水合 Pb(OH) + 在高岭石 (001) 晶面的吸附机理

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1 May 物理化学学报 (Wuli Huaxue Xuebao) Acta Phys. -Chim. Sin. 2014, 30 (5), [Article] doi: /PKU.WHXB 水合 Pb(OH) 在高岭石 (001) 晶面的吸附机理 王 1,2 娟 1,* 夏树伟 1 于良民 ( 1 中国海洋大学化学化工学院, 海洋化学理论与工程技术教育部重点实验室, 山东青岛 ; 2 青岛农业大学化学与药学院, 山东青岛 ) 摘要 : 采用密度泛函理论广义梯度近似平面波赝势法, 结合周期平板模型, 探讨了水体环境中 Pb(OH) 在高岭石铝氧八面体 (001) 晶面的吸附行为和机理, 确定了吸附配合物的结构 配位数 优势吸附位和吸附类型. 结果表明, 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. 高岭石表面存在含 平伏 氢原子的表面氧 (O l) 位和含 直立 氢原子的氧 (O u) 位, 后者更易与 Pb(OH) 单齿配位, 该吸附配合物具有较高的结合能 ( kj mol -1 ), 为优势吸附物种 ; 高岭石表面位于同一个 Al 原子上的 O uo l 位可形成双齿配合物. 表面 O l 与水分子配体形成氢键, 对配合物的稳定性起到关键作用. Mulliken 布居和态密度分析表明, 高岭石单齿配合物中 Pb O 成键机理主要为 Pb 6p 轨道与 Pb 6s O 2p 反键轨道进行耦合, 电子转移到反键轨道. 双齿配合物 Pb O l H 共配位结构中, 受配位氢原子影响, Pb O l 成键过程成键态电子填充占主导地位. 关键词 : Pb(OH) ; 高岭石 ; 化学吸附 ; 密度泛函理论 ; 配位数 中图分类号 : O647; O641 Adsorption Mechanism of Hydrated Pb(OH) on the Kaolinite (001) Surface WANG Juan 1,2 XIA Shu-Wei 1,* YU Liang-Min 1 ( 1 Key Laboratory of Marine Chemistry Theory and Technology, Ministry of Education, College of Chemistry and Chemical Engineering, Ocean University of China, Qingdao , Shandong Province, P. R. China; 2 College of Chemistry and Pharmacy, Qingdao Agricultural University, Qingdao , Shandong Province, P. R. China) Abstract: The adsorption behavior of Pb(OH) on the basal octahedral (001) surface of kaolinite has been investigated using the Perdew-Burke-Ernzerhof generalized gradient approximation (GGA-PBE) of density functional theory with periodic slab models, where the water environment was considered. The coordination geometry, coordination number, preferred adsorption position, and adsorption type were examined, with binding energy estimated. All the monodentate and bidentate complexes exhibited hemi- directed geometry with coordination numbers of 3-5. Site of O u with up hydrogen was more favorable for monodentate complex than site of O l with lying hydrogen. Monodentate complexation of O u site with a high binding energy of kj mol -1 should be the most preferred adsorption mode, while bidentate complexation on O uo l site of single Al center was also probable. The stability of adsorption complex was found closely related to the hydrogen bonding interactions between surface O l and H in aqua ligands of Pb(II). Mulliken population and density of states analyses showed that coupling of Pb 6p with the antibonding Pb 6s O 2p states was the primary orbital interaction between Pb(II) and the surface oxygen. Hydrogen complexation occupied a much large proportion in the joint coordination structure of bidentate complex, where bonding state filling predominated Received: January 21, 2014; Revised: March 20, 2014; Published on Web: March 21, Corresponding author. shuweixia@ouc.edu.cn; Tel: The project was supported by the National Natural Science Foundation of China ( ) and Natural Science Foundation of Shandong Province, China (ZR2012CQ015). 国家自然科学基金 ( ) 和山东省自然科学基金 (ZR2012CQ015) 资助项目 Editorial office of Acta Physico-Chimica Sinica

2 830 Acta Phys. -Chim. Sin Vol.30 for the Pb O l interaction. Key Words: Pb(OH) ; Kaolinite; Chemical adsorption; Density functional theory; Coordination number 1 Introduction The heavy metal elements, as one of the important pollutants in water environment, have attracted global concern due to their nonbiodegradable and persistent nature. 1,2 The particular toxicity of lead has been well established by the Agency for Toxic Substances and Disease Registry of the U.S. Department of Health and Human Services 3 since the year of Pb(II) is the common oxidation state of lead under normal environmental conditions, and hydrolysis of Pb(II) often occurs at slightly alkaline water environment, where Pb(OH) is one of the most important species. Despite of the abundant biochemical data about Pb(II), much less effort has been devoted to understanding the fundamental aqueous chemistry of Pb(OH) as well as its adsorption behavior on the mineral surface, especially from the molecular or atomic level. Kaolinite, one of the plentiful natural clays, is a 1:1 layered aluminosilicate mineral with a unit cell composition of Al 4Si 4O 10(OH) 8. It has been widely used as the adsorbent for the removal of heavy metals from contaminated groundwater 4,5 due to its high specific surface area, good chemical, and mechanical stability, etc. The removal process for heavy metals takes place either through ion exchange on the permanent negative charge sites usually caused by isomorphic replacement of Si 4 by Al 3 in the silica tetrahedrons, 6 or through adsorption on the ph-dependent variable charge sites (S(OH)) of the alumina face and the crystal edges, 7 or a combination of both. Although a variety of batch tests about Pb(II) uptake by kaolinite have been reported 8,9 with the hydrolysis of Pb(II) considered, the developed empirical or semiempirical surface complexation models are only applicable for the specific experimental conditions, and sometimes they even don't fit to the data well. Also, the individual behaviors of Pb 2 and Pb(OH) can not be distinguished, including the preferred complexation mode, adsorption position, etc. Moreover, Pb(II) usually appears as an intermediate acid (with respect to the hard and soft acids and bases (HSAB) theory 10 ), able to bind wide families of ligands in very flexible coordination modes 11 with hemi-directed or holo-directed coordination geometry. 12 These versatile characters of Pb(II) have further made the adsorption behavior of Pb(OH) on kaolinite surface complicated. Quantum chemistry calculations, especially the density functional theory (DFT) investigations, have become popular to complement experimental data recently. Although a large amount of DFT investigations on the structural and electronic properties of Pb(II) compounds have been carried out, 13 theoretical model about Pb(OH) adsorption on kaolinite surface inclusive of the species and geometry of complex has not been found yet. A first- principles density functional study of Pb(OH) adsorption on the basal octahedral Al (Al(o)) (001) surface of kaolinite was performed in this work. The water environment was considered. The monodentate and bidentate complexes, coordination geometry, and coordination number (CN) of Pb(II) as well as the two types of Al(OH) sites were examined. Feasibilities of different types of complexes were evaluated by the values of corresponding binding energies and the similarity of structure parameters to the experimental data. The related charge population and density of states (DOS) were also analyzed in detail to investigate the Pb O bonding mechanism. 2 Theoretical methods and models 2.1 Computational methods All calculations were performed with plane-wave pseudopotential DFT method utilizing the Cambridge Sequential Total Energy Package (CASTEP) code 14 in Materials Studio 5.0. Structures of the kaolinite bulk and (001) surface models were optimized with Perdew-Burke-Ernzerhof (PBE) 15 functional of the generalized gradient approximation (GGA), as it could describe molecular bonding (including hydrogen bond strength 16,17 ) to greater accuracy than the local density approximation (LDA). Vanderbilt ultrasoft pseudopotential 18 in conjunction with a cutoff energy of 380 ev was adopted throughout. Monkhorst- Pack meshes of k- point sampling 19 were generated with (4 2 3) and (2 2 1) grids for the kaolinite bulk and surface models, respectively. Higher cutoff energy or more refined k- point mesh only caused neglected changes in the results. Atomic positions were optimized by Broyden-Fletcher-Goldfarb- Shanno (BFGS) scheme with Gaussian smearing width of 0.20 ev until the total energy changed, the maximum tolerances of the force, and displacement converged to less than ev atom -1, 0.3 ev nm -1, and nm, respectively. Optimization of kaolinite bulk with chemical composition of Al 4Si 4O 10(OH) 8 was started from the experimental structure reported by Bish. 20 The resulted unit cell parameters were a= nm, b= nm, c= nm and α =91.813, β = , γ =89.778, within the error of 1.6% to the experimental values. Similar results have also been obtained by Hu and Michaelides 21 based on their DFT calculations. Structures of kaolinite and the (001) slab are shown in Fig.1 from a side view. Different from the kaolinite bulk, inner surface hydroxyls in the (001) slab could be clarified into two types: one third lying hydroxyls (O lh) oriented parallel to the kaolinite (001) surface and two thirds upright hydroxyls (O uh) oriented perpendicularly to the surface. The orientation of inner hydroxyls

3 No.5 WANG Juan et al.: Adsorption Mechanism of Hydrated Pb(OH) on the Kaolinite (001) Surface 831 Fig.1 Side view of kaolinite structure with hydroxyl positions indicated (a) and the optimized geometry of (001) slab from the same visual point (b) in the slab also changed, not parallel to the (001) surface as in the kaolinite bulk. As aqueous chemistry of Pb(OH) in solution was not well known, all possible species of Pb(OH) (H 2O) n (1 n 8) were tested in a 1.0 nm 1.0 nm 1.0 nm periodic box with GGA- PBE, ultrasoft pseudopotential, a cutoff energy of 380 ev, and k-point sampling. The same tolerances for self-consistence with total energy of ev atom -1, force of 0.3 ev nm -1, and displacement of nm were adopted to be consistent with the substrate optimization. 2.2 Surface models The kaolinite (001) slab of a single two- sheet layer can be clarified into H-O-Al-O-Si-O six atomic sublayers. Based on the comparison of calculations on one-layer to two-layer models and one-layer models with different numbers of relaxed and fixed sublayers, a (2 1 1) surface unit cell (1.046 nm nm nm) was adopted as the substrate for Pb(OH) adsorption, where the top four sublayers were relaxed and the bottom two sublayers were fixed during the optimization. The same atomic constraint has been used for uranyl adsorption study on (001) surfaces of kaolinite by Kremleva et al. 22 The slabs were repeated periodically in the z direction with a vacuum gap of 1.5 nm. Surface models with Pb(OH) adsorbed were optimized under the same conditions to the clean (001) slab. As shown in Fig.2, four types of surface complexation are explored. Monodentate complex of Pb(OH) on Al(o) surface might form on the O u site with up hydrogen or O l site with lying hydrogen of any Al center since they belong to different types of inner surface hydroxyls, corresponding to configuration A or B, respectively. As the joint coordination of Pb(II) and H to the same surface oxygen center is possible, 23 bidentate complexes located on O uo l site of single Al center (C) and two neighboring Al centers (D) are also tested, where only the joint coordination structure of Pb O l H is considered. The binding energy (ΔE bind), calculated according to the reaction (1) based on the deprotonation of surface hydroxyl groups, is defined as the energy difference between the products and reactants. Pb(OH)(H 2O) n S(OH) 3OH - [S(OH) 2(O)Pb(OH)(H 2O) m] (n-m1)h 2O (1) ΔE bind = E S/Pb (n -m 1)E -E -E -E H2 O OH - S Pb(OH)(H 2 O) n where S stands for the surface of the substrate, n and m are the numbers of H 2O ligands in aqueous Pb(OH) and the adsorption complex, respectively, E S and E S/Pb represent the total energies of clean kaolinite (001) slab and the slab with Pb(OH) adsorbed on the surface, respectively. OH - acts as an example group in alkaline aqueous system, which can react with the missed H of S(OH) 3 here. Bond length of Pb O is taking as the main criteria to evaluate the reasonability of complex structure. In four types of adsorption modes, all possible species of [S(OH) 2(O)Pb(OH) (H 2O) m] with m varying from 1 to 7 are tested. Complex from the DFT optimization having moderate Pb O length of less than 0.37 nm 24 will be retained, otherwise be ruled out. As the water environment is considered in our work, Pb(II) tends to combine aqua ligands as many as possible. So only reasonable complex with maximum value of m is adopted and analyzed. 3 Results and discussion 3.1 Geometries of Pb(OH)(H 2O) n Pb(II) has a [Xe] 4f 14 5d 10 6s 2 6p 0 electronic configuration, where the stereochemically active lone pair of electrons of 6s 2 can take up more space on a specific region of the surface of coordination sphere than a single bond does, 12 and gives rise to the uneven distribution of ligands around Pb(II) ultimately. During the periodic DFT optimizations, one or more aqua ligands of Pb(OH)(H 2O) 6-8 left the first hydration shell with R Pb O>0.37 nm Fig.2 Schematic representation of the monodentate complexes on Ou (A) and Ol (B) sites and bidentate complexes on OuOl site of single Al center (C) and two neighboring Al centers (D)

4 832 Acta Phys. -Chim. Sin Vol.30 Fig.3 Equilibrium geometries of Pb(OH)(H2O) 1-5 from periodic DFT calculations to reach the solvation sphere, indicating that the maximum value of n was 5 here. As for the geometries of Pb(OH) (H 2O) 1-5 shown in Fig.3, complexes of Pb(OH)(H 2O) 1-4 exhibited hemidirected geometry with increasing tendency to the holo-directed structure, while geometry with one upright water molecule occupying the other hemisphere of Pb(II) coordination globe was observed for Pb(OH)(H 2O) 5. Combined with our previous findings that aqueous Pb(II) mainly exists in the structure of Pb(H 2O) 6 2, 25 Pb(OH)(H 2O) 5 may be the most probable species of aqueous Pb(OH) and is used as the initial adsorbate of kaolinite throughout. 3.2 Adsorption complexes Monodentate complexes In the monodentate adsorption mode, Pb(OH) combines with the deprotonated substrate of [S(OH) 2O] -, forming the overall neutral surface complex species. With m ranging from 1 to 7, all possible monodentate complexes of [S(OH) 2(O)Pb(OH) (H 2O) m] were tested. Unreasonable structures of O u site with much longer Pb O bond length were obtained until CN of Pb(II) was lower than 6, corresponding to the maximum m value of 3 (shown in Fig.4A). Two water molecules of Pb(II) hydration shell in aqueous system were crowded out by the kaolinite (001) slab due to the steric hindrance effect. Furthermore, stability and CN of complex were found to depend strongly on the hydrogen bonding interactions between aqua ligands and surface hydroxyls, especially the strong attraction of surface O l to H of aqua ligands (forming the O l H w bond). Hydrogen bond between O of aqua (or hydroxyl) ligands and H of O u site (designated as O w H u or O H H u ) also occurred, but was relatively weak and was less of a factor for the stability of complex. As for the O l site, only one water molecule could retain in the coordination sphere of Pb(II), corresponding to the maximum CN of 3 (Fig.4B). Although atom of O l exhibited stronger affinity to Pb(II) than O u with a shorter Pb O l bond length of nm (Table 1), repulsive interactions from six surrounding O uh groups drove most of the aqua ligands of Pb(OH) (H 2O) 5 away. No hydrogen bond of O l H w formed as the distance between neighboring O l centers was about 0.52 nm, too far apart for H w to interact directly. Both monodentate complexes were in hemi-directed geometry, where bond lengths of Pb O s and Pb O H were obviously shorter than Pb O w. An even distribution of hydroxyl and aqua ligands just above the basal Al(o) (001) surface in configuration A has been found, associated with Pb-Al distance of nm and three strong O l H w bonds at distances of Fig.4 Equilibrium geometries of monodentate complexes on Ou (A) and Ol (B) sites and bidentate complexes on OuOl site of the same Al center (C) and two neighboring Al centers (D) Dashed lines denote the hydrogen bonding interactions. Table 1 Structure parameters and binding energies (ΔEbind) of monodentate and bidentate complexes of Pb(OH) on the Al(o) (001) surface of kaolinite Complex CN D(Pb Om)/nm D(Pb Os)/nm D(Pb OH)/nm D(Pb Ow)/nm D(Pb Al)/nm D(Ol H)/nm ΔEbind/(kJ mol -1 ) A: mono-ou , 0.267, , 0.159, B: mono-ol C: bi-ouol(al) , , D: bi-ouol(al) , CN: coordination number of Pb(II) in the complex; D(Pb Os), D(Pb OH), and D(Pb Ow): distances of Pb to the O centers of Al(o) surface, hydroxyl, and aqua ligands, respectively; D(Pb Om): average value of Pb O bond lengths; D(Pb Al): distance of Pb to the nearest Al center of the (001) slab; D(Ol H): hydrogen bond length between surface Ol and H of hydroxyl or aqua ligands;-: no corresponding data

5 No.5 WANG Juan et al.: Adsorption Mechanism of Hydrated Pb(OH) on the Kaolinite (001) Surface , 0.159, and nm, respectively (Table 1). The Pb O u bond length of nm was slightly lower than the corresponding EXAFS (extensive X- ray absorption fine structure) value of (0.230±0.001) nm in the Pb(II) complex on kaolinite surface, 26 but in consistent with the Pb O length of nm for Pb(II) complex on aluminum oxides from the EXAFS spectroscopy. 27 Being different, configuration B displayed the distorted trigonal pyramid geometry with Pb O l length of nm, Pb Al distance of nm, and the only one Pb O w bond of nm. Binding energies calculated from reaction (1) were and kj mol - 1 (Table 1) for configurations A and B, respectively. The negative values indicated the exothermic and favorable characteristic of the Pb(OH) adsorption process. Thus, complex of O u site seems more favorable based on the high binding energy, strong hydrogen bonding interactions, and good agreement of Pb O u bond length with experimental values as well Bidentate complexes Bidentate complexes with joint coordination structure located on single Al and two neighboring Al centers could be seen from Fig.4C and 4D, respectively. Both complexes featured hemidirected geometry with short Pb O u bonds (0.215 and nm) and slightly long Pb O l bonds (0.277 and nm). The weak Pb O l bond was mainly caused by the competition of H with Pb to the O l center. All hydroxyl and aqua ligands of Pb(II) formed hydrogen bonds with surface hydroxyls, where the O l H w type was stronger than O w H u or O H H u. Bond lengths of Pb O H (0.233 and nm) were comparable to the corresponding value in Pb(OH)(H 2O) 5, revealing that the combination of Pb(II) with Al(o) surface did not affect the bond of Pb O H significantly. The main difference was that at most two water molecules could remain in the Pb(II) coordination sphere for configuration C, whereas only one aqua ligand left in configuration D. In other words, the maximum CN of Pb(II) was 5 for bidentate complex of single Al center, and 4 for that of two neighboring Al centers. Inspiringly, the Pb-Al distance of nm in configuration C was in good agreement with the corresponding EXAFS data of nm in the Pb(II) complex on aluminum oxides 27 and nm for Pb(II) complex on iron oxides. 28 Combined with the binding energies of and kj mol -1 for respectively configurations C and D, complexation in bidentate way seems less likely to occur on two neighboring Al centers, where the distance between O u and O l might be too large for Pb(II) to interact with both the atoms simultaneously. Analysis focused on the two preferred adsorption types: monodentate complexation of O u site (A) and bidentate complexation of single Al center (C). Configuration A featured a relatively high binding energy of kj mol -1, incorporating the adsorption energy of Pb(OH), dissociation energies of two aqua ligands from Pb(OH)(H 2O) 5 and hydration energy of H from deprotonation of surface hydroxyl group. Although value of ΔE bind ( kj mol -1 ) for configuration C was slightly low, dissociation energy of one more water molecule from Pb(OH)(H 2O) 5 was incorporated as the complex has a CN of 4, lower than that of complex A. Thus, the adsorption energies of Pb(OH) in two types might be comparable. Moreover, both the Pb O bond length of nm in configuration A and Pb- Al distance of nm in configuration C were in qualitative agreement with the EXAFS data. So these two types of adsorption were all probable, with monodentate complexation more easily to occur. 3.3 Properties Mulliken population Mulliken atomic populations of atoms O u, O l, O H, and Pb before and after the adsorption are listed in Table 2, which helps to quantify the charge transfer induced by Pb(OH) adsorption. Charges for O u and O l in clean kaolinite slab without adsorbent were respectively-1.06 and-1.05, indicating the comparable electronegativity of two different types of surface oxygens. Charges for O H and Pb in Pb(OH) (H 2O) 5 before the adsorption were and 1.26, respectively. Upon values of charge difference (Δ), Pb accepted about 0.20 electrons from the surrounding oxygen ligands during the adsorption process, including O u, O l, O H, and O w. Therefore, covalency of Pb has increased after it combined with the Al(o) (001) surface. Bond population of Pb O s has also been calculated to investigate the corresponding bonding mechanism (Table 2). Bonds of Pb O u, Pb O l in monodentate complexes and Pb O u in bidentate complexes all featured negative population values, suggesting that filled antibonding interaction was the major orbital contribution. Positive values of Pb O l population in the joint coordination structure have been found, indicating the predominantly bonding orbital interactions. As values of Pb O l population were small, the formed Pb O l bonds were very weak with notably long bond lengths of about nm. So it seems that while hydrogen and Pb(II) coordinate to the same Table 2 Mulliken charges for atoms of Os, OH, Pb, and Pb Os bond population in different types of adsorption complexes Complex Ou,b Ol,b Os,a Os OH,b OH,a OH Pbb Pba Pb Pb Os A: mono-ou B: mono-ol C: bi-ouol(al) , , , 0.01 D: bi-ouol(al) , , , 0.02 The subscripts b and a represent the Mulliken charges before and after the adsorption, respectively. Os and OH, atoms of oxygen of kaolinite surface and hydroxyl ligand, respectively. Δ denotes the charge difference of atoms and is calculated according to the formula =Qa Qb. The two values in columns of Os,a, ΔOs, and Pb Os for complexes C and D are respectively corresponding to atoms of Ou and Ol, where Ou and Ol are the two kinds of surface oxygen Os.

6 834 Acta Phys. -Chim. Sin Vol.30 surface O l simultaneously, the coordination between hydrogen and O l is stonger than that of Pb(II) DOS analysis Further insight into the bonding nature of Pb(II) with hydroxyls of Al(o) (001) surface could be examined by the partial electronic densities of states (PDOS) of O u, O l and Pb in four types of adsorption complexes. As shown in Fig.5A, combination of Pb(II) with O u affected the O u 2s significantly, as it has split from one peak (a) into two narrow parts (b, c, and d), with the corresponding energy shifted from ev to a relatively high region of about ev. Peaks in dashed curve at valence band region of ev (a), as part of O u 2p contribution in the clean Al(o) surface, decreased from the highest value of ~1.10 electrons ev -1 (a) to ~0.20 electrons ev - 1 (b, c, and d), while those at the conduction band of ev increased from the highest value of ~0.02 electrons ev -1 to ~0.20 electrons ev -1 after the adsorption. These changes are mainly caused by the Pb-O u interaction, Pb 6p coupling with the Pb 6s O u 2p antibonding states. As Pb 6s and O u 2p states are located in the similar energy region, an efficient interaction that produces strongly antibonding Pb 6s O u 2p states occurs mainly at ev. Coupling of Pb 6p with the antibonding states can be obviously known from the overlaps between Pb 6s, Pb 6p, and O u 2p at ev. Antibonding orbital interaction in chemical bonding is a characteristic of Pb(II) compounds. Similar interaction has been found in PbO 29 and hydrated Pb(II). 25 Also, it has been reported that the minimization of these unfavourable covalent antibonding interactions is the driving force for structural distortion. 30 In the matter of Pb-O l interaction (shown in Fig.5B), O l 2s in configuration B (f) was obviously different from those in configurations C (g) and D (h), but close to O u 2s in configuration A (b). Thus, Pb-O s interactions in monodentate complexes were the same whatever at the O u site or O l site, different from those in bidentate complexes due to the competitive coordination. O l 2s and O l 2p in configurations C (g) and D (h) were similar to those of clean Al(o) surface (e), suggesting that O l coordinated with hydrogen to a much larger extent than Pb(II). Affected by the dominant hydrogen coordination, overlaps of bonding orbitals between Pb 6s and O l 2p at ev increased. Bond of Pb O l exhibited small positive population value ultimately. Additionally, no notable difference between two types of surface hydroxyls (O uh and O lh) has been found, as they have comparable Mulliken charges (-1.06 and-1.05) and similar DOS curves. Thus, the main reason for Pb(II) preferring to bind on the O u site with relatively high CN and large binding energy is that it has the favorable position for hydrogen bonding interaction to stabilize the structure. As hydrogen bond of the O l H w type plays a key role in determining stability of the complex, and distance between two nearest atoms of O l is too large, Pb(II) is less likely to adsorb on the O l site due to the strong repulsions of surrounding O uh to aqua ligands of Pb(II). Fig.5 Partial electronic densities of states (PDOS) of Ou in the clean Al(o) surface (a), Ou and Pb in configurations A (b), C (c), D (d) (shown in part A) and Ol in the clean Al(o) surface (e), Ol and Pb in configurations B (f), C (g), D (h) (shown in part B) 4 Conclusions The structure and mechanism of Pb(OH) adsorption on the Al(o) (001) kaolinite surface have been investigated by the plane-wave pseudopotential DFT calculations. Pb(OH)(H 2O) 5 is found as the most probable species of Pb(OH) in aqueous system and used as the initial absorbate. All complexes in monodentate and bidentate modes feature the hemi-directed geometry with low CNs of 3 to 5, where the steric hindrance effect of kaolinite acts as the major cause. Hydrogen bonding interaction of surface O l with H w of aqua ligands has been found playing a key role in determining the stability of complex. Pb(II) adsorption in monodentate way prefers the O u to O l site as it has favorable position for the O l H w interaction. Among the adsorption types examined, monodentate complex of O u site with the highest binding energy may be the major species, and bidentate complex on O uo l site of single Al center is also probable. Upon the Mulliken population and DOS analysis, Pb(II) accepts electrons from surface oxygens during the adsorption process. Pb 6p coupling with the Pb 6s O 2p antibonding states is the primary orbital interaction of Pb(II) with surface oxygen. Pb-O l interaction in the joint coordination structure features the predominantly bonding state fill-

7 No.5 WANG Juan et al.: Adsorption Mechanism of Hydrated Pb(OH) on the Kaolinite (001) Surface 835 ing due to the competitive coordination of hydrogen. Supporting Information: Atomic coordinates in initiative cell of kaolinite, equilibrium geometries of Pb(OH)(H 2O) 1-5 and [S(OH) 2(O)Pb(OH) (H 2O) m] with corresponding structural parameters have been included. The information is available free of charge via the internet at References (1) Karlsson, K.; Viklander, M.; Scholes, L.; Revitt, M. J. Hazard. Mater. 2010, 178, 612. doi: /j.jhazmat (2) Wasim Aktar, M.; Paramasivam, M.; Ganguly, M.; Purkait, S.; Sengupta, D. Environ. Monit. Assess. 2010, 160, 207. doi: /s (3) ATSDR (Agency for Toxic Substances and Disease Registry). Toxicological Profile for Lead (Update). U. S. Department of Health and Human Services, Atlanta, Georgia. cdc.gov/toxprofiles/tp.asp?id=96&tid=22 (accessed Nov 20, 2012). (4) Tarasevich, Y. I.; Klimova, G. M. Appl. Clay Sci. 2001, 19, 95. doi: /S (01) (5) Gupta, S. S.; Bhattacharyya, K. G. Phys. Chem. Chem. Phys. 2012, 14, doi: /c2cp40093f (6) Hong, H. L.; Min, X. M.; Zhou, Y. J. Wuhan Univ. Technol. 2007, 22, 661. doi: /s (7) Spark, K. M.; Wells, J. D.; Johnson, B. B. Eur. J. Soil Sci. 1995, 46, 633. doi: /ejs issue-4 (8) Srivastava, P.; Singh, B.; Angove, M. J. Colloid Interface Sci. 2005, 290, 28. doi: /j.jcis (9) Hizal, J.; Apak, R.; Hoell, W. H. Environ. Prog. Sustain. 2009, 28, 493. doi: /ep.v28:4 (10) Pearson, R. G. J. Am. Chem. Soc. 1963, 85, doi: / ja00905a001 (11) Puskar, L.; Barran, P. E.; Duncombe, B. J.; Chapman, D.; Stace, A. J. Phys. Chem. A 2005, 109, 273. doi: /jp047637f (12) Shimoni-Livny, L.; Glusker, J. P.; Bock, C. W. Inorg. Chem. 1998, 37, doi: /ic970909r (13) Hummer, K.; Grüneis, A.; Kresse, G. Phys. Rev. B 2007, 75, doi: /PhysRevB (14) Clark, S. J.; Segall, M. D.; Pickard, C. J.; Hasnip, P. J.; Probert, M. I. J.; Refson, K.; Payne, M. C. Z. Kristallographie 2005, 220, 567. (15) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1996, 77, doi: /PhysRevLett (16) Ireta, J.; Neugebauer, J.; Scheffler, M. J. Phys. Chem. A 2004, 108, doi: /jp (17) Sun, T.; Wang, Y. B. Acta Phys. -Chim. Sin. 2011, 27 (11), [ 孙涛, 王一波. 物理化学学报, 2011, 27 (11), 2553.] doi: /PKU.WHXB (18) Vanderbilt, D. Phys. Rev. B 1990, 41, doi: / PhysRevB (19) Monkhorst, H. J.; Pack, J. D. Phys. Rev. B 1976, 13, doi: /PhysRevB (20) Bish, D. L. Clay. Clay Miner. 1993, 41, 738. doi: / CCMN (21) Hu, X. L.; Michaelides, A. Surf. Sci. 2008, 602, 960. doi: /j.susc (22) Kremleva, A.; Krüger, S.; Ro sch, N. Langmuir 2008, 24, doi: /la801278j (23) Mason, S. E.; Iceman, C. R.; Tanwar, K. S.; Trainor, T. P.; Chaka, A. M. J. Phys. Chem. C 2009, 113, doi: / jp807321e (24) Gourlaouen, C.; Gerard, H.; Parisel, O. Chem. -Eur. J. 2006, 12, (25) Wang, J.; Xia, S. W.; Yu, L. M. Acta Chim. Sin. 2013, 71, [ 王娟, 夏树伟, 于良民. 化学学报, 2013, 71, 1307.] (26) Mishra, B.; Haack, E. A.; Maurice, P. A.; Bunker, B. A. Chem. Geol. 2010, 275, 199. doi: /j.chemgeo (27) Bargar, J. R.; Brown, G. E., Jr.; Parks, G. A. Geochim. Cosmochim. Acta 1997, 61, doi: /S (97) (28) Bargar, J. R.; Brown, G. E., Jr.; Parks, G. A. Geochim. Cosmochim. Acta 1997, 61, doi: /S (97) (29) Walsh, A.; Watson, G. W. J. Solid State Chem. 2005, 178, doi: /j.jssc (30) Mudring, A. V. Eur. J. Inorg. Chem. 2007, 2007 (6), 882.

8 Supplementary Information for Acta Phys. -Chim. Sin. 2014, 30 (5), doi: /PKU.WHXB 水合 Pb(OH) 在高岭石 (001) 晶面的吸附机理 王娟 1,2 夏树伟 1,* 于良民 1 ( 1 中国海洋大学化学化工学院, 海洋化学理论与工程技术教育部重点实验室, 山东青岛 ; 2 青岛农业大学化学与药学院, 山东青岛 ) Adsorption Mechanism of Hydrated Pb(OH) on the Kaolinite (001) Surface WANG Juan 1,2 XIA Shu-Wei 1,* YU Liang-Min 1 ( 1 Key Laboratory of Marine Chemistry Theory and Technology, Ministry of Education, College of Chemistry and Chemical Engineering, Ocean University of China, Qingdao , Shandong Province, P. R. China; 2 College of Chemistry and Pharmacy, Qingdao Agricultural University, Qingdao , Shandong Province, P. R. China) Corresponding author. shuweixia@ouc.edu.cn; Tel: S1

9 Cell parameters for initiative cell of kaolinite: a= nm, b= nm, c= nm; α=91.926, β= , γ= Space group: C1 Atomic coordinates: Atom x/a y/b z/c Al(1) Al(2) Si(1) Si(2) O(1) O(2) O(3) O(4) O(5) OH(1) OH(2) OH(3) OH(4) H(1) H(2) H(3) H(4) S2

10 Equilibrium geometries of Pb(OH)(H 2 O) 1 5 and the corresponding structural parameters: Pb(OH)H 2 O Pb(OH)(H 2 O) 2 Pb(OH)(H 2 O) 3 Pb(OH)(H 2 O) 4 Pb(OH)(H 2 O) 5 Fig.S1 Equilibrium geometries of Pb(OH)(H 2 O) 1 5 from periodic DFT calculations. Atoms of Pb, O and H are colored grey, red and white, respectively. Table S1 Equilibrium geometrical parameters and binding energies of PbOH(H 2 O) n n Pb O H Pb O w Pb O m /nm /nm /nm ΔE binding O H PbO w,1 O H PbO w,2 coordination /(kj mol 1 /( ) /( ) ) geometry hemi-directed , hemi directed , 0.243, hemi directed , 0.252, 0.270, hemi directed , 0.256, 0.264, 0.296, holo-directed Pb O H and Pb O w, the distances of Pb to the O centers of OH and H 2 O ligands, respectively. Pb O m, the average value of Pb O bond lengths in PbOH(H 2 O) n. O w,1 and O w,2 are the oxygen atoms of aqua ligands neighboring to the hydroxyl ligand , no corresponding datum. ΔE binding, the binding energy of PbOH(H 2 O) n, is calculated according to the reaction: Pb 2 x H 2 O = Pb(OH)(H 2 O)n (x-n-1) H 3 O, and defined as the energy difference between the products and reactants. S3

11 Equilibrium geometries of [S(OH) 2 (O)Pb(OH)(H 2 O) m ] and the corresponding structural parameters: A B C D Fig.S2 Equilibrium geometries of monodentate complexes on O u (A) and O l (B) sites and bidentate complexes on O u O l site of the same Al center (C) and two neighboring Al centers(d). AlO 6 octahedra are indicated by green polygons. Black dashed lines denote the hydrogen bonding interactions Table S2 Structure parameters and binding energies (ΔE bind ) of mono and bidentate complexes of Pb(OH) on the Al(o) (001) surface of kaolinite Complexes N Pb O m /nm Pb O s /nm Pb O H /nm Pb O w /nm Pb Al/nm O l H/nm O H H u /nm ΔE bind / (kj mol 1 ) A , 0.267, 0.167, 0.159, B C , , D , N, the coordination number of Pb(II) in the complex. Pb O s, Pb O H and Pb O w represent the distances of Pb to the O centers of Al(o) surface, hydroxyl and aqua ligands, respectively. Pb O m, average value of Pb O bond lengths in the adsorption complex. Pb Al, distance of Pb to the nearest Al center of the (001) slab. O l H, hydrogen bond length between surface O l and H of hydroxyl or aqua ligands. O H H u, hydrogen bond length between O of hydroxyl ligand and H of surface O u H., no corresponding data. S4