Materials Transactions, Vol. 54, No. 9 (2013) pp. 1809 to 1817
© 2013 The Mining and Materials Processing Institute of Japan
Characteristic Sorption of H3BO3/B(OH)4¹ on Magnesium Oxide
Keiko Sasaki1,+, Xinhong Qiu1, Sayo Moriyama1, Chiharu Tokoro2, Keiko Ideta3 and Jin Miyawaki3
1
Department of Earth Resources Engineering, Kyushu University, Fukuoka 819-0395, Japan
Department of Creative Science and Engineering, Faculty of Science and Engineering,
Waseda University, Tokyo 169-8555, Japan
3
Department of Advanced Device Materials, Institute for Materials Chemistry and Engineering,
Kyushu University, Kasuga 816-8180, Japan
2
The reaction mechanism of H3BO3/B(OH)4¹ with MgO in aqueous phase was investigated using sorption isotherm, XRD, 11B-NMR and
FTIR. Release of Mg2+ was observed soon after contact of MgO with H3BO3 and maximum released Mg2+ was proportional to the initial boron
concentration, suggesting ligand-promoted dissolution of MgO by H3BO3. The molecular form of H3BO3 was more reactive with MgO in
releasing Mg2+ ions than B(OH)4¹. 11B-NMR results indicated that trigonal B ([3]B) was predominant over tetrahedral B ([4]B) in solid residues
after sorption of H3BO3. The molar ratio of [4]B/[3]B increased with H3BO3 sorption density. XRD patterns for the solid residues were assigned
to Mg(OH)2 and peaks broadened with increasing H3BO3 sorption density, except for (hk0) planes due to c-axis lattice strain induced by
incorporation of H3BO3 between layers. These results indicated that H3BO3 interfered in the c-axis stacking of in Mg(OH)2. Molecular H3BO3
acted as a trigger when reacting with the MgO surface, releasing Mg2+ to produce an unstable complex leading to the precipitation formation of
Mg(OH)2, which is a sink for the immobilization of H3BO3/B(OH)4¹. [doi:10.2320/matertrans.M-M2013814]
(Received April 13, 2013; Accepted June 18, 2013; Published August 2, 2013)
Keywords: trigonal boron, magnesium oxide, sorption mechanism, ligand-promoted dissolution, 11B-NMR
1.
Introduction
Boron is a ubiquitous element in rocks, soil and water.
Most of the Earth’s soils have less than 10 mg/kg boron, with
high concentrations found in parts of the western United
States and in other sites stretching from the Mediterranean to
Kazakhstan. Soils typically have boron concentrations of 2 to
100 mg/kg, but the average concentration is 10 to 20 mg/kg,
with large areas of the world being boron deficient. Boron
concentrations in rocks range from 5 mg/kg in basalts to
100 mg/kg in shales, and averages 10 mg/kg in the Earth’s
crust overall. Seawater contains an average of 4.6 mg/L boron
with a range of 0.5 to 9.6 mg/L. Freshwater concentrations
normally range from <0.01 to 1.5 mg/L, with higher
concentrations found near regions of high soil boron levels.1)
Boric acid and boron salts have extensive industrial use in
the manufacture of glass and porcelain, in wire drawing, the
production of leather, carpets, cosmetics and photographic
chemicals, for fireproofing fabrics and weatherproofing wood.
Boron compounds are used in certain fertilizers for the
treatment of boron deficient soils. Boric acid, which has mild
bactericidal and fungicidal properties, is used as a disinfectant
and a food preservative. Borax is widely used in the welding
and brazing of metals, and more recently, boron compounds
have found applications in hand cleansing, high-energy fuels,
cutting fluids and catalysts.2) Dissolution of boric acid from
coal fly ash has also been an environmental issue for many
years,3­5) and the chemical states of boron in coal fly ash have
been investigated by 11B-NMR and TOF-SIMS because it
may control leaching into the environment.6­9) Additionally,
boric acid (10B) has the important ability to absorb neutrons,
known as “boron neutron capture”10) and is thus applied as an
absorbent in nuclear reactors.
Findings from human and animal experiments have shown
that boron is a dynamic trace element that can affect the
+
Corresponding author, E-mail: [email protected]
metabolism or utilization of numerous substances involved in
life processes including calcium, copper, magnesium, nitrogen, glucose, triglyceride, reactive oxygen and estrogen.11)
Through these effects, boron can affect the function or
composition of several body systems including the immune
system, blood, brain and skeleton.12) Maximum concentration
limits are regulated at 10 and 1 mg/L for industrial drainage
and drinking water, respectively, by the World Health
Organization.13)
At ambient pH, the dominant species of boron is
undissociated boric acid (H3BO3). Boron is, therefore,
difficult to immobilize at circumneutral pH values using
electrostatic interactions. Boric acid dissociates to tetrahydroxyborate, with a pKa of 9.23.14)
H3 BO3 þ H2 O BðOHÞ4 þ Hþ
pKa 9:23ð1Þ
When B concentration is less than 25 mM, only the
monomers H3BO3 and B(OH)4¹ exist. Triboric acid is
formed when B concentration is greater than 25 mM, with
pKa of 6.84:14)
3BðOHÞ3 B3 O3 ðOHÞ4 þ Hþ þ 2H2 O
pKa 6:84 ð2Þ
Triborate contains two trigonal B atoms ([3]B) and one
tetrahedral B atom ([4]B) per molecule, and can be formed by
the reaction of boric acid with tetrahydroborate:
2BðOHÞ3 þ BðOHÞ4 B3 O3 ðOHÞ4 þ 3H2 O
K 110 ð3Þ
The current primary method of boron removal is by
sorption on boron-selective resins, which have n-methyl
glucamine groups containing many hydroxyl groups that
form complexes with borate.15) These resins are relatively
expensive (³100 USD/L) and not suitable for large scale
use, for example, as reactive materials for groundwater
treatment. For selection of reactive materials for a practical
scale treatment system, the target compound reactivity,
permeability, accessibility and cost performance must be
considered.16,17)
1810
K. Sasaki et al.
70
Magnesium oxide (MgO) has been investigated as one of
the most efficient borate sorbents,18) and related compounds,
such as layered double hydroxides (LDHs) based on Mg and
their calcined products, have also been shown to be reusable
sorbents for anionic species like borate and fluoride.19)
Magnesium oxide is a basic material for reactive sorbents
before modification, and the removal of borate and fluoride
onto MgO has been previously reported.18,20­26) However,
most investigations have focused on sorption isotherms to
predict performance, and the mechanism of borate removal
by MgO has not yet been fully discussed. Garcia-Soto
and Camacho27) demonstrated three steps of reaction; (1)
hydration and formation of Mg(OH)2 gel, (2) alkalization by
acid-base reaction between MgO and water and subsequent
diffusion of B(OH)4¹ into micropores, and (3) stereo-specific
chemical reaction between B(OH)4¹ and OH¹ by a ligand
exchange model. Based on this mechanism, immobilized B
species have been regarded as B(OH)4¹.22) In the present
work, a mechanism for borate removal by MgO will be
discussed based on observation by several techniques.
Understanding the mechanism will facilitate optimization of
the properties of MgO as an industrial sorbent for borate,
because it is usually produced by calcination of natural magnesite (MgCO3) and/or hydromagnesite (Mg2(CO3)(OH)2).
B concentration/ mmol L
-1
MgO
0.10 g
0.25 g
0.50 g
1.00 g
2.00 g
60
50
40
30
20
10
(a)
0
2+
Mg concentration/ mmol L
-1
16
(b)
14
12
10
8
6
4
2
0
12
11
Experimental
0.10­2.00 g of magnesium oxide was added to 0.04 L of the
chosen concentrations (5.6­67.0 mM as B) of H3BO3 solution
in 50 mL polyethylene bottles. The initial pH was adjusted
to 9.0 « 0.2 using 1.0 mol/L NaOH, and in some cases to
6.0 « 0.2 and 11.0 « 0.2. Duplicate suspensions were shaken
on a rotary shaker at 100 rpm with a stroke of 7.5 cm and
maintained at 25°C, until equilibrium was achieved. In some
experiments, 12 mM EDTA was added as a masking reagent
for Mg2+ ions. All chemical reagents were special grade
(Wako Pure Chemical Industries, Ltd., Osaka, Japan).
At intervals, samples were removed, filtered with cellulose
acetate membrane (0.2 µm pore size), and diluted for
determination of the concentrations of dissolved B and
Mg species by ICP-AES (VISTA-MPX, Seiko Instruments,
Chiba, Japan). Samples of solid residues were lyophilized
in a freeze-dryer for more than 24 h and stored in a vacuum
desiccator prior to XRD, 11B-NMR, SEM and FTIR.
X-ray diffraction patterns of the solid residues were
collected on a RIGAKU Ultima IV XRD (Akishima, Japan)
using CuK¡ radiation (40 kV, 40 mA) at a scanning speed of
2° min¹1 and scanning step of 0.02°.
Solid-state 11B-NMR spectra for solid residues after
sorption of borate were acquired on a JEOL ECA 800
(Akishima, Japan) with Delta NMR software version 4.3
using 3.2 mm MQMAS probes and a single pulse method.
The resonance frequency for 11B was 256.6 MHz at a field
strength of 18.8 T. Typical acquisition parameters were
spinning speed 15 kHz, pulse length 2.5 µs, relaxation delay
10 s (11B) and 63­341 scans depending on B concentration.28)
The 11B chemical shift was referenced to saturated H3BO3
solution at 19.5 ppm.9) Chemical reagents H3BO3 and
Na2B4O7 (special grade, Wako Pure Chemical Industries,
Ltd., Osaka, Japan) were used as standards.
pH
2.
10
9
(c)
8
0
100
200
300
400
500
Time/ hour
Fig. 1 Changes in (a) B and (b) Mg concentrations and (c) pH for the
sorption of 67.0 mM borate on 0.10­2.00 g MgO at an initial pH of 9.0
and 25°C using various amounts of MgO. Sorption data are expressed as
averages in duplicate, and the scattering is within symbol sizes.
SEM images of the solid residues after sorption of
H3BO3/B(OH)4¹ were observed with a scanning electron
microscope (SEM; VE-9800, Keyence, Osaka, Japan) using
15 kV accelerating voltage.
Fourier-transformed infrared spectra (FTIR) for the same
solid residues were collected using an FTIR 670 Plus
(JASCO, Hachioji, Japan) in transmission mode after dilution
to 2 mass% in KBr. Chemical reagents Mg(OH)2, Na2B4O7
and H3BO3 (special grade, Wako Pure Chemical Industries,
Ltd., Osaka, Japan) were used as standards.
3.
Results and Discussion
The sorption rate of H3BO3/B(OH)4¹ on MgO was clearly
dependent on the ratio of MgO mass to solution volume,
as shown in Fig. 1(a), with a greater ratio providing faster
sorption. Mg2+ ions were released by the sorption of borate
on the MgO, as shown in Fig. 1(b). It was to be expected
that the maximum concentrations of released Mg2+ ions were
negatively correlated with the MgO mass to solution volume
Characteristic Sorption of H3BO3/B(OH)4¹ on Magnesium Oxide
70
12
(a)
30
20
-1
initial [B]
5.6 mM
11.5 mM
21.6 mM
67.0 mM
40
Q / mmol g
B concentration/ mM
initial pH 9
10
50
10
0
16
8
9.6
(0.21)
10.0
(0.17)
6
(0.27)
10.2
(0.11)
4
10.2 10.4
(0.051)(0.081)
(b)
14
2+
10.0
(0.42)
0.10 g MgO
60
Mg concentration /mM
1811
2
12
11.2
(0.0071)
10
0
8
0
5
10
6
4
initial pH 6
10.3
(0.14)
11.3
(0.0076)
15
11.3
(0.016)
initial pH 11
20 25 30
Ce / mM
35
40
45
Fig. 3 Effect of initial pH on sorption isotherm of H3BO3/B(OH)4¹ on
MgO at 25°C. MgO mass was fixed at 0.10 g. Numbers indicate
equilibrated pH and numbers in brackets indicate molar ratios of B/Mg in
the solid residues.
2
0
12
(c)
pH
11
10
9
8
0
20
40
60
80
Time/ hour
100
120
Fig. 2 Changes in (a) B and (b) Mg concentration and (c) pH in sorption of
5.6­67.0 mM borate on MgO at the initial pH 9.0 and 25°C using 0.10 g
of MgO. Sorption data are expressed as averages in duplicate, and the
scattering is within symbol sizes.
ratio in the presence of the same initial [B] (Figs. 1(a), 1(b)).
The pH increased immediately after contact of MgO with the
borate solution, and stronger alkalization occurred when a
larger mass of MgO was used (Fig. 1(c)). The molar ratios of
B/Mg in the solid residues were calculated as 0.42, 0.22,
0.12, 0.062 and 0.053 for MgO masses of 0.10, 0.25, 0.50,
1.00 and 2.00 g, respectively.
Figure 2 shows the effect of initial B concentration on
B and Mg2+ concentrations and pH in the presence of
0.10 g MgO in a 0.040 L solution. At 67.0 mM initial B
concentration, equilibrium was not achieved within 120 h, as
shown in Figs. 1(a) and 2(a). The maximum concentrations
of released Mg2+ ions clearly correlated with initial B
concentration (Fig. 2(b)). A remarkable amount of Mg2+ ions
were released within the first 10 h, while B concentration
did not change much when the initial B concentration
was 67.0 mM. These results indicate that Mg2+ ions were
extracted by B species, possibly through the formation of
surface complex species (¸MgO­Mg­B(OH)3+ in eq. (6)).
In this case, pH increased immediately to 10.0­10.4, and less
significant increases in pH were observed at higher initial B
concentration (Fig. 2(c)). Molar ratios of B/Mg in the solid
residues were calculated to be 0.42, 0.13, 0.08 and 0.04 for
initial B concentrations of 67.0, 21.6, 11.5 and 5.6 mM,
respectively.
Effect of initial pH on the sorption of H3BO3/B(OH)4¹
on MgO is shown in Fig. 3. The isotherms were obtained
using 0.10 g of MgO in 0.04 L of 1.0­67.0 mM B solution at
initial pH of 6.0­11.0. At an equilibrium concentration (Ce)
of less than 25 mM, the highest sorption density (Q) was
obtained at an initial pH of 6.0. This suggests that molecular
H3BO3 (pKa 9.23) was a trigger for reaction with the
MgO surface. However, for Ce above 25 mM, it was stereochemically difficult for the H3BO3 trimer to approach the
MgO surface to form surface complexes through triboric
acid, and the equilibrium pH was lower at an initial pH of 6.0
than at 9.0, leading to an insufficient increase in saturation
index for Mg(OH)2 and lower Q values. At an initial pH
of 11.0, the predominant species were B(OH)4¹ and/or
B3O3(OH)4¹, which are less reactive with MgO than H3BO3,
resulting in far smaller Q values compared with those
observed at initial pH of 6 and 9.
XRD patterns for solid residues recovered after the
experiments of Figs. 1 and 2 are shown in Figs. 4 and 5,
respectively. With the same initial B concentrations, it was to
be expected that the MgO phase was more retained with
increasing mass of MgO. The XRD results clearly indicated
that Mg(OH)2 was a secondarily formed major phase (JCPDS
45-946), accompanied with unreacted MgO (JCPDS 441482). Generally, MgO is slightly soluble in water (solubility
0.0086 g/L at 30°C29)) and immediately hydrates to become
Mg(OH)2 with a low product of solubility (Ksp 10¹10.9 at
25°C), as previously described.30)
MgO þ H2 O Mg2þ þ 2 OH
Mg
2þ
þ 2 OH MgðOHÞ2
ð4Þ
ð5Þ
Although most diffraction peaks for Mg7B4O13·7H2O are
overlaid by those for Mg(OH)2, a peculiar peak at 2ª = 12.7°
1812
K. Sasaki et al.
MgO 0.10 g
initial [B] = 67.0 mM
(a) 67.0 mM
(a) 0.10 g
(b) 21.6 mM
20
(a) 0.10 g
200
(III)
(IV)
222
80
(a) 0.10 g
(a) 0.10 g
(b) 0.25 g
(b) 0.25 g
(b) 0.25 g
Intensity
Intensity
(b) 0.25 g
(c) 0.50 g
(c) 0.50 g
Intensity
(a) 0.10 g
Intensity
311
110
102
111
101
100
30
40
50
60
70
Diffraction angle, 2θ / degree [Cu Kα]
Fig. 5 XRD patterns for the solid residues recovered after sorption of 5.6­
67.0 mM borate on 0.10 g MgO at an initial pH of 9.0 and 25°C, as shown
in Fig. 2.
101
(II)
100
(I)
111
Fig. 4 XRD patterns for the solid residues recovered after sorption of
67.0 mM borate on 0.10­2.00 g MgO at initial pH of 9.0 and 25°C using
various amounts of MgO, as shown in Fig. 1.
210
120?
011
001
10
80
001
222
202
311
70
(d) 5.6 mM
110
60
201
103
200
50
220
111
220
111
102
40
Diffraction angle, 2θ / degree [Cu Kα]
102
30
(e) 2.00 g
110
101
111
100
001
100 kcps
20
(c) 11.5 mM
MgO
Mg(OH)2
Mg5(CO3)4(OH)2•4H2O
Mg7B4O13•7H2O
200
MgO
Mg(OH)2
Mg5(CO3)4(OH)2•4H2O
Mg7B4O13•7H2O
Intensity
Intensity
102
210
120
(c) 0.50 g
(d) 1.00 g
10
100 kcps
011
(b) 0.25 g
(c) 0.50 g
(d) 1.00 g
(c) 0.50 g
(d) 1.00 g
(d) 1.00 g
(d) 1.00 g
(e) 2.00 g
(e) 2.00 g
(e) 2.00 g
(e) 2.00 g
31
32
33
34
35
Diffraction angle, 2θ / degree [Cu Kα]
34
36
38
40
Diffraction angle, 2θ / degree [Cu Kα]
48 49 50 51 52 53
Diffraction angle, 2θ / degree [Cu Kα]
57
58
59
60
61
Diffraction angle, 2θ / degree [Cu Kα]
Fig. 6 Narrow ranges of XRD patterns (I) 31­35°2ª, (II) 34­40°2ª, (III) 48­53°2ª, (IV) 57­61°2ª for the solid residues recovered after
sorption of 67.0 mM borate on 0.10­2.00 g MgO at initial pH of 9.0 and 25°C using various amounts of MgO, as shown in Fig. 1.
Symbols are the same as in Fig. 4.
(d 6.86 ¡) can be assigned to only Mg7B4O13·7H2O, which
increased with increase in B/Mg molar ratio in the solid
residues. Mg7B4O13·7H2O, as can be seen with Mg7(OH)7B(OH)4(B3O6(OH)3), is basically in a matrix of Mg(OH)2.
With increase in B/Mg molar ratio in the solid residues,
the structure of Mg(OH)2 is transitionally changed into
Mg7(OH)7B(OH)4(B3O6(OH)3). The d value is also close to
three times of d101 for Mg(OH)2, possibly being attributed
to long-period ordered (LPO) structures of Mg(OH)2. More
importantly, significant broadening of these and other planes
was observed with increasing B/Mg molar ratio, while the
(hk0) planes were not broadened but exhibited a small shift
to larger diffraction angles (Fig. 6). This indicated that
integration of H3BO3 interfered mainly in c-axis stacking of
Mg(OH)2. It is likely that a planar molecule of H3BO3
coordinates to Mg atoms in parallel to layer planes to disturb
Characteristic Sorption of H3BO3/B(OH)4¹ on Magnesium Oxide
1813
Fig. 7 Schematic illustration of model to immobilize H3BO3 in Mg(OH)2
phase.
regular stacking along the c-axis (Fig. 7).31) These phenomena were not observed in sorption of fluoride on MgO.26,27)
Other phases such as Mg7B4O13·7H2O (JCPDS 19-754) and
Mg5(CO3)4(OH)2·4H2O (JCPDS 25-513) can also be seen
with increasing molar ratio of B/Mg in Fig. 4, although the
reliability of identification is relatively low with these index
cards. In Fig. 5, a greater amount of residual MgO remained
and secondary phases other than Mg(OH)2 decreased with
decreasing initial B concentration.
SEM images taken at low magnification showed that
sorbent size dramatically increased after sorption of borate
(Figs. A1(a)­A1(e)), and SEM at high magnification showed
that their surface morphologies also changed after sorption,
from granular aggregated rods to honeycomb structures
(Figs. A1(f )­A1(i)). Hydration led to swelling and increase
in porosity. At lower initial B concentrations the surface of
the solid residues looked like rose petal debris (Figs. A1(g),
A1(h)), and gradually became less defined with increasing
initial B concentration (Fig. A1(j)). This suggested that
the observed honeycomb morphology is characteristic to
Mg(OH)2 precipitates at ambient temperatures26,27) and that
the morphologies were influenced by the lattice strain of
Mg(OH)2 bearing larger B contents.
Figure 8 shows 11B-NMR spectra for solid residues
recovered after sorption of 5.6­67.0 mM B on 0.10 g MgO.
The molar ratio of [4]B/[3]B was estimated based on the 11BNMR spectra of Na2B4O7 ([3]B © 2 and [4]B © 2) and H3BO3
([3]B © 1) standards. It was clear that trigonal B ([3]B) was
predominant over tetrahedral B ([4]B) in all residues, and that
the molar ratio of [4]B/[3]B gradually increased with initial
B concentration. This suggests that the molecular form of
H3BO3 is preferentially inserted between Mg(OH)2 layers,
and that co-precipitation of B(OH)4¹ with Mg(OH)2 follows
after H3BO3 integration when excess amounts of B are
present. This is a novel insight explaining the reaction
mechanism of H3BO3/B(OH)4¹ with MgO.
The above results were also supported by FTIR spectroscopy (Fig. A2). FTIR bands observed at around 1460­
1500 cm¹1 were assigned to the asymmetric B­O stretching
mode ¯3([3]B­O) in B(OH)3, the band at around 1080 cm¹1
was assigned to the asymmetric B­O stretching mode
¯3([4]B­O) in B(OH)4¹, and the band at 978 cm¹1 was
assigned to the stretching mode ¯1([4]B­O) in B(OH)4¹.31)
The bands at around 1260­1280 cm¹1 were assigned to
Abundance
[4]
[3]
(a) 5.6 mM
B/ B
0.0142
(b) 11.5 mM
0.0092
(c) 21.6 mM
0.0184
(d) 67.0 mM
0.0512
(e) Na2B4O7
(f) H3BO3
(powder)
30
20
10
0
Chemical shift/ ppm
-10
Fig. 8 11B-NMR spectra for solid residues recovered after sorption of 5.6­
67.0 mM borate on 0.10 g MgO at an initial pH of 9.0 and 25°C, and
related standards. Numbers in the figure indicate the molar ratio of
[4]
B/[3]B in the solid residues, calculated from relative intensities in the
11
B-NMR spectra.
the plane bending mode ¤(B­O­H) in both B(OH)3 and
B(OH)4¹.32) Their intensities increased with increasing
initial B concentration (Figs. A2(d)­A2(g)) and decreasing
MgO concentration (Figs. A2(h)­A2(k)). Signal intensities
observed for tetragonal [4]B at around 1080 cm¹1 were weak
and broad for solid residues recovered after sorption of
67.0 mM B on 0.1 g MgO, as shown in Fig. A2(g). There
were no IR bands assignable to Mg(OH)2 in the measured
region.
Based on the 11B-NMR results that trigonal [3]B was
the predominant species immobilized in the solid phase,
additional experiments were conducted. Figure 9(a) shows
Mg2+ ions released from MgO reagent when 0.50 g MgO was
suspended in 200 mL of 10 mM boron solution at initial pH
of 6.0 and 9.0. Under these conditions, excess amounts
of MgO were added in comparison with B concentration.
Changes in total B concentration were not observed within
3 h after contact of MgO with the boron solutions. It was
obvious that much greater amounts of Mg2+ ions were
released at an initial pH of 6.0. pH increased with elapsed
time and converged to mostly the same value after 100 min,
independent of the initial pH (Fig. 9(b)), as a result of the
reactions described in eqs. (4) and (5). The proportion of
boron species was calculated using the measured pH values
and pKa (Fig. 9(c)). The molecular form of boric acid, which
was dominant at pH 6.0, was mainly responsible for the
observed release of Mg2+.
1814
K. Sasaki et al.
5
-1
4
3 mM H3BO3 + 12 mM EDTA
2 mM H3BO3 + 12 mM EDTA
12 mM EDTA
3 mM H3BO3
2 mM H3BO3
1 mM H3BO3
(a)
6
B concentration/ mM
Mg concentration/ mmol L
7
(a) Open and closed symbols are
with and without B.
3
2
1
5
4
3
2
0
1
12
(b)
10
0
7
9
6
(b)
Mg concentration/ mM
pH
11
8
7
-
5
10
2+
[H3BO3], [B(OH)4 ]/ mmol L
-1
6
(c)
8
4
3
2
1
initial pH 6
H3BO3,
initial pH 9
H3BO3,
6
4
-
0
B(OH)4
0
50
100
0
20
B(OH)4
2
0
5
150
200
40
Time/ hour
60
80
Fig. 10 Effect of EDTA on (a) B and (b) Mg concentrations with time
during sorption of 1­3 mM boric acid on MgO with presence and absence
of 12 mM EDTA at an initial pH of 9 and 25°C. 0.01 g MgO was added to
40 mL solution including 1­3 mM H3BO3 with or without 12 mM EDTA.
Time/ minute
2+
Fig. 9 Released Mg concentration (a) from MgO and measured pH (b)
with time in 10 mM boron solution at initial pH of 6.0 ( & ) and 9.0
( & ). Proportion of molecular boric acid and borate (c) was calculated
using measured pH and pKa (9.23) for boric acid at 25°C. Open and solid
symbols in (a) and (b) indicate results with and without B, while circles
and squares denote initial pH of 6.0 and 9.0, respectively.
MgO­MgOH þ H3 BO3
MgO­Mg­BðOHÞ3 þ þ OH
MgO­Mg­BðOHÞ3 þ H2 O
MgOH þ ½MgBðOHÞ4 þ
þ
½MgBðOHÞ4 þ OH MgBðOHÞ5
MgBðOHÞ5 MgðOHÞ2 H3 BO3
Based on all of the above results, the mechanism for
the immobilization of H3BO3/B(OH)4¹ by MgO can be
proposed as follows. Hydroxyl groups (­OH) coordinate
with the surface of MgO in an aqueous system, as shown
in eq. (6). Molecular H3BO3 preferentially reacts with the
surface to form the surface complex ¸MgO­Mg­B(OH)3+,
accompanied by an increase in pH, known as ligandpromoted dissolution (eq. (6)). The surface complex
¸MgO­Mg­B(OH)3+ is dissociated into ¸MgOH and
[MgB(OH)4]+ according to eq. (7). Equations (6) and (7)
repeat until the surface is completely covered with secondary
precipitates. The complex [MgB(OH)4]+, which has a
relatively small stability constant (pK ¹1.34 ³ ¹1.63 at
25°C,14)), precipitates as MgB(OH)5 and/or Mg(OH)2 (Ksp
1.1 © 10¹11 at 25°C,33)), bearing B(OH)3 as shown in eqs. (8)
and (9).
ð6Þ
þ
ð7Þ
ð8Þ
ð9Þ
As mentioned above, the principal mechanism of H3BO3/
B(OH)4¹ immobilization is based on formation of Mg(OH)2.
This was further confirmed by the following results.
Figure 10 demonstrates changes in B and Mg concentration
in the reaction of 0.01 g MgO with 0­3 mM B at an initial pH
of 9.0 in 1.00 L, in the presence of 0­12 mM EDTA. Under
the condition the amount of MgO added is very limited and
added EDTA is enough high to complex with Mg2+, even if
added MgO is entirely dissolved. As expected in Fig. 10(b),
the equilibrium pH has fallen within 9.0 to 10.0 in all cases.
Even without EDTA the precipitation of Mg(OH)2 is
unsaturated. With EDTA only, Mg2+ did not dissolve at all.
Amarel et al.34) have reported that chelants function as
inhibitors of the hydration of MgO. To release Mg2+ ions
from MgO, H3BO3 was always necessary. In the presence of
12 mM EDTA, 0.01 g MgO was entirely dissolved within
Characteristic Sorption of H3BO3/B(OH)4¹ on Magnesium Oxide
80 h. Its hydration, as shown in eq. (8), was blocked by
EDTA as follows:
½MgBðOHÞ4 þ þ EDTA BðOHÞ4 þ ½EDTA Mg2þ ð10Þ
The Mg2+ ions were stabilized in solution as a chelate
complex with EDTA, which inhibited the precipitation of
Mg(OH)2 and led to no immobilization of B species. Thus,
it was confirmed that formation of Mg(OH)2 is principle for
immobilization of H3BO3/B(OH)4¹.
4.
Conclusions
The mechanism of H3BO3/B(OH)4¹ immobilization on
MgO was investigated by sorption data in combination
with characterization by XRD, 11B-NMR and FTIR of solid
residues recovered after sorption. 11B-NMR results indicated
that the predominant B species in solid residues after
sorption at an initial pH of 9.0 was trigonal [3]B. Dissolution
rates of Mg2+ from MgO at various pH showed that the
molecular form of H3BO3 is responsible for release of Mg2+
from MgO. Thus, the ligand-promoted dissolution of MgO
by H3BO3 was proposed as the initiation step in the reaction
of H3BO3/B(OH)4¹ with MgO. Through the surface complex ¸MgO­Mg­B(OH)3+, complex ions of [MgB(OH)4]+
are released in the aqueous phase and are immediately
hydrated, resulting in Mg(OH)2-bearing H3BO3. Formation
of Mg(OH)2 is the principle mechanism of H3BO3
immobilization, which occurs by coordination to Mg atoms
and interferes in the c-axis stacking of Mg(OH)2, with
production of some B(OH)4¹ by co-precipitation, based on
11
B-NMR and XRD results. XRD patterns for the solid
residues demonstrated more significant broadening, except
for hk0 plane reflections, with increasing B content. Because
these hk0 planes were not dependent on the c-axis, it was
proposed that the molecular H3BO3 between Mg(OH)2 layers
led to significant structural strain along the c-axis with
increasing B content of the solid residues. Effect of initial pH
on the sorption density of B species (Q) depended on the
equilibrium concentration of B (Ce). An initial pH of 6.0
provided a larger Q, less than 25 mM Ce, while an initial pH
of 9.0 provided a larger Q, more than 25 mM Ce. This was
considered to be due to the fact that at an initial pH of 6.0,
the equilibrated pH did not sufficiently rise, resulting in
under saturation for Mg(OH)2 to sorb B.
These findings are applicable to the sorption mechanism
of H3BO3/B(OH)4¹ in substances related to MgO such as
layered doubled hydroxides (LDHs) including Mg and their
calcined products, and is expected to contribute to the
development of novel sorbents for inorganic B species.
Acknowledgements
Financial support was provided to KS by Funding Program
for Next Generation of World-Leading Researchers (“NEXT
program” GR078) in Japan Society for the Promotion of
Science (JSPS). This study was partially supported by the
New Energy and Industrial Technology Development
Organization (NEDO) under the Innovative Zero-emission
Coal-fired Power Generation Project. The authors thank Dr.
Kwadwo Osseo-Asare at Pennsylvania State University for
1815
discussion on dissolution of metallic oxides using JSPS
Invitation Fellowship Program for Research in Japan FY2011
(Short Term ID S-11179).
REFERENCES
1) W. G. Woods: Environ. Health Perspect. 102 (suppl 7) (1994) 5­11.
2) S. Şahin: Desalination 143 (2002) 35­43.
3) J. A. Cox, G. L. Lundquist, A. Przyjazny and C. D. Schmulbach:
Environ. Sci. Technol. 12 (1978) 722­723.
4) W. D. James, C. C. Graham, M. D. Glascock and A. G. Hanna:
Environ. Sci. Technol. 16 (1982) 195­197.
5) A. Iwashita, Y. Sakaguchi, T. Nakajima, H. Takanashi, A. Ohki and S.
Kambara: Fuel 84 (2005) 479­485.
6) P. Burchill, O. W. Howarth, D. G. Richards and B. J. Sword: Fuel 69
(1990) 421­428.
7) S. Hayashi, T. Takahashi, K. Kanehashi, N. Kubota, K. Mizuno, S.
Kashiwakura, T. Sakamoto and T. Nagasaka: Chemosphere 80 (2010)
881­887.
8) S. Kashiwakura, T. Takahashi, H. Maekawa and T. Nagasaka: Fuel 89
(2010) 1006­1011.
9) T. Takahashi, S. Kashiwakura, K. Kanehashi, S. Hayashi and T.
Nagasaka: Environ. Sci. Technol. 45 (2011) 890­895.
10) Y. Oka, M. Akiyama and S. An: Prog. Nucl. Energy 32 (1998) 61­
70.
11) R. O. Nable, G. S. Bañuelos and J. G. Paull: Plant Soil 193 (1997) 181­
198.
12) H. F. Nielsen, D. C. Hunt, M. L. Mullen and R. J. Hunt: FASEB J 1
(1987) 394­397.
13) WHO: 1998. Environmental Health Criteria Monograph 204. IPCS,
Geneva.
14) R. L. Bassett: Geochim. Cosmochim. Acta 44 (1980) 1151­1160.
15) R. Liu, W. Ma, C. Jia, L. Wang and H. Li: Desalination 207 (2007)
257­267.
16) R. Thiruvenkatachari, S. Vigneswaran and R. Naidu: J. Ind. Eng.
Chem. 14 (2008) 145­156.
17) N. Öztürk and D. Kavak: Desalination 223 (2008) 106­112.
18) K. Sasaki, N. Fukumoto, S. Moriyama and T. Hirajima: J. Hazard.
Mater. 191 (2011) 240­248.
19) K. Goh, T. Lim and Z. Dong: Water Res. 42 (2008) 1343­1368.
20) H. Bilinski, B. Matkovic, D. Kralj, D. Radulović and V. Vranković:
Water Res. 19 (1985) 163­168.
21) H. Polat, A. Vengosh, I. Pankratov and M. Polat: Desalination 164
(2004) 173­188.
22) M. M. de La Fuente García-Soto and E. Muñoz Camacho: Sep. Purif.
Technol. 48 (2006) 36­44.
23) B. Nagappa and G. T. Chandrappa: Microporous Mesoporous Mater.
106 (2007) 212­218.
24) A. Bhatnagar, E. Kumar and M. Sillanpää: Chem. Eng. J. 171 (2011)
811­840.
25) K. Sasaki, H. Takamori, S. Moriyama, H. Yoshizaka and T. Hirajima:
J. Hazard. Mater. 185 (2011) 1440­1447.
26) K. Sasaki, N. Fukumoto, S. Moriyama, Q. Yu and T. Hirajima: Sep.
Purif. Technol. 98 (2012) 24­30.
27) M. M. de La Fuente García-Soto and E. Muñoz Camacho: Desalination
249 (2009) 626­634.
28) L. Züchner, J. C. C. Chan, W. Müller-Warmuth and H. Eckert: J. Phys.
Chem. B 102 (1998) 4495­4506.
29) Material Safety Data Sheet, ACC# 13450, http://fscimage.fishersci.
com/msds/13450.htm.
30) A. Fedoročková and P. Raschman: Chem. Eng. J. 143 (2008) 265­
272.
31) N. Jiang, D. Su and J. C. H. Spence: Ultramicroscopy 109 (2008) 122­
128.
32) D. Peak, G. W. Luther, III and D. L. Sparks: Geochim. Cosmochim.
Acta 67 (2003) 2551­2560.
33) W. Stumm and J. J. Morgan: Aquatic Chemistry the 3rd ed., (Wiley
Intersciences, 2008, New York).
34) L. F. Amaral, I. R. Oliveira, P. Bonadia, R. Salomão and V. C.
Pandolfelli: Ceram. Int. 37 (2011) 1537­1542.
1816
K. Sasaki et al.
Appendix
(a)
(f)
(b)
(g)
(c)
(h)
(d)
(i)
(e)
(j)
Fig. A1 SEM images of MgO ((a) and (f )) and solid residues (others) recovered after sorption of 5.6­67.0 mM borate on 0.10 g MgO at
an initial pH of 9.0 and 25°C at low magnification ((a)­(e)) and high magnification ((f )­(j)). Initial [B] were 5.6 mM for (b) and (g),
11.2 mM for (c) and (h), 21.6 mM for (d) and (i) and 67.0 mM for (e) and (j). Horizontal bars indicate 5.00 µm in (a)­(e) and 1.00 µm in
(f )­(j).
Characteristic Sorption of H3BO3/B(OH)4¹ on Magnesium Oxide
(k)
0.1
(j)
0.1
(i)
0.1
(h)
[4]
ν1 ( B-O)
[4]
ν3 ( B-O)
0.1
δ (B-O-H)
Absorbance/ -
[3]
ν3 ( B-O)
0.1
(g)
(f)
0.1
(e)
0.1
(d)
0.1
(c) Mg(OH)2
0.5
(b) Na 2B4O7
0.05
(a) H3BO3
0.5
1600
1400
1200
1000
-1
Wavenumber/ cm
800
Fig. A2 FTIR spectra for solid residues recovered after sorption of B on
MgO, as shown in Figs. 1 and 2 and related standards. (a) H3BO3, (b)
Na2B4O7, (c) Mg(OH)2, (d) 5.6 mM B with 0.10 g MgO, (e) 11.5 mM B
with 0.10 g MgO, (f ) 21.6 mM B with 0.10 g MgO, (g) 67.0 mM B with
0.10 g MgO, (h) 67.0 mM B with 0.25 g MgO, (i) 67.0 mM B with 0.50 g
MgO, (j) 67.0 mM B with 1.00 g MgO and (k) 67.0 mM B with
2.00 g MgO.
1817