Accepted Manuscript Title: Theoretical investigation of lead vapor adsorption on kaolinite surfaces with DFT calculations Author: Xinye Wang Yaji Huang Zhigang Pan Yongxing Wang Changqi Liu PII: DOI: Reference:

S0304-3894(15)00208-3 http://dx.doi.org/doi:10.1016/j.jhazmat.2015.03.020 HAZMAT 16668

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Journal of Hazardous Materials

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26-12-2014 15-2-2015 10-3-2015

Please cite this article as: Xinye Wang, Yaji Huang, Zhigang Pan, Yongxing Wang, Changqi Liu, Theoretical investigation of lead vapor adsorption on kaolinite surfaces with DFT calculations, Journal of Hazardous Materials http://dx.doi.org/10.1016/j.jhazmat.2015.03.020 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Title： Theoretical investigation of lead vapor adsorption on kaolinite surfaces with DFT calculations Author names: Xinye Wanga, Yaji Huanga,*, Zhigang Panb, Yongxing Wanga, Changqi Liua Affiliations: a Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education, School of Energy and Environment, Southeast University, Nanjing 210096, China; b College of Materials Science and Engineering, Nanjing Tech University, Nanjing 210009, China. Corresponding author: Yaji Huang Tel.: +86-25-83794744-808 E-mail address: [email protected] Postal address: School of Energy and Environment, Southeast University, Sipailou 2#, Nanjing, Jiangsu, China Zip code: 210096

Abstract Kaolinite can be used as the in-furnace sorbent/additive to adsorb lead (Pb) vapor at high temperature. In this paper, the adsorptions of Pb atom, PbO molecule and PbCl2 molecule on kaolinie surfaces were investigated by density functional theory (DFT) calculation. Si surface is inert to Pb vapor adsorption while Al surfaces with dehydroxylation are active for the unsaturated Al atoms and the O atoms losing H atoms. The adsorption energy of PbO is much higher than that of Pb atom and PbCl2. Considering the energy barriers, it is easy for PbO and PbCl2 to adsorb on Al surfaces but difficult to escape. The high energy barriers of de-HCl process cause the difficulties of PbCl2 to form PbO⋅Al2O3⋅2SiO2 with kaolinite. Considering the inertia of Si atoms and the activity of Al atoms after dehydroxylation, calcination, acid/alkali treatment and some other treatment aiming at amorphous silica producing and Al activity enhancement can be used as the modification measures to improve the performance of kaolinite as the in-furnace metal capture sorbent. Keywords Density functional theory, Lead vapor, Adsorption, Kaolinite, Chlorine

1. Introduction Semi-volatile toxic metals can volatilize from coal, waste or other fuels in high temperature furnace, then condense and nucleate in the flue gas after heat exchangers [1]. This process causes the migration of these metals from fuel to submicron particles which can be hardly removed efficiently by most dedusting equipments [2]. Kaolinite can be used as the in-furnace sorbent/additive to inhibit the migration of semi-volatile metals through the chemical adsorption and the physical conglutination at high temperature [3-5]. Kaolinite (Al2O3⋅2SiO2⋅2H2O) can react with semi-volatile metal vapor to form some aluminosilicate, silicate or aluminate at high temperature [3-4]. These products cause the eutectic melt of kaolinite surface which becomes sticky and can be adhered to by nanoscale metal particles [5]. It means the chemical adsorption is the premise of the physical adhesion and plays a fundamental role during the metal capture by kaolinite. In this paper, only lead (Pb) vapor adsorption was taken into consideration for the limited article length and the abundant data from previous researches on Pb adsorption, especially the kinetics data from Davis, Gale and Yao [3,6-8]. Pb vapor distributes in several species during combustion. Thermal equilibrium analysis indicates that oxide and chloride are the major species while single atom is minor [9]. At lower temperature, lead dichloride (PbCl2) vapor and lead monochloride (PbCl) vapor are formed prior to lead oxide (PbO) vapor as long as there is enough chlorine (Cl) input for the effect of direct or indirect chlorination [10-11]. Meanwhile, PbCl2 is much more than PbCl in the oxidizing atmosphere [9]. At higher temperature, a small part of PbCl2 and PbCl vapor transforms to PbO and Pb atom vapor [9,12]. According to the Pb distribution in furnace, in this paper, the adsorbates are limited to Pb atom, PbO molecule and PbCl2 molecule. Several researches have mentioned the inhibition effect of Cl on the metal capture by kaolinite. Scotto et al. injected kaolinite particles at high temperature to depress the formation of submicron Pb [13]. For Cl/Pb mole ratio of 2, kaolinite reduced 97.5% of submicron Pb, while for Cl/Pb of 17, the removal rate dropped to 28%. Linak, Davis and Mwabe reported the same effect of Cl on Na and Cd capture. They attributed the inhibition to the non-reactivity of metal chlorides and considered only metal oxides and metal hydroxides could react with kaolinite at high temperature [7,14-15]. However, Scotto’s another experiment found that kaolinite could capture PbCl2 vapor at 650 ºC and the product was PbO⋅Al2O3⋅2SiO2 [13]. Uberoi et al. and Yao et al. obtained the same results [16-18]. Scotto inferred that the high excess chlorine concentrations made kaolinite adsorption too slowly to be effective [13].

Therefore, the essential issue about the effect of Cl is the difference between the adsorptions of PbO and PbCl2 on kaolinite surface. The previous researches focus on the effect of combustion condition and fuel composition on the metal capture efficiencies of kaolinite with few attention to the chemical adsorption mechanisms, so it is difficult to understand how these factors work, such as Cl. The difficulty to study the mechanisms is how to accurately observe or quantitatively detect the chemical adsorption without the interference of some physical processes. Davis and Wendt developed the size fractionation method (SFM) to evaluate the chemical adsorption fraction, then investigated the global reaction kinetics, however, the adsorption mechanisms are still not clear and needs to explored in depth [6,9]. Quantum chemical calculation has been used to predict the adsorption structure and to investigate the adsorption characteristics in atomic scale without the interferences existing in experiments [20-22]. In this paper, we used density functional theory (DFT) to investigate the adsorption of Pb vapor on kaolinite surfaces [20-22]. The objective is to understand the adsorption characteristics of different Pb species vapor (Pb atom, PbO and PbCl2), which could explain the questions like where the active sites of kaolinite surface are for Pb vapor adsorption, how Cl effects Pb adsorption and how to enhance the adsorption performance of kaolinite. 2. Experimetal and modeling 2.1. Kaolinite Kaolinite is a two-layer clay mineral composed of the alternate layers of tetrahedral SiO4 (Si-layer) and octahedral AlO6 (Al-layer) periodically, as shown in Fig. 1 (a) and (b) [23]. Water is bonded in the form of hydroxyl groups in Al-layer. The inner hydroxyl groups lie between Al-layer and Si-layer, while the inner-surface hydroxyl groups lie between kaolinite layers and connect these layers by hydrogen bonds [24]. The kaolinite used in the experiments is of the purity higher than 99.5% with the mean diameter of 10 µm.

2.2. Drop tube furnace and sampling An electrically heated drop tube furnace (DTF) (55 mm in diameter, 1500 mm in length) was used to study the thermal dehydroxylation of kaolinite in high temperature furnace. The raw kaolinite was fed into the furnace at a rate of 0.5-1 g/min by a micro screw feeder combining with jet aerator. The gas temperature in DTF was kept at 900 ºC, and the gas residence time was 1, 3 and 5s, close to the situation in real furnace [25-26]. The products were collected by the glass fiber filter at 120 ºC in case of water vapor condensation, and then were detected by Fourier transform infrared spectroscopy (FTIR) to analyze the surface functional groups. 2.3. DFT Modeling Methodology The DFT calculations on the basis of the plane-wave pseudopotential approach were realized by using the Cambridge Sequential Total Energy Package (CASTEP) which can deal with the periodic boundary condition (PBC) efficiently [27]. The interactions between the ions and the electrons were described by the Vanderbilt ultrasoft pseudopotential [28].The exchange-correlation potentials were described by generalized gradient approximation (GGA) and the Perdew, Burke and Ernzerhof (PBE) functional [29]. The original crystal structure of kaolinite was constructed according to Young’s data [30]. Once the optimized unit cell of kaolinite was obtained, the periodic supercell (2×1×1) with a vacuum thickness of 20 Å was generated for Pb vapor adsorption. Only the surface adsorption was considered because the invasion of products from the surface to the inside layer would be another complex chemical process. Moreover, the flash calcination can separate kaoinite into single crystal platelets by open meso-pore gaps [6]. Therefore, the single platelet composed of one Si-layer and one Al-layer can meet the requirements of this study and can simplify the calculation. Two sides of the platelet were investigated, Al surface (001) and Si surface (00 1 ). The geometry optimization followed the energy cutoff of 700 eV to ensure the calculation quality high enough [31]. The transition states (TS) were searched using the complete linear synchronous transit/quadratic synchronous transit (LST/QST) method with a root-mean-square (RMS) forces on atom tolerance of 0.01 eV/Å [32]. One imaginary frequency only was required for TS to identify the saddle point on the potential energy surface. If not, the further search would be carried out. Atomic charges and

bond populations were investigated by Mulliken population analysis which is particularly suitable for analyzing the results of computations performed using well-converged planewave basis sets [33]. The adsorption energy (∆Eadsorption) was defined as follows [20]: ∆Eadsorption = Eadsorbate+kaolinite – (Eadsorbate + Ekaolinite)

(1)

where Eadsorbate+kaolinite, Eadsorbate and Ekaolinite denote the energ of the kaolinite with the adsorbed molecule, the energy of molecule and the energy of bare kaolinite, respectively. The charge density difference (∆qi) was defined as follows [20]: ∆qi = qi,after adsorption – qi,before adsorption

(2)

where qi,after adsorption and qi,before adsorption denote the charges of i-th atom in the products system and in the reactants system, respectively. 3. Results and discussion 3.1. Thermal dehydroxylation of kaolinite The structure of kaolinte in furnace can not be maintained as that of the raw kaolinite as shown in Fig. 1 for the dehydroxylation at high temperature, so it is necessary to identify the possible structures before vapor adsorption [24,34]. The thermogravimetry - differential scanning calorimeter (TG-DSC) was used to detect the thermal properties of raw kaolinite and the results are shown in Fig. 2. The weight loss during the heating process is up to 13 % and close to 13.96 % which is the theoretical moisture content of Al2O3⋅2SiO2⋅2H2O. The deviation was caused by the impurities (e.g. quartz). According to the DSC analysis, there are one endothermic peak at 506 ºC and one exothermic peak at 998 ºC. The coincidence between the endothermic range and the weight loss range indicates the dehydroxylation at 350-800 ºC. The exothermic peak indicates the formation of mullite at 998 ºC [35]. The real heating process of kaolinite during in-furnace metal capture is not soak calcination but flash calcination which has been studied as a timesaving processing to obtain metakaolinite [36-37]. But unlike producing metakaolinite (injecting kaolinite into the flame at 1000-1200 ºC), kaolinite as the metal vapor sorbent may be exposed at a lower temperature for the different fuel performance and combustion atmosphere, so there are still some possibilities of incomplete dehydroxylation [36-37]. The thermal dehydroxylation process of kaolinite was simplified in the DTF with the gas residence time (1-5 s) and gas temperature (900 ºC) similar to those in real furnace. The FTIR spectra of different samples is shown in Fig. 3. The related functional groups corresponding to the absorption bands are shown in Table 1. In the high frequency region, the adsorption peaks of hydroxyl group are weakened significantly but not disappeared with the increasing residence time. In the low frequency region, the adsorption peaks of metakaolinite structure become obvious after the flash calcination. Therefore, the existence of incomplete dehydroxylation is demonstrated and the following research should consider two kinds of kaolinite (1) partial dehydroxylation and (2) complete dehydroxylation. The extreme case of partial dehydroxylation is presented by removal of one water molecule from Al surface. Hydroxyl groups to form water molecules should obey two rules that first only hydroxyl groups located close to each other will react, and second the process of removal of hydroxyl groups follows the gradual decrease in coordination numbers of aluminum (Al) atoms from six to four [23]. The hydroxyl groups on Al ring are marked from No. 1 to No. 6 in Fig. 4 (a). The nearest pair of hydroxyl groups is No. 1 and No. 2 which are considered as the source of one water molecule to remove. There are two possible combinations that OH (No. 1) + H (No. 2) → H2O and H (No. 1) + OH (No. 2) → H2O. Considering the lower energy barrier and product energy obtained, the second combination is more reasonable and the kaolinite losing one water molecule should follow the structure showed in Fig. 4 (b). Following this mode, all the hydroxyl groups were removed to construct the metakaolinite shown in Fig 4 (c) which is similar with the reported structure [23]. Four kinds of surface should be considered to adsorb Pb species: Si surface of kaolinite losing one water molecule (Si surface-1), Si surface of metakaolinite (Si surface-2), Al surface of kaolinite losing one water molecule (Al surface-1) and Al surface of metakaolinite (Al surface-2). The dehydroxylation has little effect on the structure of Si-layer, but has significant effect on that of Al-layer in which the coordination number of Al atoms is reduced, corresponding to the charge constant of Si atoms and the

charge increase of Al atoms by the population analysis. All the calculation results indicate that there is almost no difference between the adsorptions on Si surface-1 and Si surface-2, so we chose the first one on behalf of Si surfaces. Thus, the adsorptions were considered on one Si surface and two Al surfaces. 3.2. Pb atom adsorption on kaolinte surfaces There are only silicon (Si) atoms and oxygen (O) atoms exposing on the Si surface. Considering their electronegativity, the O atoms connecting two tetrahedral SiO4 were selected as the potential active site. Pb atom adsorption on the Si surface is shown in Fig. 5(a) and (b). The Pb atom is 2.793 Å above the O atom and causes no obvious structural change of Si surface. Charge density distribution and charge density difference are usually used to evaluate the bonding between atoms, according to the rule that the overlap of charge density distribution and the charge density accumulation between atoms indicate the covalent bond character while the separation of charge density distribution and the charges transfer between atoms indicate the ionic bond character [20,41]. In Fig. 5(c) and (d), there are no obvious corresponding evidence of covalent bond between the Pb atom and the O atom. The population analysis shows no charge transfer between Pb and Si surface. There is almost no adsorption energy and energy barrier during the Pb atom adsorption on Si surface, as shown in Table 2. So it is considered that Pb atom can not be adsorbed on Si surface which has no active site for Pb atom sorption. Because of the removal of one water molecule, there are one O atom exposed without bonding with H and two V-coordinated Al atoms on Al surface-1 as the potential active sites [7,42]. Pb atom was placed above three active atoms and the optimized structure is shown in Fig. 6(a) and (b). The distance between Pb atom and active O atom is 2.320 Å, shorter than that on Si surface, as shown in Table 3. The Pb atom is about 3 Å from two V-coordinated Al atoms. There is a small overlap of charge density distribution between the Pb atom and the active O atom, but without obvious charge density accumulation between them, as shown in Fig. 6(c) and (d). The obvious charge density accumulation between the Pb atom and the V-coordinated Al atoms indicates the possibility of ionic polarization and covalent bond between them [43]. The charge transfer (0.29 e) from the Pb atom and the active O atom to two V-coordinated Al atoms illustrates the great activity of V-coordinated Al atoms. The adsorption energy is -131 kJ/mol and no TS was found. It is believed that Pb atom is attracted for the polarity of the V-coordinated Al atoms, and then forms strong covalent bonds with them. Al surface-2 has abundant active O atoms and all the Al atoms are IV-coordinated for the complete dehydroxylation, so the polarity and activity of Al surface are enhanced greatly. The Pb atom was placed above the middle of Al ring of Al surface-2 and the optimized structure in Fig. 7(a) and (b) shows that the Pb atom is close to two convex O atoms and one V-coordinated Al atom. The charge density distribution and the charge density difference here are similar to those in the adsorption on Al surface-1, but the adsorption energy is almost twice. 3.3 PbO adsorption on kaolinte surfaces PbO has two atoms to be adsorbed on kaolinite surface. For Si surface, the potential adsorption follows the corresponding relation of Pb atom to O atom (from Si surface) and O atom (from PbO) to Si atom, as shown in Fig. 8(a) and (b). Actually, the Pb-O bond is extremely weak while the O-Si bond is strong, as shown in Fig. 8(c) and (d). The O atom on Si surface shows the low activity once again, while the IV-coordinated Si atom shows the ability to increase the coordination number. However, the extremely low adsorption energy and the same energy barriers from reactant and product shown in Table 2 indicate the invalidation of this adsorption. The TS structure in Fig. 8(e) and (f) indicates the energy barrier is due to the slight structural adjustment of Si surface and the slight change of Pb-O bond length. The exposed O atom and two V-coordinated Al atoms on Al surface-1 are considered to bond with Pb atom and O atom (from PbO) respectively. The optimized structure of PbO adsorption on Al surface-1 is shown in Fig. 9(a) and (b). The Pb atom is 2.207 Å away from the active O atom, while the O atom (from PbO) is only about 1.88 Å around the active Al atoms. Considering the extent of charge density distribution overlap and the charge density accumulation, the covalent bond between the Pb atom and the active O atom in Fig. 9(c) and (d) is stronger than that of the single Pb atom adsorption on the same surface in Fig. 6(c) and (d). The O atom (from PbO) integrates into the Al-O coordination so harmoniously that a red arrow is needed to mark it out in Fig. 9(e). The O atom (from PbO) accumulates large charges to form the ionic bond with two V-coordinated Al atoms as the sixth coordination, as shown in Fig. 9(f). The original Al-O bonds of kaolinite are also ionic and the bond lengths are around 1.8 Å, close to that of the new Al-O bonds (around 1.88 Å) [21]. The adsorption energy is as high as -343

kJ/mol, however the energy barrier from reactant is very low. Consequently, it is easy for PbO to adsorb on Al surface-1 but difficult to escape. The attraction between the O atom from PbO and the V-coordinated Al atoms plays the leading role in adsorption. The TS structure in Fig. 9(e) and (f) indicates that the energy barrier is due to position change of hydroxyl groups and two V-coordinated Al atoms and the bond length increase of PbO. The PbO adsorption on Al surface-2 in Fig. 10(a) and (b) is similar to that on Al surface-1 in Fig. 9(a) and (b). The Pb atom and two O atoms connecting IV-coordinated Al atoms are joined by covalent bond, while the O atom (from PbO) and two IV-coordinated Al atoms are joined by ionic bond. O atom (from PbO) becomes the fifth coordination of two Al atoms. The adsorption energy is up to -476 kJ/mol, higher than that in the adsorption on Al surface-2, and the energy barrier is still very low. The O atom (from PbO) marked in red integrates into the Al-O coordination harmoniously and keeps these two Al atoms from exposing on the surface and weakens their attraction to Pb atom occurring in Fig. 7(d). The TS structure in Fig. 10(e) and (f) indicates that the energy barrier is due to the Al-ring structural adjustment and the bond length increase of PbO. 3.4. PbCl2 adsorption on kaolinite surfaces The potential dichloride adsorption could happen between Pb atom and O atom from Si surface, as shown in Fig. 11(a) and (b). The distance between Pb atom and O atom is up to 2.934 Å and the covalent bond between them is very weak, as shown in Fig. 11(c) and (d). There is no adsorption energy and TS, so the adsorption of PbCl2 on Si surface is invalid as that of Pb atom and PbO. The opposite direction of two Cl atoms to Si surface indicates the repellent between Cl atoms and Si surface. For the PbCl2 adsorption on Al surface-2, the possible attractions occur between Pb atom and active O atom and between Cl atoms and hydroxyl groups, as shown in Fig. 12(a) and (b). The hydrogen bonds seem to exist between Cl atom and H atoms for the skewed electron density distribution towards hydroxyl groups and the charge density accumulation [20]. However, these hydrogen bonds are very weak for the large atomic radius of Cl [44]. There is a weak covalent bond between Pb atom and active O atom as well. The adsorption energy is -127 kJ/mol and the energy barrier from reactant is very low. The TS structure in Fig. 12(e) and (f) indicates that the energy barrier is due to the position change of hydroxyl groups, and PbCl2 keeps its original structure well. The reaction of PbCl2 with kaolinte is considered to follow the description of equation 3 or equation 4 based on the X-ray diffraction (XRD) analysis of solid products after high temperature adsorption [8,13,16-17]. Al2O3⋅2SiO2⋅2H2O + PbCl2 → PbO⋅Al2O3⋅2SiO2+2HCl Al2O3⋅2SiO2 + H2O + PbCl2 → PbO⋅Al2O3⋅2SiO2 + 2HCl

(3) (4)

Here, the de-HCl process was investigated as equation 3 following the rule that Cl atom combines with the nearest H atom to form HCl. The positive adsorption energy and the high energy barrier from reactant indicate the difficult process of primary de-HCl and the more difficult process of secondary de-HCl, as shown in Table 2. The weak adsorption of PbCl2 and the difficulty of de-HCl make the reaction shown in equation 3 difficult to occur. As shown in Fig. 12(g) and (h), the de-HCl process is very complex that the H atom and Cl atom both need to get rid of the bound from the O atom and the Pb atom respectively before the formation of HCl. The actual process of de-HCl may be more complex and maybe involve the water molecule as equation 4 or oxygen molecule as equation 5. Al2O3⋅2SiO2 + O2 + PbCl2 → PbO⋅Al2O3⋅2SiO2 + Cl2 (5) During the process of geometry optimization of the PbCl2 adsorption on Al surface-2, PbCl2 was placed above the convex active O atom at first, then two Cl atoms stared to get close to Al atoms, at last PbCl2 molecule was embedded on the surface, as shown in Fig. 13(a) and (b). There are one covalent bond between Pb atom and convex O atom and two covalent bonds with some ionic properties between Cl atoms and IV-coordinated Al atoms which is similar to the bonds in AlCl3, as shown in Fig. 13(c) and (d) [45]. The distance between Pb atom and active O atom is shorter than that in the PbCl2 adsorption on Al surface-1. The increase in the covalent degree between the Pb atom and the O atom is due to the attraction between Cl atoms and Al atoms. For the interaction between Cl atoms and Al atoms, the adsorption energy is up to -247 kJ/mol, almost twice that of the PbCl2 adsorption on Al surface-1. The energy barrier from reactant is also extremely low. The TS structure in Fig. 10(e) and (f) indicates that the energy barrier is due to the Al-ring structural adjustment, the bond length increase of Pb-Cl and the angle increase of Cl-Pb-Cl.

3.5. Comparison with experiment results Because of the difficulties to obtain and observe the homogeneous adsorption product at high temperature, no direct and basic experiment results can be used to compare with the calculation results. Consequently, some macroscopical experiment data and phenomenon are used here for the comparison and discussion. The global kinetics of PbO adsorption by kaolinite has been investigated according to SFM by Davis and Wendt whose results show an activation energy of 0 kJ/mol [7]. Their kinetic model based on the product with the PbO to Al2O3·2SiO2 mole ratio of 1:1 which is equivalent to one PbO molecule in one Al ring or Si ring, similar to our adsorption structures in Fig. 8-10, so the comparability between two research results is positive. In this paper, effective absorptions of PbO on Al surface-1 and -2 are with the activation energies of 12 and 24 kJ/mol respectively which are so low that these results could be considered to agree with the experiment result. Yao and Naruse investigated the global kinetics of PbCl2 adsorption by kaolinite during the sewage sludge combustion [8]. The Cl content in sewage sludge is much higher than that of Pb, so the Pb adsorbate was considered as PbCl2 according to the thermodynamic equilibrium calculation. The adsorption product was considered as PbO·Al2O3·2SiO2 which indicates the existence of the de-HCl process. The activation energy of PbCl2 adsorption by kaolinite is 124 kJ/mol in their research. The comparable results in this paper are from the PbCl2 adsorption on Al surface-1 and the following de-HCl process. The adsorption activation energy in this paper is only 9 kJ/mol, while the de-HCl activation energies are 492 and 606 kJ/mol, much higher than Yao and Naruse’s results [8]. This disagreement could be due to (1) the PbO in sewage sludge, (2) ash content in sewage sludge and (3) the water vapor in furnace. Firstly, part of Pb element in sewage sludge should be in the form of PbO, so there are two transformation paths when PbO exposes in the high temperature atmosphere containing Cl and kaolinite. The first one is that PbO reacts with Cl to form PbCl2 then to be adsorbed on kaolinite surface with a high energy barrier and the second one is that PbO is adsorbed directly on kaolinite surface with a low energy barrier. These two paths are in the competing relationship, so it is not an adsorption of pure PbCl2 but that of the mixture of PbCl2 and PbO in Yao’s experiment [8,46]. Secondly, there are three influential contents in the ash of sewage sludge, alkali metals (Na and K), alkaline earth metals (Ca and Mg) and silica/aluminum compounds. The alkali metals and alkaline earth metals are good fluxes to promote the melt of kaolinite which is good for metal capture by changing the kaolinite surface structure or producing amorphous silica from kaolinite (it will be discussed in the next paragraph) [42-47]. The silica/aluminum compounds can be considered as another metal vapor sorbent which may be more effective than kaolinite. Thirdly, Yao and Naruse discribed the adsorption of chlorides as equation 3 while Scotto and Uberoi discribed the adsorption as equation 4 that they thought the hydrogen in the reaction is not from kaolinite but the free water vapor molecules [13,16-17]. Here, we only considered that the water was from the hydroxyls of kaolinite surface, however the real situation is not clear. Maybe the de-HCl process could be carried out in other path with lower energy barrier. 3.6 Prospects The results have shown that the deep dehydroxylation can produce more unsaturated and active Al atoms on Al surface, so calcination at high temperature as a simple processing could be used to improve the performance of kaolinite to adsorb Pb vapor [23,24]. A more radical method of acid or alkali treatment following the process of calcination → acid or alkali solution soak → calcination, which is a traditional method to increase the acidity and catalytic activity of kaolinite, can produce more unsaturated and active Al atoms on surface [48]. The activity of Si atom is depressed in raw kaolinite, so Si atom activation should be considered as another possible path to modify kaolinite. Kaolinite calcination, as we know, can also produces amorphous silica which has much stronger activity. The similar effect can be achieved by acid or alkali treatment as well. 4. Conclusions The DFT calculation of three Pb species adsorption on kaolinite surfaces provided large useful information about adsorption product structure, bonding between adsorbate and surface, adsorption energy and reaction barrier which established some in-depth understandings. The Pb vapor adsorption occurs at Al surface not Si surface because the active sites are non-VI-coordinated Al atoms and O atoms losing H atom. Pb atom can be adsorbed around active O atoms by weak covalent bond and unsaturated Al atoms by strong covalent bonds. PbO can be adsorbed not only by weak Pb-O covalent bond but also strong O-Al ionic bond which makes PbO adsorption stronger than Pb atom and PbCl2 adsorption.

Besides the Pb-O covalent bond, PbCl2 could be adsorbed by OH-Cl hydrogen bond or Cl-Al covalent bond. The high energy barriers of de-HCl process cause the difficulties of the transition from the weak chloride adsorption to the strong oxide adsorption. It is easy for PbO and PbCl2 to adsorb on Al surfaces but difficult to escape. Considering the inertia of Si atoms and the activity of Al atoms in the kaolinite after dehydroxylation, calcination, acid/alkali treatment and some other treatment aiming at amorphous silica producing and Al activity enhancement can be used as the modification measures to improve the performance of kaolinite as the in-furnace metal capture sorbent. Acknowledgments The authors gratefully acknowledge the support from the National Natural Science Foundation of China (No. 51476031) and the Doctoral Fund of Ministry of Education of China (No. 20130092110007). Special thanks to the College of Materials Science and Engineering of Nanjing Tech University for the use of CASTEP. References [1] C.Y. Wu, P. Biswas, Lead species aerosol formation and growth in multicomponent high-temperature environments, Environ Eng Sci. 17 (2000) 41-60. [2] H. Yi, J. Hao, L. Duan, X. Tang, P. Ning, X. Li, Fine particle and trace element emissions from an anthracite coal-fired power plant equipped with a bag-house in China, Fuel. 87 (2008) 2050-2057. [3] T.K. Gale, J.O.L. Wendt, In-furnace capture of cadmium and other semi-volatile metals by sorbents, Proceedings of the Combustion Institute. 30 (2005) 2999-3007. [4] J.O.L. Wendt, S.J. Lee, High-temperature sorbents for Hg, Cd, Pb, and other trace metals: Mechanisms and applications, Fuel. 89 (2010) 894-903. [5] X.Y. Wang, Y.J. Huang, Z.P. Zhong, Y.P. Yan, M.M. Niu, Y.X. Wang, Control of inhalable particulate lead emission from incinerator using kaolin in two addition modes, Fuel Processing Technology. 119 (2014) 228-235. [6] T.W. Gale, JOL, High-temperature interactions between multiple-metals and kaolinite, COMBUSTION AND FLAME 131 (2002) 299-307. [7] S.B. Davis, J.O.L. Wendt, Mechanism and kinetics of lead capture by kaolinite in a downflow combustor, Proceedings Of the Combustion Institute. 28 (2000) 2743-2749. [8] H. Yao, I. Naruse, Control of trace metal emissions by sorbents during sewage sludge combustion, Proceedings of the Combustion Institute. 30 (2005) 3009-3016. [9] L. Sørum, F.J. Frandsen, J.E. Hustad, On the fate of heavy metals in municipal solid waste combustion. Part II. From furnace to filter, Fuel. 83 (2004) 1703-1710. [10] B. Zhang, X.Y. Yan, K. Shibata, T. Uda, M. Tada, M. Hirasawa, Thermogravimetric-mass spectrometric analysis of the reactions between oxide (ZnO, Fe2O3 or ZnFe2O4) and polyvinyl chloride under inert atmosphere, Mater T Jim. 41 (2000) 1342-1350. [11] B. Nowak, S. Frías Rocha, P. Aschenbrenner, H. Rechberger, F. Winter, Heavy metal removal from MSW fly ash by means of chlorination and thermal treatment: Influence of the chloride type, Chemical Engineering Journal. 179 (2012) 178-185. [12] X.Y. Wang, Y.J. Huang, M.M. Miu, Y.X. Wang, Effect of multi-factors interaction on trace lead equilibrium during municipal solid waste incineration, J mater cycles waste. In press (2014) DOI: 10.1007/s10163-014-0332-0. [13] M.V. Scotto, M. Uberoi, T.W. Peterson, F. Shadman, J.O.L. Wendt, Metal Capture by Sorbents In Combustion Processes, Fuel Processing Technology. 39 (1994) 357-372.

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Figure and Table

Fig. 1. Detailed description of kaolinite. Framework structure of (a) the view along X-axis and (b) the view against Z-axis (yellow ball = Si, purple ball = Al, red ball = O and white ball = H). Fig. 2. TG-DSC plots of raw kaolinite at heating rate of 20 ºC/min. Fig. 3. Selected regions of FTIR spectra of raw kaolinite and flash-calcined kaolinite within 1s, 3s and 5s at 900 ºC. Fig. 4. Optimized structure of (a) kaolinite, (b) kaolinite losing one water molecule and (c) metakaolinite. Fig. 5. Pb atom adsorption on Si surface with the structure of (a) the view along X-axis and (b) the view against Z-axis, (c) charge density distribution of Pb-O slice and (d) charge density difference of Pb-O slice (black ball = Pb). Fig. 6. Pb atom adsorption on Al surface-1 with the structure of (a) the view along X-axis and (b) the view against Z-axis, (c) charge density distribution of Pb-O-Al slice and (d) charge density difference of Pb-O-Al slice.

Fig. 7. Pb atom adsorption on Al surface-2 with the structure of (a) the view along X-axis and (b) the view against Z-axis, (c) charge density distribution of Pb-O-Al slice and (d) charge density difference of Pb-O-Al slice. Fig. 8. PbO adsorption on Si surface with the structure of (a) the view along X-axis and (b) the view against Z-axis, (c) charge density distribution of O-Pb-O-Si slice and (d) charge density difference of O-Pb-O-Si slice. TS structure of (e) the view along X-axis and (f) the view against Z-axis. Fig. 9. PbO adsorption on Al surface-1 with the structure of (a) the view along X-axis and (b) the view against Z-axis, (c) charge density distribution of O-Pb-O slice; (d) charge density difference of O-Pb-O slice; (e) charge density distribution of Al-O-Al slice and (f) charge density difference of Al-O-Al slice. TS structure of (g) the view along X-axis and (h) the view against Z-axis. Fig. 10. PbO adsorption on Al surface-2 with the structure of (a) the view along X-axis and (b) the view against Z-axis, (c) charge density distribution of O-Pb-O-Al slice and (d) charge density difference of O-Pb-O-Al slice. TS structure of (e) the view along X-axis and (f) the view against Z-axis. Fig. 11. PbCl2 adsorption on Si surface with the structure of (a) the view along X-axis and (b) the view against Z-axis, (c) charge density distribution of Pb-O slice and (d) charge density difference of Pb-O slice (green ball = Cl). Fig. 12. PbCl2 adsorption on Al surface-1 with the structure of (a) the view along X-axis and (b) the view against Z-axis, (c) charge density distribution of O-Pb-Cl-OH slice and (d) charge density difference of O-Pb-Cl-OH slice. TS structure of (e) the view along X-axis and (f) the view against Z-axis. De-HCl-1 structure of (g) the view along X-axis and (h) the view against Z-axis. De-HCl-2 structure of (i) the view along X-axis and (j) the view against Z-axis. Fig. 13. PbCl2 adsorption on Al surface-2 with the structure of (a) the view along X-axis and (b) the view against Z-axis, (c) charge density distribution of O-Pb-Cl-Al slice and (d) charge density difference of O-Pb-Cl-Al slice. TS structure of (e) the view along X-axis and (f) the view against Z-axis. Table 1 Related adsorption bands and their corresponding functional groups [38-40]. Table 2 Adsorption energies and energy barriers in the adsorption processes. Table 3 Length of possible bonds between adsorbates and surface. Table 1 Related adsorption bands and their corresponding functional groups [38-40]. Band Group or structure Band Group or structure 3695 inner-surface –OH 1070 metakaolinite Al2O3⋅2SiO2 3669 inner-surface –OH 939 inner-surface –OH 3650 inner-surface –OH 915 inner –OH 3620 inner –OH 817 tetrahedral Al-O 755 octahedral O-Al-OH 699 octahedral O-Al-OH 538 octahedral O-Al-OH

Table 2 Adsorption energies and energy barriers in the adsorption processes. Adsorption Energy barrier (kJ/mol) Reaction system energy From reactant From product (kJ/mol) Pb

Si surface Al surface-1 Al surface-2

0 -131 -241

No TS No TS No TS

No TS No TS No TS

PbO

Si surface Al surface-1 Al surface-2

-2 -343 -476

26 12 74

28 355 550

PbCl2

Si surface Al surface-1 De-HCl-1 De-HCl-2 Al surface-2

0 -127 108 170 -247

No TS 9 492 606 3

No TS 137 384 436 250

Table 3 Length of possible bonds between adsorbates and surface. Reaction system

Possible bonds and bond length (Å)

Pb

Si surface Al surface-1 Al surface-2

Pb-O 2.793 Pb-O 2.320, Pb-Al1 2.952, Pb-Al2 3.015 Pb-O1 2.541, Pb-O2 2.629, Pb-Al 3.058

PbO

Si surface Al surface-1 Al surface-2

Pb-O 2.399, O-Si 1.837 Pb-O1 2.207, Pb-O2 2.535, O-Al1 1.886, O-Al2 1.878 Pb-O1 2.608, Pb-O3 2.509, Pb-O3 2.572, O-Al1 1.760, O-Al2 1.786

PbCl2

Si surface Al surface-1 Al surface-2

Pb-O 2.934 Pb-O 2.276, Cl1-OH1 2.171, Cl1-OH2 2.367, Cl2-OH3 2.188, Cl2-OH4 2.639 Pb-O 2.164, Cl1-Al1 2.400, Cl2-Al2 2.318

Graphical Abstract .