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Resources Processing, Volume 63, Issue 3
P99-P104

Resources Processing 63 : 99–104 (2016)

RESOURCES
PROCESSING

Original Paper

A Novel Anion Exchanger Composed of Formate Hydrotalcite
for Sorptive Recovery of Phosphorus
Toshiyuki TANAKA*, Ryo TSUKANE, Tomoko MATSUDA,
Mutsuharu IMAOKA and Hiroyasu TAMAI
Mechanical & Material Research Laboratory, Tottori Institute of Industrial Technology, 1247 Kusaka, Yonago,
Tottori 689-3522, Japan

Abstract
Feasibility of a renewable sorbent for phosphorus was determined by testing the anion-exchange
ability of hydrotalcite, which holds formate ions in its interlayer. The formate hydrotalcite, successfully prepared by reacting raw hydrotalcite with a formic acid/1-propanol solution, exhibited highperformance for phosphorus removal and collection based on a stoichiometric anion-exchange principle. It was estimated that the collected orthophosphate ions existed as poly-anion forms of HPO42–
and PO43– with a ratio of 67.6:32.4 in the interlayer after the sorption. The formate hydrotalcite is alsoable to revert to the starting hydrotalcite after the sorption, when treated with Na2CO3 aqueous solution, demonstrating that it can be an attractive alternative to the conventional chloride or nitrate hydrotalcite as an anion exchanger.
Key words: Formate hydrotalcite, Anion exchange, Orthophosphate ion, Sorbent

1. Introduction
Phosphorus is an important resource, because itis an essential element for living beings and isalso used industrially in manure. Since there is noprocess available that allows the simultaneous
industrial application of phosphorus and its regeneration after use, it is important to secure stable
phosphate resources. A method to secure phosphate resources is sorption from discharged and/or
environmental water by using an anion exchanger
such as hydrotalcite (HT). HT is a typical mineralwith layered double hydroxides (LDHs), which
are known as anion-exchange materials1,2. HT
consists of positively charged brucite-likeoctahedral layers formed by partial substitutionof Al for Mg and interlayer carbonate ion, andis represented by the chemical formula
Mg6Al2(OH)16CO3·4H2O. HTs are commonly synthesized in industrial fields for use in products
such as resin additives and antacids for the stomach3–5, and LDHs are often studied for applicationas scaffolds for immobilizing functional organic
Accepted 29 August, 2016
*e-mail: tanaka.toshiyuki@pref.tottori.jp

species6–8. The sorption of phosphate ions with
LDHs from aqueous solution is an attractive
method to secure phosphate resources and has
been studied by many researchers9–18. In general,these anion-exchange reactions are conducted byusing LDHs with chlorate or nitrate ions in theinterlayer, because it is difficult to exchange carbonate ions incorporated on the LDH structurewith external anions because of the strong affinityof carbonate anions toward the brucite-like layer19,20. However, the chlorate ion might releaseharmful trihalomethane, and the nitrate ion is detrimental to water treatments; therefore, an alternative to chlorate- or nitrate-LDHs is necessary todevelop an anion-exchange technology for securing phosphorus resources.
In the present work, we focused on HT with incorporated formate ions in the interlayer for theremoval of orthophosphate ions in aqueous solutions. The formate ion is the smallest carboxylate
ion and has a very simple structure like that ofcarbonate, chlorate, and nitrate ions. The loadingof short-chain carboxylate ions such as the formate ion into LDHs is regarded as difficult inaqueous media21–23, implying that formate-HT,
wherein the incorporated anion has low affinity

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supernatant was analyzed.
2.3 Equipment
X-ray powder diffraction (XRD) patterns were
measured on a Rigaku Ultima IV diffractometerattached to a D/teX Ultra semiconductor detector
with Ni-filtered Cu-Kα radiation, using an operating voltage and current of 40 kV and 40 mA, respectively. Fourier transform infrared (FTIR) spectra were measured on a Shimadzu IR Prestige-21
Fourier transform infrared spectrometer equippedwith an AIM-8800 automatic infrared microscopeby accumulating 40 scans at 4 cm–1 resolution using a diamond cell. The anions in aqueous solutions were analyzed by an ion chromatograph system equipped with a Waters 432 conductivity
detector, 515 HPLC pump, and Shodex NI-424
column using the eluent composed of 8 mmol·
dm–3 4-hydroxybenzoic acid, 2.8 mmol·dm–3
bis(2-hydroxyethyl)imino-tris(hydroxymethyl)
methane, 2 mmol·dm–3 phenylboronic acid, and
0.005 mmol·dm–3 trans-1,2-diaminocyclohexaneN,N,N′,N′-tetraacetic acid aqueous solution. A
Seiko Instruments Inc. SPS3100 inductively coupled plasma-atomic emission spectrometer and a
Kyoritsu Chemical-Check Lab. TOC-UVA totalorganic carbon analyzer were used to analyze theaqueous solutions and validate the ion chromatography results.

toward the brucite-like layers, will exhibit highperformance as an anion exchanger. Although thechemical structure of formate-HT was reported
from the viewpoint of the delamination of brucitelike layers24–26, the anion-exchange property was
not studied in this process. Anion-exchange tests
were conducted to investigate the stoichiometryof the anion exchange process and the feasibilityof the phosphorus sorbent.
2. Experimental
2.1 ReagentsSpecial grade reagents were obtained from
Tokyo Chemical Industry Co., Ltd., and Wako
Pure Chemical Industries, Ltd., Japan, and wereused as-received without further purification. Distilled water used in this work was purified by thewater passing through an Advantec GS-200
AQUARIUS automatic water distillation apparatus. The commercially available HT was used as
the starting material (DHT-6 with the theoreticalcomposition Mg6Al2(OH)16CO3·4H2O, Kyowa
Kagaku Kogyo Co., Ltd).
2.2 Sample preparation
The typical preparation of formate-HT was
conducted using the batch technique as follows.
The reaction was performed in a 1-propanol solution of formic acid27–31. The HT powder was dispersed in 50 cm3 of a formic acid/1-propanol
solution adjusted to the prescribed concentrationof formic acid, which was equivalent to 98% of
the anion-exchange capacity of the added HT,with a concentration of 80 g·dm–3, in a capped
glass bottle, and the mixture was stirred for 4 h.
The suspension was centrifuged, and the precipitate was washed with 1-propanol and dried undervacuum at room temperature. The resulting powder was referred to as formate-HT.
Subsequently, the formate-HT powder (0.5 g)
was dispersed in a single-anion aqueous solution(50 cm3) containing 100 mmol·dm–3 of KNO3,
KF, KCl, KBr, or KI in a capped Erlenmeyer
flask. The flasks were shaken for the prescribedtime at 25°C in a constant-temperature bath. Additionally, a similar method was conducted using
a 10 mmol·dm–3 of K2HPO4 aqueous solution inorder to investigate the removal behavior forphosphorus. Furthermore, the HT powder, which
incorporates orthophosphate ions, was dispersed
in a 10 mmol·dm–3 of Na2CO3 aqueous solution in
the same manner in order to check the stoichiometry of the elimination of phosphorus from the HT
in conjunction with re-carbonation of HT. The
resulting suspension was centrifuged, and the

3. Results and discussion
3.1 Preparation of formate-HT
The XRD patterns and FTIR spectra of the prepared samples are shown in Fig. 1 and Fig. 2,
respectively. The XRD pattern of formate-HTdisplayed typical diffraction peaks indexed withrhombohedral symmetry for the starting HT32,
indicating that the crystal structure of HT weremaintained after the formate acid/1-propanol
treatment. In contrast, the intensity of the absorption bands attributed to the interlayer carbonate
ion (ν3-CO32–: 1371 cm–1) decreased in formateHT, and the absorption bands attributed to carboxylate antisymmetric (νas-COO–: 1585 cm–1) andsymmetric (νs-COO–: 1381 cm–1) stretching wereobserved24–26,30. This observation is evidence ofthe substitution of carbonate ions (CO32–) by formate ions (HCOO–). For the reasons stated above,
the formate ion was considered successfully loaded on to the interlayer of HT. A sharp peak otherthan the ones listed above was observed at1346 cm–1 in the spectrum of formate-HT. This
peak could be assigned to magnesium or aluminum formate, because of dissolution and reprecipitation of the brucite-like layer by the reaction

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Fig. 1

Fig. 2

Fig. 3 Relationship between the amounts of formate ionsreleased from formate-HT and anions sorbed tothem. Data represent mean ± standard deviation.

XRD patterns of (a) HT as received and (b)formate-HT. The prime miller indices are assigned
in each pattern to a rhombohedral system.

that the amounts of formate ions released from thehydrotalcite were in fair agreement with that incorporated to hydrotalcite, irrespective of the species of the monovalent anions. Therefore, the correlation demonstrates that formate-HT can work
as an anion-exchanger. Meanwhile, the interceptof the regression line was 0.219, i.e., the amountof formate ions released was slightly larger thanthat expected from stoichiometry. This value indicates that a little amount of magnesium or aluminum formate formed during the preparation offormate-HT in 1-propanol dissolved into theaqueous solution. Fig. 3 also shows an approximate order of the monovalent anion selectivity forformate-HT through anion exchange, which is
F– > I– > Br– > Cl– > NO3–. However, this orderdoes not conform to that shown by the chlorate-,
nitrate-HT, and Ni-Al-type formate-LDHs (F– >Cl– > Br– > NO3– > I–)19,20,26. Although the reason
for the change in sequence is under investigation,
we believe that the phenomena is interesting andmight be owing to the hydrated or non-hydrated
structure of the interlayer formate ions.
3.3 Removal behavior for phosphorus
The time course of orthophosphate and formate
ion concentrations after the dispersion of formateHT in the K2HPO4 aqueous solution is shown in
Fig. 4. Formate-HT could remove phosphorus
with the release of the formate ions within 2 h ofdispersion, whereas carbonate-HT could remove
little. As mentioned before, the anion-exchange
reaction is perhaps the expected mechanism of the

FTIR spectra of (a) HT as received and (b)formate-HT. The prime assignment is shown ineach spectrum.

with formic acid in 1-propanol medium33–35.
3.2 Stoichiometry for anion exchange
Stoichiometry for the anion-exchange reactionof monovalent anions on formate-HT is shown in
Fig. 3. The correlation between the amount of theformate ions released and the anions incorporated
was tested by correlation coefficient (R) and orthogonal linear regression methods36. The relationship had a strong positive correlation (R =0.977), and the slope of the regression line was
1.007, which is close to 1. These values indicate
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Fig. 4 Time dependences of P and HCOO– concentration
after the dispersion of formate-HT (0.5 g) in
K2HPO4 aqueous solution (10 mmol·dm–3, 50 cm3,25°C).

Fig. 5 Speciation of orthophosphate ions at various pH.
This graph is drawn by solving the simultaneous
equations of acid dissociation constants and massbalance. The acid dissociation constants of orthophosphate ions are Ka1 = 7.5 × 10–3 (pKa1 = 2.1),
Ka2 = 6.2 × 10–8 (pKa2 = 7.2), and Ka3 = 4.8 × 10–13(pKa3 = 12.3).

removal. The subsequent change in the valency ofthe orthophosphate ion might be of interest to researchers. The typical speciation of the orthophosphate ions is depicted in Fig. 5. The ratio of
HPO42– to PO43– in the starting solution at pH 8.8
was estimated to be 97.4:0.01, by solving the simultaneous equations involving acid dissociation
constants and mass balance37. Meanwhile, theHPO42–:PO43– ratio of 67.6:32.4 in the interlayerof HT was estimated in the following way. First,
the net amount of formate ions contributing toward anion exchange (2.14 mmol·g–1) was calculated by subtracting the intercept of Fig. 3(0.22 mmol·g–1) from the amount of formate ionsreleased from formate-HT (2.36 mmol·g–1). Second, the average valency of the incorporated
orthophosphate ions (2.32) was calculated by
dividing the net exchanged anions amount by
the amount of incorporated phosphorus (0.92mmol·g–1). Third, the ratio of orthophosphate ions
(HPO42–:PO43–=67.6:32.4) was estimated by applying the average valency to the system depicted
in Fig. 5. This change in the ratio indicates that
deprotonation of the interlayer species may proceed through an anion-exchange reaction, since
the interlayer surface of the hydroxide layer of HT
is basic, possibly corresponding to pH 12.0. Further work is in progress to investigate the valencyof the orthophosphate ion in the interlayer. In Addition, formate-HT is expected to incorporate
orthophosphate ions up to an anion-exchange
capacity of 3.2 meq.·g–1. Based on the above facts,
it can be suggested that formate-HT could serve
as a rapid remover of phosphorus.
3.4 Regeneration of sorbent
Stoichiometry was investigated for the elimina-

Fig. 6 Equivalent exchanged anions: (a) formate ion released from formate-HT when testing removal for
orthophosphate ion, (b) formate ion released when
testing the reaction with sodium carbonate, and (c)carbonate ion removed when testing the reaction
with sodium carbonate.

tion of phosphorus from phosphate-HT in conjunction with re-carbonation of HT. The amountof phosphorus was 0.80 mmol·g–1 for the incorporation and 0.92 for the elimination after dispersionof phosphate-HT in the Na2CO3 aqueous solution.
Then, the equivalent exchanged anions other thanphosphate through the incorporation and elimination test are shown in Fig. 6. The amount of formate ions released when testing the incorporationand elimination for phosphorus (2.66 meq.·g–1)
was comparable to that of carbonate ions consumed when testing the elimination (2.73

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ly eliminated from aqueous solution by volatilization or oxidative degradation, formate-HT is
expected to be an interesting substitute forchloride- and nitrate-HT, which are commonlyused as anion exchangers. Further work is in progress to explore the removal and collection ofharmful and/or recyclable anions using formateHT.
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Fig. 7

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A novel anion exchanger, hydrotalcite with formate ions in the interlayer (formate-HT), was investigated. This material showed the ability toremove phosphorus based on an anion-exchange
principle. Furthermore, the phosphorus incorporated in HT can be collected and the formate-HT
can be renewed as the starting HT. Because of theunique feature that allows formate ions to be easiVol. 63, No. 3 (2016)

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