Adsorption Of Hydrated Lead(Ⅱ), Copper(Ⅱ) And Zinc(Ⅱ) On The Kaolinite(001) Surface

Heavy metals have become a severe public health problem recently because of theirnon-biodegradable and persistent nature, especially in the contaminated waterenvironment. Adsorption has been considered as one of the most economical andeffective methods for the removal or immobilization of heavy metal ions. Kaolinite, akind of natural clay minerals, has been widely used as adsorbent due to its low cost,large surface area, good cation exchange capacity and the presence of large amountsof active hydroxyl groups. Removal of heavy metal ions with kaolinite wasinvestigated experimentally mainly by batch tests, scarcely by spectroscopictechniques and quantum chemistry calculations. The corresponding adsorptionmechanism has not been well understood yet. A periodic density functional study ofPb(II), Cu(II) and Zn(II) adsorption on the basal octahedral Al (Al(o))(001) surfaceof kaolinite in the water environment has been explored in this work, based on thePerdew-Burke-Ernzerhof generalized gradient approximation (GGA-PBE) approachwithin the CASTEP code. The main results are as follows:1. Optimization of all possible hydrated species of Pb(II)(Pb(H2O)1-92+) shows thatthe maximum coordination number (CN) of aqueous Pb(II) is8. Determination ofPb–O bond length, binding energy and Mulliken charge population of equilibriumgeometries of Pb(H2O)1-82+indicate that the primary hydration numbers of6,7and8are all possible, and the geometry of Pb(H2O)62+is found to be the transition statebetween the holo-directed and the hemi-directed. Based on the equilibrium structuralparameters of Pb(H2O)1-82+, relaxation of Pb(H2O)52+in aqueous solution and theEXAFS (extended X-ray adsorption fine structure) data, the most probablecoordination number of the first hydration shell of aqueous Pb(II) is6of a complexwith approximately hemi-directed structure. Bonds of Pb–O in Pb(H2O)62+exhibitstrong ionicity with few covalent character. Pb6p6d coupling with the Pb6s–O2p antibonding states is the primary orbital interaction of Pb(II) with oxygen.2. Surface “Ou” site with “up” hydrogen is more favorable for Pb(II) binding thanthe “Ol” site with “lying” hydrogen. All complexes in mono-, bi-and tridentate modesexhibit hemi-directed geometry with CN of5,4and5, respectively. Monodentatecomplex of “Ou” site is found the most energetically favorable, with binding energy ofabout20kcal·mol-1higher than the values of bi-and tridentate complexes. It shouldbe the major species of Pb(II) complexes at relatively low pH region. A tentativecomparison with available EXAFS results for Pb(II) adsorption shows qualitativeagreement with the tridentate complex of “OuOuOl” site on two neighboring Alcenters, which seems the likely type at high pH conditions. The hydrogen bondinginteraction of surface “Ol” with “H” of aqua ligands acts as the key factors indetermining CN of Pb(II) and stability of complex.3. Pb(OH)(H2O)5+is found the most probable species of Pb(OH)+in slightlyalkaline aqueous system with holo-directed geometry. Pb(OH)+can bind with thekaolinite Al(o)(001) surface in mono-or bidentate way. All the mono-and bidentatecomplexes exhibit the hemi-directed geometry with coordination number of3to5.Site of “Ou” is more favorable for monodentate complex than site of “Ol”.Monodentate complexation of “Ou” site with a high binding energy of-43.68kcal·mol-1should be the most preferred adsorption mode, while bidentatecomplexation on “OuOl” site of single Al center is also probable. Stability ofadsorption complex is found closely related to the hydrogen bonding interactionbetween surface Oland H in aqua ligands of Pb(II). Mulliken population and densityof states (DOS) analysis show that Pb6p6d coupling with the Pb6s–O2p antibondingstates is the primary orbital interaction of Pb(II) with surface oxygen. Hydrogencomplexation occupies a much large proportion in the joint coordination structure ofbidentate complex, where the bonding state filling predominates for Pb–Olinteraction.4. PbCl(H2O)4+is found the most probable species of PbCl+in the waterenvironment with hemi-directed geometry. Both mono-and bidentate PbCl+complexes on kaolinite Al(o)(001) surface exhibit the hemi-directed geometry with coordination number of3to5. Monodentate complex of Pb(II) prefers the surface siteof “Ou” to “Ol”. Monodentate complexation of “Ou” site with a high binding energy of-63.87kcal·mol-1should be the most likely adsorption mode, while bidentatecomplexation on “OuOl” site of single Al center is also probable. Stability ofadsorption complex is found closely related to the hydrogen bonding interactionbetween surface Oland H of aqua ligands. Mulliken population and DOS analysisindicate that interaction of Pb(II) with strongly electronegative O or Cl produces boththe bonding orbitals and antibonding orbitals of Pb6s–O(Cl) np, and coupling of Pb6p6d with the antibonding states occurs simultaneously. Based on the value of bindingenergy, the order of Pb(II) species on the kaolinite Al(o)(001) surface is Pb2+>PbCl+> Pb(OH)+.5. Cu(H2O)62+is found the most probable species of Cu(II) in the waterenvironment with distorted octahedral geometry. Both monodentate and bidentateCu(II) complexes can be formed on the kaolinite Al(o)(001) surface, with themonodentate way more easily to occur. All complexes exhibit the CN of4withquadrangular or tetrahedronal geometry. Being different to the Pb(II) adsorptioncomplexes, sites of “Ou” and “Ol” show similar affinity to the Cu(II) ion. Two typesof hydrogen bonding interactions are formed between the aqua ligands of Cu(II) andsurface hydroxys of kaolinite, surface “Ol” with H of aqua ligands (denoted as“Ol…Hw”) and O of aqua ligands with H of surface “OuH” groups (“Ow…Hu”), wherethe “Ol…Hw” type is relatively strong and plays a key role in determining the stabilityof adsorption complex. Couplings of O2p with the sp3d2and sp3hybrid orbitals arethe main interactions of Cu(II) with surface oxygen, corresponding to respectively thequadrangular or tetrahedronal structures. The Jahn-Teller effect is involved in theCu(II) complexes as it has Cu3d9electronic configuration, which describes thegeometrical distortion of compounds from the point of decreased orbital degeneracy.6. Zn(H2O)62+is found the most probable species of Zn(II) in the waterenvironment with octahedral geometry. Zn(II) binds with the kaolinite Al(o)(001)surface in either monodentate or bidentate way, with the monodentate mode more easily to occur. Monodentate complex of “Ou” site exhibits CN of4or5withtetrahedronal and quadrangular pyramidal geometry, respectively. All the bidentatecomplexes and monodentate complex of “Ol” site feature CN of4with tetrahedronalstructure. Hydrogen bonding interactions between the aqua ligands of Zn(II) andsurface hydroxys of kaolinite act as a key factor in determining the stability ofadsorption complex. Couplings of O2p with the sp3d2and sp3hybrid orbitals are themain interactions of Zn(II) with surface oxygen, corresponding to respectively thequadrangular pyramidal or tetrahedronal structures.The heavy metal ions of Pb(II), Cu(II) and Zn(II) studied in the work are carefullyselected and very representative as they can stand for most of the divalent heavy metalions. Pb(II) has “stereochemically active lone pair of electrons” and is able to bindwide families of ligands (3-10) in very flexible coordination modes with holo-directedor hemi-directed coordination geometries. Geometries of Cu(II) complexes areaffected by the Jahn-Teller effect as they have the Cu3d9electronic configuration,while Zn(II) acts as one kind of common metal ions with all of its orbitals fully filled.Therefore, this study provides valuable insights into the structure and bondingmechanism of Pb(II), Cu(II) and Zn(II) adsorption on the Al(o)(001) surface ofkaolinite from the atomic level, which may help to interpret the experimental data andimprove our understanding of heavy metal ion adsorption. Furthermore, it may act asthe model reference for the adsorption of other metal ions on the (001) surfaces ofkaolinite.