollutants and technology on remediation of contaminated soil and waste water treatment.At present most of the adsorption researches are based on the materials with negative charged,but few researches on the interaction of heavy metal cations with positive charged materials are reported.In this article positive charged material layered double hydroxides(LDHs)are chosen as model adsorbents to study the adsorption behavior of Pb(â…¡)on LDHs.The adsorption mechanisms are discussed and we hope to obtain the influence law of structural charge on heavy metal adsorption and deepen the cognition to the adsorption mechanism.In the summary part,firstly,the related contents of heavy metal such as properties and damage are introduced;secondly,the research progresses on macro adsorption experiments and the micro mechanisms are reviewed;thirdly,the basic knowledge of layered double hydroxides is introduced.In the section one of the experiment,Mg-Fe LDH with the formula [Mg_0.72Fe_0.28(OH)₂]Cl_0.06(OH)_0.22is prepared and the sorption behaviors of Pb(â…¡)on Mg-Fe LDH are studied including adsorption isotherm,adsorption kinetics,and various factors that affect the sorption behaviors such as pH,ionic strength.The sorption amount of Pb(â…¡)on Mg-Fe LDH is high and the sorption kinetics can be described by pseudo-second order kinetic model and double exceptional function model successfully.The sorption data can be fitted by Aranovich-Donohue isotherm which belongs to S type isotherm.The Mg-Fe LDH has a pH buffer capacity and the initial pH of the Pb(â…¡)solution has no effect on the final equilibrium sorption amount and the back electrolyte NaNO₃ also has no obvious effect on the sorption amount. Combined with XRD and FT-IR,the mechanisms of Pb(â…¡)sorption on Mg-Fe LDH are surface precipitation which is referred to the basic lead carbonate precipitation and surface complexation including chemical bonding and electric bonding.In the section two of the experiment,four kinds of Mg-Al LDHs with different anions Cl^-,NO₃^-,CO₃^2-,EDTA in the interlayer are prepared.The sorption amount of Pb(â…¡)on Mg₃Al-CO₃ LDH is high and the sorption mechanism is that Pb(â…¡)acted with CO₃^2-in the interlayer in the local basic environment in the LDHs/water interface to form the basic lead carbonate.The sorption amounts of Pb(â…¡)on Mg₃Al-Cl LDH and Mg₃Al-NO₃ LDH are less than that on Mg₃Al-CO₃ LDH and the sorption data can be described by Aranovich-Donohue isotherm.The sorption mechanism is the surface complexation which refers to Pb(â…¡)reacts with the hydroxyl on the surface of LDHs samples.The mechanism of Pb(â…¡)sorption on Mg₃Al-EDTA LDH is the complexation of Pb(â…¡)with EDTA in the interlayer.In the section three of the experiment,Mg-Al-Fe LDHs with the same charge but different Fe content are prepared.The sorption data can be described by Freundlich isotherm and pseuo-first order kinetic model.The mechanism is the surface precipitation of basic lead carbonate.With the increase of Fe content the sorption rate in kinetic experiment and the equilibrium sorption amount in the isotherm experiment increase.