Mechanism of boron uptake by hydrocalumite calcined at different temperatures

Journal of Hazardous Materials 287 (2015) 268–277

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Mechanism of boron uptake by hydrocalumite calcined at different temperatures Xinhong Qiu a,∗ , Keiko Sasaki a , Yu Takaki a , Tsuyoshi Hirajima a , Keiko Ideta b , Jin Miyawaki b a b

Department of Earth Resources Engineering, Kyushu University,Fukuoka 819-0395, Japan Institute for Materials Chemistry and Engineering, Kyushu University, Kasuga 816-8180, Japan

h i g h l i g h t s

g r a p h i c a l

a b s t r a c t

• Higher sorption density of borate was observed at higher calcination temperature. • Sorption of borate by Ca-Al-LDH was mainly through DR mechanism. • Removal of borate by Ca-LDH-300 and Ca-LDH-500 were through forming of ettringite. • Boron was mainly adsorbed and intercalated into hydration of Ca-AlLDH-900.

a r t i c l e

i n f o

Article history: Received 1 October 2014 Received in revised form 26 December 2014 Accepted 29 January 2015 Available online 30 January 2015 Keywords: Boron Hydrocalumite Calcination Ettringite Mechanism

a b s t r a c t Hydrocalumite (Ca-Al-layered double hydroxide (LDH)) was prepared and applied for the removal of borate. The properties of Ca-Al-LDH calcined at different temperatures were diverse, which affected the sorption density and mechanism of boron species. The sorption density increased with increase in calcined temperature and the sample calcined at 900 ◦ C (Ca-Al-LDH-900) showed the maximum sorption density in this work. The solid residues after sorption were characterized by 11 B NMR, 27 Al NMR, SEM, and XRD to investigate the sorption mechanism. Dissolution–reprecipitation was the main mechanism for sorption of borate in Ca-Al-LDH. For Ca-Al-LDH calcined at 300 and 500 ◦ C, regeneration occurred in a short time and the newly forming LDHs were decomposed to release Ca2+ ions and formed ettringite with borate. Two stages occurred in the sorption of boron by Ca-Al-LDH calcined at 900 ◦ C. In the first stage, boron species adsorbed on the alumina gel resulting from the hydration of calcined products. In this stage, borate was included as an interlayer anion into the newly forming LDHs in the following stage, and then immobilized as HBO3 2− into the interlayer, most the LDHs. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Boron is an essential element for plants and animals, although excess intake of boron may result in human health problems, and animal and plant disease [1,2]. Although the maximum boron concentration in drinking water in World Health Organization is 2.4 mg/L, tougher standards at 1.0 mg/L were established in many

∗ Corresponding author. Tel.: +81 0928023339. E-mail address: [email protected] (X. Qiu). http://dx.doi.org/10.1016/j.jhazmat.2015.01.066 0304-3894/© 2015 Elsevier B.V. All rights reserved.

areas, such as European Union, Japan [3,4]. Those standards are well below the boron content found in water resources of many areas [5]. Therefore, inexpensive and appropriate technologies for boron removal need to be developed. Up to now, many methods have been explored for the removal of boron from water, such as coagulation and electrocoagulation, sorption, ion exchange and membrane separation [6–10]. Among these methods, sorption process is one of the most useful and methods, since it generates less waste for disposal with lower cost [11]. Therefore, as core technologies for sorption process of boron, adsorbents have been widely studied [12,13]. Among

X. Qiu et al. / Journal of Hazardous Materials 287 (2015) 268–277

them, hydroxyl-containing boron-selective adsorbents are considered to be one of the best promising materials for boron removal [8,14,15], such as the boron specific resin (such as IRA743, CRB 05), which is the most representative adsorbent for borate. However, the higher production cost of resin is a great shortage [16]. Therefore, numerous low cost adsorbents and waste products have been investigated in recent years [17]. Layered double hydroxides (LDHs) are currently receiving considerable attention for a wide variety of applications in environmental remediation. The compositions of LDHs have the general expression [M(II)1−x M(III)x (OH)2 ]x+ (An− )x /n ·mH2 O, in which M(II) and M(III) are divalent and trivalent metallic cations located in the main layer, An− is the interlayer anion, m is the number of interlayer water molecules, and x is the molar ratio of M3+ /(M2+ + M3+ ) [17–21]. Mg-Al-LDH, which is hydrotalcite-like, has been applied to removal of boron species and the sorption density is higher than boron specific resin [11,13,22]. Within the LDH family, however, there are other compositions. For representative example, hydrocalumite (Ca2 Al(OH)6 A− ), which is also called AFm in cement science literature [23,24]. Hydrocalumite can be easily developed in neutral and fly ashes when a simple lime treatment was applied and it is a type of LDH with well-ordered Ca and Al located in the hydroxide layers and 6-fold coordinated Al3+ , as in hydrotalcite. Differently from the hydrotalcite, Ca2+ is coordinated with six OH groups and the water molecules in the interlayer are also directly coordinated to Ca atoms [24]. Because of this, each interlayer water molecule occupies a certain ordered position to create a well-defined anionic interlayer. More regularly arranged anions, cations, and interlayer water molecules are observed in CaAl-LDH compared with Mg-Al-LDH [25,26]. Another noteworthy difference is that hydrocalumite can be transformed into different crystal phases, such as the calcium aluminate hydrate, CaO, and mayenite (Ca12 Al14 O33 ), depending on the calcination temperature [27,28]. In addition, strong Lewis base sites are found in calcined hydrocalumite and markedly influenced by the calcination temperature [27]. Thus, the ordering of the cations and its special thermal decomposition probably result in it having different basicity, stability, and crystal phases depending on the calcination temperature, which may give these materials their own unique adsorption properties to hydrotalcite. Unlike hydrotalcite, the main removal mechanism of anionic contaminants by hydrocalumite is dissolution-precipitation because of its higher solubility than hydrotalcite [23]. In this process, hydrocalumite is dissolved to release Ca2+ , Al(OH)4 − , and A− , and then the contaminant is precipitated with Ca2+ . Because boron species can form complexes with Ca2+ and Al(OH)4 − , it is speculated that hydrocalumite could have good performance in the removal of boron species. However, until recently, no reports have investigated the potential of hydrocalumite to remove boron, the influence of calcination temperature, and the mechanism. Hence, the optimal calcination temperatures of Ca-Al-LDH for boron removal were investigated. The mechanism of the sorption of boron on Ca-Al-LDH calcined at different temperatures was comprehensively investigated using Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), scanning electron microscopy (SEM), 27 Al and 11 B nuclear magnetic resonance (27 Al NMR and 11 B NMR) of the solid residues after the sorption.

269

Teflon vessel to introduce it into a Milestone Ethos Plus microwave oven (Milestone Inc., Shelton, USA). The temperature was increased to 150 ◦ C within 10 min and then maintained for 3 h. The cooled seriflux was processed with solid–liquid separation through supercentrifugation at 10,000 rpm for 10 min and washed several times with ultrapure water. The resulting product in this step was called Ca-Al-LDH. Ca-Al-LDH was calcined at temperatures of 300, 500, and 900 ◦ C. The Ca-Al-LDH was heated in a muffle furnace (P90G, KDF, Kyoto, Japan) up to a certain temperature (300, 500, or 900 ◦ C) at a rate of 10 ◦ C/min, maintained at the desired temperature for 3 h, and left to cool to room temperature. The calcined products at 300, 500, and 900 ◦ C were designated as Ca-Al-LDH-300, Ca-Al-LDH-500, and D-LDH-900, respectively. 2.2. Characterization The crystalline phases within the various Ca-Al-LDH samples were characterized using an X-ray diffractometer (Ultima IV, Rigaku, Tokyo, Japan) with Cu K␣ radiation. The accelerating voltage and applied current were 40 kV and 40 mA, respectively, with a scanning speed of 2◦ /min and a scanning step of 0.02◦ . Thermogravimetric analysis (TG-DTA, 2000SA, Billerica, Massachusetts, USA) was performed from room temperature to 1000 ◦ C at a heating rate of 10 ◦ C/min. FTIR were recorded on a FTIR 670 Plus spectrophotometer (JASCO, Tokyo, Japan). The morphologies of the products calcined at different temperatures were observed using a VE-9800 scanning electron microscope (Keyence, Osaka, Japan) at 20 kV accelerating voltage. 11 B NMR and 27 Al NMR spectra were collected on a JNM-ECA 800 spectrometer (JEOL, Tokyo, Japan) with Delta NMR software version 4.3. 2.3. Sorption batch tests The initial pH of the H3 BO3 solutions was adjusted to 7.0 using 1 mol/L NaOH. For the sorption experiments, 0.100 g of product was added to 40 mL of borate solution. The mixture was shaken at

2. Methods 2.1. Preparation of LDH A solution containing a specific amount of Ca(NO3 )2 ·4H2 O (3.78 g) and Al(NO3 )3 ·9H2 O (3.09 g) with the molar ratio Ca:Al = 2:1 was added to 0.5 mol/L NaNO3 (50 mL). The pH was adjusted to 12 by 2 mol/L NaOH. The resulting slurry was transferred into a

Fig. 1. XRD patterns of Ca-Al-LDH calcined at different temperatures. Symbols: , Ca-Al-LDH (interlayer anion: NO3 −); , Ca(OH)2 ; , CaO ; #Ca12 Al14 O33 .

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Fig. 2. Changes of (a) B concentration, (b) pH, (c) divalent cation concentration, and (d) Al concentration during sorption of 2.5 mM B in Ca-Al-LDH, Ca-Al-LDH-300, Ca-Al-LDH-500, and Ca-Al-LDH-900.

100 rpm with an 8 cm stroke in a shaker (TB-16R, Takasaki Kagaku, Kawaguchi, Japan) at room temperature. At the setting time, the supernatants were filtered (0.20 ␮m) and the total B, Ca, and Al concentrations were determined by inductively coupled plasmaatomic emission spectrometry (PerkinElmer 8500, Massachusetts, USA). The solid residues after sorption were collected for several characterizations.

of broad XRD peaks with weak intensities [28]. When the calcination temperature was 900 ◦ C, new XRD reflection peaks appeared, which are assigned to mayenite (Ca12 Al14 O33 ) and CaO, suggesting that Ca-Al-LDH was completely converted to the mixed oxide. These transformations were confirmed by the 27 Al MAS NMR, SEM images, TG-DTA and FTIR (see Figs. S1–S4, Supporting information part 1).

3. Results

3.2. Sorption of borate

3.1. Characterization

Fig. 2 shows typical time courses of B, Ca, Al concentrations and pH during batch sorption tests. The pseudo first-order and pseudo second-order kinetic models have been explored to express the sorption kinetics. The pseudo first-order kinetic model and pseudo second-order kinetic model are expressed as [29]:

The XRD patterns of Ca-Al-LDH and Ca-Al-LDH calcined at 300, 500, and 900 ◦ C are shown in Fig. 1. The XRD pattern of CaAl-LDH exhibits the characteristic peaks of hydrocalumite. The spacing (d) of the (002) diffraction peak of 8.67 Å represents the interlayer spacing, which is very close to the previously reported value [27]. The sharp peaks and high intensities of the characteristic peaks indicate well-developed crystals. After calcination, the hydrocalumite phase was no longer detected and other phases were found. For the calcined products at 300 ◦ C, Ca-Al-LDH was transformed into an amorphous phase and its crystalline phase was totally collapsed, as demonstrated by the broad and indistinct peaks in the XRD pattern. In this step, the possible structure may be Ca2 AlO1.5 (OH)3 ·(NO2 )2 , as suggested by Renaudin et al. [28]. When the calcination temperature was increased to 500 ◦ C, the XRD pattern of the calcined Ca-Al-LDH sample shows new diffraction peaks, which are mainly assigned to CaO and Ca(OH)2 . The main crystal phase is attributed to amorphous Ca4 Al2 O5 (OH)4 because

ln(Qe − Qt ) = lnQe − k1 t t 1 t = + Qe Qe k2 Qe2

(pseudofirst-order kinetic model) (pseudosecond-order kinetic model)

where Qe (mmol/g) and Qt (mmol/g) represent borate sorbed at time t and equilibrium time, k1 (/min) and k2 represents the sorption rate constant of pseudo first-order kinetic model and pseudo second-order kinetic model. The kinetic parameters estimated by nonlinear regression are shown in Table 1. It can be found that the correlation coefficients of pseudo second-order model was slightly better than that in pseudo first-order model, suggesting that the chemisorption might be the rate determining step and controlling the adsorption process [30]. Meanwhile, from the sorption rate

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271

Table 1 Sorption kinetic parameters for borate sorption onto Ca-Al-LDH calcined at different temperatures 3 h. Different LDHs

Pseudo first-order kinetic model

Ca-Al-LDH Ca-Al-LDH-300 Ca-Al-LDH-500 Ca-Al-LDH-900

Pseudo second-order kinetic model

Qe (mmol/g)

k1 (/min)

R2

Qe (mmol/g)

k2 (g/mmol min)

R2

0.1162 0.8753 1.0390 0.5635

0.0198 0.0006 0.0009 0.0586

0.93 0.98 0.95 0.92

0.1243 1.3270 1.4355 0.5951

0.2209 0.0003 0.0005 1.1594

0.93 0.98 0.95 0.96

Fig. 3. Sorption isotherms of B by Ca-Al-LDH calcined at different temperatures (sorbent: 2.5 g/L). Table 2 Sorption isotherm parameters for borate sorption by Ca-Al-LDH calcined at different temperatures. Different LDHs

Ca-Al-LDH Ca-Al-LDH-300 Ca-Al-LDH-500 Ca-Al-LDH-900

Langmuir isotherm model

Freundlich isotherm model

Qmax (mmol/g)

KL (L/mmol)

R2

KF (mmol1−1/n L1/n /g)

1/n

R2

2.0 2.1 3.8 7.2

0.06 0.54 0.11 0.18

0.99 0.85 0.74 0.95

0.13 0.72 0.64 1.11

0.73 0.39 0.48 0.68

0.99 0.89 0.77 0.96

constant (k2 ), the highest value was found in Ca-Al-LDH-900, suggesting that with Ca-Al-LDH-900 the sorption equilibrium could be reached faster than others. In addition, sorption rate constant of Ca-Al-LDH-300 and Ca-Al-LDH-500 were quite similar, suggesting that they have similar sorption behaviors and different from Ca-AlLDH and Ca-Al-LDH-900. Fig. 3 shows that the sorption isotherms of borate by Ca-Al-LDH calcined at 300–900 ◦ C and the obtained results were analyzed with the Langmuir and Freundlich isotherm models as follows [31]: Qe =

Qmax KL Ce 1 + KCe

(Langmuir isotherm model)

1

Qe = KF × Cen

(Freundlich isotherm model)

where Qe is the amount sorbed at equilibrium in mmol/g and Ce is the borate concentration at equilibrium and Qmax is the sorption capacity (mmol/g). KL is the adsorption equilibrium constant (L/mmol) and KF and n are Freundlich constants which are related with the adsorption capacity and the intensity of adsorption, respectively. Non-liner fitting was used and the acquired parameters of two sorption isotherm models are summarized in Table 2. From Fig. 3, it indicated that poor performance for the removal of boron species was observed with Ca-Al-LDH, and the sorption efficiency was greatly improved by calcination of Ca-AlLDH. In the calcined samples, Ca-Al-LDH-900 showed the highest

sorption density. In addition, the sorption behavior of Ca-Al-LDH300 and Ca-Al-LDH-500 were similar and could not fit to the both isothermal models (poor coefficient R2 ), which was different from the Ca-Al-LDH and Ca-Al-LDH-900. It means that the sorption mechanism of borate by Ca-Al-LDH calcined at 300 to 900 ◦ C was strongly influenced by the calcination temperature (Fig. 2(a) and Fig. 3). To elucidate the different mechanisms, the solid residues after sorption were collected for XRD, 11 B NMR, 27 Al NMR, and SEM analyses. For Ca-Al-LDH after sorption, the XRD pattern after 20 min is similar to the original pattern (Fig. 4). However, two types of CaAl-LDHs with different interlayer spacings (d) appeared after 3 h. The smaller d value of the Ca-Al-LDH at 11.58◦ represents 7.64 Å. The larger d spacing was around 8.63 Å, which is similar to the original d spacing. Because the brucite-like layer distance is reported to be 4.8 Å [32], the interlayer spaces occupied by different anions in Ca-Al-LDHs were calculated to be approximately 2.84 and 3.83 Å, suggesting that at least two kinds of anions were intercalated in the LDHs. With increasing reaction time, two types of Ca-Al-LDHs were formed and the relative intensities of the characteristic peaks in the two types of Ca-Al-LDHs were stable. To identify the major anionic species, FTIR spectroscopy was used (Fig. S5). There are no significant differences in the FTIR spectra of Ca-Al-LDH before and after sorption of borate. This suggests that the anionic species in the interlayer of Ca-Al-LDH were nitrate and OH− (Fig. S5a). Differently from uncalcined LDH, Ca-Al-LDH-300 was converted to LDHs within 20 min. In this step, there were two types of LDHs with different interlayer spacings. The values of the d spacings were 8.61 and 7.74 Å, and the intercalated anions may be NO3 − and OH− based on the results of FTIR spectroscopy (Fig. S5b). In addition, the XRD peak around 8.34◦ is assigned to CaO·Al2 O3 ·Ca(OH)2 ·18H2 O (JCPDS 00-042-0487). However, the new LDHs with larger interlayer spacing gradually disappeared after 3 h. This was determined from the elimination of the bending vibration mode of N-O in the FTIR spectrum after 48 h. In addition, two main crystal phases assigned to ettringite and LDHs appeared in the XRD patterns after 8 h. The peak intensities of the LDHs decrease with time, which is different from ettringite. In Fig. 4, the representative peak intensities assigned to ettringite at 9.6◦ , 16.10◦ , 18.14◦ , and 26.2◦ increase with decreasing borate concentration. Ca-Al-LDH-500 also transformed into LDHs within 20 min, as with Ca-Al-LDH-300. However, only one type of LDH was observed after 20 min and there is no vibration mode of N-O in the FTIR spectrum (Fig. S5c). Because the interlayer spacing was 7.63 Å, the main anionic species may be OH− . In addition, the characteristic peaks of ettringite appear at the beginning of reaction, and the peak intensities increase with time, while the intensities of the peaks assigned to LDHs decrease with time. This suggests that the sorption mechanism of Ca-Al-LDH-500 was similar to that of Ca-Al-LDH-300. For Ca-Al-LDH-900, a broad peak with a shoulder appears within 20 min, suggesting that there were two types of anions in the interlayer of the LDHs (Fig. 4). Although the interlayer spacings were 8.54 and 7.71 Å, which are similar to those in Ca-Al-LDH-300, the anions in Ca-Al-LDH-900 should be different from those in Ca-Al-LDH-300. This is because no vibration peaks corresponding to NO3 − exist in

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Fig. 4. XRD patterns of the solid residues after sorption of 2.5 mM B withinitial pH 7.0. Symbols: , Ca-Al-LDH (interlayer anion: NO3 ); , Ca(OH)2 ; , CaO; , CaO•Al2 O3 •Ca(OH)2 •18H2 O; 䊉, Ca-Al-LDH (interlayer anion: OH−); , CaCO3; , borate-containing ettringite (Ca6 [Al(OH)6 ]2[B(OH)4 ]6 ); #, Ca12 Al14 O33 ; 夽, Ca2 Al(OH)6 •Al(OH)4 ; ♦, Ca-Al-LDH (interlayer anion: HBO3 2− ).

the FTIR spectrum (Fig. S5d). From 3 to 8 h, the shoulder peak in the LDHs disappears. As the reaction progressed, LDHs with different interlayers appeared and the LDHs with an interlayer distance of 8.15 Å became more obvious at the end of the reaction. At the end of the reaction, the main coordination number of Al in Ca-Al-LDH, Ca-Al-LDH-300, Ca-Al-LDH-500 and Ca-Al-LDH-900 was found to be six in 27 Al NMR (Fig. 5). Moreover, two types of [6 ]Al exist in Fig. 5. One peak around 10 ppm assigned to[6 ]Al in the LDHs is present for all of the samples. The other peak at 15 ppm is only present for Ca-Al-LDH-300 and Ca-Al-LDH-500, and can be assigned to [6 ]Al in ettringite. 11 B NMR MAS spectra were recorded for the solid residues after sorption (Fig. 6). The chemical shifts of tetragonal boron ([4 ]B) are mainly ranged from −3 to 3 ppm, whereas the chemical shift of trigonal boron ([3 ]B) is 15–20 ppm [33,34]. The main boron species in Ca-Al-LDH-300 and Ca-Al-LDH-500 was tetragonal, while trigonal boron was dominant in Ca-Al-LDH-900. In addition, the peak

assigned to tetragonal boron in Ca-Al-LDH-900 is broader than that in Ca-Al-LDH-300 and Ca-Al-LDH-500. This is because the environment of the each boron species in Ca-Al-LDH-900 was slightly different, resulting in the difference in the electron density. This is also reflected by a fact that the boron species in Ca-Al-LDH-900 should be freely distributed inside the interlayers of the LDHs, while the sharp peaks for Ca-Al-LDH-300 and Ca-Al-LDH-500 indicate that the electron density of boron in both samples is not significantly different and boron may be immobilized in the structure of the newly formed crystal. The hexagonal sheets remained in Ca-Al-LDH after 3 and 48 h (Fig. 7a). However, morphological changes occurred in Ca-Al-LDH300 and Ca-Al-LDH-500. After 3 h, the plate-like particles present before sorption were converted to small irregular particles. After 48 h, the SEM images showed a needle-like morphology, which corresponds to ettringite entangled with small sheets in both samples (Fig. 7b and c). For Ca-Al-LDH-900, small plate-like particles appeared at the beginning of reaction, which is quite different from the starting material. At the end of the reaction, the shape and size of these sheets were more uniform than after 3 h (Fig. 7d). From

Fig. 5. 27 Al NMR spectra of Ca-Al-LDH, Ca-Al-LDH-300, Ca-Al-LDH-500, and Ca-AlLDH-900 after sorption of 2.5 mM borate at 48 h.

Fig. 6. 11 B NMR MAS spectra of Ca-Al-LDH-300, Ca-Al-LDH-500, and Ca-Al-LDH-900 after 48 h sorption.

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273

Fig. 7. SEM images of solid residues after sorption of boron at 48 h:(a) Ca-Al-LDH, (b) Ca-Al-LDH-300, (c) Ca-Al-LDH-500, and (b) Ca-Al-LDH-900. The insets show SEM images of the solid residues after 3 h (Scale bars indicate 1.00 ␮m).

the above results, the removal mechanism of boron species was dependent on the calcination temperature.

(Eq. (2)). Therefore, only a small amount of boron was removed by the LDHs. 2Ca2+ + Al(OH)4 − + 3OH−  Ca2 Al(OH)6 ·OH

4. Discussion According to Radha et al. [35], oxyanion uptake by Ca-Al-LDH should be involved in the disruption of the electrostatic interactions and hydrogen bonds between the hydroxide layers and the outgoing anions, followed by the reformation of these bonds with the incoming anions. This is called the dissolution–reprecipitation (DR) mechanism. In the first step, uncalcined Ca-Al-LDH is unstable and the following dissolution occurs [36,37]: Ca2 Al(OH)6 ·NO3  2Ca2+ + Al(OH)4 − + 2OH− + NO3 −

(1)

During this process, the pH increased from 11.5 to 12.8, and the OH− concentration was significantly larger than that of B(OH)4 − (Fig. 4a and b). Although the negative charges of OH− and B(OH)4 − are similar, the affinity of OH− and B(OH)4 − for the layers in LDHs are different, which can be explained by the electrostatic potential. From the average electrostatic potential distribution (ESP, calculated by Gaussian 09 [38] and Multiwfn 3.3 [39], method in Supporting information part 3) of the complete molecular surface of OH− and B(OH)4 − , the ESP value for B(OH)4 − was −117.76 kcal/mol which it is less negative than that of OH− (−175.74 kcal/mol). Thus, the affinity of OH− for the host layer in the Ca-Al-LDH is stronger than that of B(OH)4 − , so the sorption sites are more easily occupied by OH− inside Ca-Al-LDH in the reprecipitation process

(2)

In addition, the DR mechanism occurs more easily at the edge of the LDHs because LDHs are layered structures. The newly formed Ca-Al-LDH intercalating OH− (Ca2 Al(OH)6 ·OH) is mainly located at the edge of the original LDHs to inhibit the release of NO3 − and dissolution of Ca2 Al(OH)6 ·NO3 . Therefore, the corresponding vibration mode of NO3 − is still observed in the FTIR spectrum after 48 h reaction (Fig. S5a). In addition, because of the affinity between NO3 − and the metallic layers is weaker than that between OH− and the metallic layers [40], the d spacings of Ca2 Al(OH)6 ·NO3 and Ca2 Al(OH)6 ·OH were 8.63 and 7.63 Å. Therefore, there are two types of LDHs (Fig. 4). Moreover, hydrogen bonds form between the interlayer anions and M-OH in the host layers [41]. According to the ESP calculation, OH− (−175.74 kcal/mol) is more negatively charged than NO3 − (−133.91 kcal/mol), and the electronic density of Al increases after NO3 − is substituted by OH− . The chemical shift moved to higher field after sorption (Figs. S1 and 5). In addition, DR does not damage the crystal phases other than the LDHs, so the morphology of the solid residues was similar to that before reaction and the hexagonal sheet was maintained (Fig. 7a). The sorption mechanism of borate on Ca-Al-LDH is shown in Fig. 8a. After calcination at 300 ◦ C, Ca-Al-LDH is converted to Ca4 Al2 O3 (OH)6 ·(NO2 )2 (Ca-Al-LDH-300) [28]. Within 20 min, this product reconstructed into a layered structure in the borate solution through hydration, which can be determined from the XRD

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Fig. 8. Illustration for sorption mechanism of boron species in (a) Ca-Al-LDH, (b) Ca-Al-LDH-300 and Ca-Al-LDH-500, and (c) Ca-Al-LDH-900.

pattern (Fig. 4). Because NO2 was included in Ca-Al-LDH-300, LDHs with NO3 − as an interlayer (2Ca2 Al(OH)6 ·NO3 ) formed at the beginning. With increasing pH, the concentration of OH− increased. The affinity of OH− for the host layers in the Ca-LDH was stronger than

that of NO3 − and B(OH)4 − , so the sorption sites were more easily occupied by OH− inside the LDHs. Ca2 Al(OH)6 ·NO3 + OH− → Ca2 Al(OH)6 ·OH + NO3 −

(3)

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Thus, the XRD pattern of Ca-Al-LDH-300 shows two types of LDHs with different interlayer spacings after 20 min (Fig. 4). The OH− in the interlayer of the reconstructed LDHs became predominant with time, which is different from Ca-Al-LDH. This may be because of two reasons. Firstly, the structural stability in Ca-AlLDH-300 after regeneration was less than Ca-Al-LDH. Therefore, the NO3 − ions were more easily replaced by OH− . Secondly, anionic exchange in LDHs is easier with smaller particles [42]. Thus, NO3 − in LDHs with small particles size can be exchanged more easily. In addition, Ca-LDH dissolves in the solution because it has higher solubility (Eq. (2)). According to the above reaction, LDHs dissolve in aqueous solution and then the dissolved species can be bound with B(OH)4 − to form borate-containing ettringite (Ca6 [Al(OH)6 ]2 [B(OH)4 ]6 ) in basic condition because the borate ion has greater ability to form ettringite than other oxyanions [43,44]. 6Ca2+ + 2Al(OH)4 − + 6B(OH)4 − + 4OH−  Ca6 [Al(OH)6 ]2 [B(OH)4 ]6

(4)

Therefore, the boron concentration gradually decreased with increasing representative reflected peak intensities of ettringite. This also leads to [4 ]B being the main boron species in the 11 B NMR spectrum (Fig. 6). Moreover, LDHs and ettringite are the main crystal phases in the XRD pattern of Ca-Al-LDH-300 after 48 h. Although the coordination number of Al in both phases is six, the electronic arrangement of Al in the LDHs and ettringite is different. Therefore, two peaks with different chemical shifts are assigned to [6 ]Al in Fig. 5. In addition, CO2 from air or the sorbent dissolves to form CO3 2− . The reaction of Ca2+ released from the LDHs and ettringite with CO3 2− should occur. Formation of CaCO3 can be observed in an XRD pattern. 2+

Ca

+ CO3

2−

 CaCO3

(6)

However, the [3 ]B peak is not observed in the 11 B NMR spectrum (Fig. 6), so this reaction may not be the main mechanism for the removal of boron. Different behavior was observed for the sorption of boron to Ca-Al-LDH-300 and Ca-Al-LDH-500 at the beginning of the reaction. The chemical formula of the LDHs after calcination at 500 ◦ C was Ca4 Al2 O5 (OH)4 . This crystal phase was immediately converted to LDHs and the pH increased to about 12: Ca4 Al2 O5 (OH)4 + 5H2 O → 2Ca2 Al(OH)6 ·OH

during sorption. However, the solution pH value and the release of Ca2+ and Al(OH)4 − ions were quite similar (Fig. 2). The reason is explained in the Supporting information (Part 4). When Ca-Al-LDH-900 was added into the boric acid solution, the following reaction occurs [46,47]: Ca12 Al14 O33 + (48 + x)H2 O → 6[Ca2 Al(OH)6 ] ·[Al(OH)4 ] + Al2 O3 ·xH2 O

(8)

CaO + H2 O → Ca(OH)2

(9)

In this transformation, [Ca2 Al(OH)6 ]·[Al(OH)4 ] is in a layered structure with an Al(OH)4 − interlayer. Al2 O3 ·xH2 O is an gel form, and boron species may be rapidly adsorbed since boron species can be immobilized in Al2 O3 and Al(OH)3 [43,48]. Therefore, the concentration of boron sharply decreased at the beginning. At the same time, because of the high pH, the alumina gel transformed to Al(OH)4 − . Al(OH)4 − in [Ca2 Al(OH)6 ]·[Al(OH)4 ] and/or generated from the alumina gel can react with Ca(OH)2 to generate LDHs [49,50]. The boron species inside the aluminum gel and OH− in water can be immobilized as anions in the interlayer of LDHs: xCa(OH)2 + Al(OH)4 − + OH−  Cax Al(OH)2+2x ·OH + 2OH−

(10)

xCa(OH)2 + Al(OH)4 − + B(OH)4 −  Cax Al(OH)2+2x ·B(OH)4 + 2OH−

(11)

According to Zhang and Reardon [51], the incorporation of borate into Ca-Al-LDH also involves a change in its coordination from tetrahedral B(OH)4 − to trigonal HBO3 2− : 2xCa(OH)2 + 2Al(OH)4 − + B(OH)4 − → [Cax Al(OH)2+2x ]2 ·HBO3 + 2H2

O + 3OH−

(12)

(5)

Hemming and Hanson suggested that borate may be substituted in the CO3 2− site through the following equation [45]: CaCO3 + B(OH)4 − → Ca(HBO3 ) + HCO3 − + H2 O

275

(7)

Then, the following reactions were similar to those of Ca-AlLDH-300 (Eqs. (2), (4) and (5)). Because ettringite was formed, the rod shaped particles in the SEM images of both Ca-Al-LDH-300 (Fig. 7b) and Ca-Al-LDH-500 (Fig. 7c) are assigned to ettringite. In addition, two types of Al exist in the 27 Al NMR spectrum of CaAl-LDH-500, and the peak intensity assigned to ettringite is larger than that of the LDHs. Therefore, the molar fraction of ettringite was larger than that of the LDHs, which is different from Ca-Al-LDH-300 (Fig. 5). Because ettringite was formed from LDHs and the formation of borate ettringite was the primary mechanism for borate removal, the higher contents of ettringite should have the higher sorption densities, in good agreement with the sorption results in Fig. 3. In addition, since the sorption of borate by Ca-Al-LDH-300 and -500 were mainly through forming new crystal structure, the sorption isotherm could not fit well to Langmuir model (Table 2). The sorption mechanism of borate in Ca-Al-LDH-300 and -500 is shown in Fig. 8(b). Formation of ettringite from LDHs was observed in Ca-Al-LDH300 and Ca-Al-LDH-500, as well as Ca-Al-LDH without calcination,

Because HBO3 2− is more negatively charged than OH− , it can be more stably immobilized inside the interlayers of LDHs under high pH conditions. In addition, the ionic size of HBO3 2− is larger than OH− , so there are LDHs with different layer spacings in the XRD pattern. As the reaction progresses, the residual Ca12 Al14 O33 transformed into alumina gel and [Ca2 Al(OH)6 ]·[Al(OH)4 ], and the latter slowly transformed into Ca3 [Al(OH)6 ]2 , which is more stable. During this step, the boron species in the solution are captured by the newly generated aluminum gel and then react with Ca(OH)2 to generate LDHs. Thus, the XRD peak intensities of [Ca2 Al(OH)6 ]·[Al(OH)4 ] decrease, while the intensity of the peaks of the LDHs increase (Fig. 4). At the same time, the concentration of boron species gradually decreases (Fig. 2). Because the main boron species is HBO3 2− , [3 ]B is the main species in the 11 B NMR spectrum (Fig. 6). During the sorption of boron species on Ca-Al-LDH-900, the pH increased to 12.7. The high pH (>12.5) is unsuitable for the formation of ettringite [52]. Furthermore, because the interaction of boron species with metallic layers is mainly through hydrogen bonding, the electron density of borate ([4 ]B) in the interlayer of LDHs is different from the others because of the different location and water molecules in the interlayer. Therefore, a broad peak is observed in the 11 B NMR spectrum of Ca-Al-LDH-900 after sorption, while sharp peaks are observed in the samples of Ca-Al-LDH-300 and Ca-Al-LDH-500 because boron is present in the crystal structure (Fig. 6). The final crystal phase was LDHs, and hexagonal sheets appear in the SEM image (Fig. 7d). The sorption mechanism of borate to Ca-LDH-900 is shown in Fig. 8c. 5. Conclusion The results clearly show that the removal efficiency of boron by Ca-Al-LDH after calcination at different temperatures is higher

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than by uncalcined Ca-Al-LDH. With the aid of XRD, 11 B NMR, 27 Al NMR, and SEM, the DR mechanism is predominant for the removal of borate by Ca-Al-LDH. However, it was inhibited by OH− in the solution. Ca-Al-LDH-300 and Ca-Al-LDH-500 were immediately transformed into LDHs that were fragile in aqueous solution. The formation of borate-containing ettringite from the dissolved ions derived from the LDHs is the main removal mechanism. However, for Ca-Al-LDH-900, the boron species adsorbed to the alumina gel formed from the hydration of Ca12 Al14 O33 . This reaction acts as a sink for the attraction of H3 BO3 /B(OH)4 − into the interlayers of newly formed LDHs. Because the transformation of tetrahedral B(OH)4 − to trigonal HBO3 2− occurs inside the LDHs, boron can be stably immobilized in the interlayer of LDHs. In addition, it should be noticed that the huge amount of Al were released during the experiment (Fig. 2) so that the stability and recycling capability of Ca-Al-LDH are still need to improve. Therefore, further studies are needed before application. Acknowledgements Financial support was provided to KS by Funding Program for Progress 100 (World Premier International Researcher Invitation Program) through the operating expense grants of the Ministry of Education, Culture, Sports, Science and Technology. This work was also partially supported by the New Energy and Industrial Technology Development Organization (NEDO) under the Innovative Zero-Emission Coal-Fired Power Generation Project. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.plantsci. 2004.08.011. References [1] N. Hilal, G.J. Kim, C. Somerfield, Boron removal from saline water: a comprehensive review, Desalination 273 (2011) 23–35. [2] J. Wolska, M. Bryjak, Methods for boron removal from aqueous solutions – a review, Desalination 310 (2013) 18–24. [3] European Economic Area, The quality of water intended for human consumption, in, Council Directive, 98/83/EC (1998). [4] Norwegian Institute of Public Health, Seawater Desalination Facility on Okinawa in, 2006. [5] B.Y. Wang, X.G. Lin, P. Bai, Boron removal using chelating resins with pyrocatechol functional groups, Desalination 347 (2014) 138–143. [6] M.R. Pastor, A. Ferrándiz Ruiz, M.F. Chillón, D.P. Rico, Influence of pH in the elimination of boron by means of reverse osmosis, Desalination 140 (2001) 145–152. [7] H. Hyung, J. Kim, A mechanistic study on boron rejection by sea water reverse osmosis membranes, J. Membr. Sci. 286 (2006) 269–278. [8] H. Liu, B. Qing, X. Ye, Q. Li, K. Lee, Z. Wu, Boron adsorption by composite magnetic particles, Chem. Eng. J. 151 (2009) 235–240. [9] O.P. Ferreira, S.G. de Moraes, N. Durán, L. Cornejo, O.L. Alves, Evaluation of boron removal from water by hydrotalcite-like compounds, Chemosphere 62 (2006) 80–88. [10] R. Kunin, A.F. Preuss, Characterization of a boron-specific ion exchange resin, Ind. Eng. Chem. Res. 3 (1964) 304–306. [11] P. Koilraj, K. Srinivasan, High sorptive removal of borate from aqueous solution using calcined znal layered double hydroxides, Ind. Eng. Chem. Res. 50 (2011) 6943–6951. [12] Y. Cengeloglu, A. Tor, G. Arslan, M. Ersoz, S. Gezgin, Removal of boron from aqueous solution by using neutralized red mud, J. Hazard. Mater. 142 (2007) 412–417. [13] L. Kentjono, J.C. Liu, W.C. Chang, C. Irawan, Removal of boron and iodine from optoelectronic wastewater using Mg–Al (NO3 ) layered double hydroxide, Desalination 262 (2010) 280–283. [14] J. Wolska, M. Bryjak, N. Kabay, Polymeric microspheres with N-methyl-d-glucamine ligands for boron removal from water solution by adsorption – membrane filtration process, Environ. Geochem. Health 32 (2010) 349–352. [15] H. Liu, B. Qing, X. Ye, Q. Li, K. Lee, Z. Wu, Boron adsorption by composite magnetic particles, Chem. Eng. J. 151 (2009) 235–240. [16] A.A. Harada, T. Takagi, S. Kataoka, T. Yamamoto, A. Endo, Boron adsorption mechanism on polyvinyl alcohol, Adsrption 17 (2011) 171–178.

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