Solvent extraction of some trace metals and iron with N-octyl-N,N- bis(dihexylphosphinylmethyl)amine

Article abstract of DOI:10.1134/S1070363211100070

The processes were studied of the solvent extraction of the ions of triply-charged trace elements including scandium, indium, gallium, and yttrium, as well as iron, with N-octyl-N,N-bis(dihexylphosphinylmethyl)amine solution in toluene, chloroform or methylene chloride from hydrochloric, nitric or perchloric acids aqueous solutions. The metals extraction dependence on the acid concentration showed that the best results were reached using perchloric acid. The calculation of partition coefficients of metals allowed us to reveal a high selectivity of the scandium extraction. The prospects of using the investigated bisphosphinylamine in the technology of extraction, concentration and separation of the trace metals ions was concluded. Pleiades Publishing, Ltd., 2011.

Full text of DOI:10.1134/S1070363211100070

ISSN 1070-3632, Russian Journal of General Chemistry, 2011, Vol. 81, No. 10, pp. 2080–2087. © Pleiades Publishing, Ltd., 2011.
Original Russian Text © R.A. Cherkasov, A.R. Garifzyanov, O.B. Bazanova, S.V. Leont’eva, 2011, published in Zhurnal Obshchei Khimii, 2011, Vol. 81,
No. 10, pp. 1627–1634.
Solvent Extraction of Some Trace Metals and Iron
with N-Octyl-N,N-bis(dihexylphosphinylmethyl)amine
R. A. Cherkasov, A. R. Garifzyanov, O. B. Bazanova, and S. V. Leont’eva
Kazan (Volga Region) Federal University, ul. Kremlevskaya 18, Kazan, Tatarstan, 420008 Russia
e-mail: rafael.cherkasov@ksu.ru
Received April 28, 2011
Abstract—The processes were studied of the solvent extraction of the ions of triply-charged trace elements
including scandium, indium, gallium, and yttrium, as well as iron, with N-octyl-N,N-bis(dihexylphosphinyl-
methyl)amine solution in toluene, chloroform or methylene chloride from hydrochloric, nitric or perchloric
acids aqueous solutions. The metals extraction dependence on the acid concentration showed that the best
results were reached using perchloric acid. The calculation of partition coefficients of metals allowed us to
reveal
a high selectivity of the scandium extraction. The prospects of using the investigated
bisphosphinylamine in the technology of extraction, concentration and separation of the trace metals ions was
concluded.
DOI: 10.1134/S1070363211100070
The solvent and membrane extractions are base of
the concentration, separation and analysis of metals in
natural and technological sources, providing a
comprehensive processing of the mineral raw materials
and secondary resources, as well as solution of
important environmental problems [1–3]. In this
context, it should be emphasized that the rapid
development of the chemistry and technology of noble,
rare-earth and non-ferrous metals is largely due to the
development of the innovative extraction
methods. Recently, much attention was given to
extraction methods for extracting, separating and
analyzing the trace metals [4–6]. A variety of
extraction objects and the specificity of their
extraction, concentration and separation is a need to
find the new high-effective extractants suitable for
extraction processes for each of the target substrates
individually. Among the large set of organic reagents
used to solve these problems, the prominent place
belongs to the neutral organophosphorus com-
pounds. In the extraction technology of rare and trace
elements are used most frequently the organophos-
phorus compounds, including phosphates, phos-
phonates and phosphine oxides, in some cases
containing various additional functional groups, which
affect the coordination type of the extractant with the
substrate [7, 8]. However, the arsenal of reagents used
in the extraction technologies of the trace elements
does not practically involve such highly effective and
selective liquid membrane metal carriers as
aminophosphoryl compounds, although, as previously
shown, they can be successfully used in the process of
liquid membrane extraction of the alkali, alkaline-earth
[9], noble [10, 11] and rare-earth metal ions. Recently,
the study of the processes of the solvent and liquid
membrane extraction of scandium and other rare-earth
and trace elements was reported [12, 13]. A large
number of aminophosphoryl compounds were found to
be useful for the extraction of metal ions, in particular
scandium(III). The best results were achieved using
N-octyl-N,N-bis(dihexylphosphinylmethyl)amine
{Oct[HexP(O)CH ] N}. This work presents the results
2
2
of studying the processes of solvent extraction of
triply-charged ions of the trace elements indium,
yttrium, scandium, and gallium with diphosphinyl-
amine. In this series was included for comparison the
ferric ion, which often accompanies these metals.
The solvent extraction of the metal ions is usually
carried out from aqueous solutions of mineral acids
such as hydrochloric, nitric, or perchloric. Therefore,
the process of metals extraction may be accompanied
by the mineral acid co-extraction. As shown previously
[11], when studying the concentration dependence of
the extraction degree of these acids with the selected
reagent using toluene, chloroform or methylene
2
080

SOLVENT EXTRACTION OF SOME TRACE METALS AND IRON
chloride as diluents, the extractability of perchloric
2081
E, %
acid at the starting concentration 0.2 M increases until
a 1:1 solvate formation, and then remains unchanged
up to a concentration of 4 M At the same time, the
concentration of nitric acid in the organic phase
increases continuously with increasing its content in
the aqueous phase. The extraction isotherm of
hydrochloric acid reaches the limit at the concentration
in the aqueous phase equal to 6 M, with the formation
of the 1:1 complex. These results suggest that at low
concentrations of acids, the degree of the metals
extraction will be the maximum, since effect of the
competitive reactions of the extractant with the acid
will be insignificant The acid extractability decreases
in the following sequence: HClO > HNO > HCl,
4
3
according to the orientation of the anion hydration
energies: for perchlorate, nitrate and chloride anions
they are 209, 288 and 330 J mol , respectively [14].
c(HCl), M
–
1
Fig. 1. Dependence of the extraction degree of Ga(III),
Sc(III) and Fe(III) with 0.025 М Oct[Hex P(O)CH
toluene solution on the hydrochloric acid concentration. cFe
2
2 2
] N
Hydrochloric acid is uncommonly used in the trace
metals hydrometallurgy. Therefore, the effect of its
concentration in the aqueous phase on the extractability
with 0.025 M solution of N,N-bis(dihexyl-phos-
phinylmethyl)amine in toluene was studied only for
Ga(III), Sc(III) and Fe(III) ions. Figure 1 shows the
dependence of the ions extraction (E) on the hydro-
chloric acid concentration.
–
3
–3
–3
1
1:1.
0.28× 10 M; cGa 14×10 М; cSc 0.5×10 М; phases ratio
the inner coordination sphere of the Fe(III) the charge
transfer bands appear in the near-UV region. The
solutions were prepared with a certain concentration of
hydrochloric acid and the metal at a 1:1 ratio of the
volumes of two phases. For comparison, Fig. 2 shows
the absorption spectrum of the extract obtained by the
Fe(III) extraction with the well studied extractant
The extractability of scandium at the concentration
of hydrochloric acid up to 0.5 M exceeds the efficiency
of iron extraction, but with increasing acidity of the
aqueous phase up to 2.5 M and higher the
extractability of iron prevails. The reason for such
dependence may be associated with a change in the
extraction mechanism in the investigated range of
acidity, and attention should be paid to the similarity of
the curves character at the extraction of these metals
with tributylphosphite and trioctylamine [7, 15]. It can
be assumed that the complexation degree of these
cations differing in their atomic radius is not identical
in different acidity ranges. Under the selected condi-
tions, gallium is not practically extracted with the exa-
mined agent in the whole range of acidity of the
aqueous phase.
trioctylamine (Oct N).
3
It can be seen that the spectra of the Fe(III)
complexes with bisphosphinylamine and Oct N are
3
almost the same, suggesting the similarity of the
extraction mechanisms in a wide range of the acid
concentrations. According to [15], Oct N extracts
3
Fe(III) as an ionic associate I, formed by the
protonated trioctylamine tetrachloride and iron.
3+
–
+
+
–
4
Feaq + 4Claq + Haq + (Oct
3
N)org → [Oct
3
NH ·FeCl
]
org
.
I
Obviously, the process of Fe(III) extracting with
aminobisphosphinyl extractant at high concentrations
of hydrochloric acid can be expressed by the same
equation, leading to the appearance of the extracting
complex II.
To evaluate the nature of iron(III) complexation
with N,N-bis(dihexylphosphinylmethyl)amine at dif-
ferent concentrations of hydrochloric acid, the UV
spectra of the organic phase were examined in the field
near the maxima and minima of the iron extraction
curves (Fig. 1), corresponding to different concentra-
tions of the acid. Fe(III) chloride complexes are
colored, and with the incorporation of chloride ion into
As can be seen from Fig. 1, the curve of the
extraction of Fe dependence on the HCl concentration
is observed to decrease in the low concentration range,
then at cHCl ~2.5 M the extraction degree increases
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 81 No. 10 2011

2
082
CHERKASOV et al.
almost to 100%. Comparing this relation with the
absorption spectrum of the extracts, we can assume
that at low acid concentrations the chloride-ion does
not enter into the inner coordination sphere of the
iron. Probably, in this range the ionic associate III
containing the iron complex with bisphosphinylamine
and three chloride-ions [Eq. (1)] is transferred into the
organic phase.
P
P
O
O
_
Fe 3a +q
4Claq
H
aq
+
+
_
N
N H FeCl4
P O org
O
P
org
II
P
P
P O
O
O
_
_
(1)
3
+
Fe3+
2
N
P
Fe
3Claq
N
N
3Cl
aq
O
P
O org
O
P
org
III
The further increase in the concentration of
hydrochloric acid from 0.25 to 2.5 M causes the
accumulation of chloride iron complexes in the
aqueous phase. In the organic phase the free reagent
fraction is reduced because of co-extraction of
hydrochloric acid. The observed increase in the
extraction degree in this range can be associated with
the formation of mixed-ligand complex IV.
P
N
P
O
_
3
+
Fe
3Claq
aq
O org
_
P
N
Cl
ε
O
_
Fe3+
Cl
_
O
Cl
P
org
IV
A maximum of the band in the UV spectrum, which
arises due to the Fe→Cl charge transfer in the inner
coordination sphere, corresponds to this state of the
extracted complex. Further increase in the acid
concentration leads to the transfer of the iron ion as a
complex II into the organic phase.
λ, nm
Fig. 2. The UV spectrum of Fe(III) complexes extracted
The calculation of the metals partition coefficients
the ratio of their partition coefficients) with respect to
the most effectively extracted element scandium shows
a small value for the scandium/iron pair. The
selectivity of the same scandium/gallium pair varies
with N-octyl-N,N-bis(dihexylphosphinylmethyl)amine. Vоrg/Vaq
=
(
1
1
, с 0.025 M, diluent toluene. (1) cHCl 0.33 M, cFe 4.5×
–
4
–
4
0
M; (2) cHCl 0.87 M, cFe 4.5×10 M; (3) cHCl 1.5 M,
–4 –4
c
Fe 2.3×10 M; (4) cHCl 3.1 M, cFe 0.9×10 M; (5) cHCl
–
4
–
4
4
.5 M, cFe 0.9×10 M; and (6) cHCl 4.5 M, cFe 0.9×10 M.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 81 No. 10 2011

SOLVENT EXTRACTION OF SOME TRACE METALS AND IRON
2083
E, %
E, %
3
c(HNO ), M
3
c(HNO ), M
Fig. 3. Dependence of the extraction degree of In(III),
Ga(III), Sc(III) and Fe(III) with 0.025 M Oct[Hex P(O)CH
toluene solution on the nitric acid concentration. cIn 5×
Fig. 4. Dependence of the extraction degree of In(III),
Sc(III) and Fe(III) with 0.025 M Oct[Hex P(O)CH
methylene chloride solution on the nitric acid con-
2
2
]
2
N
2
2 2
] N
–
3
–3
–3
–3
–3
–3
1
0
M;
c
Ga 14×10 M; cSc 5×10 M; cFe 5×10 M;
centration. сIn 8.75×10 M; сSc 19.76×10 М; cFe
–3
phases ratio 1:1.
16.04×10 М, phases ratio 1:1.
greatly at different acid concentrations and increases
from 17 at cHCl 0.4 M to 627 at 0.28 M with a further
decrease to 37.8 at 1.12 M These results are well
illustrated by the dependences shown in Fig. 1. However,
as expected, the separation efficiency in hydrochloric
acid is low.
VI + HNO3aq
+
–
3
→
({Oct[Hex
2
P(O)CH
2
]
2
NH · ·2HNO
3
}·NO
)
org
.
VII
In addition, it is well known that the nitrate ion can
enter into the inner coordination sphere of the
extracted metal ion [15].
Nitric acid medium is most frequently used in
hydrometallurgical processes for obtaining and
purification of the trace elements. We investigated the
effect of nitric acid concentration on the extraction
degree of In(III), Y(III), Ga(III), Sc(III) and Fe(III)
with the reagent solutions in toluene, methylene
chloride or chloroform. These relationships are shown
in Figs. 3–5.
Not monotonous change of the extraction of metals
dependence on the nitric acid concentration indicates
E, %
Nitric acid extraction systems are the most difficult
to interpret, since in contrast to HCl, nitric acid itself is
extracted into the organic phase by three types of
extracting particles: ionic associate, formed as a result
of bisphosphinylamine V protonation, and two solvates
(H-comp-lexes) VI and VII, which are formed by the
sequential binding of nitric acid with the phosphine
oxide groups [11].
+
–
{
→
Oct[Hex
{Oct[Hex
2
P(O)CH
2
]
2
N}org + Haq + NO3aq
3
c(HNO ), M
+
–
3
2
P(O)CH
]
2 2
NH ·NO
}
org
,
Fig. 5. Dependence of the extraction degree of Y(III),
Sc(III) and Fe(III) with 0.025 M Oct[Hex P(O)CH
chloroform solution on the nitric acid concentration. с
V
2
2 2
] N
+
–
3
V + HNO3aq → ({Oct[Hex
2
P(O)CH
2
]
2
NH ···HNO
3
}·NO
)
org
,
Y
–
3
–3
–3
9
.8×10 M; сSc 19.76×10 М; cFe 16.04×10 М, phases
VI
ratio 1:1.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 81 No. 10 2011

2
084
CHERKASOV et al.
of extraction from the aqueous nitric acid media is also
E, %
lower than from hydrochloric acid. However, it is
characteristic that, as in hydrochloric acid media, at the
high acid concentrations the Fe(III) ion begins to be
extracted better than Sc(III). The similarity of these
two acidic media is found in the calculation of the
metals partition coefficients relative to scandium: they
are small in almost all used range of acidity in a
scandium/iron pair and reach significant values (100–
3
00) in the range of moderate acidity for the three
other studied elements, indium, gallium and yttrium, in
all three diluents.
As shown earlier, the highest efficiency and
selectivity of the liquid-liquid extraction with
aminophosphinyl extractants can be achieved with use
of perchloric acid aqueous solutions as a releasing
phase [11]. The wide use of perchlorate aqueous media
in the hydrometallurgical extraction processes is due to
the fact that the perchlorate ion being the least
hydrophilic inorganic anion provides a relatively high
degree of the metal ions transfer into the organic
phase. We studied the effect of perchloric acid
concentration on the extraction degree of the studied
triply-charged cations. The dependences obtained are
shown in Figs 6–8.
4
c(HClО ), M
Fig. 6. Dependence of the extraction degree of In(III),
Y(III), Ga(III), Sc(III) and Fe(III) with 0.025
Oct[Hex P(O)CH N toluene solution on the perchloric
acid concentration. сIn 3.42×10 M; с
M
2
2 2
]
–
3
–3
Y
1.02×10 M; сGa
–3
–
3
–3
8
.25×10 M; сSc 9.8×10 M; с(Fe) 14.54×10 M, phases
ratio 1:1.
the complexity of the process occurrence at different
pH values of the aqueous phase. This confirms the
above assumption about the probable structure of the
complexes extracted under these conditions. We can
assume that in general the mechanism of extraction
from nitric acid medium involves the formation of the
complex particles with the same stoichiometry as for
hydrochloric acid: at low acid concentrations the metal
The extractability sequence Sc > In > Fe > Y > Ga
in the range of low acidity does not coincide with a
change in the ionic radii of extractable ions, which are
3
+
(
Å): 0.89, 0.94, 0.79, 1.04, 0.76 [16]. In the high
structures are probably formed [M {Oct[Hex P(O)·
2
3
+
–
3
acidity range the elements of a smaller radius are
somewhat better extracted: Sc > Fe > Ga > Y >
In. This tendency is characteristic for the metals
extraction with neutral organophosphorus reagents [7].
CH ] N} ] ·3NO ; in the media with moderate acidity
2
2
2
3
+
(
1–4 M) the mixed-ligand complexes [M {Oct[Hex ·
2
–
3 3
P(O)CH ] N}(NO ) ], and in the strongly acidic media
2
2
the more complex ionic associate (Oct[Hex P(O)·
2
+
3+
–
–
CH ] N·2HNO ) ·[M (NO ) ] .
2
2
3
3 4
These results demonstrate the similarity in the
nature of the dependence between the degree of ions
extraction and perchloric acid concentration in various
diluents, suggesting the unity of the extraction
mechanisms for all the used organic phases. The
degree of scandium extraction in all cases is extremely
high in toluene and methylene chloride in whole of
acidity range of the aqueous phase. In chloroform it
increases with the increasing concentration of
perchloric acid. Noteworthy that in these conditions, in
contrast to hydrochloric and nitric acid media, there is
a very noticeable extraction degree for indium and
gallium, which reaches 30–50%. For all the
investigated metals the best extractability was ob-
served in the less polar toluene as compared to
methylene chloride and chloroform.
The strong influence of the diluent polarity should
be noted: in the non-polar toluene the extractability
increases at low acidity, and it falls at the higher
acidity. In the more polar methylene chloride a smooth
decrease in the extraction degree of scandium and the
abrupt increase in the extraction degree of iron were
observed with increasing nitric acid concentration in
the aqueous phase. In chloroform, the extraction
degree of these metals increases as acid concentration
increases. The available data do not allow us to
interpret confidently the nature of the solvate effects.
One can only state the growth f the ion associates
extractability using polar diluents. The extraction
degree of the In(III), Y(III) and Ga(III) ions is much
lower than that for Sc(III) and Fe(III). The efficiency
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 81 No. 10 2011

SOLVENT EXTRACTION OF SOME TRACE METALS AND IRON
2085
E, %
E, %
4
c(HClО ), M
4
c(HClО ), M
Fig. 7. Dependence of the extraction degree of In(III),
Y(III), Ga(III), Sc(III) and Fe(III) with 0.025
Oct[Hex P(O)CH N methylene chloride solution on the
perchloric acid concentration.
Fig. 8. Dependence of the extraction degree of In(III),
Y(III), Ga(III), Sc(III) and Fe(III) with 0.025
Oct[Hex P(O)CH N chloroform solution perchloric acid
concentration. сIn 6.83×10 M; с
8.25×10 M; сSc 19.77×10 M; сFe 14.54×10 M, phases
ratio 1:1.
M
M
2
2
]
2
2
2 2
]
–
3
–3
–3
–3
с
In 8.19×10
M;
с
Y
Y
10.25×10 M; сGa
–3
–
3
–3
–3
–3
1
1
0.25×10 M; сGa 8.25×10 M; сSc 19.77×10 M; сFe
4.54×10 M, phases ratio 1:1.
–
3
Based on these experimental results, were
calculated the partition coefficients of the metals in
relation to the scandium in a wide range of perchloric
acid concentrations in the aqueous phase for all the
used diluents. The highest selectivity was observed for
the scandium in a water–toluene system (Table 1) at
EXPERIMENTAL
The diluents, nitrates and chlorides of the
investigated metals of chemical or analytical grade
were used. The extractant solutions in the organic
solvents were prepared using the accurately weighted
c(HClO ) 0.05–0.1 M, reaching the highest values in
4
the Sc(III)/Ga(III) pair. In the same concentration
range of perchloric acid the higher selectivity relative
to the scandium was revealed in methylene chloride.
At high acid concentrations (~2 M) there is a
selectivity growth in the Sc(III)/M(III) pairs for all the
investigated cations (Table 2). In chloroform the
selectivity (2–200) for all the Sc(III)/M(III) is
relatively low, and only at high concentrations of
perchloric acid (0.6–2 M) a moderate selectivity (200–
Table 1. Partition coefficients [Kpart = DMet/DMet] of the
metals at the extraction with 0.025 M solution of
N-octyl-N,N-bis(dihexylphosphinylmethyl)amine in toluene
(Vaq/Vorg = 1)
c(HClO
4
),
Sc(III)/
In(III)
232
Sc(III)/Fe(III)
Sc(III)/Y(III) Sc(III)/Ga(III)
M
0
0
0
0
0
.0049
22.7
110
3400
150
174
950
.0107
.0214
.0488
.108
12.5
17.6
296
6
00) was achieved.
330
347
908
The obtained results indicate a high efficiency of
77000
81300
1260
337
7230
77000
2110
2470
5150
1390
2890
194000
116000
6310
3300
1380
379
extraction with N-octyl-N,N-bis(dihexylphosphinyl-
methyl)amine, which is suitable for the extraction of
the studied trace elements ions from the aqueous nitric
and perchloric acid media with an enhanced selectivity
toward scandium. Taking into account the relative
synthetic availability of this aminophosphine oxide
reagent and its hydrolytic and thermal stability, it can
be use in the processes of extraction, concentration and
separation of the trace metal ions.
4980
259
0.191
0
0
1
1
.243
.954
.33
331
183
1520
1220
3850
113
.91
172
594
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 81 No. 10 2011

2
086
CHERKASOV et al.
Table 2. Partition coefficients [Kpart = DMet/DMet] of the
metals at the extraction with 0.025 M solution of N-octyl-
N,N-bis(dihexylphosphinylmethyl)amine methylene chloride
valent amount of alkali. The titration curves were
plotted in integral and differential forms, and on the
basis of the acid concentration data in the both phases
the extraction isotherms were plotted.
(Vaq/Vorg = 1)
c(HClO
4
), Sc(III)/
Fe(III)
The metal ions extraction was performed at a 1:1
ratio of the phases, 2 ml each phase. in glass tubes. The
mixing was carried out mechanically, after stirring for
Sc(III)/In(III) Sc(III)/Y(III) Sc(III)/Ga(III)
M
0
0
0
0
0
0
0
1
1
.0049
4.92
7.63
40.1
16.8
7.97
69.6
162
437
133
3
0 min the samples of the aqueous and organic phases
.0195
.0488
.0977
.243
.572
.954
.52
were acquired and centrifuged for 5 min.
12.0
16.5
51.5
122
225
375
The method for the synthesis and characterization
of N-octyl-N,N-bis(dihexylphosphinylmethyl)amine is
described in [18].
33.7
30.5
235
185
135
8770
208
95.4
411
42200
91700
200
67600
ACKNOWLEDGMENTS
11.4
17.7
57.0
This work was financially supported by the Russian
Foundation for Basic Research (grant no. 10-03-
676
154
0
0580).
.91
3720
64100
11500
REFERENCES
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determined by the complexometric titration [17]. To
produce the required acidity in aqueous solution was
added hydrochloric, perchloric or nitric acid of
chemically pure grade. The working solutions were
prepared in distilled water.
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4
The experimental studies were performed using an
AVU 6c mechanical shaking devise, an OP 8UKhL4.2
centrifuge and a 3070E Rigaku X-ray fluorescence
spectrometer (Japan). The extraction degree was
determined by measuring the metals concentration in
aqueous and organic phases by X-ray fluorescence
spectroscopy using the method given in [11].
5
6
7
The extraction of the inorganic acid solution was
carried out with N-octyl-N,N-bis(dihexylphosphinyl-
methyl)amine in chloroform, methylene chloride and
toluene (0.05 M) using glass test tubes at the stirring
for 30 min. The phase separation was carried out by a
five-minute centrifugation at 2000 rpm. To a 2 ml
aliquot in a glass cell was added 20 ml of water–
isopropanol mixture (1:3). The acid concentration in
the organic phase was determined by the
potentiometric titration with 0.03 M alkali solution in a
water–isopropanol mixture on the calculated equi-
1
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aminami,
solyami
aminov
i
chetvertichnykh
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