Removal mechanism of polymeric borate by calcined layered double hydroxides containing different divalent metals

Colloids and Surfaces A: Physicochem. Eng. Aspects 482 (2015) 702–709

Contents lists available at ScienceDirect

Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa

Removal mechanism of polymeric borate by calcined layered double hydroxides containing different divalent metals Xinhong Qiu a,b , Keiko Sasaki b,∗ a b

School of Chemistry and Environmental Engineering, Wuhan Institute of Technology, Wuhan 430073, PR China Department of Earth Resources Engineering, Kyushu University, Fukuoka 819-0395, 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

• Removal mechanism of polymeric borate by different CLDHs has been discussed. • Regeneration was inhibited and borate could not be immobilized effectively by Mg-CLDH. • Intercalation of polymeric borate is main removal mechanism of boron by Zn-CLDH. • Formation of ettringite and CaCO3 was the main reasons for boron removal by Ca-CLDH.

a r t i c l e

i n f o

Article history: Received 17 May 2015 Received in revised form 10 July 2015 Accepted 14 July 2015 Available online 19 July 2015 Keywords: Boron LDHs Anion exchange Complex Intercalation Mechanism

a b s t r a c t The removal mechanism of polymeric borate by calcined layered double hydroxides is not clear. In this work, layered double hydroxides containing different divalent metals were synthesized and calcined to produce calcined layered double hydroxides (Zn-, Mg-, and Ca-CLDH). Then, Zn-, Mg-, and Ca-CLDH were applied to remove polymeric borate. Zn-CLDH showed better performance for the removal of borate than Ca-CLDH, and hardly any borate was removed by Mg-CLDH. Based on the characterization results, the detailed removal process of polymeric borate by different calcined layered double hydroxides is discussed. Because there is little H3 BO3 that can act as a trigger, and ligand promoted dissolution of the complex H3 BO3 and MgO is prevented. Therefore, Mg-CLDH could not transform to the layered structure to immobilize the borate. For the Zn-CLDH, Zn-CLDH transformed into Zn-LDH, and polymeric borate was absorbed into the interlayer of layered double hydroxides, which is the dominant mechanism of borate removal by Zn-CLDH. Reconstruction of the Ca-LDH from the Ca-CLDH was more rapid than the other calcined layered double hydroxides. However, formation of borate-containing ettringite was the main removal mechanism in the first stage. With increasing reaction time, the reaction between CO3 2− and Ca2+ released from ettringite, and the regeneration of Ca-LDH to form CaCO3 were the main reasons for borate removal in the second stage. © 2015 Elsevier B.V. All rights reserved.

1. Introduction

∗ Corresponding author. E-mail address: [email protected] (K. Sasaki). http://dx.doi.org/10.1016/j.colsurfa.2015.07.036 0927-7757/© 2015 Elsevier B.V. All rights reserved.

Boron is widely distributed in the aqua sphere and lithosphere of the earth [1]. Because of its unique properties, it is extensively used in the manufacture of semiconductors, glass wools, and flame retardants [2]. Although boron is an essential micronutrient that is important for the growth and development of animals and plants,

Q. Xinhong, K. Sasaki / Colloids and Surfaces A: Physicochem. Eng. Aspects 482 (2015) 702–709

703

hexahydrate (Zn(NO3 )2 ·6H2 O), aluminum nitrate nonahydrate (Al(NO3 )3 ·9H2 O), urea (CON2 H4 ), boric acid (H3 BO3 ), and sodium hydroxide (NaOH) were of special grade and used as received from Wako (Osaka, Japan) without purification.

excess intake of boron has adverse effects on organisms. Based on the analysis of the toxicokinetics of boron, the World Health Organization proposed a guideline of 2.4 mg boron/L drinking water in 2011 [3]. Therefore, the development of effective methods for the removal of boron species from water will undoubtedly have many applications [4]. Because sorption is considered to be one of the most effective methods for borate removal [5], as a core component of the sorption process, sorbents with low cost have been rapidly developed in recent years, such as active carbon [6], red mud [7], dolomite [8], and mineral materials [9]. Among these materials, layered double hydroxides (LDHs), a type of highly efficient and practical anion adsorbent, have been widely applied to anion removal [10]. They have high sorption efficiency for the sorption of borate, especially calcined LDHs (CLDHs) [11,12]. Many publication have been proved that the sorption density of borate by CLDHs was higher that of LDHs [2,12]. Differing from the uncalcined product, CLDHs are considered to be bimetallic oxides after calcination at around 500 ◦ C [10,13]. According to Hibino and Tsunashima [14] and Millanged et al. [13], the difference between CLDHs and the unique stratified structure of LDHs is that CLDHs are a type of solid solution that is formed by the dissolution of trivalent metals into the lattices of divalent metallic oxides. This type of structure is capable of hydrating in aqueous solution to regenerate electropositive metallic hydroxide layers that can capture anions, thereby removing anions from solution [15]. In 2013, Theiss et al. [4] suggested that this process of structural regeneration and capture of anions is the primary mechanism for the high-efficiency absorption of borate in CLDHs. However, in 2014, Isaacs-Paez et al. [16] suggested that this process of high-efficiency absorption of borate in CLDHs is not a simple process of electrostatic sorption after structural regeneration, and another reactive mechanism exists between CLDHs and borate in the process of structural regeneration. Based on the sorption mechanisms in the reports of Demetriou et al. [17] and Sasaki et al.[18], we discovered that the reactions between Zn-Al-CLDH, Mg-Al-CLDH, and Ca-Al-CLDH with borate were actually a complicated process that involves the interactions among CLDHs, newly generated LDHs, borate, and OH− [19]. However, the above finding is primarily based on a low concentration of boron. When the concentration of boron is <25 mM, only the monomers of H3 BO3 and B(OH)4 − exist. When the concentration of boron is >25 mM, borate exists in the form of polymers [1,20]:

The hydrolysis reactions of ammonium ions to give ammonia and carbonate to give hydrogen carbonate result in a pH of about 9 (Carbonate is also used as an anion in the formation of LDHs) [1]. This pH is stabilized and is suitable for precipitating a large number of metal hydroxides. Therefore, different type of LDHs with well crystallized and hexagonally shaped. Because Ca-LDH could not be synthesized by the urea method, the microwave-assisted method was used. A solution containing Ca(NO3 )2 ·4H2 O (3.78 g) and Al(NO3 )3 ·9H2 O (3.09 g) in molar ratio 2:1 was added to 50 mL of 0.5 mol/L NaNO3 solution, and the pH was adjusted to 12 with 2 mol/L NaOH. The resulting slurry was transferred to a Teflon vessel and placed in a Milestone Ethos Plus microwave oven (Sorisole, Italy). The temperature was increased to 150 ◦ C within 10 min, maintained at this temperature for 3 h, and then naturally cooled to room temperature. The cooled slurry was subjected to solid–liquid separation by super-centrifugation at 10,000 rpm for 10 min and then washed several times with ultrapure water. The solid residues were then dried overnight in a vacuum freeze–drier. Before the sorption experiments, all of the LDHs were calcined at 500 ◦ C for 3 h.

3B(OH)3  B3 O3 (OH)4 − + H− + 2H2 O

(1)

2.3. Sorption experiments

2B(OH)3 + 2B(OH)4 –  B4 O5 (OH)4 2− + 5H2 O

(2)

Although the boron concentration in natural water is considerable less than 25 mM, in the boron-containing waste water discharged during manufacturing processes, the concentration of borate is higher than this value [21]. Therefore, it is necessary to investigate the sorption mechanism between CLDHs and polymeric borate. Considering that CLDHs can be decomposed by different divalent metals, and given that CLDHs formed by different divalent metals have different properties [22,23], sorption of polymeric borate must have a different sorption mechanism to monomeric borate. In this study, the sorption of polymeric borate in CLDHs containing different divalent metals is investigated, and the interaction mechanisms between polymeric borate and the different types of CLDHs are proposed and then discussed. 2. Experimental methods 2.1. Chemicals Magnesium cium nitrate

nitrate hexahydrate (Mg(NO3 )2 ·6H2 O), caltetrahydrate (Ca(NO3 )2 ·4H2 O), zinc nitrate

2.2. Preparation of LDHs with different divalent metals To synthesize LDHs with the divalent metals Zn and Mg (Zn-LDH and Mg-LDH), Al(NO3 )3 ·9H2 O (3.09 g), M2+ salt (Mg(NO3 )2 ·6H2 O, 4.29 g; Zn(NO3 )2 ·6H2 O, 4.98 g) and urea (4.90 g) were dissolved in ultrapure water in a beaker to form a clear solution with a total volume of 50 mL. The solution was then transferred to a Teflon vessel, which was placed in an oven at 100 ◦ C for 36 h. The resulting slurry was then separated by centrifugation at 10,000 rpm for 10 min. The products were rinsed with deionized water, and then freeze–dried overnight. In this method, hydrolysis of urea was included two steps, the formation of ammonium cyanate (NH4 CHO), with subsequent fast hydrolysis of cyanate to ammonium carbonate: CO(NH2 )2 → NH4 CNO

(3) +

NH4 CNO + 2H2 O → 2NH4 + CO3

2−

(4)

Sorption experiments were performed with the calcined products in 30 mmol/L H3 BO3 . The initial pH was adjusted to 7.00 using 1 mol/L NaOH. For the sorption experiments, 0.100 g of the calcined products were added to 40 mL borate solutions, followed by shaking at 15,650 g and room temperature using a shaking incubator TB-16R (Takasaki Kagaku, Kawaguchi, Japan). At certain time intervals, the supernatants were filtered (0.20 ␮m) to determine the total concentrations of B, Ca, Zn, Mg, and Al by inductively coupled plasma atomic emission spectrometry (ICP-AES; Optima 8300, PerkinElmer, MA, USA). 2.4. Characterization The crystalline phases of the various LDHs were determined 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−1 and a scanning step of 0.02. Brunauer–Emmett–Teller (BET) surface area measurements were performed using an MX-6 surface analyzer (Bel Japan Inc., Toyonaka, Japan) at three points by the nitrogen adsorption method. The morphologies of the solid products before

704

Q. Xinhong, K. Sasaki / Colloids and Surfaces A: Physicochem. Eng. Aspects 482 (2015) 702–709

the corresponding sizes of the Ca-CLDH flakes are larger than those of Mg-CLDH. The size and the shape of Zn-CLDH are totally different to Mg-CLDH and Ca-CLDH. The surface of Zn-CLDH is rough and flakes are present in each particle. These different structures are correlated with the results of the BET surface areas. Based on their representative results, the specific surface area of Zn-CLDH is 99.9 m2 /g, which is much larger than the values for Mg-CLDH (4.5 m2 /g) and Ca-CLDH (3.6 m2 /g).

3.2. Sorption of polymeric borate

Fig. 1. XRD patterns of Mg-LDH, Zn-LDH and Ca-LDH after calcination. Symbols: ♦ Mg-CLDH;  Ca-LDH; 䊐 Ca(OH)2 CaO

CaAl2 O4 .

and after sorption of borate were observed using scanning electron microscopy (SEM) with a VE-9800 spectrometer (Keyence, Osaka, Japan) at 20 kV accelerating voltage. 11 B NMR spectra were collected on a JNM-ECA 800 spectrometer (JEOL, Tokyo, Japan) with Delta NMR software (version 4.3). The metal content (Ca, Zn, Mg, and Al) of each CLDH was measure by ICP-AES. Before measurement, the different CLDHs were dissolved in 0.1 mol/L HCl and then diluted. 3. Results 3.1. Characterizations Fig. 1 shows X-ray diffraction (XRD) patterns of CLDHs with different divalent metals. For Mg-CLDH and Zn-CLDH, the diffraction peaks in the respective patterns are all magnesium oxide (MgO) [24] and zinc oxide (ZnO) [25]. However, differing from the MgCLDH and Zn-CLDH, structures, the diffraction peaks in the Ca-CLDH pattern are mainly CaO and Ca(OH)2 . Furthermore, a broad peak was found at around 26–36◦ . Similar phenomenon can also be found in the work of Lopez-Salinas et al. and Renaudin et al. [26,27]. Refer to the work of Renaudin et al., it may indicate an amorphous structure of Ca4 Al2 O5 (OH)4 . To deduce the mainly structures of the different samples, the metal contents in the different CLDHs were determined by ICPAES and the content of oxygen was calculated by ignoring the content of hydrogen since hydrogen has small mass and metal oxide (or surface hydrated metal oxide) were the main compounds after calcination. The molar ratios of the divalent metal to the trivalent metal in the three types of CLDH were about 2. For MgCLDH and Zn-CLDH, the molar ratios of metal to oxygen were 0.73 and 0.71, suggesting that the formulas of Mg-CLDH and Zn-CLDH may be Mg2 AlO4.1 and Zn2 AlO4.2 . For Ca-CLDH, the molar ratio of metal to oxygen was around 0.60, and the possible formula may be Ca4 Al2 O5 (OH)4 [26–28]. Fig. 2 shows SEM images of the different LDHs before and after calcination. After calcination, the unique hexagonal sheets of the LDHs are still present for both Mg-CLDH and Ca-CLDH, in which

Sorption of borate in different CLDHs was carried in 30 mmol/L borate solution with pH 7.00, as shown in Fig. 3. The maximum sorption of borate was found in Zn-CLDH, while only a small amount of borate was removed by Ca-CLDH. No significant removal of borate was found for Mg-CLDH. In addition, at the beginning of the reaction, sorption of borate in Ca-CLDH was more rapid than the other CLDHs, while sorption of borate in Zn-CLDH occurred 8 h later during the reaction. By comparing these results with previous results obtained with a lower borate concentration (2.5 mmol/L, Fig. S2) [19], for a high concentration of borate, these three materials showed different behavior in the removal of boron species, especially Mg-CLDH where a higher borate concentration had a great effect on the removal of borate. To investigate the different sorption mechanisms, the solid residues of the CLDHs after sorption were collected for XRD and 11 B NMR analyses. From the XRD patterns of Mg-CLDH at different reaction times (Fig. 4a), the MgO-like structure was maintained throughout the reaction process. This suggests that Mg-CLDH did not transform to the Mg-LDH structure in the presence of a high concentration of borate within 48 h. However, when the concentration of borate was 2.5 mmol/L, Mg-CLDH transformed to the LDH structure within 20 h with a relatively large quantity of borate sorption (Fig. S3a). For ZnCLDH, when the concentration of borate was 2.5 mM, it only took 40 min for it to transform to the LDH structure (Fig. S3b). When the concentration was increased to 30 mM, it took nearly 8 h to transform to the LDH structure (Fig. 4b). After 8 h, Zn-LDHs with different interlayer spacings occurred in the XRD pattern. For the first type of Zn-LDH, the (003) peak located at 8.09◦ represents an interlayer spacing of around 10.97 Å. For the second type of Zn-LDH, the (0 0 3) diffraction peak is at 11.48◦ and the interlayer spacing was calculated to be 7.71 Å. The distance of the brucite-like layer is 4.8 Å [29]. Therefore, the spaces occupied by anions in the different types of Zn-LDH were approximately 6.2 and 2.9 Å, respectively. Considering the molecular sizes of both monomeric borate and polymeric borate, boron species exist in LDHs mainly in the form of polymers at 8 h. After 20 h reaction time, the characteristic diffraction peak of the LDH structure with relatively large interlayer spacing is more striking, indicating that the boron species still existed inside the LDHs mainly in the form of polymers. However, after 48 h reaction time, the intensity of the characteristic diffraction peak of the LDH structure with relatively large interlayer spacing decreased, while the peak of the relatively small interlayer spacing became stronger (Fig. 4b). Differing from Mg-CLDH and Zn-CLDH, when the reaction time was 80 min, the Ca-CLDHs had already transformed into the LDH structure (Fig. 4c). In addition, peaks for ettringite and CaCO3 can also be found in the XRD patterns [30,31]. With increasing reaction time, the characteristic diffraction peak of the LDH structure gradually decreased, while the relative intensities of the ettringite and CaCO3 peaks remained the same. However, after 20 h reaction time, the relative intensity of the Ca-LDH and ettringite peaks started to decrease while that of CaCO3 increased. In particular, for 48 h reaction time, the peaks of ettringite and the LDH essentially disappeared, and CaCO3 became the main structure in the XRD patterns (Fig. 4c).

Q. Xinhong, K. Sasaki / Colloids and Surfaces A: Physicochem. Eng. Aspects 482 (2015) 702–709

705

Fig. 2. SEM image of (a) Mg-CLDH, (b) Zn-CLDH and (c) Ca-CLDH. 11 BMQ-MAS NMR spectra were collected for the different samples after borate sorption to more deeply and clearly understand the mechanisms, as shown in Fig. 5. According to previous reports, the chemical shifts of tetrahedral borate ([4] B) mainly range from −3 to 3 ppm. Correspondingly, the chemical shifts of trigonal boron ([3] B) are about 15–20 ppm [32,33]. Therefore, [4] B and [3] B existed in all of the samples. For Mg-CLDH, after 3 h reaction time, the molar ratio of [3] B/[4] B in the solid residuals was 1.48 (Fig. 5a). Similarly, the molar ratio of [3] B/[4] B was 1.53 at the end of the reaction. This suggests that the [3] B was the main species throughout the reaction, and it is different from the results found in 2.5 mmol/L (Fig. S4a). For 2.5 mmol/L borate, [3] B was the main boron species in the beginning period of the reaction, while [4] B became the dominant boron species at 48 h. For Zn-CLDH, the 11 B NMR results were similar to Mg-CLDH. The molar ratios of [3] B/[4] B were 1.29 and 0.83 after 180 min and 48 h, respectively (Fig. 5b). This result is different to that with a lower concentration of borate (Fig. S4b). For 2.5 mmol/L borate, the molar ratio of [3] B/[4] B was 0.92 after 180 min, and this value continuously increased to 1.4 at the end of the reaction. Differing from Mg-CLDH and Zn-CLDH, the molar ratio of [3] B/ [4] B increased from 0.49 to 1.36 during the reaction and [3] B was the main boron species at the end of the reaction (Fig. 5c). However, for 2.5 mmol/L borate, although [4] B and [3] B both existed in CaCLDH (Fig. S4c), the amount of [4] B was always higher than that of [3] B. From the above result, for high borate concentration, we can conclude that the sorption mechanism of polymeric borate in these three materials is different to the absorption mode of monomeric borate.

structure can be hydrated to regenerate the Mg-LDH structure in borate solution, and then reacts with borate through the following process [19]. At the beginning of the reaction, hydration occurs to form Mg OH, Al OH, Mg OH2 + , and Al OH2 + on the surface of Mg-CLDH. Then, a reaction between Mg OH and B(OH)3 occurs [19]:

4. Discussion

Based on the above reaction, the formation of Mg(OH) 2 ·H3 BO3 by the reaction between MgO and H3 BO3 in this stage acts as a trigger and sink to aid regeneration of the Mg-LDH structure, and attracts borate into the newly formed interlayer of Mg-LDH. However, when the boron concentration is >25 mmol/L, the predominant boron species in solution is B3 O3 (OH)4 − at this pH (Fig.

4.1. Removal mechanism of borate by Mg-CLDH After calcination, Mg-CLDH has a MgO-like structure with Al ions dissolved in the lattice to form a solid solution. This type of

Mg O Mg(OH) + B(OH)3 

MgO Mg O(H+ ) B(OH)2 + OH−

(3)

The surface complex of MgO Mg O(H+ ) B(OH)2 releases complex ions through ligand promoted dissolution in the aqueous phase. These complex ions are immediately hydrated, resulting in the formation of H3 BO3 -containing Mg(OH)2 (Mg(OH)2 ·H3 BO3 ). With increasing reaction time, Mg(OH) 2 gradually reacts with aluminum hydroxide, and borate is captured with the formation of a new layer structure as follows [34]: xMg(OH)2 + Al(OH)3 + B(OH)4 −  Mgx Al(OH)2+2x ·B(OH)4 + OH− (4)

xMg(OH)2 + Al(OH)4 − + B(OH)4 −

 Mgx Al(OH)2+2x ·B(OH)4 + 2OH−

(5)

706

Q. Xinhong, K. Sasaki / Colloids and Surfaces A: Physicochem. Eng. Aspects 482 (2015) 702–709

Fig. 3. Changes of (a) B and (b) pH during sorption of 30 mM B on Zn-CLDH, Mg-CLDH and Ca-LDH. Amount of sorbent: 2.5 g/L.

Fig. 4. XRD patterns of (a) Mg-CLDH, (b) Zn-CLDH, and (c) Ca-CLDH after sorption of 30 mM B under pH 7.0. Symbols: , Mg-LDH; ♦, Mg-CLDH; ,Zn-LDH with larger interlayer; 夽 Zn-CLDH; 䊏 Zn-LDH; , Ca-LDH; 䊐, Ca(OH)2 ;

, CaO; , Ca12 Al14 O33 ; 䊉 Borate-containing ettringite (Ca6 [Al(OH)6 ]2 [B(OH)4 ] 6 ); , CaCO3 .

Q. Xinhong, K. Sasaki / Colloids and Surfaces A: Physicochem. Eng. Aspects 482 (2015) 702–709

707

high concentration borate solution. At 180 min reaction time, B3 O3 (OH)4 − in the solution was electrostatically adsorbed on the surface of Zn-CLDH: Zn(Al)OH2 + + B3 O3 (OH)4 − →

Zn(Al)OH2 + + B3 O3 (OH)4 −

(9)

After 20 h reaction time, differing from the surface adsorption at 180 min, Zn-LDH formed and the boron was immobilized by intercalation in the form of B3 O3 (OH)4 − and B(OH)4 − [35]: Zn2 AlO4.2 + B3 O3 (OH)4 − + 4.2H2 O  Zn2 Al(OH)6 ·B3 O3 (OH)4 + 8.4OH−

(10)

Zn2 AlO4.2 + B(OH)4 − + 4.2H2 O  Zn2 Al(OH)6 ·B(OH)4 + 8.4OH−

(11)

The length of the B3 O3 (OH)4 − and B(OH)4 − molecules were calculated to be around 4.2 and 2.6 Å (Gaussian 09, computational details are in the Supporting information). Therefore, there are two types of Zn-LDH with different interlayer spacings in the XRD patterns (Fig. 4b). After B3 O3 (OH)4 − was immobilized into the structure of LDHs, the concentration of boron decreased and became <25 mM (Fig. 3a). Therefore, B3 O3 (OH)4 − could be decomposed into B(OH)3 and B(OH)4 − . Then, B(OH)4 − was immobilized in the layer by electrostatic sorption, while B(OH)3 may be adsorbed in Zn-LDH by an irreversible reaction with Al–OH [36] and/or formation of a hydrogen bond with the metallic layer of Zn-LDH [37]: B3 O3 (OH)4 − + 3H2 O  3B(OH)3 + OH− −

B(OH)3 + OH  B(OH)4

−

(12) (13)

S1), and the pH value is inappropriate for the regeneration of the Mg-LDH structure (Fig. 3b). Thus, the conversion of Mg-CLDH to Mg-LDH is suppressed. However, hydration on the surface of MgCLDH is still possible, so B3 O3 (OH)4 − is electrostatically adsorbed on Mg-CLDH by the following equations:

Thus, the amount of Zn-LDH with relatively large interlayer spacing decreased with reaction time (Fig. 4b). Following this process, the molar ratio of boron [3] B/[4] B was <1.0 in the 11 B NMR spectrum at equilibrium (Fig. 5b). The sorption mechanism of borate in Zn-CLDH is summarized in Fig. 6b. However, it should be noticed that the concentration of borate in the solution was lower than 25 mM and the borate was existed in the form of monomer after 20 h. Since the sorption data and characterization data were mostly from samples collected when the concentration of borate was higher than 25 mM, the conclusions for Zn-CLDHs can still be used. For example, if the concentration of borate was not lower than 25 mM during the sorption, the space distance of regenerative Zn-LDH shall mainly be 10.97 Å which may similar to the results of 20 h.

3B(OH)3  B3 O3 (OH)4 – + H+ + 2H2 O

4.3. Removal mechanism of borate by Ca-CLDH

Fig. 5. 11 B NMR MAS spectra of (a) Mg-CLDH, (b) Zn-CLDH and (c) Ca-CLDH after sorption of polymeric borate under 30 mmol/L.

−

B(OH)3 + OH  B(OH)4

−

Mg(Al)OH2 + + B3 O3 (OH)4 − →

(6) (7)

Mg(Al)OH2 + + B3 O3 (OH)4 −

(8)

In addition, small amounts of B(OH)4 − formed from B(OH)3 may also be present and immobilized on the surface of Mg-CLDH. Therefore, the molar ratio of [4] B to [3] B is >1.0 in the 11 B NMR spectra (Fig. 5a). Moreover, the specific surface area of Mg-CLDH was small, resulting in less reactive sites on the surface, and the amount of borate removal was unsatisfactory. The mechanism of borate removal by Mg-CLDH under various conditions is schematically illustrated in Fig. 6a.

Soon after adding the Ca-CLDH to the borate solution, the pH increased to around 9 (Fig. 3b). At this pH, the main boron species are B4 O5 (OH)4 2− (Fig. S1). Because of the high alkalinity, the transformation of Ca-CLDH to Ca-LDH occurred more rapidly than for the other CLDHs in the high concentration borate solution (Fig. 4c). However, at the beginning, the pH value in solution was still not favorable for regeneration of Ca-LDH from Ca-CLDH. Therefore, Ca2+ and Al3+ species released from Ca-CLDH reacted with B(OH)4 − to form borate-containing ettringite (Ca6 [Al(OH)6 ]2 [B(OH)4 ]6 ), which is the main removal mechanism of borate at this stage and the concentration of boron species decreased (Fig. 3a) [38]:

4.2. Removal mechanism of borate by Zn-CLDH

B4 O5 (OH)4 2− + 5H2 O  2B(OH)3 + 2B(OH)4 −

Differing from the behavior in 2.5 mmol/L borate solution (Fig. S3b), the transformation of Zn-CLDH to Zn-LDH was slow in the

6Ca2+ + 2Al(OH)4 − + 6B(OH)4 − + 4OH−

(14)

708

Q. Xinhong, K. Sasaki / Colloids and Surfaces A: Physicochem. Eng. Aspects 482 (2015) 702–709

Fig. 6. Schematic illustrations of sorption mechanism of polymeric borate onto (a) Mg-CLDH, (b) Zn-CLDH and (c) Ca-CLDH.

 Ca6 [Al(OH)6 ]2 [B(OH)4 ]6

(15)

With increasing reaction time, Ca-LDH slowly formed and appeared in the XRD patterns (Fig. 4c). However, for the Ca-CLDH obtained at 500 ◦ C, a strong band at 1418 cm−1 is present in the Fourier transform infrared spectroscopy spectrum (Fig. S5), indicating the formation of certain carbonate-containing minerals [28]. Therefore, during the reaction, CO3 2− from Ca-CLDH may compete with B4 O5 (OH)4 2− for the sorption sites inside the interlayer of the new LDHs. Because of its higher electrostatic potential (−181.621 kcal/mol for B4 O5 (OH)4 2− and −262.46 kcal/mol for CO3 2− , determined by Gaussian 09 and Multiwfn 3.3), the affinity of CO3 2− for Ca-LDH is higher than that of B4 O5 (OH)4 2− . Therefore, the interlayer anions in the LDHs were occupied by CO3 2− : Ca4 Al2 O5 (OH)4 + 5H2 O + CO3 2− → [Ca2 Al(OH)6 ]2 CO3 + 2OH− (16)

With increasing reaction time, the intensity of the peaks assigned to ettringite and LDHs decreased and CaCO3 became the dominant structure. This is because Ca-LDH and ettringite can dissolve and release Ca2+ ions into the solution. Dissolved Ca2+ ions react with CO3 2− to form CaCO3 , which is stable in alkaline conditions. The reaction can also be clearly confirmed by the XRD pattern at 48 h (Fig. 4c): Ca2+ + CO3 2−  CaCO3

(17)

Therefore, boron species in the solution may lead to precipitation of CaCO3 . Meanwhile, CaCO3 can react with boric acid through the following process [39]: CaCO3 + B(OH)4 −  Ca(HBO3 ) + HCO3 − + H2 O

(18)

This may be the main mechanism for boron removal at the end of the reaction. Because of this, [3] B became the predominant boron species, as shown in the 11 B NMR spectra (Fig. 5c). Because polymeric borate was the main boron species in the solution and the concentration of B(OH)4 − was limited, the sorption density of boron in Ca-CLDH was low. The sorption mechanism of borate in Ca-CLDH is shown in Fig. 6c. 5. Conclusions The removal mechanism of polymeric borate by CLDHs with different divalent metals has been investigated. The results of sorption experiment showed that the maximum sorption of borate occurred in Zn-CLDH, and borate sorption rate in Ca-CLDH was rapid at the beginning of the reaction. However, higher initial boron concentration inhibited the sorption of borate in Mg-CLDH. Combining XRD with 11 B NMR spectroscopy, it was found that the predominant removal mechanism of polymeric borate by Zn-CLDH was anion exchange, where the polymeric borate was intercalated in the interlayers of Zn-LDH. Because of the pH and borate species, the surface reaction between Mg-CLDH and H3 BO3 was inhibited and Mg-CLDH could not be effectively transformed into the Mg-LDH structure. Therefore, the removal of polymeric borate by Mg-CLDH was mainly by electrostatic adsorption. Differing from Zn-CLDH and Mg-CLDH, the removal of polymeric borate by Ca-CLDH includes

Q. Xinhong, K. Sasaki / Colloids and Surfaces A: Physicochem. Eng. Aspects 482 (2015) 702–709

two steps. At the beginning of the reaction, Ca2+ and Al3+ species are released from Ca-CLDH and react with borate, which was ionized from polymeric borate, to form borate-containing ettringite. This is the main removal mechanism of borate by Ca-CLDH in the first step. In the second step, Ca2+ is released from ettringite and the newly generated Ca-LDH, and reacts with CO3 2− to form CaCO3 . Boron species may precipitate with CaCO3 and/or react with CaCO3 to form Ca (HBO3 ) at the end of the reaction.

[12] [13] [14] [15] [16]

Acknowledgement

[20]

Financial support was provided to KS by Funding Program for Progress 100 in Kyushu University and this work was also supported by Natural Science Foundation of Hubei Province (No. 2015CFB506).

[17] [18] [19]

[21] [22] [23] [24] [25]

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.colsurfa.2015.07. 036 References [1] N. Hilal, G.J. Kim, C. Somerfield, Desalination 273 (2011) 23–35. [2] J. Jiang, Y. Xu, K. Quill, J. Simon, K. Shettle, Ind. Eng. Chem. Res. 46 (2007) 4577–4583. [3] World Healthe Organization, Guidelines for Drinking-water Quality (Fourth edition), 2011. [4] F.L. Theiss, G.A. Ayoko, R.L. Frost, J. Colloid Interface Sci. 402 (2013) 114–121. [5] H.N. Liu, B.J. Qing, X.S. Ye, Q. Li, K. Lee, Z.J. Wu, Chem. Eng. J. 151 (2009) 235–240. ´ M.D. Ristic, ´ Carbon 34 (1996) 769–774. [6] L.V. Rajakovic, [7] Y. Cengeloglu, A. Tor, G. Arslan, M. Ersoz, S. Gezgin, J. Hazard. Mater. 142 (2007) 412–417. [8] K. Sasaki, X.H. Qiu, Y. Hosomomi, S. Moriyama, T. Hirajima, Micropor. Mesopor. Mater. 171 (2013) 1–8. [9] R. Keren, U. Mezuman, Clays Clay Miner. 29 (1981) 198–204. [10] K. Goh, T.T. Lim, Z.L. Dong, Water Res. 42 (2008) 1343–1368. [11] X.H. Qiu, K. Sasaki, T. Hirajima, K. Ideta, J. Miyawaki, Chem. Eng. J. 225 (2013) 664–672.

[26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39]

709

P. Koilraj, K. Srinivasan, Ind. Eng. Chem. Res. 50 (2011) 6943–6951. F. Millange, R.I. Walton, D. O’Hare, J. Mater. Chem. 10 (2000) 1713–1720. T. Hibino, A. Tsunashima, Chem. Mater. 10 (1998) 4055–4061. X. Duan, D.G. Evans, Layered Double Hydroxides, Springer, 2006. E.D. Isaacs-Paez, R. Leyva-Ramos, A. Jacobo-Azuara, J.M. Martinez-Rosales, J.V. Flores-Cano, Chem. Eng. J. 245 (2014) 248–257. A. Demetriou, I. Pashalidis, A.V. Nicolaides, M.U. Kumke, Desalin. Water Treat. 51 (2013) 6130–6136. K. Sasaki, X.H. Qiu, S. Moriyama, C. Tokoro, K. Ideta, J. Miyawaki, Mater.s Trans. 54 (2013) 1809–1817. X.H. Qiu, K. Sasaki, K. Osseo-Asare, T. Hirajima, K. Ideta, J. Miyawaki, J. Colloid Interface Sci. 445 (2015) 183–194. D. Pear, G.W. Luther, D.L. Sparks, Geochim. Cosmochim. Ac. 67 (2003) 2551–2560. L. Kentjono, J.C. Liu, W.C. Chang, C. Irawan, Desalination 262 (2010) 280–283. C.M. Jinesh, C.A. Antonyraj, S. Kannan, Appl. Clay Sci. 48 (2010) 243–249. H. Yan, M. Wei, J. Ma, F. Li, D.G. Evans, X. Duan, J. Phys. Chem. A 113 (2009) 6133–6141. F. Bruna, R. Celis, I. Pavlovic, C. Barriga, J. Cornejo, M.A. Ulibarri, J. Hazard. Mater. 168 (2009) 1476–1481. X. Cheng, X.R. Huang, X.Z. Wang, D.Z. Sun, J. Hazard. Mater. 177 (2010) 516–523. E. López-Salinas, M.E.L. Serrano, M.A.C. Jácome, I.S. Secora, J. Porous Mater. 2 (1996) 291–297. G. Renaudin, J. Rapin, B. Humbert, M. Fran, C.C. Ois, Cem. Concr. Res. 30 (2000) 307–314. J. Tian, Q.H. Guo, J. Chem. 2014 (2014) 1–8. J. Choy, E. Jung, Y. Son, M. Park, J. Phys. Chem. Solids 65 (2004) 509–512. Y. Hiraga, N. Shigemoto, J. Chem. Eng. Jpn. 40 (2012) 1269–1271. M. Zhang, E.J. Reardon, Environ. Sci. Technol. 37 (2003) 2947–2952. T. Takahashi, S. Kashiwakura, K. Kanehashi, S. Hayashi, T. Nagasaka, Environ. Sci. Technol. 45 (2010) 890–895. S. Kashiwakura, T. Takahashi, H. Maekawa, T. Nagasaka, Fuel 89 (2010) 1006–1011. Z.P. Xu, G.Q. Lu, Chem. Mater. 17 (2005) 1055–1062. Z.M. Ni, S.J. Xia, L.G. Wang, F.F. Xing, G.X. Pan, J. Colloid Interface Sci. 316 (2007) 284–291. M. Konstantinou, G. Kasseta, I. Pashalidis, Int. J. Environ. Technol. Manage. 6 (2006) 466–479. P. Koilraj, R.S. Thakur, K. Srinivasan, J. Phys. Chem. C 117 (2013) 6578–6586. Y. Hiraga, N. Shigemoto, J. Chem. Eng. Jpn. 43 (2010) 865–871. N.G. Hemming, G.N. Hanson, Geochimica et Cosmochimica Acta 56 (1) (1992) 537–543.