Mechanochemistry-Based Synthesis of Highly Crystalline γ-Zirconium Phosphate for Selective Ion Exchange

Article abstract of DOI:10.1021/acs.inorgchem.7b03202

Highly crystalline γ-zirconium phosphate has been synthesized by a novel minimalistic approach and investigated as a selective ion exchanger for cesium, ammonium and potassium. In contrast to current solution-based preparations, the mechanochemistry-based synthesis provides easy access to γ-zirconium phosphate with short synthesis times and low crystallization temperature. The addition of NaF as a mineralizer increases the crystallinity of γ-zirconium phosphate, which forms micrometer-sized uniformly shaped rectangular platelets. The crystalline material has extremely high selectivity to cesium even in the presence of 1000- or 500-fold excess Na⁺ or Ca²⁺, respectively. The removal efficiency was >98% in the pH range of 2-5.5. As an ion exchanger for purification of dialysate, crystalline γ-zirconium phosphate shows higher uptake of ammonium and potassium ions than the amorphous gel compound currently used in sorbent cartridges. This sustainable protocol opens up opportunities for many practical applications of γ-zirconium phosphate.

Full text of DOI:10.1021/acs.inorgchem.7b03202

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Mechanochemistry-Based Synthesis of Highly Crystalline
γ‑Zirconium Phosphate for Selective Ion Exchange
Yu Cheng,‡ Xiao Dong (Tony) Wang,§ Stephan Jaenicke,‡ and Gaik-Khuan Chuah*,‡
‡Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543
§Bruker Singapore, 11 Biopolis Way, #10-10, Helios, Singapore 138667
S
* Supporting Information
ABSTRACT: Highly crystalline γ-zirconium phosphate has
been synthesized by a novel minimalistic approach and
investigated as a selective ion exchanger for cesium, ammonium
and potassium. In contrast to current solution-based prepara-
tions, the mechanochemistry-based synthesis provides easy
access to γ-zirconium phosphate with short synthesis times
and low crystallization temperature. The addition of NaF as a
mineralizer increases the crystallinity of γ-zirconium phosphate,
which forms micrometer-sized uniformly shaped rectangular
platelets. The crystalline material has extremely high selectivity
to cesium even in the presence of 1000- or 500-fold excess Na+
or Ca2+, respectively. The removal efficiency was >98% in the
pH range of 2−5.5. As an ion exchanger for purification of dialysate, crystalline γ-zirconium phosphate shows higher uptake of
ammonium and potassium ions than the amorphous gel compound currently used in sorbent cartridges. This sustainable
protocol opens up opportunities for many practical applications of γ-zirconium phosphate.
The synthesis of α-zirconium phosphate (α-ZrP), a
monohydrate with formula α-Zr(HPO4)2·H2O, was first
reported by Clearfield and Stynes in 1964.19 The crystalline
compound was obtained by refluxing the amorphous zirconium
phosphate gel in excess phosphoric acid. In α-ZrP, coplanar
zirconium atoms are sandwiched between two layers of
monohydrogen phosphate groups (Figure 1a).20 Within the
1. INTRODUCTION
Zirconium phosphates have been investigated for applications
as ion exchangers,1−3 catalysts,4−7 in drug delivery systems,8−10
fuel cells,11−13 nanocomposites,14−16 and for hydrogen
storage.17 Zirconium phosphates are stable to high temper-
atures, ionizing radiation and acidic conditions. These proper-
ties make them uniquely suitable for applications in the
recovery and separation of radioisotopes and treatment of
radioactive wastewater. The cation exchange capacity of
zirconium phosphate is also utilized in sorbent systems for
dialysis.18 Zirconium phosphates exist in the amorphous as well
as various crystalline forms including Zr(HPO4)2·H2O (α-
ZrP),19,20 Zr(HPO4)2·8H2O (θ-ZrP),21 Zr(PO4)(H2PO4)·
2H2O (γ-ZrP),22,23 and τ-Zr(HPO4)2.24 Unlike the crystalline
zirconium phosphates, the amorphous gel is easily prepared
from precipitation of zirconium salts and phosphoric acid or
phosphate salts. However, for reproducible performance, the
crystalline form is highly preferred. In particular, the positioning
of the phosphate groups in the crystal lattice depends on the
degree of crystallinity.1 With lower crystallinity, more
phosphate groups are irregularly positioned in the crystal
lattice resulting in cavities with a range of sizes, in contrast to
the uniform cavities of the crystalline compound.25 Character-
ization of the amorphous gel is difficult as changes resulting
from small variations in the starting materials or in the synthesis
may remain undetected. Crystalline zirconium phosphates are
also less prone to hydrolysis than the amorphous gel,
permitting us to apply the material in strongly acidic as well
as fairly alkaline solutions.
−
HPO4 , three of the oxygen atoms are coordinated to
zirconium whereas the fourth oxygen, which bears a hydrogen,
points toward the interlayer space. The spacing between the
layers is 7.6 Å and access to the microporous cavities is limited
by the maximum opening of 2.62 Å between Zr−O from two
adjacent layers.26 This is adequate for unhydrated lithium,
sodium and potassium ions but to accommodate the larger
cesium or rubidium ions, the interlayer distance must first be
enlarged by addition of a base to neutralize the lattice
protons.27
Subsequently, two additional crystalline phases, the anhy-
drous β- and dihydrate γ-zirconium phosphate, were synthe-
sized. Differing in the water content, they are easily
interconvertible under mild conditions of controlled humidity
at room temperature.22 Unlike α-ZrP, γ-ZrP does not contain
−
any HPO4 groups but rather, phosphates (PO43−) and
−
dihydrogen phosphates (H2PO4 ). Hence, it is expressed as
Zr(PO4)(H2PO4)·2H2O. The presence of these groups has
been confirmed by 31P MAS NMR studies where two
Received: December 23, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.7b03202
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Inorganic Chemistry
Article
Figure 1. Structures of (a) α-ZrP and (b) γ-ZrP.
Table 1. Reported Procedures to Form γ-ZrP and γ-NH4ZrP
solution
method
P/Zr
F/Zr
ref
1
2
3
4
5
6
1 M ZrOCl2·8H2O (0.1 mol) added to refluxing 10 M NaH2PO4·H2O (2 mol) in refluxed for 25 h
20
22
34
23
36
30
29
3 M HCl
1 M ZrOCl2·8H2O (0.715 mol) added to boiling solution of 6 M NaH2PO4 (14 refluxed for 6 weeks
mol) in 2 M H3PO4
27.4
27.5
12
1 M ZrOCl2·8H2O (0.24 mol) added to refluxing solution of 10 M NaH2PO4
(4.91 mol) in 3 M H3PO4 (1.58 mol)
refluxed for 72 h, then hydrothermal
treatment at 190 °C for 120 h
1 M ZrOCl2·8H2O (0.1 mol) added to boiling 6 M NaH2PO4·2H2O, pH adjusted sealed in Pyrex glass tubes, heated in
to 2 using conc. HCl autoclave at 190 °C for 1 week
2 M NH4H2PO4 (0.9 mol) mixed with aqueous solution of 1.3 M ZrOCl2·8H2O heated on a water bath at 80 °C for 5 days 13.8
(0.065 mol) and 8 M HF
6.15
10
7.4 M NH4F (0.3 mol) solution added to 0.5 M ZrOCl2·8H2O (0.03 mol), pH heated at 60 °C for 7 days
adjusted to 2 with 2 N HCl followed by addition of 85% H3PO4
2
7
8
9
5 mmol ZrOCl2·8H2O, 42−84 mmol NaH2PO4 in 5 cm3 H2O
5 mmol ZrOCl2·8H2O, 20−83 mmol NH4H2PO4 in 5 cm3 H2O
50 mmol ZrOCl2·8H2O, 400 mmol NH4H2PO4 in 100 cm3 H2O
sealed tube in autoclave at 250 °C for 96 h
sealed tube in autoclave at 250 °C for 96 h 16.7
autoclaved at 150−160 °C for 18−54 h
8.3−16.7
37
37
37
8
resonances at −27.4 and −9.4 ppm could be observed.28 The
stoichiometric amounts or with slight excess in their normal
state without the use of solvents.38 This “dry” green protocol
was easily scalable and gave >90% yields of highly crystalline α-
ZrP with new morphologies and controllable particle sizes. In
view of the useful properties of γ-ZrP, we were interested if the
minimalistic protocol can be applied for its synthesis. We found
that unlike α-ZrP, the synthesis of γ-ZrP cannot be directly
synthesized using H3PO4 but requires sodium or ammonium
dihydrogen phosphates. Therefore, an additional step is
required to exchange the cation against H+. This work reports
on the synthesis parameters necessary to make highly
crystalline γ-ZrP and a study into its ion-exchange properties.
zirconium atom is octahedrally coordinated to four oxygen
3−
−
atoms of the PO4 and two oxygens of the H2PO4 groups
(Figure 1b).23,29,30 The remaining two oxygen atoms of H2PO4
are bound to hydrogen atoms and project into the interlayer
space where they interact with the water molecules through
hydrogen bonding. The basal spacing in γ-ZrP is 12.25 Å, and
subtracting an average layer thickness of 9.3 Å leaves an
interlayer space of 2.95 Å.31 Because of the larger interlayer
space compared to ∼1 Å for α-ZrP,10 γ-ZrP can sorb larger ions
and also has faster ion exchange rates.32−35
Despite these advantages, γ-ZrP has been much less studied
than α-ZrP. This is largely due to the difficulties in obtaining
highly crystalline γ-ZrP. Table 1 lists the methods that have
been reported for forming crystalline γ-phase zirconium
phosphate.22,23,29,30,34,36,37 Solutions containing ZrOCl2·8H2O
and MH2PO4 (M represents NH4, Na, or K) are refluxed or
hydrothermally treated for days to weeks to achieve
crystallinity. An excess of MH2PO4 and additives, HF or
NH4F, are typically used. This leads to large amounts of wastes
that require proper treatment and disposal. In an attempt to
develop an industrial method, Kobayashi conducted the
synthesis in an autoclave to enable higher temperatures so
that the reaction time could be shortened (Table 1, entry 9).37
Despite using a Hastelloy-B autoclave, the synthesis could only
be carried out at 150 °C as corrosion of the steel occurred at
higher temperatures. Hence, the energy and material costs in
synthesizing γ-ZrP is high.
2. EXPERIMENTAL SECTION
2.1. Chemicals. Zirconium(IV) oxide chloride octahydrate (AR
grade), sodium fluoride (AR grade) and strontium nitrate (>99%)
were supplied by Merck. Orthophosphoric acid (85 wt %), sodium
dihydrogen phosphate monohydrate, ammonium dihydrogen phos-
phate, cesium chloride (99.9%), ammonium nitrate (>99.5), and
magnesium nitrate hexahydrate (99%) were purchased from Sigma-
Aldrich. Potassium chloride (extra pure) and sodium chloride (extra
pure) were obtained from GCE Laboratory Chemicals. Calcium
chloride dihydrate (>99%) was from Riedel-deHaen. These chemicals
were used as received.
2.2. Synthesis. In a typical synthesis, ZrOCl2·8H2O (3.3 mmol),
xNaH2PO4·H2O (x = 2−5) and yNaF (y = 0, 0.3) were placed in an
agate mortar and ground for ∼15 min before transferring to a 30 cm3
polypropylene container. The addition of NaF was found to form
highly crystalline α-ZrP,38 hence, its effect on the synthesis of γ-ZrP
was investigated. The mixture was transferred into a polypropylene
container and heated at 120 °C for different times. After cooling, the
solid was washed with deionized water and changed to the protonated
form by treatment with HCl solutions of different concentrations
We have recently synthesized highly crystalline α-ZrP with
variable morphologies by adopting a minimalistic approach.
The reactants, ZrOCl2·8H2O and 85% H3PO4, were used in
B
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Figure 2. (A) XRD patterns of sodium hydrogen zirconium phosphate synthesized using ZrOCl2·8H2O: 4NaH2PO4·H2O:0.3NaF at 120 °C for (a,
b) 24 and (c) 48 h; (B) corresponding γ-ZrP after treatment in 0.5 M HCl. Red lines in B indicate peak positions for γ-ZrP (ICDD 04−011−1172).
(0.5−2 M). About 1 g of the solid was placed in 50 cm3 of HCl
solution and stirred for 6 h at room temperature. The sample was
washed with copious amounts of deionized water to remove any
remaining HCl and finally dried at 50 °C. Another series of γ-ZrP was
prepared using NH4H2PO4 as the phosphate source.
adsorbed onto the zirconium phosphate was calculated from the
concentration difference in the solution before and after the ion
exchange.
The removal efficiency (R) of a given cation was calculated
according to
For comparison of ion-exchange properties, gel ZrP was synthesized
by precipitation of an aqueous solution of 0.2 M ZrOCl2·8H2O (30
cm3) with an equal volume of 0.5 M H3PO4 solution under vigorous
magnetic stirring at room temperature. After aging at room
temperature for 6 h, the resulting gelatinous zirconium phosphate
was vacuum filtered and thoroughly washed with 0.1 M H3PO4
aqueous solution until free of chloride ions. α-ZrP was prepared
using 85% H3PO4 instead of NaH2PO4.38 The ZrOCl2·8H2O and
H3PO4 mixture (1:3 ratio) was heated at 120 °C for 24 h, washed with
deionized water and dried at 50 °C.
2.3. Characterization. Powder X-ray diffraction (XRD) patterns
were measured using a Bruker D8 Advance diffractometer (40 kV, 40
mA) equipped with a copper anode (λ = 1.5406 Å) and a LynxEye XE
detector. The 2θ range was from 5° to 60° with a step size of 0.02° and
a dwell time of 0.5 s/step. To minimize preferential orientations, some
of the samples were either backloaded and measured in the Bragg−
Brentano geometry or filled into 0.3 mm ϕ capillary tubes and
measured in the Debye−Scherrer geometry. The sample holder was
spun at 30 rpm. Thermogravimetric analysis (TGA) was carried out on
a Dupont SDT 2960 analyzer with a heating ramp of 10 °C min−1 to
1000 °C under nitrogen flow at 25 cm3 min−1. The morphology was
observed by scanning electron microscopy (SEM) using a JEOL JSM-
6701F instrument. The sample was coated with platinum prior to
observation. Transmission electron microscopy (TEM) and selected
area electron diffraction (SAED) were carried out using a JEOL JEM-
3010 instrument operated at 300 kV. The sample (∼10 mg) was
dispersed in isopropanol (10 cm3) and, after sonication, two drops of
the supernatant were placed onto a carbon-coated copper grid and
dried overnight at room temperature.
Ci − C
Ci
R(%) =
e 100%
(1)
where Ci is the initial concentration and Ce is the equilibrium
concentration. The distribution coefficient (Kd) was also determined.
It is defined as the ratio of the concentration of ions adsorbed per
gram of sorbent to the concentration of ions in one cm3 of solution,
and expressed mathematically as39
Kd⎜ 3 ⎟ =
⎛
⎞
Ci − Ce
Ce
cm
g
V
m
⎝
⎠
(2)
where V is the volume of solution and m is the mass of sorbent.
3. RESULTS AND DISCUSSION
3.1. Synthesis Using NaH2PO4·H2O. Sodium dihydrogen
phosphate was used as the phosphate source in the synthesis of
γ-ZrP. The synthesis gel consisted of ZrOCl2·8H2O, NaH2PO4·
H2O and NaF. The molar ratio of ZrOCl2·8H2O:NaH2PO4·
H2O was varied from 1:2 to 1:5, keeping NaF/Zr at 0.3. The
product formed with a stoichiometric molar ratio of 1:2 for
ZrOCl2·8H2O: NaH2PO4·H2O was of low crystallinity. Hence
higher ratios were used. The X-ray diffractograms of the solids
obtained using a molar ratio of 1:4 (Figure 1) and 1:5 (Figure
S2A) showed several sharp peaks. Based on the database,
hydrated sodium zirconium hydrogen phosphates with different
water content were formed (Figure S1). These were identified
as NaHZr(PO4)2·xH2O (ICDD 00−033−1311) and NaHZr-
(PO4)2·2.5H2O (ICDD 00−032−1217). Half sodium-ex-
changed forms of γ-zirconium phosphate have been reported
previously.35,36 The peaks at 2θ ≈ 7.66° (d ≈ 11.5 Å) and 8.56°
(d ≈ 10.3 Å) correspond to the basal spacing (Figure 2A).
Samples dried at 50 °C may show only one peak at 2θ ≈ 7.66°
or both peaks. Their relative intensity varied depending on the
state of hydration. For instance, after drying at 100 °C for 5 h,
the peak at 2θ ≈ 7.66° was no longer visible, and only that at 2θ
≈ 8.56° could be seen (Figure 2A(b)). The desorption of water
from the layered structure leads to a smaller basal spacing.
Regardless of the state of hydration of the sodium zirconium
hydrogen phosphate, ion exchange with dilute HCl easily forms
2.4. Ion-Exchange Properties. The following experiments were
conducted: (a) relative affinity of γ- and α-ZrP for alkali and alkaline-
earth metal ions (b) uptake of Cs+ from 0.1 to 10 mM, in the pure
solution and in the presence of 0.1 M NaCl and 0.05 M CaCl2 (c)
uptake of 1 mM Cs+ as a function of pH and (d) uptake of K+ and
NH4+ ions from 0.1 to 20 mM. For (a), three solutions containing (i)
Na+, K+, Cs+ (ii) Mg2+, Ca2+, Sr2+ and (iii) Cs+, Sr2+ with each ion
concentration at 1 mM were prepared. For (c), the pH of the solution
was adjusted to the desired value by addition of HCl solution.
+
About 20 mg (100 mg for K+ and NH4 uptake) of the material
were dispersed into 25 cm3 of the mixed ion solution and gently stirred
at room temperature for 20 h. The concentration of the ions in the
solution was measured by ion chromatography using a Dionex ICS-
1100 equipped with a CS12A-4 mm column. The amount of ions
C
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γ-ZrP. A simple treatment with 0.5 M HCl for 6 h was sufficient
to form pure phase γ-ZrP. The XRD pattern agrees well with
that in the PDF database for Zr(PO4)(H2PO4)·2H2O (ICDD
04−011−1172). The first reflection at 2θ ≈ 7.21° corresponds
to the basal distance of 12.25 Å (Figure 1B).
As expected from formula, two moles of water were lost per
mole of γ-ZrP when the sample was heated under nitrogen flow
during thermogravimetric analysis (Figure 3). The loss from
Figure 4. Scanning electron micrographs of (a, c) sodium zirconium
phosphate and corresponding (b, d) γ-ZrP synthesized for (a, b) 24
and (c, d) 48 h at 120 °C.
S5). Extending the synthesis time from 24 to 48 h resulted in
slightly bigger platelets (Figure 4c, d). Transmission electron
microscopy also corroborated the tetragonal morphology of γ-
ZrP (Figure 5a). The selected area electron diffraction (SAED)
indexing confirmed that the crystal growth proceeded along the
a and b directions (Figure 5b).
Figure 3. TGA curves of γ-NaZrP (red lines: sample a in Figure 2A)
and corresponding γ-ZrP (black lines).
the interlayer space occurs below 160 °C and is associated with
the formation of anhydrous β-ZrP which can easily revert back
to the γ-form under 90% relative humidity at room temper-
ature.30 A further weight loss occurred at higher temperatures.
About 0.67 and 0.33 mol of water were lost per formula unit
from 340−420 °C and from 700−860 °C, respectively. This
evolution of water has been attributed to condensation of
hydroxyls to form a layered γ-zirconium pyrophosphate
followed by its transformation to the cubic form.30,40 In
comparison, a typical thermogram for sodium zirconium
hydrogen phosphate shows a two- rather than a three-step
weight loss (Figure 3). Below 160 °C, removal of water from
the interlayer space gives rise to the first weight loss of 8.2%.
From the slope of the TGA curve, the initial rate of water loss
was high but decreased above ∼100 °C as more strongly
hydrogen-bonded water molecules were lost. This agrees with
the XRD results showing a decrease in the d(001) basal distance
after drying at 100 °C. The water content in the sodium
zirconium hydrogen phosphate sample is estimated to be ∼1.5
mol, which is variable depending on the state of drying. From
∼350−450 °C, structural reorganization results in a smaller
second weight loss, 3.0%. XRD of the sample after calcination
at 500 °C for 5 h shows that NaZr2(PO4)3, ZrP2O7, and NaPO3
were formed (Figure S3).
Figure 5. (a) TEM image and (b) SAED of γ-ZrP crystal lying flat on
Cu grid.
The synthesis of sodium zirconium hydrogen phosphate was
also investigated in the absence of the fluoride mineralizer. A
longer heating time of 3 days (instead of 1 day) at 120 °C was
required. The XRD peaks were broader, indicating that the
crystallites were smaller and of lower crystallinity (Figure 6).
This agrees well with the scanning electron micrographs of γ-
ZrP nanoplatelets, which were smaller and more irregular in
shape.
Scanning electron micrographs revealed that the sodium
zirconium hydrogen phosphate formed rather regular micron-
sized platelets. The morphology was squarish to tetragonal with
thickness of ∼50−60 nm (Figure 4a). No obvious change in
the morphology or size of the platelets was observed after the
HCl treatment to form γ-ZrP (Figure 4b). The morphology
was similar for γ-ZrP formed using P/Zr ratio of 5 although the
platelets were slightly thicker, ∼70 nm (Figure S4). Although
NaF and HCl were successively used during the synthesis, no
signals of these elements were detected by energy dispersive X-
ray spectroscopy, attesting to the purity of the samples (Figure
3.2. Synthesis Using NH4H2PO4. Besides NaH2PO4·H2O,
dihydrogen ammonium phosphate, NH4H2PO4, was inves-
tigated for the synthesis of γ-ZrP. Whereas with the sodium salt,
a molar ratio of 4 was required, a molar ratio of P/Zr of 3 was
sufficient with the ammonium phosphate to form compounds
of high crystallinity. The X-ray diffractogram of the as-
synthesized material confirmed that monoammonium γ-
zirconium phosphate, γ-Zr(PO4)(NH4HPO4), was formed
(ICDD 04−014−2214). The first peak at 2θ ≈ 7.85°
corresponds to a basal spacing of 11.25 Å, which is slightly
smaller than the basal spacing of 11.53 Å observed for the
D
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Figure 6. XRD and SEM of materials synthesized without NaF at NaH2PO4·H2O/ZrOCl2·8H2O ratio of (a) 4 and (c) 5; (b, d) after treatment in
0.5 M HCl.
Figure 7. XRD and SEM of (a) γ-Zr(PO4)(NH4HPO4) synthesized using ZrOCl2·8H2O:3NH4H2PO4:0.3NaF and (b) after ion exchanging with 2
M HCl solution; red lines, fitting with database (ICDD 04−014−2214).
sodium zirconium hydrogen phosphate (Figure 7). The
ammonium ions in γ-Zr(PO4)(NH4HPO4) can undergo
hydrogen bonding with the neighboring oxygens. This can
lower the free energy of formation so that a smaller P/Zr ratio
is sufficient, compared to that required for the synthesis of the
sodium product.29
phlogopites. However, in all cases, the distribution coefficients
decreased substantially for cesium concentration above 1
μM.41−43 Furthermore, some of these materials, e.g., the
metal ferrocyanides, suffer from slow kinetics and instability
below pH 2.44 γ-ZrP was previously reported to show high
selectivity to Cs+ in the presence of other ions like Na+, Ca2+,
and Sr2+.45 However, no details of the crystalline state was
given. Therefore, it is of interest to study the ion exchange
properties of the highly crystalline γ-ZrP synthesized by the
present method. A comparison is made with crystalline α-ZrP
formed by the same method (XRD and SEM in Figure S6).
Table 2 shows the removal efficiency (R) and distribution
coefficient (Kd) for alkali and alkaline earth ions in mixed
solutions. The removal efficiency for an ion is calculated from
the difference in its initial (1 mM in 25 cm3) and final
concentration after equilibration with 20 mg ZrP as ion-
exchanger/sorbent. The selective pickup of a particular ion is
reflected in a high Kd value. Both α-ZrP and γ-ZrP show the
same uptake sequence. For solution 1, the order was Cs+ > K+
> Na+; for solution 2, Ca2+ ≈ Sr2+ > Mg2+ and for solution 3,
Cs+ > Sr2+. With the exception of Cs+ which is unhydrated, this
order follows the trend of the hydrated ionic radii (Table S1).46
To form dihydrogen γ-ZrP, the concentration of the acid is
important. A minimum of 2 M HCl is necessary as lower acid
concentrations of 0.5−1 M led to incomplete transformation.
This could be due to hydrogen bonded ammonium ion, making
it more difficult to exchange with the proton.29 The SEM
showed that monoammonium γ-zirconium phosphate also
formed rectangular platelets of micron-size, similar to the
sodium zirconium hydrogen phosphate. Again, the morphology
was retained after ion-exchange with HCl to yield γ-ZrP.
3.3. Ion-Exchange Properties. The uptake of selected
ions by crystalline γ-ZrP was investigated. Cesium-selective
sorbents are of interest as cesium is a major nuclear fission
product found in circulating water of nuclear reactors. A
number of materials have been investigated for this purpose,
including potassium cobalt hexacyanoferrate, zeolites, crystal-
line silicotitanates, biotite micas, pharmacosiderites, and
E
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substantially higher than the total exchange capacity of
smectites (0.8−1.2 mequiv/g) and some zeolites.50,51
Table 2. Removal Efficiency (R) and Distribution Constant
(Kd) of α-ZrP and γ-ZrP
In the presence of 10−1000 molar excess of Na+ and Ca2+,
the adsorption of Cs+ was only slightly reduced despite the
much higher concentration of the competing ions (Figure 8).
The presence of Ca2+ has a smaller effect on Cs+ adsorption
than the Na+. This is likely due to a more hindered access to
the interlayer space for the larger hydrated Ca2+ ion (4.12 Å) as
compared to the smaller Na+ (3.58 Å). Moreover, at the highest
equilibrium concentration of ∼9 mM Cs+, the adsorbed
amount was very close to that without the added ions. In
contrast, α-ZrP shows much lower uptake of Cs+, with only
∼0.32 mequiv/g vs 1.69 mequiv/g for γ-ZrP at ∼9 mM Cs+
concentration (Figure S7). In the presence of competing ions
like Ca2+, its adsorption was further decreased.
R (%)
Kd (cm3/g)
solution
1
cations
γ-ZrP
α-ZrP
γ-ZrP α-ZrP
Na+
K+
3.03
12.7
97.3
3.04
27.9
26.4
94.7
15.1
2.56
3.63
9.27
1.11
6.96
5.70
5.82
3.37
39
182
33
47
Cs+
Mg2+
Ca2+
Sr2+
Cs+
Sr2+
49400
39
128
14
2
3
483
94
450
76
22400
223
77
44
The larger the size of the hydrated ion, the lower the
adsorption.47 A striking feature of γ-ZrP is its very pronounced
selectivity to Cs+. In the presence of Na+ and K+, the Cs+
removal efficiency, R, was 97.3% with an extremely high
distribution coefficient, Kd, of 49 400 cm3/g.
A unique feature of γ-ZrP is the stability under acidic
conditions. The uptake of Cs+ was investigated in the pH range
of 1.2 to 5.5 (Figure 9). The removal efficiency by γ-ZrP was
γ-ZrP has a much higher uptake capacity than α-ZrP for all
ions investigated, although the latter has smaller crystallites
(Figure S6b) and a larger surface area (33 vs 6.7 m2/g). Hence,
the higher uptake by γ-ZrP has to be attributed to other factors,
in particular, the presence of dihydrogen phosphate and the
larger interlayer distance. The dissociation constant at 25 °C for
H2PO4− is 6.31 × 10−8, which is much larger than 4.22 × 10−13
48
2−
−
for HPO4
.
Therefore, the proton in H2PO4 should be
much more readily exchangeable with other ions. In contrast, α-
ZrP does not contain dihydrogen phosphate groups but rather
HPO42−, resulting in a much lower ion exchange.
The adsorption isotherm of Cs+ at pH 5.5 was determined
for γ-ZrP (Figure 8). The high uptake at low Cs+ concentration
■
●
Figure 9. Removal efficiency of cesium by ( ) α-ZrP and ( ) γ-ZrP
versus pH.
>98% for pH > 2 and decreased to 81% for pH 1.2. This can be
attributed to the high H+ concentration which competes with
Cs+ so that at very low pH, the ion exchange equilibrium H+ ⇌
Cs+ will be in favor of the protonated γ-ZrP. The Cs+ removal
efficiency by α-ZrP was much poorer, with <10% for pH >3
and falling to almost zero at pH 1.2.
Besides its cation exchange property, zirconium phosphate
also has the advantage of being biocompatible because of the
low toxicity of zirconium.52 Zirconium phosphate is used in
sorbent cartridges to purify the spent dialysate from renal
treatment.53,54 Such sorbent cartridges are an important
component of wearable and portable dialysis devices that
allow renal patients to have more flexibility in their lifestyle and
have better health outcomes due to the continuous removal of
wastes from the body.55 Currently, amorphous zirconium
phosphate gel is loaded in these sorbent systems. We evaluated
the sorption capacity of the crystalline γ-ZrP and the
+
■
Figure 8. Adsorption isotherms of Cs on γ-ZrP ( ) in aqueous CsCl
●
▲
)
solutions (pH 5.5), ( ) in the presence of 0.1 M NaCl (pH 5.5), (
in the presence of 0.05 M CaCl2 (pH 5.1).
indicates a very strong interaction between Cs+ and γ-ZrP. Such
an adsorption isotherm can be classified as a H-type (high
affinity).49 For Cs+ concentration >1.5 mmol/L, the uptake
increased very gradually to 1.69 mequiv/g. This is somewhat
larger than the value of 1.35 mequiv/g reported previously for
γ-ZrP.45 However, as γ-ZrP has a theoretical exchange capacity
of 6.27 mequiv/g, the uptake involved about 27% of the
exchangeable protons. Hence, slightly more than 50% of the
+
amorphous gel ZrP for NH4 and K+, two cations typically
found in the spent dialysate. At low concentrations, both gel
and γ-ZrP show similar uptake of these ions (Figure 10).
However, above 0.5 mmol/L, the uptake of K+ and NH4 was
+
significantly higher for γ-ZrP than the gel. These results clearly
show the benefits of having high crystalline zirconium
phosphate with uniform cavities as compared to a range of
cavity sizes in the amorphous gel. Hence, at the same removal
−
protons from the first ionization of H2PO4 are exchanged.
Even at this level, the practical exchange capacity of γ-ZrP is
F
DOI: 10.1021/acs.inorgchem.7b03202
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support from the Academic Research Grant, R-143-
000-667-114, National University of Singapore, is gratefully
acknowledged. Y.C. thanks NUS-IPP program for award of a
research scholarship.
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Figure 10. Uptake by γ-ZrP (blue) and gel ZrP (black) for NH4 ( )
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Highly crystalline γ-ZrP was synthesized via a liquid-assisted
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.7b03202.
■
S
XRD and SEM of samples synthesized at different ratios
of NaH2PO4/Zr; XRD of 500 °C-calcined sodium
zirconium phosphate; XRD and SEM of α-ZrP,
elemental analysis by EDS, and adsorption isotherms of
cesium on α-ZrP (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail: chmcgk@nus.edu.sg.
■
ORCID
(16) Wong, M.; Ishige, R.; White, K. L.; Li, P.; Kim, D.;
Krishnamoorti, R.; Gunther, R.; Higuchi, T.; Jinnai, H.; Takahara,
A.; et al. Large-Scale Self-Assembled Zirconium Phosphate Smectic
Layers Via a Simple Spray-Coating Process. Nat. Commun. 2014, 5,
3589.
Gaik-Khuan Chuah: 0000-0002-4739-9405
Author Contributions
The manuscript was written through contributions of all
authors.
G
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