Surface Properties Of Na-montmorillonite (001) And Its Adsorption Of Pb(Ⅱ) Complex By Molecular Simulation

The pollution of some major rivers,such as Hanjiang and Lianjiang,in Chaoshan area has caught many researchers’concerns.Heavy metal pollution that threatened locals’health is the worst part among those issues of river pollution.Montmorillonite,which is a kind of most common clay mineral widely in natural soils and with high specific surface area and remarkable cations exchange capability,is now diffusely used as block layer in nuclear leak area and absorbent in contaminated water with heavy metal.So many methods like x ray diffraction（XRD）,atomic force microscope（AFM）,and surface potentiometric titration（SPT）used in obtaining microcosmic information about montmorillonite structure and the process of absorbing heavy metal cations on its surface.Due to the low resolution of those indirect methods,one can neither acquire transport information in atomic level nor understand the microscopic mechanism about the absorption within montmorillonite.With the rapid development of computer,molecular simulation experiments in computer enable us to obtain micro-dynamic information of the adsorption process of montmorillonite from the most basic principles,such as quantum mechanics.Density functional theory（DFT）that creatively solved many-body Schr（?）dinger equation by substituting electron density for electron wave function and making some outstanding approximations give us a solid method to harvest all electronic structures and its properties without annoying empirical parameters.This article calculated two different kinds of Na-montmorillonites using molecular simulation packages based on DFT.After validating some settings on molecular simulation packages,we also finished lots of geometry optimization task about four different surface structures cleaving from the 001 face on montmorillonite,numbers of Pb²⁺-n H₂O complex structures,and six groups of these former structures within one adsorption model.We can draw three conclusions by means of electronic structures,Mulliken charges,and energies,our work and outcomes are stated as following:1.Build montmorillonite models by using pyrophyllite crystal structure which is the same TOT model;use the outcome of lattice constants of pyrophyllite crystal to test different sets of cutoff energy in molecular simulation packages and final obtain E_cut=410 e V,which is a reasonable set for calculations and can save us a large amount of time;calculate geometry optimization task about TS-MMT and OS-MMT bulk structure and get the lowest energy structure that are little impacted by cations isomorphous substitution on MMT;Na⁺adsorbed onto TS-MMT surface more closer;the bandgap of two kinds of montmorillonites are larger than 3 e V,so they are viewed as insulator.2.In order to investigate the existence form of heavy metal cation Pb²⁺in ideal aqueous solution,we calculate 9 kinds of random structure containing one Pb²⁺cation and different number of water molecules which range from 1 to 9.We figure it out that when water molecules are 5,there have a constable complex structure between Pb²⁺and five H₂O;and when that are 6,[Pb6H₂O]²⁺are inclined to forming double layer complex structure.3.Combine the[Pb5H₂O]²⁺model with the surface models and calculate the lowest energy structures.These models come into 4 categories with 6 groups.We find out that the existence of Na⁺will hinder the adsorption of Pb²⁺complex structure onto the surface,and even help break down the monolayer complex structure into double-layer ones because of the competition between Na⁺and Pb²⁺with Oxygen in water molecules.Under no Na⁺conditions,no matter which kinds of surface are easily binding with the complex structure and have the tendency of increasing the maximal number of coordinated positions.