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Sorption properties of boron on Mg–Al bimetallic oxides calcined at different temperatures
Separation and Purification Technology 152 (2015) 192–199
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Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Sorption properties of boron on Mg–Al bimetallic oxides calcined at different temperatures Sayo Moriyama a,⇑, Keiko Sasaki b, Tsuyoshi Hirajima b, Keiko Ideta c, Jin Miyawaki c a
Fukuoka Institute of Health and Environmental Sciences, Dazaifu 818-0135, Japan Department of Earth Resource Engineering, Kyushu University, Fukuoka 819-0395, Japan c Department of Advanced Device Materials, Institute for Materials Chemistry and Engineering, Kyushu University, Kasuga 816-8180, Japan b
a r t i c l e
i n f o
Article history: Received 9 June 2015 Received in revised form 13 August 2015 Accepted 14 August 2015 Available online 15 August 2015 Keywords: Boron Sorption Bimetallic oxides Hydrotalcite 11 B NMR
a b s t r a c t Mg–Al bimetallic oxides produced via the calcination of hydrotalcite-like compounds ([(Mg0.75Al0.25(OH)2](An)0.75/nmH2O, where A is an anionic species) exhibited high potential for the removal of boron from aqueous solutions. X-ray diffraction patterns for the produced bimetallic oxides revealed that MgO was the primary phase within the range of investigated calcination temperatures. In addition, 11B NMR spectral analyses indicated that the Mg–Al bimetallic oxides captured trigonal B ([3]B) and tetrahedral B ([4]B) after the sorption of boron, regenerating hydrotalcite-like compounds. As the initial concentration of boron increased, the percentage of tetrahedral [4]B in solid residues after the sorption of boron increased. The [4]B/[3]B ratios in the solid residues increased with time along with the regeneration of hydrotalcite-like compounds. Furthermore, the Mg–Al bimetallic oxides produced from hydrotalcite-like compounds were more favorable than other bimetallic oxides and effective than single-phase MgO produced from MgCO3 at the same temperature, indicating that Mg–Al bimetallic oxides are stable materials with the potential for use in the remediation of contaminated sites and water. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction
using electrostatic interactions. Boric acid is dissociated to tetrahydroxyborate with a pKa of 9.24 [10] (Eq. (1)):
Boron is an essential micronutrient for plants, but the difference in the necessary intake level and the level at which boron becomes toxic is minimal. Therefore, in some cases, boron retards plant growth [1–3]. In particular, damage to vegetation due to high levels of boron has been observed in arid and semi-arid areas in South Australia, Turkey, North America, and Chile [4,5]. Excess intake of boron also affects the human nervous system [6–8]. Hence, regulations have recently been introduced by the World Health Organization (WHO) that has set maximum concentration limits for boron to be less than 1 mg/L for drinking water [9]. The primary causes of boron contamination can be divided into natural and anthropogenic categories. The former includes topographical factors such as mining and irrigation. The latter includes the manifold applications of boron in various industries such as in the manufacture of glass products, preservatives, audio equipment, and semiconductor processing [4]. At ambient pH, boron typically exists as undissociated boric acid; thus, it is difficult to immobilize at circumneutral pH values
H3 BO3 þ H2 O¡BðOHÞ4 þ Hþ
⇑ Corresponding author. E-mail address: [email protected] (S. Moriyama). http://dx.doi.org/10.1016/j.seppur.2015.08.023 1383-5866/Ó 2015 Elsevier B.V. All rights reserved.
ð1Þ
As a result, the current conventional method for removing boron from solution involves the use of costly boron-specific resins with n-methyl glucamine substituents containing many hydroxyl groups [11–14]. Therefore, the development of a sorbent that meets requirements for cost effectiveness, reactivity, availability, and permeability is necessary. Recently, MgO is been found to be an effective and promising sorbent for boron removal [15–17]. Hydrotalcite adopts a layered, double-hydroxide structure and n III has a general chemical formula [MII1x )x/nmH2O, 1 M2 x(OH2)](A where MII1 indicates a divalent cation (Mg2+, Zn2+, Ni2+, Co2+, Mn2+, 3+ 3+ 3+ n or Cd2+), MIII rep2 indicates a trivalent cation (Al , Fe , or Cr ), A resents an interlayer anion with a valence of n, and x corresponds to [MnIII]/[MnII] + [MnIII]. Hydrotalcite-like compounds transform to bimetallic oxides by thermal decomposition and the produced bimetallic oxides regenerate hydrotalcite-like compounds again in aqueous solutions. Hydrotalcite-like compounds and bimetallic oxides can immobilize anionic species by ion-exchange and intercalation in the interlayer, respectively. As a consequence of its unique structure, hydrotalcite and its calcined products have been widely investigated as sorbents for harmful anions such as Cl, F, Br, I,
S. Moriyama et al. / Separation and Purification Technology 152 (2015) 192–199 3 2 3 SO2 4 , B(OH)4 , NO3 , AsO3 , CrO4 , VO4 , dodecylbenzylsulfonate, 2,4,6-trinitrophenol [18–24]. In the present study, Mg-bearing bimetallic oxides prepared from hydrotalcite produced via a co-precipitation method were used as sorbents for boron removal with the objective of identifying more stable and effective alternatives. The efficacy of Mg–Al bimetallic oxides calcined at different temperatures for the immobilization of boron was investigated. The main purposes of this study include (1) the evaluation of the sorption capacity of boron onto Mg–Al bimetallic oxides produced at different temperatures and (2) the determination of the mechanism of boron sorption onto the Mg–Al bimetallic oxides. The capacities of single-phase MgO and the Mg–Al bimetallic oxides for the removal of boron from aqueous solutions were also compared.
2. Materials and methods 2.1. Synthesis and characterization of the Mg–Al bimetallic oxides A hydrotalcite-like compound with the above-described chemical formula in which x was approximately 0.25 was synthesized according to a previously reported method as the starting material for the Mg–Al bimetallic oxides [21,24]. The hydrotalcite-like compound produced via co-precipitation was calcined at 873 K, 1073 K, and 1273 K for 3 h in order to obtain the Mg–Al bimetallic oxides Mg–Al873, Mg–Al1073, and Mg–Al1273, respectively. X-ray powder diffraction patterns for the Mg–Al bimetallic oxides were collected using a diffractometer (Multi Flex, Rigaku, Tokyo, Japan) with CuKa (k = 1.5406 Å) radiation at 20 mA and 40 kV and a scanning rate of 2°/min from 5° to 85°. The powder diffraction pattern analysis software PDXL (Rigaku, Tokyo, Japan) was used for the Rietveld analysis. The crystal size was estimated from the characteristic peak at 2h = 11° (corresponding to the 003 plane) using the Halder–Wagner method [25]. XRD patterns for the solid residues obtained after sorption of borate were also collected under the same conditions. Temperature programmed desorption curves (TPD, BELCAT-B, BEL JAPAN Inc., Toyonaka, Japan) were obtained to evaluate the basicity and number of basic sites in bimetallic oxides. The CO2-TPD curves for the reagents MgO (Wako, special grade, Japan) and Al2O3 (Wako, special grade, Japan) were also obtained. After pre-treatment at 773 K, CO2-TPD was performed at 973 K for 50 min, and the quantity of desorbed CO2 was measured from room temperature to 973 K. Peak separations were performed using ChemMaster 1.2.0.2 software (BEL JAPAN Inc., Japan) to evaluate the basic sites in the bimetallic oxides. Detailed characterization of the starting materials and Mg–Al bimetallic oxides can be found elsewhere [21]. Briefly, specific surface areas were determined using the Brunauer–Emmett–Teller (BET) method [26], Raman and X-ray photoelectron spectra (XPS) were obtained to determine the surface molar ratios nAl/nMg, scanning electron microscopy (SEM; VE-9800, Keyence, Tokyo, Japan), and transmission electron microscopy (TEM; TECNAI F20, Phillips, Eindhoven, Netherlands) were performed to observe surface morphologies.
193
polypropylene vials. Duplicate suspensions were shaken on a rotary shaker at 100 rpm and 298 K for 120 h. Subsamples (1 ml) of the supernatants were taken at intervals and filtered through a membrane filter with a pore size of 0.22 lm. The concentrations of the remaining B and dissolved total Mg and Al species were determined using inductively couple plasma atomic emission spectroscopy. Mass of the B (Q) on the solid phase per unit mass of solid phase at equilibrium (mM/g) were calculated (Eq. (2)), where Ci and Ce are the initial metal concentration and concentration after the sorption of B (mM), respectively, V is the volume used (L), and M is the mass (g).
Qðmmol=gÞ ¼
ðC e C i ÞV m
ð2Þ
pH was also determined. After the sorption of B, the solid residues were collected and lyophilized to provide specimens for XRD and Fourier transform infrared (FTIR) (FT/IR-670 Plus, JASCO, Tokyo, Japan) analyses. 2.3. Characterization of the solid residues after the sorption of boron XRD patterns for solid residues after the sorption of B from solutions with initial B concentrations of 5.68 mM, 24.04 mM, 32.39 mM, and 51.33 mM were collected in the same manner as described for the Mg–Al bimetallic oxides. Solid-state 11B NMR spectra for the solid residues after the sorption of B were acquired on a JEOL ECA 800 (JEOL RESONANCE Inc., Tokyo, Japan) with 3.2 mm Multiple Quantum Magic Angle Spinning (MQMAS) probes using the single pulse method and Delta NMR software version 4.3. The resonance frequency for 11B was 256.6 MHz at the field strength of 18.8 T. Typical acquisition parameters were as follows: spinning speed was 15 kHz; pulse length was 2.5 ls; relaxation delay was 10 s (11B); and the total number of scans was 63,341 depending on the B concentration [27]. The 11B chemical shift was referenced to that of a saturated H3BO3 solution at 19.5 ppm [28]. The chemical reagents H3BO3 and Na2B4O7 (special grade, Wako Pure Chemical Industries, Ltd., Osaka, Japan) were used as standards. FTIR spectra were recorded on an FTIR spectrometer (FT/IR-670, JASCO, Japan) using KBr pellets. The solid residues obtained after the sorption of B on the Mg–Al bimetallic oxides were diluted with KBr at a rate of 10% and examined over the range from 4000 to 400 cm1. 2.4. Change in the
[4]
B/[3]B molar ratio with time
To investigate the temporal changes in the [4]B/[3]B ratios in the solid residues after the sorption of B, 0.2 g of Mg–Al873 was added to 24.04 mM B solution and the solid residues were filtered and lyophilized at intervals of 0.5, 1, 3, 6, 10, 24, and 48 h. Then, the 11 B NMR spectrum of each sample was obtained. 3. Results and discussion 3.1. Characterization of Mg–Al bimetallic oxides
2.2. Boron sorption Boron (B) solutions were prepared using H3BO3 (special grade; Wako Pure Chemical Industries, Ltd., Osaka, Japan) and high purity water (MQ water, <18.2 MX/cm) to afford initial concentrations of 1.00–51.33 mM. The initial pH of each solution was adjusted to 9.0 ± 0.05 using 1 M NaOH (special grade; Wako Pure Chemical Industries, Ltd., Osaka, Japan). Some quantity (0.100 g) of the Mg–Al bimetallic oxides was added to each B solution in
The XRD pattern for the precipitated starting material (Mg– Al373) was similar to that of hydrotalcite (JCPDS data base; Pattern number 48-0601) (Fig. 1). After the calcination of Mg–Al373 at 873–1073 K, the peaks for hydrotalcite in the XRD patterns of the calcined products disappeared and those for an MgO (JCPDS data base; Pattern number 45-946) phase appeared (Fig. 1(a)–(c)). In the material calcined at 1273 K, the XRD patterns indicated the formation of 60.8% MgO and 30.2% MgAl2O4 (JCPDS data base; Pattern
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(a)
(29.34%) (42.59%)
0.25 uV m
-2
(b)
reports that described the formation of more highly crystallized inorganic materials (MgO, dolomite, hydroxyapatite) at higher temperatures [29–31]. The results of BET, XPS, and Raman analyses were fully discussed in a previous report [21]. Fig. 2 shows the CO2-TPD curves for Mg–Al873, Mg–Al1073, Mg–Al273, and the MgO and Al2O3 reagents. The CO2-TPD curves were used to determine the quantity of adsorbed CO2 per specific area. Three CO2 desorption peaks were observed between 300 and 750 K. Reportedly, MgO and Mg–Al bimetallic oxides have weak (OH), moderate (M–O), and strong (O2) basic sites. In the present study, the CO2 desorption peak near 410–420 K was attributed to the weak basic sites, while those near 455–465 K and 520–540 K were attributed to the moderate and strong basic sites, respectively[32,33]. The values of the desorbed mass of CO2 for the MgO and Al2O3 reagents were 2.72 and 0.022 mmol/m2, respectively. The values for the bimetallic oxides were less than those for MgO at 0.15 mmol/m2 for Mg–Al873, 0.14 mmol/m2 for Mg– Al1073, and 0.34 mmol/m2 for Mg–Al1273, which are likely due to the presence of Al. These values were also less than those of MgO obtained via the calcination of MgCO3 at 873 K, 1073 K, and 1273 K (hereafter these MgO products are referred to as MgO873, MgO1073, and MgO1273, respectively). The total mass of desorbed CO2 for the calcined MgO species was 0.6753, 1.252, and 1.558 mmol/m2 for Mg–Al873, MgO1073, and MgOl1273, respectively [34]. Supplementary material Table S1 summarizes the results of the BET [21,34], crystal size [21,29], and CO2-TPD analyses for calcined MgO [29], Mg–Al bimetallic oxides, and MgO and Al2O3 reagents.
(c)
Al2O3 reagent 2
(28.07%)
(37.96%)
MgO reagent
-2
(20.18%)
(0.0220 mmol/m )
5.0 uV m
(41.87%)
2
(2.72 mmol/m )
Mg-Al 1273 2
1.0 uV m
-2
(0.343 mmol/m ) (34.00%)
(47.64%)
Mg-Al 1073
(16.43%)
number 21-1152), as estimated via Rietveld analysis. The crystallite sizes of the bimetallic oxides calculated using the Halder–Wagner equation [25] were 96.2, 136, and 302 nm for Mg–Al873, Mg–Al1073, and Mg–Al1273, respectively [21]. Thus, the crystallite size of the bimetallic oxides increased as the calcination temperature increased. These results are in good agreement with previous
(52.02%)
(31.55%)
Moderate (29.09%)
Weak (16.16%)
0.5 uV m
Fig. 1. XRD patterns for the starting material (Mg–Al373), calcined products, (Mg–Al873, Mg–Al1073, and Mg–Al1273), and solid residues after sorption of 5.68–51.33 mM B on (a) Mg–Al873, (b) Mg–Al1073, and (c) Mg–Al1273. Symbols: d, hydrotalcite (JCPDS 48-0601); N, MgO (JCPDS 45-946); 4, MgAl2O4 (JCPDS 211152).
-2
0.5 uV m
-2
Desorption of CO2
(18.36%)
300
Mg-Al 873 Strong (54.75%)
400
500
2
(0.147 mmol/m )
600
2
(0.159 mmol/m )
700
800
Temperature (K) Fig. 2. CO2-TPD curves and peak separations for Mg–Al bimetallic oxides and the MgO and Al2O3 reagents. The numbers in brackets indicate the total basicities expressed as the mass of carbon dioxide sorbed per unit surface area. The percentages indicate the relative areas of the peaks after deconvolution.
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3.2. Boron sorption The sorption of B by the bimetallic oxides from borate solutions with initial concentration of 5.68 mM is shown in Fig. 3(a). The sorption of B was stable, and B was not released from the solid phase for the duration of the experiments. The Mg–Al873 sample adsorbed B most rapidly (620 h), and the lowest equilibrium concentration (at 120 h) was achieved for Mg–Al1273. In addition, the quantities of B adsorbed on Mg–Al873 (1.58 mmol/g) and Mg– Al1073 (1.78 mmol/g) were approximately 2 times greater than those on Mg–Al1073 (0.78 mmol/g). An explanation for the more rapid sorption on Mg–Al1073 is its specific surface area, which was greater than those of other materials. Note that the sorption rate correlated well with the pH of the solution (Fig. 3(d)). Release of Mg and Al into the solution and subsequent resorption onto the solid phase were observed for all of the bimetallic oxides (Fig. 3 (b) and (c)). During the adsorption of B from the solutions, the maximum concentrations of released Mg and Al were in the order Mg–Al873 (0.47 mM) > Mg–Al1073 (0.15 mM) > Mg–Al1273 (0.082 mM) and Mg–Al1273 (0.57 mM) > Mg–Al1073 (0.94 mM) > Mg–Al873 (2.23 mM), respectively. Thus, the bimetallic oxides calcined at higher temperatures released much higher quantities of Al and lower quantities of Mg. The Al released from Mg–Al873 and Mg–Al1073 was resorbed into the solid phase, but for Mg– Al1273, 66% of the Al remained in the solution. The difference in behavior may be explained by different nAl/nMg surface molar ratios for the bimetallic oxides. A previous XPS analysis revealed that nAl/nMg increased as the calcination temperature increased (the nAl/nMg ratios of for Mg–Al873, Mg–Al1073, and Mg–Al1273
were 0.33, 0.43, and 0.53, respectively) [21], indicating that calcination at higher temperatures produced an Al-rich surface on the bimetallic oxides. Batch sorption experiments were performed in order to create sorption isotherms. Each of the Mg–Al bimetallic oxides was placed in contact with solutions containing various concentrations of B (1–53.3 mM). Notably, the concentration of released Mg never exceeded the concentration of the released Al. In addition, Mg– Al1273 released a much greater quantity of Al at higher concentrations, and the released Al was not resorbed onto the solid phase (see Supplementary material Figs. S2 and S3). The equilibrium pH decreased as the initial B concentration increased and as the calcination temperature increased. This decrease in the equilibrium pH with increasing concentration was not observed for F sorption on the bimetallic oxides (see Supplementary material Fig. S4). These results imply that the alkalinity of the solution insufficient for the regeneration of hydrotalcite-like compounds for solutions with higher B concentrations and the Mg–Al bimetallic oxides calcined at higher temperatures. The formation of hydrotalcite-like compounds generally occurs when the pH is >10.5. In addition, the regeneration of hydrotalcite-like compounds from bimetallic oxides is related to the following equation (Eq. (3)) [35]:
Mg1x Alx O1þx=2 þ ðx=nÞAn þ ðm þ 1 þ x=2 þ yÞH2 O ! Mg1x Alx ðOHÞ2 An x=n mH2 O þ xOH
The sorption isotherms for B on the Mg–Al bimetallic oxides are summarized in Fig. 4, which also included the results shown in
6
0.6
B concentration (mM)
(a)
(b) Mg concentration (mM)
Mg-Al 1273 Mg-Al 1073 Mg-Al873
5 4 3 2 1
0.5 0.4 0.3 0.2 0.1
0
0.0 0
20
40
60
80
100
120
0
20
40
60
80
100
120
Time (h)
Time (h) 3.0
12
2.5
11
2.0
10
pH
Al concentration (mM)
(c)
1.5
(d)
9
1.0
8
0.5
7
0.0
6 0
20
40
60
Time (h)
80
100
120
ð3Þ
0
20
40
60
80
100
120
Time (h)
Fig. 3. Changes in the concentration of (a) B, (b) Mg, and (c) Al, and the (d) pH during the sorption of 5.68 mM B on Mg–Al bimetallic oxides.
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12
8
6 24.04 mM
4
Mg-Al873 Mg-Al1073 Mg-Al1273
2.5
Q (mmol/g)
Q (mmol/g)
10
3.0
(a)
Mg-Al1273 Mg-Al1073 Mg-Al873 MgO1073
32.39 mM
13.15 mM
MgO873 MgO1073 MgO1273
(b)
2.0 5.68 mM
1.5
1.0
5.68 mM
51.33 mM
2
0
0.5
0
10
20
30
40
Ce (mM)
0.0
0
1
2
3
4
Ce (mM)
Fig. 4. Sorption isotherms for B on each Mg–Al bimetallic oxide and MgO calcined at 1073 K. The region with less than 4.0 mM Ce in (a) is expanded in (b). The numbers (mM) in both figures indicate the initial concentrations of B.
Fig. 3. The region where Ce < 4.0 mM is in (a) is expanded in (b). The numbers (mM) in both figures indicate the initial B concentrations. Mg–Al1073 exhibited the highest sorption density for B when Ce = 4.0 mM. In addition, the Q values for Mg–Al873 and Mg– Al1073 were higher than the B sorption capacities for MgO obtained from MgCO3 via calcination at 873–1073 K for 1 h. It has been previously suggested that MgO immobilizes B as a coprecipitate of Mg(OH)2 [15,16,29] and that MgO calcined at higher temperatures has greater basicity per unit surface area and thus a greater B sorption density [29]. However, in the present study, the Mg–Al bimetallic oxides with lower basicities per unit surface area (as indicated by the CO2-TPD results) than calcined MgO (see Supplementary material Table S1) exhibited greater B sorption densities. When MgO was used as the sorbent for the immobilization of B, the greater basicity per unit surface area was more effective for the immobilization of B; however, the basicity per unit surface area of the bimetallic oxides did not influence the B sorption density. That is, despite the presence of a MgO phase in the Mg–Al bimetallic oxides as indicated by the XRD patterns, the mechanism of the immobilization of B on the bimetallic oxides differed from that for single-phase MgO. However, the maximum sorption capacity for B on each of the Mg–Al bimetallic oxides, which was achieved for an initial B concentration of 24.04 mM, gradually decreased as the initial B concentration decreased. This decrease in the Q values was consistent with a decrease in the equilibrium pH. H3BO3 and B(OH) 4 typically exist as monomers at low concentration of B (625 mM), whereas polymeric B such as B3O3(OH)2 5 , B4O5(OH)2 4 , B5O6(OH)4 can be formed at high concentration of B (P25 mM) [10,36]. The dissociation of these polymeric B into H3BO3/B(OH) 4 depends on pH value [37,38]. Therefore, it may be difficult to immobilize these polymeric B compounds, which have large molecular and steric structures at lower pH value. On the other hand, when Ce 5 30 mM, the Q values for the Mg–Al bimetallic oxides and MgO were reversed, indicating that the MgO immobilized the polymeric B compounds via co-precipitation. The B sorption density on Mg–Al1273 was clearly less than that on Mg–Al873 and Mg–Al1073. The lower sorption may be due to the presence of the MgAl2O4 in the Mg–Al1273. Since no evidence for B sorption by MgAl2O4 has been previously reported, XRD analyses of the solid residues after B sorption were performed and are discussed below. Notably, Mg–Al1873 and Mg–Al1073 exhibited remarkable efficacy for achieving the reduction of B concentration to 1 ppm (0.92 mM), as required by WHO regulations [9].
3.3. Characterization of solid residues XRD patterns for the solid residues after sorption of B from solutions with various B concentrations are shown in Fig. 1. Hydrotalcite-like compounds regenerated from Mg–Al873 for the entire range of initial B concentrations. Broader peaks were observed as the initial B concentration increased, indicating a reduction in the crystallinity of the hydrotalcite-like compounds after sorption of B. Hydrotalcite-like compounds regenerated from Mg–Al1073 on B solutions with concentrations ranging from 5.68 to 51.33 mM, and a MgO phase was observed when the B concentration exceeded 32.39 mM. Less crystalline hydrotalcite-like compounds regenerated from Mg–Al1273 in solutions with B concentrations ranging from 5.68 to 24.04 mM, and MgAl2O4 was observed in all of the solid residues after sorption of B from these solutions. Notably, the intensity of the MgAl2O4 diffraction peaks remained unchanged after sorption of B. These results indicate that the MgAl2O4 was stable in the alkaline solutions. In addition, the lack of B sorption by the MgAl2O4 resulted in the lowest B sorption density for Mg–Al1273. Furthermore, no peaks assignable to hydrotalcite were detected in the diffraction patterns of the Mg– Al1273 solid residues after the sorption of B from solutions with B concentrations 32.39–51.33 mM. As described above, polymeric B species that form in borate solutions with concentrations >25 mM were difficult to immobilize, particularly by Mg–Al1273. Regeneration of hydrotalcite-like compounds was an important factor for obtaining greater B sorption densities. As mentioned above, the Mg–Al bimetallic oxides were more effective at B sorption than MgO when Ce P 4.0 mM. Therefore, the intercalation of B in the interlayers of the hydrotalcite-like compounds is thought to be more effective than the co-precipitation of B with Mg(OH)2. Fig. 5 shows FTIR spectra for the solid residues after sorption by the bimetallic oxides of B from solutions containing 5.68– 51.33 mM B.borate. The broad peaks at 3800–3000 cm1 were assigned to the O–H stretching mode m(O–H) [20,39], while the relatively small peaks near 1640 cm1 were assigned to the HOH water deformation mode d(H–O–H) [20,39,40]. Additional FTIR bands observed for Mg–Al373 at 1400–1500 cm1 were assigned [3] 2 to the asymmetric CO2 3 mode v3( CO3 ) [40]. Weak peaks near 1280 cm1 and 1450 cm1 in the spectra shown in (f)-(j) were assigned to the d (B–O–H) bending mode and the v3([3]B–O) [10,41] asymmetric stretching mode. These bands were incorporated into the asymmetric CO2 band in the spectra for 3
197
[4]
(b)
Mg-Al 1273 [4]
[3]
[3]
B/ B=0.37
B/ B=0.30
[3]
B/ B=0.32
Mg-Al 1073 [4]
[3]
B/ B=0.64
(j) (i)
(i)
[4]
Mg-Al 873
Intensity
(II)
Mg-Al 1273
δ (B−OH)
(j)
(a)
Mg-Al 1073
νasy (CO32-)
(I)
δ (H−O−H)
ν (O−H)
νasy ( [3]B−O)
S. Moriyama et al. / Separation and Purification Technology 152 (2015) 192–199
[4]
[3]
B/ B=0.26
(h)
(h)
Mg-Al 873
(g)
[4]
Na2BO4
(g)
[3]
B/ B=0.73
(f) (f)
H3BO3
(e)
(e)
30
20
10
0
-10 30
20
10
0
-10
Chemical Shift (ppm) (d)
(d) (c)
Fig. 6. 11B NMR MAS spectra for (a) H3BO3, Na2B4O7, and the solid residues after sorption of 5.68 mM B on Mg–Al873, Mg–Al1073, and Mg–Al1273 and (b) the solid residues after sorption of 24.04 mM B on Mg–Al873, Mg–Al1073, and Mg–Al1273.
(c)
(b) (b)
(a) (a) 0.1 cps
4000
0.1 cps
3500
3000
1800
1600
1400
1200
Wave number (cm−1) Fig. 5. FTIR spectra (I) from 3000 to 4000 cm1 and (II) from 1200 to 1800 cm1 for (a) Mg–Al373 and for the solid residues after sorption of 5.68, 24.04, and 51.33 mM B on (b), (c), and (d) Mg–Al873, (e), (f), and (g) Mg–Al1073, and (h), (i), and (j) Mg– Al1273, respectively.
Mg–Al873 and Mg–Al1073 after B sorption from 5.68 mM B solutions (Fig. 5(II) (b) and (e), respectively). The peaks assigned to the OH stretching mode (3000–3800 cm1) and CO2 asym3 metric stretching mode including the B–O stretching mode (1400–1500 cm1) sharpened after B sorption from the 5.68 mM B solution by Mg–Al873 and Mg–Al1073. On the other hand, these peaks broadened as initial B concentration decreased. The similarity of the spectra for the solid residues on Mg–Al1273 and the solid residues after the sorption of high concentrations of B (24.04 and 53.5 mM) by other bimetallic oxides calcined at lower temperatures indicates that interlayers were not completely formed in the hydrotalcite-like compounds. Importantly, these FTIR results were in agreement with the XRD results.
3.4. The 11
[4]
B/[3]B ratios in the solid residues
B NMR spectra for the solid residues after B sorption from solutions with initial B concentrations of 5.68 and 24.04 mM and those for H3BO3 and Na2B4O7, are shown in Fig. 6. The peaks near 19 and 0 ppm are due to trigonal borate (H3BO3) and tetrahydrox-
yborate (B(OH) 4 ) species, respectively. After the sorption of B from a 5.68 mM solution, trigonalborate ([3]B) was the predominant species, although the concentrations of tetrahedralborate ([4]B) were higher than those of [3]B in the solutions according to equilibrium pH values and Eq. (1). The tetrahedroxyborate/trigonalborate ([4]B/[3]B) ratios in these solid residues were in the range 0.26– 0.32. On the other hand, after the sorption of B from a solution with an initial B concentration of 24.04 mM, the ratio of [4]B/[3]B increased for all the solid residues. In addition, the bimetallic oxides calcined at lower temperature had larger [4]B/[3]B ratios, indicating that the intercalation of [4]B increased the B sorption density. The [4]B/[3]B ratio for Mg–Al1273 after the sorption of 24.04 mM B was much smaller than those for Mg–Al873 and Mg–Al1073, which implies that the intercalation capacity of Mg– Al1273 was lower than those of Mg–Al873 and Mg–Al1073. Furthermore, the immobilization of [4]B increased more than that of [3] B for Mg–Al873 and Mg–Al1073 compared with Mg–Al1273 when the initial B concentration was 24.04. Thus, highly crystalline hydrotalcite-like compounds was more likely to immobilize [4]B via intercalation. It is important to note that during B sorption on bimetallic oxides, there is a limit to the amount [3]B can be intercalated in the interlayers of the hydrotalcite-like compounds. Temporal changes in the B NMR spectra of the solid residues after sorption in a 24.04 mM B by Mg–Al873 is shown in Fig. 7. The calculated [4]B/[3]B ratios in the solid residues at each sampling time were 0.33–1.59 for 0.5–48 h, respectively. These results revealed that B was initially sorbed as [3]B during the regeneration of the hydrotalcite-like compounds; then, [4]B gradually intercalated in the interlayers of the hydrotalcite-like compounds. During the regeneration of the hydrotalcite-like compounds, the formation of positively charged metal layers in the hydrotalcite-like compounds enabled the adsorption of [4]B. These sorption mechanisms for [4]B and [3]B are different from the sorption mechanism for B on MgO. Previous reports proposed co-precipitation with Mg(OH)2 as the principle mechanism for immobilizing B using MgO, and the [4]B/[3]B ratio of in solid residues using MgO were much smaller than those obtained in the present study for the bimetallic oxides. Specifically, the [4]B/[3]B ratios were 0.0142,
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Energy and Industrial Technology Development Organization under the Innovative Zero-emission Coal-fired Power Generation Project. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.seppur.2015.08. 023.
48 h
1.59 References
Intensity
24 h
1.08
1.00
10 h
0.76
6h
0.67
3h [4]
1h
30
20
10
B/[3]B 0.24
0
-10
Chemical shift (ppm) Fig. 7. 11B NMR spectra and changes in the [4]/B/[3]B ratio as a function of time for the solid residues after sorption of 24 mM B.
0.0184, and 0.0512 after the sorption of B by MgO for 120 h from solutions with initial B concentrations of 5.6 mM, 21.6 mM, and 67.0 mM, respectively [15]. In summary, controlling the basicity per unit surface area is an important factor for increasing the B sorption density on MgO but not for Mg–Al bimetallic oxides. For the latter, regeneration of hydrotalcite-like compounds is the key to increasing the B sorption density. 4. Conclusions We can conclude that Mg–Al bimetallic oxides are promising as stable sorption materials for the remediation of B solutions to the levels required by the WHO regulations for drinking water. The Mg–Al873 and Mg–Al1073 samples were more effective than single-phase MgO obtained via the calcination of MgCO3 with higher B sorption densities when Ce P 4 mM. Moreover, the capture of trigonalborate ([3]B) and tetrahedralborate ([4]B) is an advantage of Mg–Al bimetallic oxides. Further research is needed to test the recovery and recyclability of the bimetallic oxides. Acknowledgements Financial support was provided to KS by the JSPS Funding Program for Next-Generation World Leading Researchers (NEXT Program) GR078. This study was partially supported by the New
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Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Sorption properties of boron on Mg–Al bimetallic oxides calcined at different temperatures Sayo Moriyama a,⇑, Keiko Sasaki b, Tsuyoshi Hirajima b, Keiko Ideta c, Jin Miyawaki c a
Fukuoka Institute of Health and Environmental Sciences, Dazaifu 818-0135, Japan Department of Earth Resource Engineering, Kyushu University, Fukuoka 819-0395, Japan c Department of Advanced Device Materials, Institute for Materials Chemistry and Engineering, Kyushu University, Kasuga 816-8180, Japan b
a r t i c l e
i n f o
Article history: Received 9 June 2015 Received in revised form 13 August 2015 Accepted 14 August 2015 Available online 15 August 2015 Keywords: Boron Sorption Bimetallic oxides Hydrotalcite 11 B NMR
a b s t r a c t Mg–Al bimetallic oxides produced via the calcination of hydrotalcite-like compounds ([(Mg0.75Al0.25(OH)2](An)0.75/nmH2O, where A is an anionic species) exhibited high potential for the removal of boron from aqueous solutions. X-ray diffraction patterns for the produced bimetallic oxides revealed that MgO was the primary phase within the range of investigated calcination temperatures. In addition, 11B NMR spectral analyses indicated that the Mg–Al bimetallic oxides captured trigonal B ([3]B) and tetrahedral B ([4]B) after the sorption of boron, regenerating hydrotalcite-like compounds. As the initial concentration of boron increased, the percentage of tetrahedral [4]B in solid residues after the sorption of boron increased. The [4]B/[3]B ratios in the solid residues increased with time along with the regeneration of hydrotalcite-like compounds. Furthermore, the Mg–Al bimetallic oxides produced from hydrotalcite-like compounds were more favorable than other bimetallic oxides and effective than single-phase MgO produced from MgCO3 at the same temperature, indicating that Mg–Al bimetallic oxides are stable materials with the potential for use in the remediation of contaminated sites and water. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction
using electrostatic interactions. Boric acid is dissociated to tetrahydroxyborate with a pKa of 9.24 [10] (Eq. (1)):
Boron is an essential micronutrient for plants, but the difference in the necessary intake level and the level at which boron becomes toxic is minimal. Therefore, in some cases, boron retards plant growth [1–3]. In particular, damage to vegetation due to high levels of boron has been observed in arid and semi-arid areas in South Australia, Turkey, North America, and Chile [4,5]. Excess intake of boron also affects the human nervous system [6–8]. Hence, regulations have recently been introduced by the World Health Organization (WHO) that has set maximum concentration limits for boron to be less than 1 mg/L for drinking water [9]. The primary causes of boron contamination can be divided into natural and anthropogenic categories. The former includes topographical factors such as mining and irrigation. The latter includes the manifold applications of boron in various industries such as in the manufacture of glass products, preservatives, audio equipment, and semiconductor processing [4]. At ambient pH, boron typically exists as undissociated boric acid; thus, it is difficult to immobilize at circumneutral pH values
H3 BO3 þ H2 O¡BðOHÞ4 þ Hþ
⇑ Corresponding author. E-mail address: [email protected] (S. Moriyama). http://dx.doi.org/10.1016/j.seppur.2015.08.023 1383-5866/Ó 2015 Elsevier B.V. All rights reserved.
ð1Þ
As a result, the current conventional method for removing boron from solution involves the use of costly boron-specific resins with n-methyl glucamine substituents containing many hydroxyl groups [11–14]. Therefore, the development of a sorbent that meets requirements for cost effectiveness, reactivity, availability, and permeability is necessary. Recently, MgO is been found to be an effective and promising sorbent for boron removal [15–17]. Hydrotalcite adopts a layered, double-hydroxide structure and n III has a general chemical formula [MII1x )x/nmH2O, 1 M2 x(OH2)](A where MII1 indicates a divalent cation (Mg2+, Zn2+, Ni2+, Co2+, Mn2+, 3+ 3+ 3+ n or Cd2+), MIII rep2 indicates a trivalent cation (Al , Fe , or Cr ), A resents an interlayer anion with a valence of n, and x corresponds to [MnIII]/[MnII] + [MnIII]. Hydrotalcite-like compounds transform to bimetallic oxides by thermal decomposition and the produced bimetallic oxides regenerate hydrotalcite-like compounds again in aqueous solutions. Hydrotalcite-like compounds and bimetallic oxides can immobilize anionic species by ion-exchange and intercalation in the interlayer, respectively. As a consequence of its unique structure, hydrotalcite and its calcined products have been widely investigated as sorbents for harmful anions such as Cl, F, Br, I,
S. Moriyama et al. / Separation and Purification Technology 152 (2015) 192–199 3 2 3 SO2 4 , B(OH)4 , NO3 , AsO3 , CrO4 , VO4 , dodecylbenzylsulfonate, 2,4,6-trinitrophenol [18–24]. In the present study, Mg-bearing bimetallic oxides prepared from hydrotalcite produced via a co-precipitation method were used as sorbents for boron removal with the objective of identifying more stable and effective alternatives. The efficacy of Mg–Al bimetallic oxides calcined at different temperatures for the immobilization of boron was investigated. The main purposes of this study include (1) the evaluation of the sorption capacity of boron onto Mg–Al bimetallic oxides produced at different temperatures and (2) the determination of the mechanism of boron sorption onto the Mg–Al bimetallic oxides. The capacities of single-phase MgO and the Mg–Al bimetallic oxides for the removal of boron from aqueous solutions were also compared.
2. Materials and methods 2.1. Synthesis and characterization of the Mg–Al bimetallic oxides A hydrotalcite-like compound with the above-described chemical formula in which x was approximately 0.25 was synthesized according to a previously reported method as the starting material for the Mg–Al bimetallic oxides [21,24]. The hydrotalcite-like compound produced via co-precipitation was calcined at 873 K, 1073 K, and 1273 K for 3 h in order to obtain the Mg–Al bimetallic oxides Mg–Al873, Mg–Al1073, and Mg–Al1273, respectively. X-ray powder diffraction patterns for the Mg–Al bimetallic oxides were collected using a diffractometer (Multi Flex, Rigaku, Tokyo, Japan) with CuKa (k = 1.5406 Å) radiation at 20 mA and 40 kV and a scanning rate of 2°/min from 5° to 85°. The powder diffraction pattern analysis software PDXL (Rigaku, Tokyo, Japan) was used for the Rietveld analysis. The crystal size was estimated from the characteristic peak at 2h = 11° (corresponding to the 003 plane) using the Halder–Wagner method [25]. XRD patterns for the solid residues obtained after sorption of borate were also collected under the same conditions. Temperature programmed desorption curves (TPD, BELCAT-B, BEL JAPAN Inc., Toyonaka, Japan) were obtained to evaluate the basicity and number of basic sites in bimetallic oxides. The CO2-TPD curves for the reagents MgO (Wako, special grade, Japan) and Al2O3 (Wako, special grade, Japan) were also obtained. After pre-treatment at 773 K, CO2-TPD was performed at 973 K for 50 min, and the quantity of desorbed CO2 was measured from room temperature to 973 K. Peak separations were performed using ChemMaster 1.2.0.2 software (BEL JAPAN Inc., Japan) to evaluate the basic sites in the bimetallic oxides. Detailed characterization of the starting materials and Mg–Al bimetallic oxides can be found elsewhere [21]. Briefly, specific surface areas were determined using the Brunauer–Emmett–Teller (BET) method [26], Raman and X-ray photoelectron spectra (XPS) were obtained to determine the surface molar ratios nAl/nMg, scanning electron microscopy (SEM; VE-9800, Keyence, Tokyo, Japan), and transmission electron microscopy (TEM; TECNAI F20, Phillips, Eindhoven, Netherlands) were performed to observe surface morphologies.
193
polypropylene vials. Duplicate suspensions were shaken on a rotary shaker at 100 rpm and 298 K for 120 h. Subsamples (1 ml) of the supernatants were taken at intervals and filtered through a membrane filter with a pore size of 0.22 lm. The concentrations of the remaining B and dissolved total Mg and Al species were determined using inductively couple plasma atomic emission spectroscopy. Mass of the B (Q) on the solid phase per unit mass of solid phase at equilibrium (mM/g) were calculated (Eq. (2)), where Ci and Ce are the initial metal concentration and concentration after the sorption of B (mM), respectively, V is the volume used (L), and M is the mass (g).
Qðmmol=gÞ ¼
ðC e C i ÞV m
ð2Þ
pH was also determined. After the sorption of B, the solid residues were collected and lyophilized to provide specimens for XRD and Fourier transform infrared (FTIR) (FT/IR-670 Plus, JASCO, Tokyo, Japan) analyses. 2.3. Characterization of the solid residues after the sorption of boron XRD patterns for solid residues after the sorption of B from solutions with initial B concentrations of 5.68 mM, 24.04 mM, 32.39 mM, and 51.33 mM were collected in the same manner as described for the Mg–Al bimetallic oxides. Solid-state 11B NMR spectra for the solid residues after the sorption of B were acquired on a JEOL ECA 800 (JEOL RESONANCE Inc., Tokyo, Japan) with 3.2 mm Multiple Quantum Magic Angle Spinning (MQMAS) probes using the single pulse method and Delta NMR software version 4.3. The resonance frequency for 11B was 256.6 MHz at the field strength of 18.8 T. Typical acquisition parameters were as follows: spinning speed was 15 kHz; pulse length was 2.5 ls; relaxation delay was 10 s (11B); and the total number of scans was 63,341 depending on the B concentration [27]. The 11B chemical shift was referenced to that of a saturated H3BO3 solution at 19.5 ppm [28]. The chemical reagents H3BO3 and Na2B4O7 (special grade, Wako Pure Chemical Industries, Ltd., Osaka, Japan) were used as standards. FTIR spectra were recorded on an FTIR spectrometer (FT/IR-670, JASCO, Japan) using KBr pellets. The solid residues obtained after the sorption of B on the Mg–Al bimetallic oxides were diluted with KBr at a rate of 10% and examined over the range from 4000 to 400 cm1. 2.4. Change in the
[4]
B/[3]B molar ratio with time
To investigate the temporal changes in the [4]B/[3]B ratios in the solid residues after the sorption of B, 0.2 g of Mg–Al873 was added to 24.04 mM B solution and the solid residues were filtered and lyophilized at intervals of 0.5, 1, 3, 6, 10, 24, and 48 h. Then, the 11 B NMR spectrum of each sample was obtained. 3. Results and discussion 3.1. Characterization of Mg–Al bimetallic oxides
2.2. Boron sorption Boron (B) solutions were prepared using H3BO3 (special grade; Wako Pure Chemical Industries, Ltd., Osaka, Japan) and high purity water (MQ water, <18.2 MX/cm) to afford initial concentrations of 1.00–51.33 mM. The initial pH of each solution was adjusted to 9.0 ± 0.05 using 1 M NaOH (special grade; Wako Pure Chemical Industries, Ltd., Osaka, Japan). Some quantity (0.100 g) of the Mg–Al bimetallic oxides was added to each B solution in
The XRD pattern for the precipitated starting material (Mg– Al373) was similar to that of hydrotalcite (JCPDS data base; Pattern number 48-0601) (Fig. 1). After the calcination of Mg–Al373 at 873–1073 K, the peaks for hydrotalcite in the XRD patterns of the calcined products disappeared and those for an MgO (JCPDS data base; Pattern number 45-946) phase appeared (Fig. 1(a)–(c)). In the material calcined at 1273 K, the XRD patterns indicated the formation of 60.8% MgO and 30.2% MgAl2O4 (JCPDS data base; Pattern
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(a)
(29.34%) (42.59%)
0.25 uV m
-2
(b)
reports that described the formation of more highly crystallized inorganic materials (MgO, dolomite, hydroxyapatite) at higher temperatures [29–31]. The results of BET, XPS, and Raman analyses were fully discussed in a previous report [21]. Fig. 2 shows the CO2-TPD curves for Mg–Al873, Mg–Al1073, Mg–Al273, and the MgO and Al2O3 reagents. The CO2-TPD curves were used to determine the quantity of adsorbed CO2 per specific area. Three CO2 desorption peaks were observed between 300 and 750 K. Reportedly, MgO and Mg–Al bimetallic oxides have weak (OH), moderate (M–O), and strong (O2) basic sites. In the present study, the CO2 desorption peak near 410–420 K was attributed to the weak basic sites, while those near 455–465 K and 520–540 K were attributed to the moderate and strong basic sites, respectively[32,33]. The values of the desorbed mass of CO2 for the MgO and Al2O3 reagents were 2.72 and 0.022 mmol/m2, respectively. The values for the bimetallic oxides were less than those for MgO at 0.15 mmol/m2 for Mg–Al873, 0.14 mmol/m2 for Mg– Al1073, and 0.34 mmol/m2 for Mg–Al1273, which are likely due to the presence of Al. These values were also less than those of MgO obtained via the calcination of MgCO3 at 873 K, 1073 K, and 1273 K (hereafter these MgO products are referred to as MgO873, MgO1073, and MgO1273, respectively). The total mass of desorbed CO2 for the calcined MgO species was 0.6753, 1.252, and 1.558 mmol/m2 for Mg–Al873, MgO1073, and MgOl1273, respectively [34]. Supplementary material Table S1 summarizes the results of the BET [21,34], crystal size [21,29], and CO2-TPD analyses for calcined MgO [29], Mg–Al bimetallic oxides, and MgO and Al2O3 reagents.
(c)
Al2O3 reagent 2
(28.07%)
(37.96%)
MgO reagent
-2
(20.18%)
(0.0220 mmol/m )
5.0 uV m
(41.87%)
2
(2.72 mmol/m )
Mg-Al 1273 2
1.0 uV m
-2
(0.343 mmol/m ) (34.00%)
(47.64%)
Mg-Al 1073
(16.43%)
number 21-1152), as estimated via Rietveld analysis. The crystallite sizes of the bimetallic oxides calculated using the Halder–Wagner equation [25] were 96.2, 136, and 302 nm for Mg–Al873, Mg–Al1073, and Mg–Al1273, respectively [21]. Thus, the crystallite size of the bimetallic oxides increased as the calcination temperature increased. These results are in good agreement with previous
(52.02%)
(31.55%)
Moderate (29.09%)
Weak (16.16%)
0.5 uV m
Fig. 1. XRD patterns for the starting material (Mg–Al373), calcined products, (Mg–Al873, Mg–Al1073, and Mg–Al1273), and solid residues after sorption of 5.68–51.33 mM B on (a) Mg–Al873, (b) Mg–Al1073, and (c) Mg–Al1273. Symbols: d, hydrotalcite (JCPDS 48-0601); N, MgO (JCPDS 45-946); 4, MgAl2O4 (JCPDS 211152).
-2
0.5 uV m
-2
Desorption of CO2
(18.36%)
300
Mg-Al 873 Strong (54.75%)
400
500
2
(0.147 mmol/m )
600
2
(0.159 mmol/m )
700
800
Temperature (K) Fig. 2. CO2-TPD curves and peak separations for Mg–Al bimetallic oxides and the MgO and Al2O3 reagents. The numbers in brackets indicate the total basicities expressed as the mass of carbon dioxide sorbed per unit surface area. The percentages indicate the relative areas of the peaks after deconvolution.
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3.2. Boron sorption The sorption of B by the bimetallic oxides from borate solutions with initial concentration of 5.68 mM is shown in Fig. 3(a). The sorption of B was stable, and B was not released from the solid phase for the duration of the experiments. The Mg–Al873 sample adsorbed B most rapidly (620 h), and the lowest equilibrium concentration (at 120 h) was achieved for Mg–Al1273. In addition, the quantities of B adsorbed on Mg–Al873 (1.58 mmol/g) and Mg– Al1073 (1.78 mmol/g) were approximately 2 times greater than those on Mg–Al1073 (0.78 mmol/g). An explanation for the more rapid sorption on Mg–Al1073 is its specific surface area, which was greater than those of other materials. Note that the sorption rate correlated well with the pH of the solution (Fig. 3(d)). Release of Mg and Al into the solution and subsequent resorption onto the solid phase were observed for all of the bimetallic oxides (Fig. 3 (b) and (c)). During the adsorption of B from the solutions, the maximum concentrations of released Mg and Al were in the order Mg–Al873 (0.47 mM) > Mg–Al1073 (0.15 mM) > Mg–Al1273 (0.082 mM) and Mg–Al1273 (0.57 mM) > Mg–Al1073 (0.94 mM) > Mg–Al873 (2.23 mM), respectively. Thus, the bimetallic oxides calcined at higher temperatures released much higher quantities of Al and lower quantities of Mg. The Al released from Mg–Al873 and Mg–Al1073 was resorbed into the solid phase, but for Mg– Al1273, 66% of the Al remained in the solution. The difference in behavior may be explained by different nAl/nMg surface molar ratios for the bimetallic oxides. A previous XPS analysis revealed that nAl/nMg increased as the calcination temperature increased (the nAl/nMg ratios of for Mg–Al873, Mg–Al1073, and Mg–Al1273
were 0.33, 0.43, and 0.53, respectively) [21], indicating that calcination at higher temperatures produced an Al-rich surface on the bimetallic oxides. Batch sorption experiments were performed in order to create sorption isotherms. Each of the Mg–Al bimetallic oxides was placed in contact with solutions containing various concentrations of B (1–53.3 mM). Notably, the concentration of released Mg never exceeded the concentration of the released Al. In addition, Mg– Al1273 released a much greater quantity of Al at higher concentrations, and the released Al was not resorbed onto the solid phase (see Supplementary material Figs. S2 and S3). The equilibrium pH decreased as the initial B concentration increased and as the calcination temperature increased. This decrease in the equilibrium pH with increasing concentration was not observed for F sorption on the bimetallic oxides (see Supplementary material Fig. S4). These results imply that the alkalinity of the solution insufficient for the regeneration of hydrotalcite-like compounds for solutions with higher B concentrations and the Mg–Al bimetallic oxides calcined at higher temperatures. The formation of hydrotalcite-like compounds generally occurs when the pH is >10.5. In addition, the regeneration of hydrotalcite-like compounds from bimetallic oxides is related to the following equation (Eq. (3)) [35]:
Mg1x Alx O1þx=2 þ ðx=nÞAn þ ðm þ 1 þ x=2 þ yÞH2 O ! Mg1x Alx ðOHÞ2 An x=n mH2 O þ xOH
The sorption isotherms for B on the Mg–Al bimetallic oxides are summarized in Fig. 4, which also included the results shown in
6
0.6
B concentration (mM)
(a)
(b) Mg concentration (mM)
Mg-Al 1273 Mg-Al 1073 Mg-Al873
5 4 3 2 1
0.5 0.4 0.3 0.2 0.1
0
0.0 0
20
40
60
80
100
120
0
20
40
60
80
100
120
Time (h)
Time (h) 3.0
12
2.5
11
2.0
10
pH
Al concentration (mM)
(c)
1.5
(d)
9
1.0
8
0.5
7
0.0
6 0
20
40
60
Time (h)
80
100
120
ð3Þ
0
20
40
60
80
100
120
Time (h)
Fig. 3. Changes in the concentration of (a) B, (b) Mg, and (c) Al, and the (d) pH during the sorption of 5.68 mM B on Mg–Al bimetallic oxides.
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12
8
6 24.04 mM
4
Mg-Al873 Mg-Al1073 Mg-Al1273
2.5
Q (mmol/g)
Q (mmol/g)
10
3.0
(a)
Mg-Al1273 Mg-Al1073 Mg-Al873 MgO1073
32.39 mM
13.15 mM
MgO873 MgO1073 MgO1273
(b)
2.0 5.68 mM
1.5
1.0
5.68 mM
51.33 mM
2
0
0.5
0
10
20
30
40
Ce (mM)
0.0
0
1
2
3
4
Ce (mM)
Fig. 4. Sorption isotherms for B on each Mg–Al bimetallic oxide and MgO calcined at 1073 K. The region with less than 4.0 mM Ce in (a) is expanded in (b). The numbers (mM) in both figures indicate the initial concentrations of B.
Fig. 3. The region where Ce < 4.0 mM is in (a) is expanded in (b). The numbers (mM) in both figures indicate the initial B concentrations. Mg–Al1073 exhibited the highest sorption density for B when Ce = 4.0 mM. In addition, the Q values for Mg–Al873 and Mg– Al1073 were higher than the B sorption capacities for MgO obtained from MgCO3 via calcination at 873–1073 K for 1 h. It has been previously suggested that MgO immobilizes B as a coprecipitate of Mg(OH)2 [15,16,29] and that MgO calcined at higher temperatures has greater basicity per unit surface area and thus a greater B sorption density [29]. However, in the present study, the Mg–Al bimetallic oxides with lower basicities per unit surface area (as indicated by the CO2-TPD results) than calcined MgO (see Supplementary material Table S1) exhibited greater B sorption densities. When MgO was used as the sorbent for the immobilization of B, the greater basicity per unit surface area was more effective for the immobilization of B; however, the basicity per unit surface area of the bimetallic oxides did not influence the B sorption density. That is, despite the presence of a MgO phase in the Mg–Al bimetallic oxides as indicated by the XRD patterns, the mechanism of the immobilization of B on the bimetallic oxides differed from that for single-phase MgO. However, the maximum sorption capacity for B on each of the Mg–Al bimetallic oxides, which was achieved for an initial B concentration of 24.04 mM, gradually decreased as the initial B concentration decreased. This decrease in the Q values was consistent with a decrease in the equilibrium pH. H3BO3 and B(OH) 4 typically exist as monomers at low concentration of B (625 mM), whereas polymeric B such as B3O3(OH)2 5 , B4O5(OH)2 4 , B5O6(OH)4 can be formed at high concentration of B (P25 mM) [10,36]. The dissociation of these polymeric B into H3BO3/B(OH) 4 depends on pH value [37,38]. Therefore, it may be difficult to immobilize these polymeric B compounds, which have large molecular and steric structures at lower pH value. On the other hand, when Ce 5 30 mM, the Q values for the Mg–Al bimetallic oxides and MgO were reversed, indicating that the MgO immobilized the polymeric B compounds via co-precipitation. The B sorption density on Mg–Al1273 was clearly less than that on Mg–Al873 and Mg–Al1073. The lower sorption may be due to the presence of the MgAl2O4 in the Mg–Al1273. Since no evidence for B sorption by MgAl2O4 has been previously reported, XRD analyses of the solid residues after B sorption were performed and are discussed below. Notably, Mg–Al1873 and Mg–Al1073 exhibited remarkable efficacy for achieving the reduction of B concentration to 1 ppm (0.92 mM), as required by WHO regulations [9].
3.3. Characterization of solid residues XRD patterns for the solid residues after sorption of B from solutions with various B concentrations are shown in Fig. 1. Hydrotalcite-like compounds regenerated from Mg–Al873 for the entire range of initial B concentrations. Broader peaks were observed as the initial B concentration increased, indicating a reduction in the crystallinity of the hydrotalcite-like compounds after sorption of B. Hydrotalcite-like compounds regenerated from Mg–Al1073 on B solutions with concentrations ranging from 5.68 to 51.33 mM, and a MgO phase was observed when the B concentration exceeded 32.39 mM. Less crystalline hydrotalcite-like compounds regenerated from Mg–Al1273 in solutions with B concentrations ranging from 5.68 to 24.04 mM, and MgAl2O4 was observed in all of the solid residues after sorption of B from these solutions. Notably, the intensity of the MgAl2O4 diffraction peaks remained unchanged after sorption of B. These results indicate that the MgAl2O4 was stable in the alkaline solutions. In addition, the lack of B sorption by the MgAl2O4 resulted in the lowest B sorption density for Mg–Al1273. Furthermore, no peaks assignable to hydrotalcite were detected in the diffraction patterns of the Mg– Al1273 solid residues after the sorption of B from solutions with B concentrations 32.39–51.33 mM. As described above, polymeric B species that form in borate solutions with concentrations >25 mM were difficult to immobilize, particularly by Mg–Al1273. Regeneration of hydrotalcite-like compounds was an important factor for obtaining greater B sorption densities. As mentioned above, the Mg–Al bimetallic oxides were more effective at B sorption than MgO when Ce P 4.0 mM. Therefore, the intercalation of B in the interlayers of the hydrotalcite-like compounds is thought to be more effective than the co-precipitation of B with Mg(OH)2. Fig. 5 shows FTIR spectra for the solid residues after sorption by the bimetallic oxides of B from solutions containing 5.68– 51.33 mM B.borate. The broad peaks at 3800–3000 cm1 were assigned to the O–H stretching mode m(O–H) [20,39], while the relatively small peaks near 1640 cm1 were assigned to the HOH water deformation mode d(H–O–H) [20,39,40]. Additional FTIR bands observed for Mg–Al373 at 1400–1500 cm1 were assigned [3] 2 to the asymmetric CO2 3 mode v3( CO3 ) [40]. Weak peaks near 1280 cm1 and 1450 cm1 in the spectra shown in (f)-(j) were assigned to the d (B–O–H) bending mode and the v3([3]B–O) [10,41] asymmetric stretching mode. These bands were incorporated into the asymmetric CO2 band in the spectra for 3
197
[4]
(b)
Mg-Al 1273 [4]
[3]
[3]
B/ B=0.37
B/ B=0.30
[3]
B/ B=0.32
Mg-Al 1073 [4]
[3]
B/ B=0.64
(j) (i)
(i)
[4]
Mg-Al 873
Intensity
(II)
Mg-Al 1273
δ (B−OH)
(j)
(a)
Mg-Al 1073
νasy (CO32-)
(I)
δ (H−O−H)
ν (O−H)
νasy ( [3]B−O)
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[4]
[3]
B/ B=0.26
(h)
(h)
Mg-Al 873
(g)
[4]
Na2BO4
(g)
[3]
B/ B=0.73
(f) (f)
H3BO3
(e)
(e)
30
20
10
0
-10 30
20
10
0
-10
Chemical Shift (ppm) (d)
(d) (c)
Fig. 6. 11B NMR MAS spectra for (a) H3BO3, Na2B4O7, and the solid residues after sorption of 5.68 mM B on Mg–Al873, Mg–Al1073, and Mg–Al1273 and (b) the solid residues after sorption of 24.04 mM B on Mg–Al873, Mg–Al1073, and Mg–Al1273.
(c)
(b) (b)
(a) (a) 0.1 cps
4000
0.1 cps
3500
3000
1800
1600
1400
1200
Wave number (cm−1) Fig. 5. FTIR spectra (I) from 3000 to 4000 cm1 and (II) from 1200 to 1800 cm1 for (a) Mg–Al373 and for the solid residues after sorption of 5.68, 24.04, and 51.33 mM B on (b), (c), and (d) Mg–Al873, (e), (f), and (g) Mg–Al1073, and (h), (i), and (j) Mg– Al1273, respectively.
Mg–Al873 and Mg–Al1073 after B sorption from 5.68 mM B solutions (Fig. 5(II) (b) and (e), respectively). The peaks assigned to the OH stretching mode (3000–3800 cm1) and CO2 asym3 metric stretching mode including the B–O stretching mode (1400–1500 cm1) sharpened after B sorption from the 5.68 mM B solution by Mg–Al873 and Mg–Al1073. On the other hand, these peaks broadened as initial B concentration decreased. The similarity of the spectra for the solid residues on Mg–Al1273 and the solid residues after the sorption of high concentrations of B (24.04 and 53.5 mM) by other bimetallic oxides calcined at lower temperatures indicates that interlayers were not completely formed in the hydrotalcite-like compounds. Importantly, these FTIR results were in agreement with the XRD results.
3.4. The 11
[4]
B/[3]B ratios in the solid residues
B NMR spectra for the solid residues after B sorption from solutions with initial B concentrations of 5.68 and 24.04 mM and those for H3BO3 and Na2B4O7, are shown in Fig. 6. The peaks near 19 and 0 ppm are due to trigonal borate (H3BO3) and tetrahydrox-
yborate (B(OH) 4 ) species, respectively. After the sorption of B from a 5.68 mM solution, trigonalborate ([3]B) was the predominant species, although the concentrations of tetrahedralborate ([4]B) were higher than those of [3]B in the solutions according to equilibrium pH values and Eq. (1). The tetrahedroxyborate/trigonalborate ([4]B/[3]B) ratios in these solid residues were in the range 0.26– 0.32. On the other hand, after the sorption of B from a solution with an initial B concentration of 24.04 mM, the ratio of [4]B/[3]B increased for all the solid residues. In addition, the bimetallic oxides calcined at lower temperature had larger [4]B/[3]B ratios, indicating that the intercalation of [4]B increased the B sorption density. The [4]B/[3]B ratio for Mg–Al1273 after the sorption of 24.04 mM B was much smaller than those for Mg–Al873 and Mg–Al1073, which implies that the intercalation capacity of Mg– Al1273 was lower than those of Mg–Al873 and Mg–Al1073. Furthermore, the immobilization of [4]B increased more than that of [3] B for Mg–Al873 and Mg–Al1073 compared with Mg–Al1273 when the initial B concentration was 24.04. Thus, highly crystalline hydrotalcite-like compounds was more likely to immobilize [4]B via intercalation. It is important to note that during B sorption on bimetallic oxides, there is a limit to the amount [3]B can be intercalated in the interlayers of the hydrotalcite-like compounds. Temporal changes in the B NMR spectra of the solid residues after sorption in a 24.04 mM B by Mg–Al873 is shown in Fig. 7. The calculated [4]B/[3]B ratios in the solid residues at each sampling time were 0.33–1.59 for 0.5–48 h, respectively. These results revealed that B was initially sorbed as [3]B during the regeneration of the hydrotalcite-like compounds; then, [4]B gradually intercalated in the interlayers of the hydrotalcite-like compounds. During the regeneration of the hydrotalcite-like compounds, the formation of positively charged metal layers in the hydrotalcite-like compounds enabled the adsorption of [4]B. These sorption mechanisms for [4]B and [3]B are different from the sorption mechanism for B on MgO. Previous reports proposed co-precipitation with Mg(OH)2 as the principle mechanism for immobilizing B using MgO, and the [4]B/[3]B ratio of in solid residues using MgO were much smaller than those obtained in the present study for the bimetallic oxides. Specifically, the [4]B/[3]B ratios were 0.0142,
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Energy and Industrial Technology Development Organization under the Innovative Zero-emission Coal-fired Power Generation Project. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.seppur.2015.08. 023.
48 h
1.59 References
Intensity
24 h
1.08
1.00
10 h
0.76
6h
0.67
3h [4]
1h
30
20
10
B/[3]B 0.24
0
-10
Chemical shift (ppm) Fig. 7. 11B NMR spectra and changes in the [4]/B/[3]B ratio as a function of time for the solid residues after sorption of 24 mM B.
0.0184, and 0.0512 after the sorption of B by MgO for 120 h from solutions with initial B concentrations of 5.6 mM, 21.6 mM, and 67.0 mM, respectively [15]. In summary, controlling the basicity per unit surface area is an important factor for increasing the B sorption density on MgO but not for Mg–Al bimetallic oxides. For the latter, regeneration of hydrotalcite-like compounds is the key to increasing the B sorption density. 4. Conclusions We can conclude that Mg–Al bimetallic oxides are promising as stable sorption materials for the remediation of B solutions to the levels required by the WHO regulations for drinking water. The Mg–Al873 and Mg–Al1073 samples were more effective than single-phase MgO obtained via the calcination of MgCO3 with higher B sorption densities when Ce P 4 mM. Moreover, the capture of trigonalborate ([3]B) and tetrahedralborate ([4]B) is an advantage of Mg–Al bimetallic oxides. Further research is needed to test the recovery and recyclability of the bimetallic oxides. Acknowledgements Financial support was provided to KS by the JSPS Funding Program for Next-Generation World Leading Researchers (NEXT Program) GR078. This study was partially supported by the New
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