Effect of natural organic matter model compounds on the structure

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Effect of natural organic matter model compounds on the structure memory effect of different layered double hydroxides Shuang Xu, Jiawen Zhao, Qianqian Yu, Xinhong Qiu, and Keiko Sasaki ACS Earth Space Chem., Just Accepted Manuscript • DOI: 10.1021/ acsearthspacechem.9b00175 • Publication Date (Web): 30 Jul 2019 Downloaded from pubs.acs.org on August 2, 2019

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ACS Earth and Space Chemistry

Effect of natural organic matter model compounds on the structure memory effect of different layered double hydroxides Shuang Xu1, Jiawen Zhao1, Qianqian Yu2, Xinhong Qiu1*, Keiko Sasaki3,4*

1School

of Chemistry and Environmental Engineering, Wuhan Institute of Technology, Wuhan, China

2School

of Earth Science, China University of Geosciences, Wuhan, 430074, China

3Department 4Kyushu

of Earth Resources Engineering, Kyushu University, Fukuoka, Japan

University Institute for Asian and Oceanian Studies, Fukuoka, Japan

E-mail: [email protected] (Xinhong Qiu), [email protected] (Keiko Sasaki)

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Abstract The structural transformation of layered double hydroxides (LDHs) plays an important role in a variety of natural environmental behaviors. As one of the important way of structure transformation in LDHs, structure memory effect is closely interacted with humic acid (HA). However, the role of functional groups in HA is unclear since its structure is relatively complex. Therefore, seven representative small molecules of single or double aromatic compounds were selected to explore their influence on the regeneration of different LDHs, thereby revealing key function groups in HA. As a result, Ortho-benzene compounds could more effectively affect the regeneration of LDHs than other model compounds. For these compounds, the aliphatic chain length and the aromatic ring size have little effect on regeneration of LDHs. However, increasing the number of carboxyl or hydroxyl groups affected the results, though all of them accelerated the regeneration rate of LDHs. Additionally, LDHs during the regeneration have a positive adsorption effect on both o-carboxyl and o-hydroxyl aromatic compounds, however they have shown different effects on the regeneration of LDHs. Phthalic acid revealed no effect on the structure and crystallinity of regenerated LDH, while catechol inhibited the regeneration process and decreased the crystallinity of the regenerated LDHs. The effects of o-hydroxyl aromatic compounds like catechol, are also dependant on the type of LDHs. They have an inhibitory effect on the crystallinity of the regenerated Mg-LDH within a short time or with a high concentration, but have no effect on its morphology. Although o-hydroxyl aromatic compounds have no effect on the crystallinity of regenerated Ca-LDH, the crystal

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growth was suppressed. This is due to the difference in the regeneration processes between two LDHs. These results provide a more detailed theoretical basis for understanding the influence of HA on the structural transformation of LDHs, or even other minerals like hydroxides in the environments. Keywords: hydrotalcite and hydrocalumite; CLDHs; structural regeneration; humic acid; structural features;

1. Introduction Layered double hydroxides (LDHs) are anionic clay minerals1 with diverse properties and a wide range of applications.2,3 Siebecker et al.4 found that LDHs formed in soil play an important role in the migration of metallic ions through adsorption and their structural transformation. Moreover, Sahoo et al.5 reported that magnesium-rich LDHs could interact with atmospheric CO2, indicating that LDHs have the potential to store and/or separate CO2. This provided a new pathway in the sequence of the global carbon cycle. Some scholars even believe that the structural characteristics of hydrotalcite minerals have an important connection with the origin of life.6,7 LDHs can lose their layered structure at a certain calcination temperature and restore their original structure in aqueous solution8 and even humid atmosphere,9 which is called the structural memory effect. This process would produce new minerals depending on the types of regenerated LDHs10 and interlayer anion species.11,12,13 Therefore, these phenomena should inevitably affect the interaction between LDHs and the natural environment.

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Furthermore, humic acid (HA) is widely present in soil zone and aqueous phase of the natural environment. It behave as an important natural ligand to migrate aromatic pollutants in the epigenetic environment. It gives an extremely important influence on the surface properties of minerals, as well as on the migration and transformation of metals and pollutants.14,15,16,17 It has been confirmed that reaction with natural organic matter (NOM), particularly humic substances, can significantly alter the surface properties of minerals.18,19,20 Moreover, the interaction of HA with mineral surfaces is very common in the natural environment. Based on this, we have previously reported the effective mechanisms of different types of natural HA on the regeneration of LDHs. We found that HA indeed affected the structural regeneration of LDHs, and changed the physical and/or chemical properties of the material.21 These effects lead to the changes in the role of LDHs and other natural environment factors. In fact, HA does not have the specific molecular structure, but contains a large number of reactive functional groups. The coordination and content of different functional groups in HA have also been confirmed to be closely related to mineral interactions.22 Evanko et al.23 used goethite to adsorb a variety of aromatic compounds, and found that the adsorption performance is affected by the coordination, quantity, and type of these reactive functional groups. Liang et al.24 found that the differences in the above factors affected the degradation of carbon tetrachloride (CCl4) by green rust. Moreover, the amounts of these reactive functional groups in different sources of HA are not the same,21 which results in HA in different regions having different effects on minerals. It is therefore meaningful to understand the interactions of simple aromatic

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compounds with LDHs. However, the effects of functional groups in the several simplified models to simulate HA on LDH regeneration have not yet been reported in detail. Thus, this study not only provides a theoretical basis for the interactions between LDHs and HA in their actual environments, but also has a certain guiding significance for the transformation and other environmental effects of other minerals. In addition, considering that calcination of LDHs containing different divalent metallic ions produced different products, the functional groups in HA tend to have different interactions with them. Of these, MgAl-LDH has been extensively studied as a typical hydrotalcite.25,26 Due to the instability of CaAl-LDH, new mineral can be formed to achieve the purpose of removing contaminants, and Ca-LDH has various calcined products at different calcination temperature, which also leads to different regeneration processes.11,27. Therefore, in this paper, we studied the regeneration processes and mechanisms between two LDHs and seven simplified and well-characterized molecules containing the characteristic functional groups in HA, as shown in Fig. 1. We also investigated the effects of the type, coordination, and quantity of specific functional groups in HA on the surface properties, crystal structures, and morphologies of LDHs during regeneration.

2. Experimental methods 2.1 Chemicals Special grade magnesium nitrate hexahydrate (Mg(NO3)2·6H2O), calcium nitrate tetrahydrate (Ca(NO3)2·4H2O), aluminum nitrate nonahydrate (Al(NO3)3·9H2O), urea

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(CON2H4), sodium nitrate (NaNO3), sodium hydroxide (NaOH), benzoic acid (C6H5COOH), phenol (C6H5OH), phthalic acid (C8H6O4), catechol (C6H6O2), 2hydroxynaphthalene (C10H8O), 1-naphthoic acid (C11H8O2), and 4-phenylbutyric acid (C10H12O2) were obtained from WAKO (Osaka, Japan), and used without purification.

2.2 Preparation of LDHs To acquire high crystallinity of LDHs, the urea method was introduced for the preparation of MgAl-LDH. Al(NO3)3·9H2O (3.09 g), Mg(NO3)2·6H2O (4.29 g), and urea (4.90 g) were dissolved in 50mL of deionized water. The solution was then transferred into a Teflon vessel, which was placed in an oven at 100 oC for 36 h. After that, the slurry was separated by centrifugation at 9000 rpm for 5 min. The products were rinsed with deionized water, and then dried in a 50 oC oven. A high alkaline condition (pH > 11.5) was required for the Ca-LDH, and it could therefore not be synthesized by the urea method. Instead, hydrothermal co-precipitation methods were used for the synthesis of Ca-LDH. A solution containing a certain amount of Ca(NO3)2·4H2O (3.78 g) and Al(NO3)3·9H2O (3.09 g) at a molar ratio of 2:1 was added to dissolve in 50 mL of 0.5 mol/L NaNO3 solution; the pH was adjusted to 12 with 2 mol/L NaOH. The resulting slurry was transferred into a Teflon vessel, then sealed with a metallic holder and placed in a constant-temperature oven at 100 oC for 36 h. The cooled slurry was subjected to solid-liquid separation via super-centrifugation at 9,000 rpm for 5 min, and washed at several times with deionized water and ethanol. The solid residues were then dried in a vacuum drying oven. Before sorption experiments, two types of LDHs were calcined at 500 oC for 3 h, which were named as Mg-CLDH and

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Ca-CLDH, respectively. As mentioned by Sideris et al.,28 the arrange of metal ions in the metal layer are depended on the molar ratio of divalent and trivalent metals. When the molar ratio was 2:1, the the arrange of metal ions (Mg2+ and Al3+) in the layer is regular. In addition, base on previous work, the ratio was 2:1, the adsorption performance is higher than that of 3:1.29 Thus, the ratio 2:1 was adopted in this article.

2.3. Effect of HA models on the regeneration of LDHs Seven humic acid (HA) modeled solutions were prepared with 25mg/L, which were the same as those used in a previous article21, and 0.100 g of CLDHs was added to the 40 mL solution. Its initial pH was adjusted to ∼7.00, and the suspensions were shaken at 100 rpm at room temperature using a shaking incubator. At intervals, supernatants were filtered (0.45 µm) to provide for the determination of the total Ca, Mg, and Al concentrations via inductively coupled plasma-atomic emission spectrometry (ICP-MS, Optima 8300, Perkin Elmer, Massachusetts, USA). The concentrations of the aromatic model substances in the solution were determined by an ultraviolet-visible (UV-vis) spectrophotometer L5 (INESA, Shanghai, China). Solid residues after sorption were collected and characterized using the XRD, SEM, FTIR, and TEM.

2.4. Characterization The crystalline phase of the nanoparticles was determined using a D8 Advance X-ray diffractometer (Bruker, Germany) employing Cu Kα radiation with a scanning speed of 2/ min and scanning step of 0.02. The accelerating voltage and applied current were 30 kV and 20 mA, respectively. The morphologies of different samples

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were observed using a JSM-5510LV scanning electron microscope (SEM) (JEOL, Japan). Fourier transform infrared (FTIR) spectra were recorded on a JASCO FTIR spectrometer (FT/IR-670 Plus, Japan). The size and morphology of the nanoparticles after sorption were examined via a JEM-2100 transmission electron microscope (TEM) (JEOL, Tokyo, Japan)

2.5. Computational details Density functional theory (DFT) with the three-parameter hybrid functional (B3LYP) were used for geometrical optimization and the full electron basis sets, 631G(d), were used for B, O, and H, respectively. Gaussian 09 program was used for geometrical optimization and quantitative analysis of electrostatic potential on molecular surface was performed by Multiwfn 3.3.

3. Results and discussion 3.1 Powder XRD Analysis As determined by previous studies,21 HA can affect the regeneration rate of LDHs. Here, the XRD results can allow for rapid understanding of the effects of each aromatic compound models on regeneration of LDHs. The XRD patterns of the regenerations of Mg-LDH and Ca-LDH in deionized water are presented in Fig. S1. According to the structural memory effect of LDHs, the CLDHs can undergo structural regeneration in aqueous solution and return to the original layered structure. As can be seen in Fig. S1a, the crystalline structure of Mg-CLDH hardly changed after 3 h, but the characteristic diffraction peaks of Mg-LDH appeared after 48 h. However, after acting alone with seven modeled aromatic compounds, Mg-LDH was substantially

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regenerated from Mg-CLDH after 3 h, as shown in Fig. 2a. Catechol had an inhibitory effect on the regeneration of Mg-LDH compared with other models, and the content of MgO was 50.89%, which was higher than in the presence of other models (Table 1) (calculated using Maud software).30 Nevertheless, regenerated products in the presence of other models consisted of almost 75-98% of LDH (Table 1). In addition, Mg-CLDH was completely converted to MgAl-LDH after 48 h of regeneration in the presence of seven models (Fig. 2b). Only in the presence of catechol contained 0.2% of MgO, whereas 12.57% of MgO was not converted in deionized water (Table 1). In order to more clearly verify the effect of each modeled aromatic compounds on LDH regeneration, with increase in the concentration of the models, it was found that catechol showed a strong inhibitory effect after 48 h, and 75% of MgO remained, as shown in Fig. 2c and Table 1. It can be presumed that the specific chemical structure of catechol caused the difference from other models. In addition, when Ca-CLDH was regenerated in deionized water, the Ca-LDH was formed after 3 h, and 52.32% of CaCO3 was formed (Fig. S1b and Table 2). A similar result was obtained at 48 h. However, as shown in Fig. 3a, after Ca-CLDH reacted alone with the seven modeled aromatic compounds, there was no significant difference among the models, and the results were similar to those obtained in deionized water. Only the content of CaCO3 was reduced to about 10% (Table 2). When increasing the reaction time (Fig. 3b) or the concentration of the models (Fig. 3c), there was no significant difference among them. However, when regenerated in catechol, the crystal size was 228 Å, which was smaller than in other models. Additionally, at a high

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concentration, the crystal size was 291 Å and the content of CaCO3 was 93.63% (Table 2).

3.2 FTIR spectroscopy FTIR spectra for the solids after the reactions of the two CLDHs with the models were collected to clarify their component and structural characteristics, as presented in Figs. S2-3. The FTIR spectra of the solids after the interaction between the Mg-CLDH and 25 mg/L models are provided in Fig. S2a. It can be seen that the results after interaction with the seven models were substantially similar to that in deionized water. The peaks around 3400 cm-1 were related to the stretching vibration mode of the -OH groups in the lattice structural water and the structural -OH groups in the LDHs. The vibration around 1630 cm-1 can be assigned to the bending vibration mode of the HO– H that usually exists in interlayer water molecules.31 The peak at 1360 cm-1 is attributable to CO32- by the incorporation of CO2 during the reaction period.32 The spectral band at 1063 cm-1 also corresponded to the interlayer carbonate.33 There were wide absorption bands near the regions of 500-800 cm-1, which were generally ascribed to the lattice vibration mode of metal-oxygen bonds.34 Similar results also appeared in the reactions of Mg-CLDH and 600 mg/L models (Fig. S2b). However, there are two types of model compounds that exhibited new vibrations mode. One is phthalic acid; the newly emerged peak of 1553 cm-1 can be attributed to C-C stretching vibration mode of benzene ring.35 Another is catechol; there were three new peaks of 1579 cm-1, 1493 cm-1, and 1258 cm-1, respectively, that can be assigned to an benzene ring.36 FTIR spectra for the reaction products between Ca-CLDH and 25 mg/L models

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are provided in Fig. S3a, which were similar to the products without aromatic compound models. The FTIR spectra showed a broad and strong band in the range of 3400–3800 cm-1 that was due to the O–H stretching vibration mode of the inorganic layers and interlayer water molecules. Another commonly observed wave number for LDH-like material was a band of 1630 cm-1, which was assigned to the bending vibration mode of interlayer water molecules. The shoulder at 1410 cm-1 was attributed to CaCO3 due to the reaction of CO32- with the dissolved Ca2+, and it easily overlapped with the vibration peak at 1380 cm-1, which was caused by carbonate.37 Correspondingly, the newly emerged adsorption band at 1046 cm-1 was attributed to the CO32- intercalated in the Ca-LDH. A band at 500-800 cm-1 in the lower wave number region corresponded to the lattice vibration mode, such as Ca-O, Mg-O, etc. Analogous changes have also occurred in the results of Ca-CLDH and 600 mg/L models (Fig. S3b). There was no significant difference between the other models, and the results were similar to those of 25 mg/L modeled aromatic compounds. However, new peaks appeared in the resultant products in the presence of phthalic acid and catechol. For phthalic acid, a new peak of 1560 cm-1 can be attributed to the C-C of benzene ring. Additionally, two peaks of 1491 cm-1 and 1257 cm-1 in the catechol results can be assigned to the benzene ring.

3.3 SEM observation SEM images were observed in order to further understand the effect of each model on the regeneration of two LDHs. The SEM images of the products from Mg-CLDH reactions with phthalic acid and catechol are presented in Fig. 4. The results indicated

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that the morphology was mostly an anomalistic lamellar structure with a coarse surface, which formed a certain agglomeration that was different from the original Mg-CLDH. It can also be seen that when the concentration increased, the solid morphology exhibits a smooth hexagonal layered structure after the reaction with catechol, which was different from the results of the other models. This difference requires further exploration. Fig. S4 presented the SEM images of Mg-CLDH reactions with other models. After the low concentration models reacted with Mg-CLDH, the solid also exhibited a sheet-like topography with a rough surface, and there was almost no difference between the models. Analogous results also appeared in the high concentration models. The SEM images after reaction of Ca-CLDH with the seven modeled aromatic compounds are provided in Figs. 5 and S5. Fig. 5 demonstrates that their morphologies after the reaction were irregular, when phthalic acid or catechol concentrations were 25 mg/L. The typical LDH hexagonal structure can be roughly seen, and there were many fragments. When the concentration increased, it was found that the sheet structure became small fragments after the reaction with phthalic acid, and there was a tooth-like morphology after interaction with catechol. However, when Ca-CLDH reacted with the other models, regardless of the concentration, it still had the hexagonal sheet structure typical of LDHs (Fig. S5). Similar to Mg-CLDH, the presence of catechol also changed the morphology of Ca-CLDH after regeneration, while the other models did not differ significantly.

3.4 TEM and SAED observation 11


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The morphologies and crystal phases of the solid products after the interaction of two CLDHs with the modeled aromatic compounds were confirmed by TEM and SAED. Figs. 6 and S6 exhibited for the reaction products between Mg-CLDH and the models. As shown in Fig. 6a, the nanosheets regenerated in deionized water had a complete hexagonal structure, and there were many needle-like solids on the surface due to the structural regeneration of CLDH that vertically developed a new, small sheet of LDHs on its surface.21 The corresponding selected area electron diffraction (SAED) patterns of individual sheets exhibited hexagonally arranged spots, confirming its single-crystal nature.38 A clear and even arrangement also indicated that the regenerated LDHs were in a high crystallinity. When regeneration occurred in phthalic acid and catechol (Figs. 6b and 6c), their morphologies after the reaction were almost the same as those in deionized water. The corresponding SAED results demonstrated that the crystallinity of the regenerated LDH was high, which was also in line with the XRD results (Fig. 2). This indicates that regeneration in phthalic acid and catechol solutions has little effect on the morphology and crystallinity of LDH. However, the LDHs regenerated in other modeled aromatic compounds were different, as shown in Fig. S6. Of these, the solid products regenerated in the presence of benzoic acid also showed partially similar results to those regenerated in phthalic acid. Large, rod-like solid in several hundreds nm of the length appeared on the surface. The morphology that were regenerated in other modeled aromatic compounds was incomplete or even bursting, and the surface was partially covered with fine, needle-like solids. Their corresponding SAED patterns were not so much clean

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compared with phthalic acid and catechol. TEM images for the reaction products of Ca-CLDH with the modeled compounds are shown in Figs. 7 and S7. After regeneration in deionized water, the LDH sheet was structurally intact but partially accumulative, resulting in an increase of diffraction spots in the SAED pattern. But the results of phthalic acid was different, in which the regenerated solid showed a complete hexagonal sheet-like structure with few fragments. Although there was a certain degree of overlap and adhesion of small-sized solids, combined with the results of SAED, phthalic acid had little effect on the morphology and crystallinity of the regenerated Ca-LDH. However, the solid products in the presence of catechol consisted of small particles with rough surface which were stacked. There were almost no large regular hexagonal sheet structures developed, which could also be further confirmed from the SAED patterns where their poor crystallinity is characterized. In addition, the TEM/SAED results of the products after reaction with other modeled aromatic compounds were mostly similar (Fig. S7). They all had a relatively complete sheet-like structure, but there were also small, irregular solids that heaped up.

3.5 Regeneration of LDHs with the modeled aromatic compounds Based on the above characterizations, it is evident that the regeneration of LDHs in the presence of phthalic acid and catechol is different from the regeneration by the other modeled aromatic compounds. The concentrations of the modeled aromatic compounds and cations, and the pH of the solutions after Mg-CLDH interacted with the modeled compounds, are shown

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in Fig. 8. As plotted in Fig. 8a, when the initial concentration of modeled aromatic compounds was 25 mg/L, the concentrations of phthalic acid and catechol were significantly reduced after the same reaction time, whereas the concentrations of the other models fluctuated around 25 mg/L. This indicated that Mg-CLDH strongly reacted with phthalic acid and catechol, whereas it had little effect on the others. The release of Mg2+ during the interaction is presented in Fig. 7c. Similarly to the result of the reaction in deionized water, the Mg2+ concentration first increased to the maximum, and then gradually decreased (Fig. S8a). However, in the presence of catechol, the maximum amount of released Mg2+ reached 0.5 mmol/L, which was higher than that in the deionized water (Fig. S8a), indicating that the presence of catechol enhanced the dissolution of Mg2+. Because catechol as a bis-hydroxyl compound has high ionization constant, enabling it to induce more metal ions. When the alkalinity of the solution reached a certain value, the metal ions would gradually transform into MgLDH. In this process, the consumption of metal ions was constant, so that the ions concentration remaining in the solution was higher. On the contrary, other models had the same effect on the release of Mg2+, as in deionized water. However, a maximum Mg2+ concentration was achieved in the model solution faster than in the deionized water. The Al3+concentration also first increased to a maximum, then gradually decreased (Fig. 8d), just as in the deionized water (Fig. S8a). Similarly, the presence of catechol also increased the concentration of Al3+ in the solution, which was twice that in deionized water. Other models still had no significant effect on the maximum release of Al3+, but their concentrations eventually remained between 0.1 and 0.3

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mmol/L, unlike that in the deionized water. However, during the reaction, the pH of the solution rose to ~10.5 and maintained the equilibrium, as shown in Fig. 8b. This is slightly higher than the pH balance in the deionized water (Fig. S8c), indicating that the aromatic model compounds enhanced dissolution of CLDH to release OH−. Although Mg-CLDH had a similar sorption effect on phthalic acid and catechol, the releasing behaviour of cation shows that the way in which they act with Mg-CLDH is different. Additionally, there are no significant differences among the other models. Fig. 9 displays the results of the interactions between Ca-CLDH and the modeled aromatic compounds. Similarly to the results of Mg-CLDH, Ca-CLDH also had a significant effect on phthalic acid and catechol, as shown in Fig. 9a. However, their final concentrations were higher than those after the reactions with Mg-CLDH. The concentrations of the other models remained essentially constant, and the concentration of Ca2+ released remained between 3 and 6 mmol/L during the reactions with the seven models (Fig. 9c). Compared with deionized water (Fig. S8b), the addition of the models reduced the concentration of Ca2+ in the solutions, indicating that the addition of the models changed the ion dissolution rule, and would inevitably influence the regeneration process of LDHs. In addition, the dissolution rule of Al3+ was also different compared to that in deionized water, and the concentration was fluctuated below 3 mmol/L (Fig. 9d), which was lower than that in deionized water (Fig. S8b). The pH of solution attained ~12.5 (Fig. 9b), which was slightly higher than that in deionized water (Fig. S8c). This also indicates that aromatic model compounds caused to release the OH− in the solution. However, the ionic dissolution rules from

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Ca-CLDH in the presence of phthalic acid and catechol are not different, and there was no difference with the other models, which is unlike the results of Mg-CLDH. For Ca-CLDH, since the conversion of LDH occurred from the beginning, the addition of catechol only accelerated the process of dissolution and re-precipitation, and this process was dynamically balanced. Even if more metal ions were generated, they would quickly re-precipitate into Ca-LDH or CaCO3. So, the concentration of metallic ions in the presence of catechol would not higher than that of other models.

3.6 Mechanism for the regeneration of Mg-LDH and Ca-LDH 3.6.1 Effect of o-benzene on regeneration of Mg-LDH The dissolution-reprecipitation (DR) process of metal ions is very important for the structural regeneration of LDHs.39 When Mg-CLDH was added to deionized water, ≡ Mg-O−, ≡ Al-O−, ≡ Mg-OH, ≡ Al-OH, ≡ Mg-OH2+, and ≡ Al-OH2+ should be generated on its surface,27 and the dissolution of Mg2+ and Al3+, and the generation of OH–, also occurred. This phenomenon explains the increase in the pH of the solution and the concentration of metallic ions in the early stage of the reaction (Fig. S8). When the pH reaches a certain value, the alkaline conditions allow Mg2+ and Al3+ to react with the OH− in the solution again. This process causes the Mg2+ concentration to decrease, and the Al(OH)4 concentration to increase. As the reaction progresses, the positive charge of the metal plate layer causes Al(OH)4 and Mg(OH)2 to react to form LDH with a CO32− or OH− interlayer (Eqs. 1 and 2).25 xMg(OH)2 + Al(OH)4− + OH− ⇄ MgxAl(OH)2+2x·OH− + 2OH− xMg(OH)2 + Al(OH)4 − + 1/2CO32− ⇄ MgxAl(OH)2+2x·(CO32−)1/2 + 2OH−

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(1) (2)


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Because Mg-CLDH proceeds with a dissolution-regeneration process, small sheets were formed to appear on the surface of the large hexagonal plates (Fig. 3). However, when Mg-CLDH was added into the phthalic acid or catechol solution, the process changed. The surface of Mg-CLDH was also hydrated, and produced ≡ Mg-O−,≡Al-O−,≡Mg-OH, ≡Al-OH, ≡Mg-OH2+, and ≡Al-OH2+. The dissolution of Mg(OH)+ and Al(OH)4− also occurred. The phthalic acid or catechol reacted with the dissolved metal ions, resulting in an increase in the metal dissolution, and the maximum metal concentration was reached within 5 min (Fig. 8) (Eqs. 3-6). OH

2

+ M

M

+ 2H+

O

OH COOH

2

O

O 2+

O

COO

+

M2+

(3)

OOC

M

+ 2H+

COO

COOH

OH

OOC

(4)

O

+ M2+

M + 2H+ O

OH

(5)

O COOH

C

+ M

O

2+

M C

COOH

+ 2H+

O

O

(6)

The released ions precipitates to form LDH under alkaline conditions, and the concentration of metallic ions in the solution was not much higher than that in the deionized water. This also sped up the regeneration of Mg-LDH. However, there were some differences in the case of catechol. During the regeneration process, new LDHs require anions to balance the charge to stabilize the layered structure, but the solution primarily had OH− and ionized catechol. According to the average electrostatic

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potential distribution (ESP, calculated by Multiwfn 3.340 and Gaussian 09,41 methods: Density functional theory (DFT) with the three-parameter hybrid functional (B3LYP)42 were used for geometrical optimization and the full electron basis sets, 6-31G(d)43) of the complete molecular surface, the ESP of OH− (-175.74 kcal/mol) is larger than that of catechol (-198.92 kcal/mol). This makes catechol more easier to be adsorbed onto the metallic oxide surface, or intercalated between the layers of LDH. Because of its high acid dissociation constant (pKa1 = 9.45, pKa2 = 12.8), when it was attracted to the vicinity of the metallic oxide, catechol can be more easily complexed with metal ions, resulting in a higher metallic ion concentration in the solution (Figs. 8c and d). The results of XRD demonstrate that the interlayer spacing after regeneration in deionized water was 7.61 Å, but that in catechol was 7.51 Å (Table 1). The change indicates that a certain amount of catechol was incorporated in the interlayer of LDH after the reaction and interacted with the hydroxyl groups on the laminate, which resulted in small interlayer spacing. However, as is evident from Figs. 2a and 2c, the crystallinity after the reaction with catechol was poor, and the regeneration of LDH was inhibited to some extent. It can be considered that catechol complexes with the surface of the metallic oxide, inhibiting further dissolution of metallic ions. And from Table 1, it can be found that the crystal size of Mg-LDH in the presence of catechol was little smaller than others. For the regenerated Mg-LDH, its surface also had hydroxyl group that could complex with catechol, which was bound to inhibit the growth of Mg-LDH to some extent. But the regeneration process of Mg-LDH gradually occurred from the surface to the inside of the oxide, so when the concentration of catechol was not high,

18



the inhibitory effect was not very obvious. Because the concentration of the modeled aromatic compounds was too low, the presence of the corresponding group would not be detected in the FTIR spectra even if they are adsorbed (Fig. S2a). However, the results of high-concentration catechol showed the vibration peaks both of the aromatic ring skeleton and of the phenolic hydroxyl group, further confirming the above conclusion. The carbonate stretching vibration mode at 1385 cm−1 suggests that catechol entered into the interlayer of LDH and interacted with CO32− via electrostatic attraction.36 The SEM images of high-concentration catechol exhibited a hexagonal flaky morphology with a smooth surface (Fig. 4b2). Combined with the corresponding XRD patterns (Fig. 2c) and FTIR spectra (Fig. S2b), it can be determined that when MgCLDH was regenerated in high-concentration catechol, a large amount of catechol completely occupied the adsorption site of the metallic oxide surface and the surface adsorption site of the transiently regenerated LDH, thereby suppressing the regeneration of Mg-LDH. It is known that calcined MgAl-LDH exhibits a hexagonal morphology with smooth surfaces,21 being consistent with the results of the reaction at high concentrations of catechol. A schematic diagram of the effect of catechol on the regeneration of Mg-LDH is illustrated in Fig. S9. As for phthalic acid, although its electrostatic potential (-177.58 kcal/mol) is similar to that of OH− (-175.74 kcal/mol), its ionization constant is small (pKa1 = 2.89, pKa2 = 5.51), resulting in its ability to be adsorbed on the metallic oxide, though it is less likely to bond with the metallic oxide surface and the regenerated LDH surface.

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Therefore, it was preliminarily believed that the strong adsorption of phthalic acid by Mg-CLDH was because it was primarily incorporated in the interlayers of the new LDH, and not on the laminate. Its layer spacing was 7.49 Å (Table 1), indicating that it was adsorbed in the interlayers of LDHs. However, the crystallinity after the reaction with phthalic acid was high, and did not resemble that after the reaction with catechol (Fig. 2a and 2c). This indicates that the process of adsorbing phthalic acid did not deteriorate the crystal phase of the regenerated LDH. Because of the strong adsorption to phthalic acid, the vibration peak of the aromatic ring skeleton at 1553 cm-1 can also be found in the high-concentration FTIR spectra (Fig. S3b). This distinction from catechol also indicates that two aromatic compounds were incorporated in LDH with different ways. It can also be observed from the SEM images (Fig. 4) that the reaction products between Mg-CLDH and phthalic acid exhibit similar morphological characteristics as those in deionized water, rather than those generated from the reaction with catechol. The TEM images (Fig. 4) also revealed that the solid morphology was relatively complete, and that almost no small fragments were produced. Combined with the above analysis, it can be conjectured that because the electrostatic potential of phthalic acid is basically equivalent to that of OH−, the regeneration environment was similar to that in deionized water. Therefore, it can be determined that the phthalic acid mainly entered the interlayer of Mg-LDH, and the crystallinity (Fig. 2) and morphology (Fig. 4) after the reaction were not affected. A schematic diagram of the effect of phthalic acid on the regeneration of Mg-LDH is

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depicted in Fig. S10.

3.6.2 Effect of o-benzene on regeneration of Ca-LDH Unlike Mg-CLDH, which produces reactive groups on the surface of deionized water, when Ca-CLDH was added into deionized water, it reacted directly with the water, or even the CO32− in the solution, to form LDH (Eqs. 7 and 8).44 Ca4Al2O5(OH)4 + 5H2O→2Ca2Al(OH)6·OH Ca4Al2O5(OH)4 + 5H2O + CO32−→[Ca2Al(OH)6]2·CO3 + 2OH−

(7) (8)

However, Ca-LDH is unstable, leading to the following dissolution.45 Ca2Al(OH)6·OH ⇄ 2Ca2+ +Al(OH)4−+ 3OH− [Ca2Al(OH)6]2·CO3 ⇄ 4Ca2+ +2Al(OH)4−+ 4OH- + CO32−

(9) (10)

The dissolved Ca2+ can react with CO32− in the solution to form a more stable CaCO3, and the CaCO3 is therefore present in the XRD pattern (Fig. S8). Therefore, in deionized water, the regeneration of LDHs proceeds as described above. When the regeneration process takes place in the model solutions, however, the situation becomes different. First, because the modeled aromatic compounds were capable of complexing with the eluted metal ions, the concentrations of ions in the solutions (Fig. 9) were lower than in the deionized water (Fig. S8b). As determined from the XRD patterns in the presence of catechol, the interlayer spacing was all ~7.4 Å, which was less than that in deionized water (Table 1). This indicates that catechol entered into the interlayer of the regenerated LDH. However, the XRD results were substantially the same at low concentrations (Figs. 2a and 2b), which was dissimilar to the results of Mg-CLDH. It is known that when Mg-CLDH is added

21


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to the solution, some reactive groups are first produced, which allows catechol, with high acid dissociation constant, to be adsorbed to the surface, inhibiting the further reaction of the metallic oxide. However, Ca-CLDH first reacts with the solution and converts to Ca-LDH (Eqs. 7 and 8), followed by the dissolution and re-precipitation processes (Eqs. 9 and 10). Therefore, when Ca-CLDH was regenerated, part of the catechol was first fixed in the interlayer of the new Ca-LDH, which resulted in the solid crystallinity being unaffected when regeneration occurred in a short amount of time. Due to its electrostatic potential and ionization constant, the remaining catechol will be complexed with the surface of the regenerated LDH, inhibiting the size of the Ca-LDH that has already formed. This is confirmed by Table 2. This was consistent with the results of the TEM observation (Figs. 7c1 and 7c2). The size of the regenerated sheet topography was significantly smaller than that of the other models, although the observed SEM images are not obvious (Figs. 5b1 and 5b2). Additionally, the vibrational peak of the aromatic ring skeleton and the phenolic hydroxyl group appeared in the FTIR spectra (Fig. S3b), which was also the same as the result of the Mg-CLDH. Therefore, considering the above results, it can be determined that in a high-concentration solution, a large amount of catechol was complexed with the metallic ions in the solution to adhere to the surfaces of the formed LDH and the unreacted metallic oxide, which changed the original sheet morphology (Fig. 5b2). Therefore, the catechol blocked the growth of LDH, this also means that catechol hindered the metallic ions from continuing to react to form LDH on this basis.

22



And there was a certain amount of CO32- in the solution due to the CO2 contained in the air and the solution itself. It is known that the Ksp of CaCO3 is 4.96*10-9, when the ions concentration in the solution reached a certain value, it would self-generate into CaCO3. Therefore, in a high concentration catechol solution, the solid product was mainly CaCO3, and a small amount of Ca-LDH, which was consistent with the XRD results (Fig. 2c). A schematic diagram of the effect of catechol on the regeneration of Ca-LDH is presented in Fig. S11. As for the results of the reaction with phthalic acid, it can be seen that the adsorption result was substantially the same as that of catechol. According to Table 2, the layer spacing after regeneration in the presence of phthalic acid was about 7.43 Å, which was less than that in deionized water, indicating that phthalic acid entered the interlayer. The results of Fig. 2 and Table 2 show that at low concentrations, the content of CaCO3 decreased with increase in the reaction time, indicating that phthalic acid can inhibit the formation of CaCO3 during regeneration. Considering the above analysis and the results of XRD, SEM, and FTIR, a large amount of phthalic acid was adsorbed between the interlayers of Ca-LDH during the regeneration and did not affect the crystallinity and morphology of the regenerated LDH, which is similar to the results of Mg-CLDH. A schematic diagram of the effect of phthalic acid on the regeneration of Ca-LDH is provided in Fig. S12.

3.6.3 Effect of non-o-benzene on regeneration of Mg-LDH and CaLDH The effect on the regeneration of Mg-LDH was almost the same for the other five

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models. As seen in Fig. 8, Mg-CLDH exhibited little adsorptive effect on them. Because its electrostatic potential was larger than that of OH− (the ESP of benzoic acid:94.75 kcal/mol, phenol:-102.44 kcal/mol, 2-hydroxynaphthalene:-88.53 kcal/mol, 1naphthoic acid:-82.52 kcal/mol, and 4-phenylbutyric acid:-158.04 kcal/mol; the pKa of benzoic acid: 4.19, phenol: 9.98, 2-hydroxynaphthalene: 9.34, 1-naphthoic acid: 4.17, and 4-phenylbutyric acid: 4.76 (25℃)) it was easier to fix OH− to the interlayer during regeneration.13 And their interlayer spacing after the reaction was smaller than that in deionized water (Table 1), suggesting that a small number of organic models entered the interlayer or acted at the edge to reduce the interlayer spacing. The regeneration rate of Mg-LDH can be accelerated in the model solutions (Figs. 2 and S1) because these models can also complex with the dissolved metallic ions in small amounts. The results of FTIR (Fig. S2) and SEM (Fig. S4) indicate that the presence of these models had little effect on the structure and morphology of regenerated Mg-LDH. Moreover, because there was a large amount of un-adsorbed organic matter in the solution, they could better promote dissolution of LDHs, form complexes, and eventually rupture the partial sheet structure (Fig. S6). Similarly to Mg-CLDH, Ca-CLDH also exhibited almost no adsorptive effect on these five models (Fig. 9). However, their interlayer spacing was different (Table 2). After reaction in the benzoic acid, 2-hydroxynaphthalane, 1-nnaphthoic acid, and 4phenylbutyric acid solution, it was found that the solids had interlayer spacing of ~7.92 Å and ~7.5 Å, respectively. The small layer spacing was thought to be caused by OH− or CO32−. The large layer spacing was due to the intercalation of the model. This was

24



different from the results of Mg-CLDH, indicating that their interlayer molecular arrangements were different. For the solid residues after reaction with phenol, there was just one LDH with an interlayer spacing of 7.44 Å. For phenol, it had a high ionization constant (pKa=9.98) that let it can coordinate with the hydroxyl group on the laminate, resulting the interlayer spacing became smaller. But for benzoic acid (pKa=4.19), 1naphthoic acid (pKa=4.17) and 4-phenylbutyric acid (pKa=4.76), their ionization constants were small. The force formed by bonding with the hydroxyl groups on the laminate was not sufficient to allow the interlayer spacing smaller, and CO32− and/or OH− were present in the solution, so that the Ca-LDH with two interlayer spacing were formed. In addition, although the ionization constant of 2-hydroxynaphthalane (pKa=9.34) was similar with that of phenol, the structure of the biphenyl ring made its molecular size larger than that of phenol, so the layer spacing eventually increased. In addition, the results of FTIR (Fig. S3) and SEM (Fig. S5) indicate that the presence of these models had little effect on the structure and morphology of Ca-LDH, although they also resulted in some of the sheet structures being incomplete (Fig. S7), as in Mg-CLDH. A schematic diagram of the effects of other models on the regeneration of Mg-LDH and Ca-LDH is illustrated in Fig. S13.

3.7 Environmental implications LDHs have a wide range of applications in environmental management due to their ability to remove contaminants and their unique mechanisms, and CLDHs are especially notable for these characteristics.46,47 These processes are closely related to their structural transformation. In addition, it has been gradually discovered that their

25


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structural characteristics are also closely related to the metal migration of soil and groundwater environments,4 the global atmospheric carbon cycle,5 and even the

early

life origin.6 However, the transformation of these hydrotalcite structures is closely related to environmental factors. The previous researches show that HA can indeed affect the regeneration of hydrotalcite structures,21 and can even affect the mechanism by which they remove pollutants.48 However, this research has been conducted at the macromolecular level, and does not use a simple functional group model to further explore its interaction with hydrotalcite. In the present work, we demonstrated that aromatic compounds to model and simplify the HA can complex with dissolved metallic ions during the structural transformation of LDHs, which are likely to affect the mechanism by which they remove contaminants, and can even affect other environmental behaviors. The ohydroxyl functional group can inhibit the regeneration of LDHs, which may result in a poor ability to treat organic/inorganic contaminants. This is also related to the type of LDH; different LDHs have different physical and chemical properties, which also provides a theoretical basis for the environmental behavior of different LDHs. However, the o-carboxyl functional group is beneficial to the structural transformation of LDHs to some extent. Therefore, the ratio of o-hydroxyl functional groups to o-carboxyl functional groups in HA affects the effect of LDH structural regeneration. This may lead environmental behaviors of LDHs depends on the regions where humic substances are abundant. However, the actual environment also contains a variety of minerals

26



which include analogue to LDHs. This research can also provide a theoretical basis for the structural transformation of these analogical minerals and certain environmental behaviors. Therefore, it is of great environmental significance to investigation the effects of functional groups in HA on the regeneration of LDHs via simplified aromatic compound models.

4. Conclusions The effect of organic models on LDH regeneration was influenced by the type and the number of functional groups, as well as by the type of LDHs. Compared with organics containing a single functional group, only o-benzene organics were well adsorbed by CLDHs, and only they affected the crystallinity and morphology of regenerated LDH, though all the models could accelerate the rate of regeneration. Additionally, o-carboxylic aromatic compounds had no effect on the regenerated LDH structure and crystallinity, while o-hydroxyl aromatic compounds had a significant influence on the structure and morphology of the regenerated LDHs. Moreover, due to the different regeneration processes of Mg-LDH and Ca-LDH, the o-hydroxyl aromatic compounds had different effects on their regeneration. They had an inhibitory effect on the crystallinity of the regenerated Mg-LDH within a short time and with a high concentration, but had no effect on its morphology. However, the o-hydroxyl aromatic compounds had no effect on the crystallinity of Ca-LDH after regeneration, but the size of LDH after regeneration was suppressed. The results demonstrate that HA macromolecules do affect the structural regeneration of LDHs by certain functional groups that have influence depending on the types of CLDHs. These results not only

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provide a more systematic theoretical basis for the structural regeneration behavior of LDH in a wide range of environments, but also provide a robust research basis for the behavior of analogical minerals of LDHs in the environments.

Supporting Information Detailed information on XRD and ion change plots for regeneration of LDH in deionized water, and FTIR, SEM, TEM results of regenerated LDHs in the presence of other models, and the schematic illustration of the reaction process between CLDHs and modeled compounds.

Acknowledge This work was supported by National Key R&D Program of China (No. 2018YFC1802701). This study was also support to KS by Japan society for the promotion of science (JSPS) KAKENHI Grant Number JP19H00883. National Natural Science Foundation of China (41703119, 51504170 and 51374157). Nature Science Foundation of Hubei Province of China (2017CFB139) and Graduate Education Innovation Foundation of Wuhan Institute of Technology (CX2018157).

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Page 32 of 44

Page 33 of 44 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure

Fig.1 Selected aromatic compounds to model the natural organic matter.

32



Fig.2 XRD pattern of regenerated Mg-LDH in the presence of different aromatic compounds (ACs). (a) ACs concentration: 25 mg/L, reaction time: 3 h; (b)ACs concentration: 25 mg/L, reaction time: 48 h; (c) ACs concentration: 600 mg/L, reaction time: 48 h.

Fig.3 XRD pattern of regenerated Ca-LDH in the presence of different aromatic compounds (ACs).(a) ACs concentration: 25 mg/L, reaction time: 3 h; (b) ACs concentration: 25 mg/L, reaction time: 48 h; (c) ACs concentration: 600 mg/L, reaction time: 48 h. 33


Page 34 of 44

Page 35 of 44 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(a1)

(a2)

(b1)

(b2)

Fig.4 SEM images of regenerated products from Mg-CLDH after 48h with (a1) 25 mg/L phthalic acid; (a2) 600 mg/L phthalic acid; (b1) 25 mg/L catechol; (b2) 600 mg/L catechol. Scale bars indicate 1μm in all.

(a1)

(a2)

(b1)

(b2)

Fig.5 SEM images of regenerated products from Ca-CLDH after 48h with (a1) 25 mg/L phthalic acid; (a2) 600 mg/L phthalic acid; (b1) 25 mg/L catechol; (b2) 600 mg/L catechol. Scale bars indicate 1μm in all. 34



Fig.6 TEM images of regenerated products from Mg-CLDH after 48 h in (a1-a2) deionized water; (b1-b2) 25mg/L phthalic acid; (c1-c2) 25mg/L catechol. Inset is the SAED result. Scale bars indicate 500nm in all.

35


Page 36 of 44

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Fig.7 TEM images of regenerated products from Ca-CLDH after 48 h in (a1-a2) deionized water; (b1-b2) 25 mg/L phthalic acid; (c1-c2) 25 mg/L catechol. Inset is the SAED result. Scale bars indicate 500nm in all.

36



Fig.8 Change of (a) modeled aromatic compounds concentration, (b) pH, (c) Mg2+ concentration and (d) Al3+ concentration during the reaction with Mg-CLDH. The initial models concentration is 25 mg/L.

37


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Fig.9 Change of (a) modeled aromatic compounds concentration, (b) pH, (c) Ca2+ concentration and (d) Al3+ concentration during the reaction with Ca-CLDH. The initial models concentration is 25 mg/L.

38



Page 40 of 44

Table 1 Crystal parameters of the regenerated phases from Mg-CLDH in the presence of different modeled aromatic compounds solutions and deionized water. Crystal

Lattice

Lattice

Weight ratio

Weight ratio

size(Å)

parameters a(Å)

parameters c(Å)

of LDH (%)

of MgO (%)

_

_

_

_

_

100

benzoic acid

7.60

60

3.04

23.06

98.77

1.23

MgAl-500

phenol

7.62

47

3.03

23.13

77.35

22.65

MgAl-500

phthalic acid

7.64

45

3.02

23.19

75.68

24.32

MgAl-500

catechol

_

_

3.03

23.39

49.11

50.89

MgAl-500

2-hydroxynaphthalene

7.58

57

3.04

22.99

89.14

10.86

MgAl-500

1-naphthoic acid

7.57

52

3.04

23.10

80.78

19.22

MgAl-500

4-phenylbuthric acid

7.58

63

3.03

88.1

11.99

T=48 h

Solution/25 mg/L

d003(Å)

Crystal

Lattice

Lattice

Weight ratio

Weight ratio

size(Å)

parameters a(Å)

parameters c(Å)

of LDH (%)

of MgO (%)

MgAl-500

deionized water

7.61

78

3.04

22.72

87.43

12.57

MgAl-500

benzoic acid

7.45

76

3.04

22.89

100

_

MgAl-500

phenol

7.52

81

3.04

22.85

100

_

MgAl-500

phthalic acid

7.49

74

3.04

22.87

100

_

MgAl-500

catechol

7.51

66

3.04

22.91

99.8

0.2

MgAl-500


7.51

74

3.04

22.8

100

_

MgAl-500

1-naphthoic acid

7.56

87

3.04

22.81

100

_

MgAl-500


7.58

86

3.04

22.81

100

_

T=48 h

Solution/600mg/L

d 003(Å)

Crystal

Lattice

Lattice

Weight ratio

Weight ratio

size(Å)

parameters a(Å)

parameters c(Å)

of LDH (%)

of MgO (%)

MgAl-500

benzoic acid

7.55

96

3.04

22.83

100

_

MgAl-500

phenol

7.55

92

3.04

22.83

100

_

MgAl-500

phthalic acid

7.54

100

3.04

22.84

100

_

MgAl-500

catechol

_

_

3.04

23.46

25

75

T=3 h

Solution/25 mg/L

d003(Å)

MgAl-500

deionized water

MgAl-500

23.02

39


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MgAl-500


7.52

89

3.04

22.83

100

_

MgAl-500

1-naphthoic acid

7.54

88

3.04

22.83

100

_

MgAl-500


7.58

84

3.04

22.84

100

_

40



Page 42 of 44

Table 2 Crystal parameters of the regenerated phases from Ca-CLDH in the presence of different modeled aromatic compounds solutions and deionized water. Crystal

Lattice

Lattice

Weight ratio

Weight ratio

size(Å)

parameters a(Å)

parameters c(Å)

of LDH (%)

of CaCO3 (%)

7.49

252

9.97

15.29

47.68

52.32

benzoic acid

7.91/7.48

221/145

10.01

16.39

88.32

11.68

CaAl-500

phenol

7.43

280

9.93

15.77

85.79

14.21

CaAl-500

phthalic acid

7.44

237

9.98

15.85

89.64

10.36

CaAl-500

catechol

7.44

230

9.95

15.76

87.47

12.53

CaAl-500


7.84

238

9.99

16.34

87.54

12.46

CaAl-500

1-naphthoic acid

7.79

258

10.02

16.37

89

11

CaAl-500


7.93/7.48

218/133

9.97

16.36

90.26

9.74

T=48 h

Solution/25 mg/L

d002(Å)

Crystal

Lattice

Lattice

Weight ratio

Weight ratio

size(Å)

parameters a(Å)

parameters c(Å)

of LDH (%)

of CaCO3 (%)

CaAl-500

deionized water

7.49

336

9.96

15.45

51.57

48.43

CaAl-500

benzoic acid

7.90/7.46

243/142

9.99

16.35

89.9

10.1

CaAl-500

phenol

7.44

382

10.08

16.23

100

0

CaAl-500

phthalic acid

7.43

326

9.97

15.80

100

0

CaAl-500

catechol

7.41

228

9.97

15.84

92.53

7.47

CaAl-500


7.87

274

10.01

16.40

86.98

13.02

CaAl-500

1-naphthoic acid

7.86/7.48

359/252

10

16.23

62.97

37.03

CaAl-500


7.87/7.47

235/201

10.06

16.30

86.58

13.42

T=48 h

Solution/600 mg/L

d002(Å)

Crystal

Lattice

Lattice

Weight ratio

Weight ratio

size(Å)

parameters a(Å)

parameters c(Å)

of LDH (%)

of CaCO3 (%)

CaAl-500

benzoic acid

7.90

407

10

11.56

75.42

24.58

CaAl-500

phenol

7.44

421

9.96

11.40

50.76

49.24

CaAl-500

phthalic acid

7.44

299

10

11.39

63.25

36.75

CaAl-500

catechol

7.45

291

10.02

11.50

6.37

93.63

T=3 h

Solution/25 mg/L

d002(Å)

CaAl-500

deionized water

CaAl-500

41


Page 43 of 44 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


CaAl-500


7.68

333

9.99

11.53

67.19

32.81

CaAl-500

1-naphthoic acid

7.89

422

10

11.56

71.89

28.11

CaAl-500


7.67

322

9.98

11.54

68.39

31.61

42



447x273mm (96 x 96 DPI)


Page 44 of 44

Effect of natural organic matter model compounds on the structure

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