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Structural memory effect of Mg-Al and Zn-Al layered double hydroxides in the presence of different natural humic acids: Process and mechanism Zhisheng Gao, Keiko Sasaki, and Xinhong Qiu Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b00059 • Publication Date (Web): 23 Apr 2018 Downloaded from http://pubs.acs.org on April 24, 2018

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Structural memory effect of Mg-Al and Zn-Al layered double hydroxides in the presence of different natural humic acids: Process and mechanism Zhisheng Gao1, Keiko Sasaki2, Xinhong Qiu1, 2* 1

School of Chemistry and Environmental Engineering, Wuhan Institute of Technology, Wuhan, China 2

Department of Earth Resources Engineering, Kyushu University, Fukuoka 819-0395, Japan Tel & Fax: (86) 27 8719 5680

*E-mail: [email protected]; [email protected]

ACS Paragon Plus Environment 1

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Abstract The structural memory effect of layered double hydroxides (LDHs) is one of the important reasons for their extensive use in environmental remediation. In this study, humic acid (HA) was extracted from black soil and sediments and characterized to determine their structures. The regeneration mechanisms of calcinated LDHs (CLDHs) including different divalent metals (Mg-CLDH and Zn-CLDH) in deionized water and different HA solutions were carefully elucidated, and the reasons for the behavior differences in the two materials were explained. The presence of the HAs significantly increased the dissolution rate of Mg2+ ions from Mg-CLDHs and subsequent regeneration of Mg-LDH. Because of the diverse functional groups in the HAs, these groups were complexed with metallic ions such as Mg2+ on the surface of Mg-CLDH in the beginning. During the process, the HAs adsorbed the regenerated LDHs on the surfaces. Therefore, the crystallinity, morphology, and specific surface area of the regenerated Mg-LDH significantly changed, especially in the presence of high concentrations of HA. In the case of Zn-CLDH, the regeneration rate of the LDH increased in the presence of HA, but the surface of Zn-CLDH was covered with regenerated Zn-LDH and HA. Then, the inside of the particles could not transform to LDH, leading to poor crystallinity and a significant increase in the ZnO content of the HA system. Keywords: Structure memory effect, humic acids, layered double hydroxides, hydration.


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1. Introduction Layered

double

hydroxides

(LDHs)

represented

by

hydrotalcite-like

compounds1 have been extensively used in the environmental sorption field, especially their calcinated products (CLDHs). LDHs lose their layered structure when calcined at 450–500 °C, producing bimetallic oxides. Valente et al.2 and Millanged et al.3 reported that only the MgO phase interacts with Al cations and is evenly distributed throughout the structure, creating a solid solution (Mg(Al)O). This calcined product can be transformed into the original layer structure through hydration; the positive charges of host layer could immobilize various hazardous anions for balancing the charge4. Therefore, the structural memory effect is the one of the important reasons for the extensive use of LDHs in environmental remediation. The reconstruction of LDHs after the calcination is generally believed to be a rehydration process from the mixed oxides to bimetallic hydroxide5, 6. Recent studies have enriched the theories of the structure memory effect of LDHs with regard to the sorption of anions; they also clearly indicate the importance of the structure memory effect of LDHs on environmental sorption7. However, it is necessary to perform further studies and investigate the effect of key environmental factors before applying the structure memory effect of LDHs to environmental remediation. One unavoidable factor is humic acid (HA)8, 9, as an important natural ligand and vehicle of pollutants in the supergene environment, which significantly affect the surface properties of minerals and the migration and transformation of metals and pollutants. Because the regeneration of calcined LDH is a rehydration process, many hydroxyl


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and carboxyl groups in HA could form complexes with common metal ions, e.g., magnesium, zinc and aluminum10, 11, dissolved during the structural transformation of CLDHs to LDHs. This complex formation may affect the recrystallization of its transformed structure to LDH. Furthermore, the functional groups in HA such as hydroxyl, carboxyl, and phenolic groups12 may combine with the surface hydroxyl groups of CLDHs or regenerated LDHs, or may enter the anionic layer of LDHs to form complexes and alter the surface physicochemical properties of the material, affecting the stability of the LDH. Despite many relevant reports13,14, LDHs and CLDHs could remove the HAs from water; however, these reports did not specifically determine how HAs influence the structural memory effect of LDHs. In addition, LDHs can be prepared from diverse M(II) and M(III) cation pairs15. Depending on the metallic species, different bimetallic oxides can be formed from different LDHs after the calcination, i.e., the properties (e.g., electronegativity and alkalinity) of the metallic oxide depend on the metals16,17. CLDHs with different metal ions nevitably leads to different hydration processes of metal oxide and reactions of HAs with metal ions, thus affecting the regeneration degree of the LDHs. Therefore, the objectives of this work are as follows: (1) to investigate the effect of the HAs on the structural memory effect of typical LDHs, (2) to evaluate the reactivity of different metallic LDHs with HAs derived from different sources with different numbers of functional groups, and (3) provide a theoretical surface reaction for the environmental application of LDHs. 2. Materials and Methods


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2.1 Chemicals All Chemicals are listed in Text S1 in Supporting Information. 2.2 Methods The extraction of HAs and the preparation of different CLDHs are described in detail in Text S2 and Text S3 in Supporting Information, respectively. 2.2.3 Effect of HAs on the regeneration of different LDHs The HAs extracted from sedimentary and black soils are labeled HA-S and HAB. The HAs were dissolved in deionized water to prepare a 25 mg/L HA solution, and its initial pH was adjusted to ∼7.00. Mixtures of 20 mL HA solution and 0.05 g CLDHs were stirred at 25 °C under 100 rpm in a shaker. To avoid the entry of CO2, we used ultrapure water to prepare the humic acid solution. During the preparation process, before transferring the humic acid solution into a tube for reaction, N2 was continuously injected. After adding the solution and the CLDHs, the tubes were covered with a lid and sealed with a sealing film (Since there is no atmosphere glove boxes, it is inevitable that there was a small amount of CO2 entered). Samples of the suspension were collected, and the residual HAs and metal ions were measured. The solid residue was filtered and soon measured by various characterization techniques (Text S4). Three parallel experiments were conducted for each time point, and the average data were shown. 3. Results and Discussion 3.1 Characterization of the HAs from different areas The content of the H/C atomic ratios of the HAs (Table S1) was approximately


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1.0, which presumed a chemical structure consisting predominantly of an aromatic framework16. The UV adsorption curves (Fig. S1 and Text S5) indicate that HA-B contains more aliphaticity than does HA-S; the contents of hydroxyl, carboxyl and carbonyl substituents on the aromatic ring in HA-S are higher than in HA-B. The FTIR spectra of both the HAs were similar and showed similar spectroscopic features (Fig. S2), where the existence of -OH and -COOH was found (Text S6). From the solid-state NMR measurements (DEPTH 2)17, although the content of aromatic carbon is the highest in both the HAs, there are some differences in the functional groups of the HAs due to different origins (Fig. 1 and Table 1). In HA-B, the second highest carbon content was aliphatic, while in HA-S, it was carboxyl. In addition, the aromatic carbon content of HA-B was similar to its aliphatic carbon content, but these carbon contents were different in HA-S. A lower aliphatic content was observed in HA-S, indicating poor humification of the sediment. Thus, HAs from different regions may vary in the degree of aromaticity and polymerization as well as the relative content of substituents. Thus, they should have different influences on the memory effect of different LDHs. 3.2 Effect of HA on the regeneration of LDHs In Fig. S3a, the sorption of HAs by Mg-CLDH was faster than that of ZnCLDH, and the sorption efficiency of HA-S by two CLDHs was slightly higher than that of HA-B. This phenomenon was more pronounced in high-concentration HA solutions (Fig. S5). Moreover, the trend of the pH changes was also slightly different. For Zn-CLDH, the pH changes in HA-B and HA-S stabilized more quickly and


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reached equilibrium at 480 min, until 10.88 and 10.48, respectively. Fig. S3b shows that the pH of the HA solution containing Mg-CLDH was higher and required a longer time to reach an equilibrium compared to the pH of the HA solution containing Zn-CLDH. Further, the pH of the HA-B solution was significantly higher than that of HA-S. Similar to the HA solution, the pH also rapidly increased in the early stage of reaction during Mg-CLDH and Zn-CLDH regeneration in deionized water (Fig. S4a). After 48 h of reaction, their pH values in solution were 10.0 and 9.63, which is lower than that in the HA solution. This effect can be attributed to the effect of HA on structural regeneration. In both HA solutions, the Mg and Al ions of Mg-CLDH significantly dissolved at 5 min of reaction, and then their contents decreased at 40 min. However, the content of Mg ions slightly increased again at 180 min and then decreased and became difficult to detect due to its low content in the solution. The Al content increased after 180 min and became slightly steady (∼0.8 mmol/L). The concentration of both the metal ions showed two fluctuations from the start to the end of the reaction, indicating different stages of regeneration. For Zn-CLDH, the contents of Zn and Al rapidly increased in the beginning of the reaction, and then after 40 min, reached equilibrium. Unlike the dissolution trend of metal ions in the HA solution, the dissolution of Mg-CLDH in deoxidized water was slower (Fig. S4). The dissolution trend of Mg and Al ions increased in the beginning, reached the maximum at 180 and 480 min, respectively, and then slowly decreased over time. However, for Zn-CLDH, the dissolution of Zn and Al ions in deionized water was


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maintained in a low range and was difficult to detect. The difference in ion dissolution indicates the effect of the HA solution on the structural regeneration of different CLDHs. In Fig. 2a b, Mg-CLDH was transformed from a MgO-like structure to MgLDH in both the HA solutions, but the trend was slightly different. In HA-B, the structure was mainly MgO within 40 min. At 180 min, some peaks assigned to the LDH appeared on the XRD pattern. For HA-S, at 180 min, the characteristic peaks of LDH were weak, and the main crystal phase was still MgO. After 6 h, a peak assigned to LDH was clearly observed. In other words, the regeneration of MgCLDH in HA-S was slower than that in HA-B. Later, at 48 h, the MgO peak of MgCLDH in HA-B almost disappeared, and at this point, the MgO content was 1.55% (Table 2) (calculated using Maud software)

18

. However, a small amount of MgO

was present in the Mg-CLDH regenerated in HA-S, and the ratio was 6.65%. Compared to the structural regeneration of Mg-CLDH, the structural transformation of Zn-CLDH was very fast (Fig. 2c and d), and the d-spacing (003) remained unchanged during the regeneration. After 5 min of the reaction, the characteristic peaks assigned to Zn-LDH appeared, and peaks ascribed to ZnO also were evident. The mass ratio of ZnO was 48.96% and 42.98% in HA-B and HA-S, respectively. The composition ratio of ZnO in solid Zn-LDH gradually decreased over time. At 48 h, the mass ratio of ZnO in Ha-B was 35.38%. This ratio was 38.69% for Zn-CLDH regenerated in HA-S. Compared to that at 5 min, the ratio of LDH slightly increased, indicating that Zn-CLDH in the HA solution almost transformed


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to LDH in the beginning of the reaction. Unlike the HA solution, for Mg-CLDH in deionized water (Fig. S6a), the weak characteristic peaks of LDH appeared only after 8 h of the reaction, and the crystal phase assigned to MgO still existed after 48 h. Although the crystal-phase LDH was mainly shown on the pattern, the weight ratio of MgO was 12.57% (Table 2). Similarly, the regeneration rate of Zn-CLDH in deionized water (Fig. S6b) was also slower than that in the HA solution, but it was completely transformed compared to the HA solution. At 40 min of reaction, the two crystal phases ZnO and Zn-LDH still were evident in the XRD pattern, and the ZnO content gradually decreased over time. At 48 h, the ZnO content (17.16%) was lower than that in the HA solution. The XRD results show that HA can promote the structural regeneration of different LDHs, but a high HA concentration affected the structural regeneration. When the HA concentration was 600 mg/L (Fig. S7), Mg-CLDHs and Zn-CLDHs could still be transformed into an LDH structure. However, the crystallinity of the regenerated Mg-LDH was obviously low, but no characteristic peak of MgO was observed in the XRD pattern, even in HA-S. Similarly, the crystallinity of Zn-LDH was also relatively low, and a large amount of ZnO was observed, i.e., ∼54.46% and 57.58% in HA-B and HA-S, respectively. Rietveld refinements of the XRD patterns of Mg-CLDH and Zn-CLDH after 48 h reaction are shown in Table 2. The interlayer spacing for the Mg-LDH regenerated in deionized water was 7.61 Å, and that for Mg-CLDH regenerated in 25 mg/L of different HA solutions was 7.56 Å. This decrease in the interlayer spacing can be


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attributed to a certain amount of HA that adsorpted on the surface. Because the HAs have different functional groups (e.g., carboxyl), electrostatic attractions occur between the metal layers and HA. When the concentration of HA-B was increased to 200 mg and 600 mg/L, the interlayer spacing of Mg-LDH slightly increased to 7.63 Å and 7.75 Å, respectively. This may be the HA adsorpeted on the surface influenced the electrostatic attraction between the anions and metal layer in the interlayer. Similarly, the interlayer spacing of the regenerated Mg-CLDH increased to 7.73 Å in HA-S when the concentration was increased to 600 mg/L. Notably, the lattice parameter a of the regenerated Mg-LDH in the HA solution was slightly smaller than that in deionized water, while the trend of lattice parameter c was the same as that of the interlayer spacing. Similarly, the interlayer changes in the regenerated Zn-LDH in water were similar to those in Mg-CLDH, but the interlayer spacing of Zn-LDH in the high-concentration HA solution was significantly smaller than that of Mg-LDH. In addition, the crystal size of Mg-CLDH was calculated, and the value in different systems clearly changed. This value was 467.63 Å before the calcination and changed to 137.39 Å after the regeneration in deionized water, but in the presence of HA, especially HA-S, its crystal size clearly decreased to 43.34 Å and 36.87 Å in 600 mg/L HA-B and HA-S, respectively. However, the crystal sizes of Zn-LDH regenerated under different conditions other than the high-concentration system were similar to each other. As shown in Fig. 3, the Mg-CLDH obtained after calcination showed smooth hexagonal platelets. After regeneration in deionized water, the Mg-CLDH


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maintained a hexagonal plate morphology, but a scale-like structure appeared on the surface and at the edges. This effect can be attributed to the dissolved and recrystallized LDH. The morphologies of Mg-CLDH regenerated in both the HA solutions showed the same changes, i.e., they transformed from smooth to rough structures, and the scale-like structure was denser. Especially, after the regeneration in the high-concentration HA solution, the layered plates of Mg-LDH were difficult to identify and appeared as small bonded pieces. As shown in Fig. 4, the original ZnCLDH exhibited a spherical structure of large grains stacked by small spherical grains19, 20, 21. The surface morphology of Zn-LDH regenerated in deionized water was different, and the spherical structure was stacked by small plates. After the regeneration in different HA solutions, the morphology also demonstrated a rough spherical structure, but the small spherical structure on the surface became irregular and binding. With an increase in the HA concentration to 600 mg/L, the morphology clearly changed; the spherical structure consisted of irregular filaments in HA-B and irregular sheets in HA-S. Moreover, the thickness of the sheet was thinner than that in deionized water. The surface area, total pore volume, BJH pore size (Table S3) of Mg-CLDH regenerated in deionized water were 71.23 m2/g, 0.18 cm3/g, 23.25 nm, and 22.22 nm, respectively. After the regeneration in HA-B and HA-S, the surface properties increased. Similarly, the surface property values of Zn-CLDH regenerated in the HA solution were greater than those of Zn-CLDH regenerated in deionized water. The BET surface area in deionized water increased from 69.75 to 71.10 m2/g in HA-S,


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and the total pore volume increased from 0.11 in the former to 0.12 cm3/g in the latter. These results indicate that the pore structures of Mg-CLDH and Zn-CLDH were adjusted by the HA. To determine the elemental status on the surface, XPS analysis was performed (Fig. S8). By comparing the binding energies of Mg 2p3/2, Zn 2p3/2, and Al 2p3/2 with those reported previously,22, 23, 24 it was observed that the surfaces of Mg-CLDH and Zn-CLDH were covered with Mg-LDH and Zn-LDH, respectively, after the reactions. Therefore, considering the XRD results, the existing MgO and ZnO were inside the particles. In addition, it should be noted that the binding energies of Mg 2p3/2 and Al 2p3/2 in Mg-CLDH regenerated in the HA solution were higher than those in deionized water, while the binding energies of Zn 2p3/2 and Al 2p3/2 in ZnCLDH regenerated in the HA solution were less than those in deionized water. This finding indicates that there was a different interaction between the HA and metal atoms. After the reaction, the FTIR spectra of Mg-CLDH and Zn-CLDH (Fig. S9) did not show the obvious vibration peaks of HA, but the vibration peak of C–O (1380 cm−1) was observed in both hydroxides, probably originating from the carbonate25. Because the interlayer spacing of the LDHs with OH− and CO32− was from 7.5 Å to 7.6 Å26, the OH− and CO32− may both exist in the interlayer of the Mg-LDH and ZnLDH regenerated in the HA and water solutions. However, the content of CO32− in the regenerative LDH was less than in the original LDH since both the original Zn-

LDH and Mg-LDH exhibit a remarkably stronger peak at approximately 1380


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cm-1, which is attributed to the carbonate. 4. Discussion 4.1 Influencing mechanism of HA on the structural memory of Mg-LDH When Mg-CLDH was added to deionized water, ≡Mg–OH, ≡Al–OH, ≡Mg–OH2+, and ≡Al–OH2+ were generated on its surface27 along with the dissolution of Mg2+ and Al3+ and the generation of OH– (Eqs. 1 and 2) MgO + H2O → Mg2+ + 2OH−

(1)

Al2O3 + H2O → 2Al3+ + 6OH−

(2)

This phenomenon explains the increase in the pH of the solution and concentration of metal ions in the early stage of the reaction (Fig. S3). However, the concentration of metal ions increased over time. The concentration of Mg2+ was the maximum at 180 min and then started to decrease. This finding can be attributed to the conversion of Mg2+ to its hydroxides because the pH increased (Eq. 3). Mg2+ + 2OH− ⇄ Mg(OH)2

(3)

The concentration of Al3+ was the same as Mg2+ during the early stage. As the pH increased over time after the reaction, Al3+ gradually converted to Al(OH)4−. Therefore, the concentration of Al3+ ion continuously increased. This reaction consumed OH−, and the pH of the solution started to decrease. Until 480 min, the pH was the lowest. Because the metal plate layer was positively charged, Al(OH)4− gradually reacted with Mg(OH)2 to generate LDHs with CO32− or OH− interlayer (Eqs. 4 and 5)28. xMg(OH)2 + Al(OH)4− + OH− ⇄ MgxAl(OH)2+2x·OH− + 2OH−


(4)

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xMg(OH)2 + Al(OH)4− + 1/2CO32− ⇄ MgxAl(OH)2+2x·(CO32−)1/2 + 2OH− (5) H2CO3 formed from CO2 and dissociated as follows: H2CO3 ⇄ H+ + HCO3- pKa1=6.3

(6)

HCO3- ⇄ H+ + CO32-

(7)

pKa2=10.3

Therefore, the concentration of Al3+ decreased, and the pH continuously increased until equilibrium at 48 h. Because Mg-CLDH proceeds with a dissolution– regeneration process, small sheets formed and appeared on the surface of the large hexagonal plates. This appearance can be attributed to the regeneration of LDH (Fig. 3). When Mg-CLDH was added to the HA solution, the reaction process changed. First, the surface of Mg-CLDH was also hydrated to produce ≡Mg–OH, ≡Al–OH, ≡Mg–OH2+, and ≡Al–OH2+, along with the dissolution of Mg2+ and Al3+. The released Mg2+ and Al3+ reacted with HA to form metal–HA complexes (M-HA, Eqs. 8 and 9)10, 11. Mg2++HA ⇄Mg2+-HA

(8)

Al3++HA ⇄Al3+-HA

(9)

Therefore, Eqs. 1 and 2 significantly occurred in the HA solution. The concentration of metal ions (free and complexed) in the solution reached a higher level at 5 min compared to the ions in deionized water, and the pH of the solution also rapidly reached ∼9.5 (Fig. S3). Then, the M-HA compounds were adsorbed by hydrate CLDHs because they contain carboxyl and hydroxyl groups (Eq. 10). ≡Mg(Al)-OH2+ + M-HA ⇄ ≡Mg(Al)-OH2+ - HA-M


(10)

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Thus, the concentration of HA in the solution rapidly decreased. Because the ratio of hydroxyl and carboxyl groups in HA-S was higher, the absorption rate of HA-S was higher (Fig. S3). The absorption reaction reached a complete equilibrium at 180 min, and most HA complexes in the solution were absorbed on the surface of complex oxide solids, thus decreasing the concentration of the divalent and trivalent metal ions in the solution. With an increase in the pH, the Mg2+ ions in M-HA may react with OH− and form Mg(OH)2. This formed Mg(OH)2 was similar to a strap; a part of HAs was co-precipitated with Mg(OH)2 (Eq. 11). In addition, Al3+ ions were also released and changed to Al(OH)4−. Thus, the concentration of metal ions and pH decreased again. Mg2+-HA + 2(OH)− → Mg(OH)2-HA

(11)

Because the precursors for the LDHs were present, LDHs were formed (Eqs. 4 and 5). With the conversion of hydroxides into LDH and the continuous dissolution of the oxide, more OH− ions were produced, and the pH of the solution increased. Therefore, the concentration of Al3+ increased. At 180 min, the diffraction peaks for the LDH appeared in the XRD pattern (Fig. 2a). In HA-S, the Mg-CLDHs were converted slowly owing to their strong complexation property, but clear LDH diffraction peaks also appeared at 300 min (Fig. 2b). Dissolution and recrystallization occurred on the surface of Mg-CLDH, generating many scale-like structures on the solid surface (Fig. 3). After 48 h, the structural transformation of Mg-CLDH was mostly completed; at this point, the MgO content in all the structures was less than that regenerated in deionized water. According to the XPS results, the


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residual MgO remained inside the grains, and the outer surfaces were mainly in the form of LDH. Therefore, the hydration and regeneration must be an outside-in process. Moreover, the HAs absorbed on the outer surfaces were both complexed with Mg and Al in the regenerated layer, thus shifting the binding energies of Mg and Al in the LDH regenerated in HA to a higher area. Owing to the presence of the HA solution, the dissolution of metal ions was accelerated. Thus, the regeneration rate of LDH in HA was accelerated as well. However, when the HAs had a stronger capacity of complexing with metals or the concentration of HA was higher, they may affect the conversion of metal ions to hydroxides, thus generating LDH. This is probably the reason why the regeneration rate of Mg-CLDH in HA-S was slower than that in HA-B. The high concentration of HA significantly improved the dissolution of metals but provided more hydroxyl and carboxyl groups for complexation with metal ions, thus limiting the conversion of metal ions to LDH. Thus, the characteristic diffraction peaks of MgO in highconcentration HA almost disappeared, but the crystallinity of LDH was relatively low, and its morphology was irregular. Moreover, because the absorption was already completed before the generation of LDH, the HAs on the surface of LDH affected the crystal growth and arrangement. Thus, the crystal grains of the regenerated Mg-LDH clearly decreased with an increase in concentration, and the morphology also changed. This morphology change increased the surface area and decreased the pore volume (Table S3). Moreover, with the incorporation of additional HA in the regenerated LDH interlayer and the larger molecular size of HA


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than that of the anions, the interlayer spacing of LDH in high-concentration HA increased. The proposed mechanism of Mg-CLDH regenerated in different solutions is schematically shown in Fig. 5. 4.1 Influencing mechanism of HA on the structural memory of Zn-LDH Unlike Mg-CLDH, when Zn-CLDH was added into deionized water, Zn2+ and Al3+ were also released into the solution, but the concentration of Zn2+ remained at a relatively low level. This low level is because the pKa of Zn(OH)2 was 6.1, which is lower than that of Mg(OH)2 (pKa=9.5). Therefore, Zn(OH)2 was easily formed at a low pH, thus increasing the regeneration rate of Zn-CLDH. The characteristic diffraction peaks of Zn-LDH were generated at 40 min (Fig. S6). According to the interlayer spacing (Table 2) and FTIR results (Fig. S9), the anions in the interlayer were OH and CO32− (Eqs. 12 and 13) 29. Zn2AlO3.5 + 3.5 H2O ⇄ Zn2Al(OH)6·OH−

(12)

Zn2AlO3.5 + 1/2CO32− +3.5 H2O ⇄ Zn2Al(OH)6·(CO32−)1/2 + OH−

(13)

As time progressed, ZnO was gradually converted to LDH over time. During this, although the pH changes were the same as those of Mg-CLDH in deionized water, the final pH was slightly lower than that of Mg-CLDH. This lower pH is related to the alkalinity of Mg and Zn because the alkalinity of Mg is higher

27

. In

addition, the concentrations of Zn2+ and Al3+ throughout the structural transformation of Zn-CLDH in deionized water were maintained at extremely low levels (Fig. S4). When Zn-CLDH was added to the HA solution, the dissolution of Zn2+ and Al3+ in the solution was higher than that in deionized water. The mechanism is probably


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associated with an increase in the dissolution rate of metal ions by HA, similar to Mg-CLDH (Eqs. 9, 14, and 15). Zn2AlO3.5 + 3.5H2O → 2Zn2+ + Al3+ + 7OH−

(14)

Zn2+ + HA ⇄ Zn2+-HA

(15)

Al3+ + HA ⇄ Al3+-HA

(9)

Because of Eq. 14, the pH increased rapidly, and the condition is suitable for forming LDH. The Zn2+ and Al3+ ions complexed with HA changed into their hydroxide forms and then rapidly reacted with each other, forming LDH (Eqs. 16 and 17). 2Zn2+ + Al3+ + 7OH− ⇄ Zn2Al(OH)6·OH− 2Zn2+ + Al3+ + 6OH− + 1/2CO32− ⇄ Zn2Al(OH)6·(CO32−)1/2

(16) (17)

Therefore, a clear LDH phase appeared at 5 min (Fig. 2c and d). At this time, almost 50% of the HAs in solution was absorbed, and the absorption of HA-S was higher than that of HA-B. This finding can be attributed to the ratio of functional groups in HA-S. As time progressed, the relative intensity and area of the characteristic peaks of both the crystal phases were unchanged. At 48 h, the absorption amount of HA by Zn-CLDH was lower than that by Mg-CLDH (Figs. S3 and S5), and the XRD pattern also shows that the ZnO content was obviously higher than the regenerated material in deionized water, especially HA-S (Table 2). This phenomenon was more obvious in the high-concentration HA solution and is probably related to the structure of Zn-CLDH. Unlike the exposed plate structure of Mg-CLDH, Zn-CLDH has a spherical


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structure. According to the XRD and XPS results (Fig. 2 and Fig. S8), it was inferred that the surface grains in the different systems were LDH. This result indicates that the regenerated LDH covered the surface of spherical grains. The regenerated LDH on the surface was positively charged, and a part of the HA was absorbed on the surface and occupied the sorption site so that it was difficult to adsorb the remainder of the HA onto the surface. Additionally, to was difficult for the HA to enter into the inside of the sphere because of steric hindrance. Because the sorption site on the surface was limited and the inner site could not be used, the sorption performance of Zn-CLDH was poor. Moreover, because the surface was covered with regenerated LDH and because of the steric hindrance of the HA, additional pores formed, and the surface area increased. However, the inner ZnCLDH was difficult to hydrate because of the steric hindrance; therefore, the ZnO content was relatively stable in the subsequent process, especially at the higher concentrations. Moreover, the HAs present on the surface, and the formation of LDH was regulated by HA. Thus, the morphology of Zn-CLDH after the regeneration in a high-concentration HA solution was different from that in a low-concentration solution and deionized water (Fig. 4). The structure of HA-S was different from that of HA-B; HA-S was more likely to be complexed with metals. Thus, the regulating effect was different. Therefore, the morphology, pore structure, and oxide content of the particles regenerated in HA-S and HA-B were diverse (Tables 2 and S3). In addition, owing to the rapid hydration and transformation of a part of Zn2+ and Al3+


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into LDH before the reaction with HA, the HAs were absorbed onto the surface of the spherical grains, even in high-concentration HA. Thus, because the HAs exist on the surface of Zn-CLDH, they regulate the regeneration of Zn-CLDH. The proposed mechanism of Zn-CLDH regenerated in different solutions is schematically shown in Fig. 6. From the above results, the HAs affect different CLDHs in different ways, and the effect of HA-S on the structure memory was stronger than the effect of HA-B because of the higher content of carboxyl groups in HA-S that can form complexes with metal ions. In addition, the reasons for the different behaviors of the LDHs with different divalent metals in the HA solution and the difference in the adsorption efficiency of HA are below. (1) During the process of formation, the pKsp values for Zn-LDH and Mg-LDH are 20.80 and 25.43, respectively,30. Therefore, during regeneration, the interaction between Zn-CLDH and humic acid is different than that between Mg-CLDH and HA. (2) Generally, a spherical adsorbent has better adsorption efficiency

because of its better dispersibility. A free sedimentation experiment was conducted with a spectrophotometer to determine the sedimentation properties of the materials31, and found that the suspension property of Zn-CLDH is better than that of Mg-CLDH (Fig. S10). However, Mg-CLDH has a sheeting structure, and its surface is fully exposed during the conversion process, and thus, this structure can make full use of its own adsorption sites. However, the spherical Zn-LDH structure is actually an accumulation of multiple sheets although it is similar to a spherical structure. During the adsorption process, as


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the humic acid, previously covered on the surface, is absorbed, the internal adsorption sites of Zn-CLDH are not fully used. Therefore, the adsorption capacity of Mg-CLDH is larger than that of Zn-CLDH. Meanwhile, the alkalinity of both materials was measured by CO2-TPD. In the Fig. S11, the peak position of Mg-CLDH was similar to that of Zn-CLDH, but the peak areas of Mg-CLDH were slightly larger than those of Zn-CLDH, suggesting that the basicity in the Mg-type is stronger than in the Zn-type. For the adsorption of humic acid, Mg-CLDH demonstrated a higher adsorption performance. (3) The adsorption of humic acid occurred at the beginning, and Mg-CLDH

and Zn-CLDH were not completely transformed. Compounds of MgO and LDH or ZnO and LDH existed. Comparing the zeta potential of the four materials (Fig. S12), the potential of MgO and Mg-LDH were higher than that of ZnO and Zn-LDH. (4) After being transformed into LDH, Mg-LDH exhibited different electrostatic potentials than Zn-LDH. Theoretical calculations were carried out on the [M2Al(OH2)9(OH)4]3+ (M=Mg and Zn) cluster32. Density functional theory (DFT) with the three-parameter hybrid functional B3LYP33 was used for geometrical optimization. The effective core potential (ECP) LANL2DZ34, 35 was employed for the divalent metal ions, and the full electron basis sets 6-31G(d)36 were used for Al, O, and H. The Gaussian 09 program37 was used for the geometrical optimization, and a quantitative analysis of the electrostatic potential on the molecular surface was performed by Multiwfn 3.3

38

(Computational model and optimized geometries of


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Mg-LDH and Zn-LDH are shown in Fig. S13). The values of the ESP (electrostatic potential) for the Zn-LDH and Mg-LDH clusters were 222.11 and 241.78 kcal/mol, respectively. These data are consistent with the zeta results reported earlier27. After ionization, humic acid is electronegative, and the two types of LDHs have different ESP. Consequentially, their interactions with humic acid after regeneration are different, and higher sorption efficiency was found in Mg-CLDH. 5. Conclusions Based on the characterization results, the HAs affected different CLDHs in different ways, and the Mg-CLDH exhibited higher adsorption efficiency than ZnCLDH due to different surface properties (e.g., electrostatic potential, morphology and Ksp). Because of the same reason, the regeneration process and mechanism for Mg-CLDH and Zn-CLDH were different. For Mg-CLDH, the presence of HA accelerated the conversion of CLDH into LDH by affecting the dissolution of its metal ions. However, owing to the complexing capacity of HA with metal ions, the crystallization of LDH regenerated in the HA solution was relatively weak, and the morphology and pore structure also changed, especially in the highconcentration HA solution. Moreover, because of the low regeneration rate of MgCLDH, Mg(OH)2 acts as a trap to transport the HA into the interlayer of the regenerated LDH. Although the regeneration rate of Zn-CLDH, was higher than that of Mg-CLDH, the regenerated Zn-LDH covered the surface of Zn-CLDH due to the regulating role of HA, thus affecting the subsequent conversion of CLDH into LDH. Therefore, the morphology, pore structure, and weight ratio of ZnO


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were different from those regenerated in deionized water. Additionally, in the high-concentration HA solution, the crystallinity weakened along with a significant increase in the ratio of ZnO.

Acknowledgment The study was supported by the National Natural Science Foundation of China (No. 51504170) and the Nature Science Foundation of the Hubei Province of China (2015CFB506).


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References: (1) Cadars, S.; Layrac, G.; Gérardin, C.; Deschamps, M.; Yates, J. R.; Tichit, D.; Massiot, D. Identification and Quantification of Defects in the Cation Ordering in Mg/Al Layered Double Hydroxides. Chem Mater 2011, 23 (11), 2821-2831. (2) Valente, J. S.; Figueras, F.; Gravelle, M.; Kumbhar, P.; Lopez, J.; Besse, J. P. Basic Properties of the Mixed Oxides Obtained by Thermal Decomposition of Hydrotalcites Containing Different Metallic Compositions. J Catal 2000, 189 (2), 370-381. (3) Millange, F.; Walton, R. I.; O'Hare, D. Time-resolved in situ X-ray diffraction study of the liquidphase reconstruction of Mg-Al-carbonate hydrotalcite-like compounds. J Mater Chem 2000, 10 (7), 1713-1720. (4) Theiss, F. L.; Ayoko, G. A.; Frost, R. L. Removal of boron species by layered double hydroxides: A review. J Colloid Interface Sci 2013, 402, 114-121. (5) Rocha, J.; del Arco, M.; Rives, V.; A. Ulibarri, M. Reconstruction of layered double hydroxides from calcined precursors: a powder XRD and 27Al MAS NMR study. J Mater Chem 1999, 9 (10), 2499-2503. (6) Stanimirova, T.; Kirov, G. Cation composition during recrystallization of layered double hydroxides from mixed (Mg, Al) oxides. Appl Clay Sci 2003, 22 (6), 295-301. (7) Duan, X.; Evans, D. G. Layered double hydroxides; Springer Science & Business Media2006; Vol. 119. (8) He, M.; Shi, Y.; Lin, C. Characterization of humic acids extracted from the sediments of the various rivers and lakes in China. Journal of Environmental Sciences 2008, 20 (11), 1294-1299. (9) Peña-Méndez, E. M.; Havel, J.; Patočka, J. Humic substances–compounds of still unknown structure: applications in agriculture, industry, environment, and biomedicine. Journal of Applied Biomedicine 2005, 3 (1), 13-24. (10) Mantoura, R. F. C.; Dickson, A.; Riley, J. P. The complexation of metals with humic materials in natural waters. Estuarine and Coastal Marine Science 1978, 6 (4), 387-408. (11) Yang, R.; van den Berg, C. M. G. Metal Complexation by Humic Substances in Seawater. Environ Sci Technol 2009, 43 (19), 7192-7197. (12) Guan, X.-h.; Chen, G.-h.; Shang, C. ATR-FTIR and XPS study on the structure of complexes formed upon the adsorption of simple organic acids on aluminum hydroxide. Journal of Environmental Sciences 2007, 19 (4), 438-443. (13) Wang, R.-X.; Wen, T.; Wu, X.-L.; Xu, A.-W. Highly efficient removal of humic acid from aqueous solutions by Mg/Al layered double hydroxides-Fe3O4 nanocomposites. RSC Adv 2014, 4 (42), 21802-21809. (14) Zhang, G.; Wu, T.; Li, Y.; Huang, X.; Wang, Y.; Wang, G. Sorption of humic acid to organo layered double hydroxides in aqueous solution. Chem Eng J 2012, 191, 306-313. (15) Goh, K.-H.; Lim, T.-T.; Dong, Z. Application of layered double hydroxides for removal of oxyanions: A review. Water Res 2008, 42 (6), 1343-1368. (16) Kim, H.-C.; Yu, M.-J. Characterization of aquatic humic substances to DBPs formation in advanced treatment processes for conventionally treated water. J Hazard Mater 2007, 143 (1), 486493. (17) Nishiyama, Y. T., JP), Frey, Michael H. (Peabody, MA, US). NMR Measurement Method. United States Patent2014.


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(18) Lutterotti, L. Total pattern fitting for the combined size–strain–stress–texture determination in thin film diffraction. Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms 2010, 268 (3), 334-340. (19) Song, J.; Leng, M.; Fu, X.; Liu, J. Synthesis and characterization of nanosized zinc aluminate spinel from a novel Zn–Al layered double hydroxide precursor. J Alloy Compd 2012, 543, 142-146. (20) Liu, J.; Song, J.; Xiao, H.; Zhang, L.; Qin, Y.; Liu, D.; Hou, W.; Du, N. Synthesis and thermal properties of ZnAl layered double hydroxide by urea hydrolysis. Powder Technol 2014, 253, 41-45. (21) Wang, T.; Li, J.; Su, Y.; Wang, C.; Gao, Y.; Chou, L.; Yao, W. The tuning of pore structures and acidity for Zn/Al layered double hydroxides: The application on selective hydrodesulfurization for FCC gasoline. Journal of Energy Chemistry 2015, 24 (4), 432-440. (22) Fleutot, S.; Dupin, J.-C.; Renaudin, G.; Martinez, H. Intercalation and grafting of benzene derivatives into zinc-aluminum and copper-chromium layered double hydroxide hosts: an XPS monitoring study. Phys Chem Chem Phys 2011, 13 (39), 17564-17578. (23) Wu, X.; Tan, X.; Yang, S.; Wen, T.; Guo, H.; Wang, X.; Xu, A. Coexistence of adsorption and coagulation processes of both arsenate and NOM from contaminated groundwater by nanocrystallined Mg/Al layered double hydroxides. Water Res 2013, 47 (12), 4159-4168. (24) Zhu, Y.; Laipan, M.; Zhu, R.; Xu, T.; Liu, J.; Zhu, J.; Xi, Y.; Zhu, G.; He, H. Enhanced photocatalytic activity of Zn/Ti-LDH via hybridizing with C60. Molecular Catalysis 2017, 427, 54-61. (25) Rao, M. M.; Reddy, B. R.; Jayalakshmi, M.; Jaya, V. S.; Sridhar, B. Hydrothermal synthesis of Mg–Al hydrotalcites by urea hydrolysis. Mater Res Bull 2005, 40 (2), 347-359. (26) Miyata, S. Anion-exchange properties of hydrotalcite-like compounds. Clay Clay Miner 1983, 31 (4), 305-311. (27) Qiu, X.; Sasaki, K.; Osseo-Asare, K.; Hirajima, T.; Ideta, K.; Miyawaki, J. Sorption of H3BO3/B(OH)4− on calcined LDHs including different divalent metals. J Colloid Interface Sci 2015, 445, 183-194. (28) Xu, Z. P.; Lu, G. Q. Hydrothermal Synthesis of Layered Double Hydroxides (LDHs) from Mixed MgO and Al2O3: LDH Formation Mechanism. Chem Mater 2005, 17 (5), 1055-1062. (29) Ni, Z.-M.; Xia, S.-J.; Wang, L.-G.; Xing, F.-F.; Pan, G.-X. Treatment of methyl orange by calcined layered double hydroxides in aqueous solution: Adsorption property and kinetic studies. J Colloid Interface Sci 2007, 316 (2), 284-291. (30) Johnson, C. A.; Glasser, F. P. Hydrotalcite-like minerals (M2Al(OH)6(CO3)0.5.XH2O, where M = Mg, Zn, Co, Ni) in the environment: Synthesis, Characterization and Thermodynamic Stability. Clay Clay Miner 2003, 51 (1), 1-8. (31) Yang, Z.; Qiu, X.; Fang, Z.; Pokeung, T. Transport of nano zero-valent iron supported by mesoporous silica microspheres in porous media. Water Sci Technol 2015, 71 (12), 1800-1805. (32) Yan, H.; Wei, M.; Ma, J.; Li, F.; Evans, D. G.; Duan, X. Theoretical Study on the Structural Properties and Relative Stability of M(II)−Al Layered Double Hydroxides Based on a Cluster Model. The Journal of Physical Chemistry A 2009, 113 (21), 6133-6141. (33) Becke, A. D. Density‐functional thermochemistry. III. The role of exact exchange. The Journal of Chemical Physics 1993, 98 (7), 5648-5652. (34) Hay, P. J.; Wadt, W. R. Ab initio effective core potentials for molecular calculations. Potentials for the transition metal atoms Sc to Hg. The Journal of Chemical Physics 1985, 82 (1), 270-283. (35) Hay, P. J.; Wadt, W. R. Ab initio effective core potentials for molecular calculations. Potentials for K to Au including the outermost core orbitals. The Journal of Chemical Physics 1985, 82 (1), 299-


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310. (36) Petersson, G. A.; Al-Laham, M. A. A complete basis set model chemistry. II. Open-shell systems and the total energies of the first-row atoms. The Journal of Chemical Physics 1991, 94 (9), 60816090. (37) Lin, Y.; Wang, J.; Evans, D. G.; Li, D. Layered and intercalated hydrotalcite-like materials as thermal stabilizers in PVC resin. J Phys Chem Solids 2006, 67 (5), 998-1001. (38) Lu, T.; Chen, F. Multiwfn: A multifunctional wavefunction analyzer. J Comput Chem 2012, 33 (5), 580-592.


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Fig. 1 13C NMR spectra of HA-B and HA-S.


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Fig. 2 XRD patterns of (a) (b) MgAl-CLDH and (c) (d) ZnAl-CLDH after regeneration in the HA solution. The initial concentration of (a) (c) HA-B and (c) (d) HA-S is 25 mg/L; initial pH: 7.25; and amount of CLDHs: 2.5 g/L.


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Fig. 3 SEM images of (a) Mg-CLDH and (b) Mg-CLDH regenerated in deionized water, (c) Mg-CLDH regenerated in 25 mg/L HA-B, (d) Mg-CLDH regenerated in 25 mg/L HA-S, (e) Mg-CLDH regenerated in 600 mg/L HA-B and (f) Mg-CLDH regenerated in 600 mg/L HA-S.


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Fig. 4 SEM images of (a)Zn-CLDH and (b) Zn-CLDH regenerated in deionized water, (c) Zn-CLDH regenerated in 25 mg/L HA-B, (d) Zn-CLDH regenerated in 25 mg/L HA-S, (e) Zn-CLDH regenerated in 600 mg/L HA-B, and (f) Zn-CLDH regenerated in 600 mg/L HA-S.


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Fig. 5 Schematic illustrations of the regeneration mechanism of Mg-CLDH in the presence of different HAs.

31

ACS Paragon Plus Environment

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Fig. 6 Schematic illustrations of the regeneration mechanism of Zn-CLDH in the presence of different HAs.


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Tables Table 1 Relative intensity of the total area for the 13C NMR signals of the humic acids and the total aromaticity. Relative intensity of functional group (%) Carbonyl

Carboxyl

Aromatic

Carbohydrate

Aliphatic

190-220

160-190

110-160

50-110

0-50

ppm

ppm

ppm

ppm

ppm

HA-B

3.27

13.08

42.76

19.49

HA-S

3.92

18.82

53.92

10.36

33


Aromaticity

Aliphaticity

%

%

21.4

51.11

48.89

12.98

69.79

30.21

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Table 2 Crystal parameters of the different CLDHs after regeneration in different HA solution and deionized water. Solution

d-spacing 003 (Å)

Crystal size (Å)

Lattice Parameters a (Å)

Lattice Parameters c (Å)

Weight ratio of LDH (%)

Weight ratio of MgO or ZnO (%)

Mg-LDH before calcination

--

7.45

467.62

3.03

22.45

100

--

Mg-CLDH

Deionized water

7.61

137.40

3.04

22.72

87.43

12.57

Mg-CLDH

HA-B 25 mg/L

7.56

64.92

3.03

22.70

98.45

1.55

Mg-CLDH

HA-S 25 mg/L

7.56

57.62

3.03

22.70

93.35

6.65

Mg-CLDH

HA-B 200 mg/L

7.63

52.65

3.03

22.81

100

--

Mg-CLDH

HA-B 600 mg/L

7.75

43.34

3.02

22.84

100

--

Mg-CLDH

HA-S 600 mg/L

7.73

36.87

3.02

22.85

100

--

Zn-LDH before calcination

--

7.63

199

3.07

22.75

100

--

Zn-CLDH

Deionized water

7.56

280

3.07

22.61

82.84

17.16

Zn-CLDH

HA-B 25 mg/L

7.55

271

3.07

22.52

64.62

35.38

Zn-CLDH

HA-S 25 mg/L

7.55

255

3.07

22.53

61.31

38.69

Zn-CLDH

HA-B 200 mg/L

7.55

264

3.07

22.59

59.20

40.80

Zn-CLDH

HA-B 600 mg/L

7.55

253

3.07

22.60

45.54

54.46

Zn-CLDH

HA-S 600 mg/L

7.57

234

3.07

22.61

42.42

57.58

34


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1


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