导　　师： 夏树伟

学科专业： 0707

授予学位： 博士

作　　者： ;

机构地区： 中国海洋大学

摘　　要： 重金属离子具有生物累积性，很难被代谢降解，易通过食物链富集，尤其是通过污染的水体环境，直接威胁人类的健康和生存。吸附法是处理含重金属离子水体的一种经济有效的方法。天然矿物高岭石具有较大的比表面积，且表面含有大量活性基团和永久性负电荷，是去除重金属有害元素较为理想的低成本吸附剂。当前有关重金属离子在高岭石表面的吸附研究主要以实验为主，相关的光谱学数据和量子化学计算报道较少，具体的吸附机理尚不明确。本文采用CASTEP软件，基于密度泛函理论的广义梯度近似平面波赝势法，结合周期平板模型，通过计算重金属吸附物种的结合能以及与实验光谱数据进行比较，研究了水体环境中三种典型重金属离子Pb/(II/)、Cu/(II/)和Zn/(II/)在高岭石（001）晶面的化学吸附产物的结构和稳定性，并探索了相应成键机理，主要研究结果如下： 1.第一性原理密度泛函理论（DFT）研究表明，Pb/(II/)的最大可能水合数为8；其中水合数6为半方位（Hemi-directed）构型和全方位（Holo-directed）构型的过渡配位数；由键长、结合能以及Mulliken电荷分析，可知水合数6，7和8均有可能存在。结合第一性原理分子动力学的Pb–O径向分布函数以及实验扩展X射线吸收精细结构（EXAFS）谱数据得出， Pb/(II/)在水溶液中水合物种主要以Pb/(H2O/)62+形式的偏半方位构型存在，Pb/(H2O/)62+中Pb–O键的共价性较弱，离子性较强，成键机理主要为Pb6p6d与Pb6s–O2p反键态轨道进行耦合。 2.高岭石铝氧八面体（001）表面含“直立”氢原子的氧位（Ou）较含“平伏”氢原子的氧位（Ol）更有利于Pb/(II/)的吸附配位；高岭石表面氧原子可参与形成单齿、双齿和三齿配合物，Pb/(II/)配合物配位数分别为5、4和5，均为半方位构型；当环境pH稍低时，位于“Ou”位的单齿配合物为主要吸附物种，该配合物具有较高的结合能，约比双//三齿吸附配合物结合能高20kcal·mol-1；当环境pH稍高时，位于相邻两个Al原子之上“OuOuOl”位的三齿吸附配合物为主要吸附物种，该吸附物种的结构参数基本与EXAFS数据吻合；高岭石表面“Ol”与Pb/(II/)的水分子配体形成氢键，对配合物的配位数和稳定性起到关键作用。 3.弱碱性水体环境中Pb/(OH/)+水合结构的主要存在形式为Pb/(OH/)/(H2O/)5+，全方位构型。Pb/(II/)与高岭石铝氧（001）面的氧原子形成单齿或双齿配合物，其配位数为3~5，均为半方位构型。高岭石表面“Ou”较“Ol”位更易与Pb/(OH/)+单齿配位，该吸附配合物具有较高的结合能（-43.68kcal·mol-1），为优势吸附物种；高岭石表面位于同一个Al原子上的“OuOl”位可形成双齿配合物。表面Ol与水分子配体形成氢键，对配合物的稳定性起到关键作用。高岭石单齿配合物中Pb–O成键机理主要为Pb6p6d轨道与Pb6s–O2p反键轨道进行耦合，电子转移到反键轨道。双齿配合物“Pb–Ol–H”共配位结构中，受配位氢原子影响， Pb–Ol成键过程成键态电子填充占主导地位。 4.水体环境中PbCl+水合结构的主要存在形式为PbCl/(H2O/)4+，半方位构型。PbCl+与高岭石铝氧（001）晶面的单齿吸附和双齿吸附配合物的配位数为3~5，均为半方位配位构型；高岭石表面“Ou”吸附位较“Ol”位更有利于PbCl+吸附反应的进行，相应吸附配合物具有较高的结合能（-63.87kcal·mol-1），为主要的吸附物种，位于同一个Al原子之上的“OuOl”位双吸附同样可能发生；Pb/(II/)与O或Cl等电负性较强的原子发生作用时，会同时形成Pb6s–O/(Cl/) np成键轨道和反键轨道，并且Pb6p6d与Pb6s–O/(Cl/) np反键轨道之间存在强烈的耦合作用。与铅的其他存在形式Pb2+和Pb/(OH/)+相比，Pb2+在高岭石表面吸附配合物的结合能最高，其次是PbCl+，最后是Pb/(OH/)+。 5.水体环境中Cu/(II/)水合结构的主要存在形式为Cu/(H2O/)62+，为拉长的八面体构型。高岭石铝氧（001）晶面对Cu/(II/)的单齿吸附和双齿吸附均可发生，以单齿吸附为优势吸附构型；单齿和双齿吸附配合物的配位数均为4，视吸附位不同呈平面四边形和四面体两种配位构型；与Pb/(II/)吸附配合物不同，Cu/(II/)在高岭石“Ou”和“Ol”位吸附没有明显差别；Cu/(II/)的水分子配体与高岭石表面铝羟基存在氢键作用，生成“Ol…Hw”和“Ow…Hu”两类氢键，前者对吸附配合物的稳定性起到关键作用；Cu/(II/)与表面O原子的成键机理主要是Cu的sp3d2杂化（平面四边形）或sp3杂化（四面体），杂化轨道进一步与O2p轨道耦合，Cu3d轨道视配位构型不同发生不同方式的能级分裂，Cu/(II/)吸附配合物构型表现出姜-泰勒效应。 6.水体环境中Zn/(II/)水合结构的主要存在形式为Zn/(H2O/)62+，为正八面体构型。高岭石铝氧（001）晶面对Zn/(II/)的单齿吸附和双齿吸附均可能发生，以单齿吸附发生可能性较大；Zn/(II/)在高岭石表面“Ou”位单齿吸附配合物为4或5配位，“Ol”位单齿配合物以及所有可能双齿配合物均为4配位，Zn/(II/)的五配位结构为四棱锥构型，四配位结构为四面体构型；Zn/(II/)的水分子配体与高岭石表面羟基存在氢键作用，对吸附配合物的稳定性起到关键作用；Zn/(II/)与表面O原子的成键机理主要是Zn的sp3d2杂化（五配位，四棱锥构型）或sp3杂化（四配位，四面体构型），杂化轨道进一步与O2p轨道耦合。 论文选取的Pb/(II/)有较宽的配位数范围3~10，具有孤对电子立体活性效应，其配合物存在半方位和全方位两种构型，选取的Cu/(II/)的3d轨道含9个电子，其配合物具有姜-泰勒效应，而Zn/(II/)所有轨道满电子，为一般常见重金属离子。因此，对三种典型重金属离子在高岭石（001）晶面吸附行为的理论研究，有助于为绝大多数重金属离子在高岭石表面的吸附行为提出合理的解释，从而对实验给予有利的指导和参考，具有重要的现实意义。 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.

关 键 词： 高岭石 重金属离子 化学吸附 密度泛函 态密度

分 类 号： [P578.964 X52]

领　　域： [天文地球] [天文地球] [环境科学与工程]