Model studies on the separation of Ca2+ and Nd3+ ions using ethylenediaminetetraacetic acid
Model studies on the separation of Ca2+ and Nd3+ ions using ethylenediaminetetraacetic acid
Z. Jagoda 0 1
A. Pelczarska 0 1
I. Szczygieł 0 1
Separation 0 1
0 Department of Inorganic Chemistry, Faculty of Engineering and Economics, Wrocław University of Economics , Komandorska 118/120, 53-345 Wrocław , Poland
1 & I. Szczygieł
Studies on the separation of calcium and neodymium ions by using ethylenediaminetetraacetic acid (H4EDTA) as a complexing agent were performed. This research was undertaken due to the possibility of H4EDTA applying to isolate rare earth elements from the solution after acidic leaching of phosphogypsum, and because of the similarity of coordination properties of calcium and lanthanides ions. The experiment was carried out in model systems containing Ca2? and Nd3? ions in hydrochloric or sulphuric acid. The content of calcium and neodymium metals, phase composition and thermal behaviour of the obtained products were determined by ICP-OES, FTIR, XRD and TG/DTA techniques. During the separation process, the precipitates of a light pink colour were obtained. The obtained results show that the neodymium ethylenediaminetetraacetate has been successfully formed and that the isolation of neodymium ions was more efficient in chloride medium. The precipitate included 72.2 and 3.9% of the starting amount of neodymium and calcium used in the experiment, respectively. However, in sulphates medium, these amounts were equal to 73.8 and 53.5%, respectively. Moreover, the obtained powder was polluted with sulphates. The addition of the EDTA in an excess (15%) contributed only to an increase in calcium content in the complex.
Rare earth elements; EDTA
Rare earth elements (RE) constitute a group of great
importance for technology. There has been a considerable
increase in applications of RE due to their desirable
chemical, catalytic, electrical, magnetic and optical
properties. These applications widely range from polishing
] through lasers [
] and magnets [
batteries  to modern technologies such as those of solar
], high–temperature superconductors  and
plasma display panels [
The RE, in contrast to the designation, in fact, are not
uncommon—for instance, the cerium content in the Earth’s
crust is similar to that of copper and nickel [
the problem appears with their large dispersion which
makes the mining and separation be difficult and
inefficient. These elements occur in nature mostly in the form of
phosphates and silicates in so-called rare earth minerals
(e.g. monazite, rhabdophane, xenotime). Currently, most
exploited deposits of the elements are those in China.
However, the China export used to be limited to 40% in
2012. This may cause serious problems for technologies
outside of China, hence alternative RE sources, are being
searched for [
]. One of the possible sources can be
phosphogypsum which is a by-product (End-of-Life
material) in the process of phosphoric acid production from
phosphate rocks, phosphorites and apatites, by the wet
method. Phosphogypsum contains RE metals in an amount
dependent on the origin of rocks used in the process,
usually, there is * 0.5–1% [
]. The largest content is
observed for lanthanum, cerium and neodymium [
Attempts to obtain RE from the waste materials of
phosphate fertilisers production were made in the past [
Those ways chiefly consist in the leaching of
phosphogypsum with an inorganic acid to be followed by
precipitation of the RE salts from the obtained solution.
Usually rare earth elements are recovered by precipitation
as RE oxalates or sodium-RE double sulphates, separated
by hydrofluoric acid and then converted into desired
commercial rare earth salts. Unfortunately, the precipitates
contain not only lanthanides but large amounts of calcium
the primary component of phosphogypsum. Due to a
similar chemical reactivity and coordination properties of Ca
and RE, separation of those elements is difficult.
Therefore, in the present investigations, we described
the first step leading to separation of calcium and
neodymium ions by using H4EDTA as the strongly chelating
agent and the difference in the stability of Ca(II) and
Nd(III) with ethylenediaminetetraacetic acid complexes.
Citric acid and amino acids similar to EDTA were found to
separate some REE but could not separate as many as
]. Ethylenediaminetetraacetic acid (EDTA)
may be an improvement over the use of other chelating
agents. The experiment was performed in model systems
which consisted of calcium and neodymium in sulphates or
chlorides medium. The task was undertaken in view of the
possibility to apply H4EDTA to the selective separation of
RE from the solution after leaching process of
phosphogypsum as well as for the known similar coordination
properties of calcium and neodymium ions. Chemical
effect of Nd ions in examined model systems should be
typical of whole RE group elements owing to their electron
configuration (n–1) d1ns2. The choice of the salts was made
for the possibility of using sulphuric(VI) or hydrochloric
acids as the phosphogypsum leaching agent. The studied
Nd and Ca separation process is considered to be applied in
the future for the recycling technology of RE elements
from the waste phosphogypsum in Poland.
Materials and methods
The following reagents, commercially available, were used
without any additional purification: calcium chloride
hexahydrate (analytically pure), ammonium sulphate
(analytically pure), ethylenediaminetetraacetic acid
(H4EDTA; C 99.0%), neodymium(III) oxide (99 ?),
sulphuric(VI) acid (96%), hydrochloric acid (35-38%).
Dihydrate calcium sulphate(VI) was obtained via reaction
of calcium chloride hexahydrate with ammonium sulphate
and its purity was confirmed by XRD.
In the first stage of the study, the calcium and
neodymium complexes with H4EDTA were obtained by treating
the relevant sulphates or chlorides with
Test S1: CaSO4 2H2O water suspension (pHS1 = 5.7)
was mixed with H4EDTA in the molar ratio 1:1 with a
small amount of water. Then the mixture was heated and
refluxed for 2 h. The precipitate was filtered, washed with
distilled water and dried in the air at room temperature.
Test C1 was carried out in a similar way, with a solution
of CaCl2 6H2O and at pHC1 = 5.2.
In the S2 and C2 test, neodymium oxide was dissolved
in the stoichiometric amount of 1 M H2SO4 (pHS2 = 6.5)
and 1 M HCl (pHC2 = 2.5), respectively. Then the
H4EDTA acid was added ensuring the molar ratio
Nd3?:H4EDTA to amount 1:1. The mixtures were refluxed for 2 h.
The precipitates were filtered, washed and dried.
The next stage was an attempt to separate calcium and
neodymium ions through chelating of sulphates (test S3)
and chlorides (test C3). Neodymium oxide was dissolved in
a similar way as in the case of tests S2 and C2. The
obtained solutions were treated with the appropriate
calcium salt (pHS3 = 6.5; pHC3 = 2.3) and H4EDTA. The
molar ratio Nd3?:Ca2?:H4EDTA was 2:1:2 and 1:1:1 in
tests S3 and C3, respectively.
The content of C, H, N, S and Cl was determined by atomic
absorption spectrometry (AAS) using the analyser CHNS
Vario EL III (Elementar). Ca and Nd analysis was
performed by the inductively coupled plasma-optical emission
spectrometry (ICP-OES) using the apparatus ARL3410
ICP with argon plasma excitation. Moreover, the obtained
products were characterised by Fourier transform infrared
(FTIR) spectroscopy, powder X-ray diffraction (XRD), and
thermal analysis (TGA/DTA). The FTIR spectra were
measured with a Perkin-Elmer System 2000 FTIR
spectrophotometer in the medium IR range (4000–400 cm-1) at
room temperature. The samples were prepared in the form
of KBr pellets. The XRD measurements were performed
with a SIEMENS D5000 diffractometer (copper X-ray
tube) in the range 2h of 5–50 with a 0.04 step and at least
2 s per step. The TGA/DTA analyses were carried out
using a derivatograph 3427 (MOM, Hungary) in a
temperature range of 20–1000 C (heating rate 7.5 C/min,
platinum crucible, air atmosphere).
Results and discussion
To obtain RE from the leaching solution, chelating agents
can be used. One of the widely applied agents is
ethylenediaminetetraacetic acid (H4EDTA). This agent is
capable of complexing the ions of most metals; it is
applied, in the form of sodium salts, in quantitative
chemical analysis owing to a simple stoichiometry of the
reaction with metals. The capability of H4EDTA agent to
chelate metal ions depends on the pH value of the solution,
which affects the protonation of H4EDTA acid as well as
the equilibrium of complexes formation. In a neutral or
slightly alkaline medium, an HEDTA3- ion will be the
prevalent form of the acid. Complexing will proceed
according to equation:
Mnþ þ HEDTA3
$ ½MEDTA n 4þ Hþ
The formation reaction of metal complexes with
H4EDTA at the pH in the range from 3.5 to 5.5 can be
Mnþ þ H2EDTA2
$ ½MEDTA n 4þ 2Hþ
At lower pH, the H4EDTA occurs in the form of
H3EDTA- ions as well as the non-dissociate H4EDTA (with a
pH less than 2), which is manifested as metal complexing
Mnþ þ H3EDTA
$ ½MEDTA n 4þ 3Hþ
Mnþ þ H4EDTA $ ½MEDTA n 4þ 4Hþ
According to the above reactions, lowering of pH shifts
an equilibrium to the left and reduces the stability of the
metal–EDTA complexes. The stability of metal–EDTA
complexes is characterised by the value of stability
constant b which lowers with decreasing of the solution pH.
The higher b value of complex then the lower pH of a
solution can be for complex creation. The difference in the
b values of Ca–EDTA (logb = 10.65) and Nd–EDTA
(logb = 16.51) complexes can be used for separation of
these coordination compounds because the calcium
complex is stable at pH above 8 and dissociates in a neutral or
acidic medium. In the case of solutions containing Ca and
Nd ions, carrying out the complexing in an acidic medium
should ensure an efficient separation of these elements.
Formation and satisfying stability of the Nd–EDTA
complex in an acidic medium are confirmed by the results of
From the above reasons, it was decided to use the
complexing agent in the acid form—H4EDTA which forms
with neodymium ions stable and insoluble in water
coordination compound. The use of sodium
ethylenediaminetetraacetate Na4EDTA is pointless because it forms
with both calcium and neodymium stable and water soluble
complex. In this case, both ions would be coordinated and
the separation will not occur.
In the case of metal complexes with
ethylenediaminetetraacetic acid, the most typical band of an IR
spectrum is that corresponding to the asymmetric
stretching vibration of carboxyl groups masCOO. When the
carboxyl groups are non-ionised and non-coordinated, this
signal appears at 1750–1700 cm-1. As a result of metals
coordination, the band is shifted towards low frequencies to
the 1650–1590 cm-1 range. In the range from
1630 to 1575 cm-1, bands of free and ionised carboxyl
groups occur [
An FTIR spectrum of a precipitate obtained in the C1
test is shown in Fig. 1a. In the figure, a band of stretching
vibration at the wave number 1697 cm-1 is visible. This
means that the carboxyl groups of H4EDTA were
noncoordinated to the metal. In fact, Fig. 1a shows an FTIR
spectrum of the free ligand—ethylenediaminetetraacetic
]. The result of the S1 test (Fig. 1b) was similar;
any calcium complex with H4EDTA was not obtained.
Two absorption bands are observed in the FTIR spectrum
in the range 1600–1700 cm-1. One of them, at 1687 cm-1,
is originated from the masCOO vibration and indicates the
presence of non-coordinated protonated carboxyl groups of
the ligand. Bending vibration of dO–H in hydration
molecules in CaSO4 2H2O combined with sulphate ions via
hydrogen bond [
] are responsible for the occurrence of
absorption at 1622 cm-1. The presence of sulphate ions is
confirmed by a broad band in the range 1225–1030 cm-1
and two bands at 667 and 602 cm-1 originated from the
stretching vibration of SO42- ions [
]. The FTIR spectrum
showed on Fig. 1b indicates that the S1 attempt did not
result in coordinating the metal to a ligand.
Hence, no calcium complexes with H4EDTA were
obtained in the course of C1 and S1 tests. The unreacted
H4EDTA and CaSO4 2H2O (in test C2) are present in the
obtained precipitates. Calcium chloride used in the C1 test
as a water soluble compound remains in the solution and is
not observed in the FTIR spectrum. These conclusions
were also confirmed by XRD analysis of the C1 precipitate.
The XRD pattern showed only H4EDTA to be present in
1400 1200 1000
the powder. This is in accordance with the literature data
on the conditions for calcium complexes formation with
A different situation is observed for the C2 and S2 tests
which aimed at obtaining neodymium complexes with
H4EDTA. In both cases, the precipitate was a light pink
colour typical of neodymium compounds. Analysis of the
1750–1575 cm-1 range in the FTIR spectra (Fig. 2) leads
to the following conclusions. For both cases, an absorption
band occurs at the same frequency corresponding to
1670 cm-1. This band does not appear in the case of
anhydrous complexes, according to the data of Ref. [
whose authors have studied different forms of hydrated
europium complexes with H4EDTA. Hence, the observed
band should be connected with the presence of hydration
water and the deformation vibration dO–H. Other
absorption bands are those at frequencies of 1598 cm-1 (C2,
Fig. 2a) and 1599 cm-1 (S2, Fig. 2b). Their origin can be
ascribed to the asymmetric valence vibrations of
metalcoordinated carboxyl groups. Because those bands are
located at the lower limit of a theoretical range of
1650–1590 cm-1, quite likely is the occurrence of
hydrogen bonds in the crystal lattice of the compound and/or
ionic character of the metal–ligand bonding rather than a
covalent one. The bands overlap the succeeding ones of
absorption at 1580 cm-1 (C2) and 1581 cm-1 (S2). The
presence of the latter one absorption indicates that one or
more carboxyl groups do not form a bond with the metal.
Consequently, the free, ionised groups COO- are present
in a molecule of the complex. Based on the analysis of the
region 1350–1450 cm-1, it can be noted that the products
of C2 and S2 experiments were single hydrates because, in
the case of the hydrated and anhydrous complexes,
respectively, three and two bands were observed in the
range under discussion [
]. In view of the high
coordination number of neodymium (9 [
] or 10 [
can be assumed that water molecule will coordinate the
metal in addition to H4EDTA. However, a further X-ray
study on monocrystals is needed to acquire detailed
information on the coordination environment of the central
ion. The lack of absorption bands in the range
1225–1030 cm-1 for the S2 test indicates the absence of
sulphate ions, whereas the 1102 cm-1 band should be
ascribed to the valence vibration of the mCN ligand
The results described above are satisfactory from the
viewpoint of the question how to separate neodymium
from calcium ions by using EDTA. Carrying out separation
process in the sulphuric or hydrochloric acid medium, one
may expect that the complex will be formed by neodymium
ions, while calcium ions remain non-coordinated.
Tests C3 and S3 also resulted in a precipitate that was a
light pink colour which indicated the presence of Nd3?
compounds. As shown in Fig. 3 FTIR spectra indicate that
a neodymium complex with EDTA was formed in both
cases. Both precipitates exhibit bands at a frequency of
1600 cm-1 which originate in the masCOO vibrations of
carboxyl groups. In contrast with the S2 product, the S3
product shows contamination with sulphate(VI) ions,
which is indicated by a broad band at 1225–1030 cm-1 and
two distinct signals at 666 cm-1 and 601 cm-1 connected
with bending vibration of sulphate(VI) ions derived from
calcium sulphate(VI). The presence of H[Nd(EDTA)] H2O
complex and unreacted EDTA in the C3 and S3 samples as
well as of calcium sulphate(VI) in the S3 precipitate, is
confirmed by the result of XRD (Fig. 4) and ICP-OES
(Table 1) analysis.
The ICP-OES results show that the complex included
72.2% of the neodymium relative to that contained in the
substrates used to carry out experiment C3. Moreover, the
ICDD 07-0752 H[Nd(EDTA]
ICDD 33-1672 H4[EDTA
ICDD 33-0311 CaSO4*2H2O
C3 product contained 3.9% of the starting amount of
calcium used in the experiment. For the S3 sample, 73.8% of
neodymium was chelated, although the product contained
53.5% of the calcium initial amount. The increased content
of C, N, and H in the C3 product could be a result of the
presence of unreacted EDTA. The accompanying lower
content of neodymium is also meaningful.
The C3 and S3 precipitates were subjected to the
thermal analysis by TGA/DTA (Fig. 5a, b). A multistep
decomposition of the C3 sample (Fig. 5a) started at 210 C.
In contrast to the results of Ref. [
], an endothermic
effect appears on the DTA curve at the onset temperature
of 210 C. Consequently, complex dehydration process
starts at this temperature, which is accompanied by a mass
loss of about 10% visible on TG curve. The high
dehydration temperature may be explained by coordination of
metal ions with a water molecule. A multistage exothermic
decomposition of organic residues of the obtained complex
and non-chelated EDTA starts at a temperature of
approximately 290 C. The related thermal effects are
observed on the DTA curve at onset temperatures of 290,
340 and 400 C, with the summary mass loss about 57%.
Decomposition of the precipitate in the test S3 runs in a
similar way (Fig. 5b). Two endothermic effects occur on
DTA 97° 187°
%0 TGA 100
Fig. 6 XRD patterns of the C3 product after annealing at 800 and
1000 C (a). XRD patterns of the S3 product after annealing at 800
and 1000 C (b)
the DTA curve at onset temperatures of 100 and 190 C,
respectively. The accompanying mass loss amounts to
about 5%. As it was shown above (Table 1; Fig. 4), the
sample S3 was contaminated with CaSO4 2 H2O. Since the
two-step dehydration of gypsum to anhydrous CaSO4
proceeds in the 100–200 C temperature range [
changes of the DTA curve fragment in question can be
attributed to the gypsum dehydration process. It can be
assumed that dehydration of the complex acquired in test
S3 will proceed at a higher temperature (190 C) due to
coordination water molecule to the metal ion. The DTA
curve in Fig. 5b shows exothermic effects at onset
temperatures of 260, 355 and 480 C, which are ascribed to the
decomposition process of an organic ligand. These effects
were accompanied by a considerable mass loss of about
45% (see TG curve).
In the case of the C3 and S3 samples, a slight
endothermic effect was also observed at onset temperature
of about 610 C, which was not accompanied by a clear
ICDD 41-1089 A-Nd2O3
ICDD 41-1089 A-Nd2O3800 °C
ICDD 21-0579 C-Nd2O3
ICDD 37-1496 CaSO4
mass loss. This is just the polymorphic transition
temperature for neodymium oxide A-Nd2O3 ? C-Nd2O3 [
The sample’s mass became stable at a temperature of
approximately 650 C.
The products acquired in tests C3 and S3 were heated in
air at 800 C for 6 h. The compounds changed their colour
into blue during sintering. The XRD analysis of C3 powder
(Fig. 6a) showed the presence of neodymium oxide
crystallized in the trigonal system (A–Nd2O3) and also revealed
the presence of cubic neodymium oxide as well. The
sample annealed at 1000 C was a monophase cubic form.
For the S3 product, after heating at 800 C, the following
phases were found: anhydrous calcium sulphate(VI) (as an
insoluble anhydrite, Anhydrite II), and two modifications
of neodymium oxide, trigonal A–Nd2O3 and cubic
C-Nd2O3 (Fig. 6b). The presence of anhydrite in the
sample after sintering at 800 C was independently confirmed
by the FTIR technique (spectrum not shown). Heating of
S3 sample at 1000 C caused the disappearance of
reflections typical for cubic neodymium oxide.
In order to gain a higher efficacy of calcium and
neodymium ion separation, the C3 experiment was repeated
with a small excess (15%) of EDTA. As a result, the
neodymium chelating efficiency somewhat increased (from
72.2 to 73.7%), which was not accompanied by a
noticeable enhancement in separation of neodymium and calcium
ions. The ICP-OES analysis showed the lowering of
neodymium content in the sample to 25.15% and also a nearly
twofold increase of calcium concentration (0.74%).
Thereby the final product contained 7.8% of the original
The results of this study indicate that the separation of
neodymium and calcium ions from the hydrochloric acid
solution in the presence of complexing agent like
ethylenediaminetetraacetic acid, i.e.: EDTA is possible,
even with a good efficiency. A small amount of
contamination in the form of non-coordinated EDTA is of no
importance from the viewpoint of the issue of lanthanides
recovery from phosphogypsum. In order to receive rare
earth metals in the desired form of oxides, the complex
Me–EDTA should be treated with high temperatures
([ 800 C), at which its organic component will undergo a
complete decomposition into gaseous products. The use of
an excess of complexing agent contributes to deterioration
of selectivity of the separation process and, consequently,
makes the calcium concentration be increased in the final
product. The further research on effect of the other ions
present in the actual leach solution during the separation of
metals ions from such system is needed.
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