The sorption properties of carbon nanotubes modified with tetraphenylmethylenediphosphine dioxide in nitric acid media

Article abstract of DOI:10.1134/S0036024408130116

The distribution of microamounts of La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and Y nitrates between aqueous solutions of HNO₃ and multiwalled carbon nanotubes noncovalently modified with tetraphenylmethylenediphosphine dioxide (L) wa

Full text of DOI:10.1134/S0036024408130116

ISSN 0036-0244, Russian Journal of Physical Chemistry A, 2008, Vol. 82, No. 13, pp. 2223–2226. © Pleiades Publishing, Ltd., 2008.
PHYSICAL CHEMISTRY
OF NANOTECHNOLOGIES
The Sorption Properties of Carbon Nanotubes Modified
with Tetraphenylmethylenediphosphine Dioxide
in Nitric Acid Media
A. N. Turanova, V. K. Karandashevb, N. K. Evseevaa, N. N. Kolesnikova, and D. N. Borisenkoa
a Institute of Solid-State Physics, Russian Academy of Sciences, Chernogolovka, Moscow oblast, 142432 Russia
b Institute of Microelectronic Technology and Ultra-High-Purity Materials, Russian Academy of Sciences,
Chernogolovka, Moscow oblast, 142432 Russia
e-mail: turanov@issp.ac.ru
Received October 1, 2007
Abstract—The distribution of microamounts of La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and Y
nitrates between aqueous solutions of HNO3 and multiwalled carbon nanotubes noncovalently modified with
tetraphenylmethylenediphosphine dioxide (L) was considered depending on the concentration of HNO3 in the
aqueous phase and L in the sorbent phase. Ln(III) ions were shown to pass to the sorbent phase in the form of
solvated nitrates Ln(NO3)3L3. The effectiveness of the extraction of such complexes decreases along the series
of rare-earth metals as the atomic number of the element in the Periodic Table increases.
DOI: 10.1134/S0036024408130116
INTRODUCTION
anchoring tetraphenylmethylenediphosphine dioxide
[11] and diphenyl(dibutylcarbamoylmethyl)phosphine
oxide [12] on the surface of fullerene black, an amor-
phous material prepared from the condensed product of
electric arc vaporization of graphite after the extraction
of fullerenes from it.
In recent years, the interest of researchers in the
physicochemical properties of carbon nanotubes
(CNTs) has increased. This is related to prospects for
using them in various science and technology areas, in
particular, as matrices for the preparation of catalysts
and sorbents [1]. The chemical modification of CNTs
during their oxidation causes coating of their surface
with carboxyl, carbonyl, and hydroxyl groups in a ratio
close to 4 : 2 : 1 [2]. This determines their sorption abil-
ity with respect to metal ions, which increases as the
concentration of acid groups on the surface of CNTs [3]
and pH of the aqueous phase [4] grow, as is character-
istic of the cation exchange mechanism of sorption of
metal ions. The use of complex-forming sorbents with
phosphonate, carbamoylmethylphosphonate, and other
groups offers promise for the extraction and concentra-
tion of transplutonium and rare-earth metals from
strongly acid solutions with a complex salt composi-
tion, including industrial radiochemical wastes [5–7].
The chemical modification of CNTs with such groups
is a complex task. A much simpler method for obtain-
ing complex-forming sorbents is noncovalent anchor-
ing of ligands with the corresponding functional groups
on the surface of CNTs.
The purpose of this work was to study the sorption
ability of CNTs noncovalently modified with tetraphe-
nylmethylenediphosphine dioxide with respect to
Ln(III) ions in nitric acid media.
EXPERIMENTAL
We used tetraphenylmethylenediphosphine dioxide
Ph2P(O)CH2P(O)Ph2 (L) synthesized according to [13].
Multiwalled carbon nanotubes were prepared as
described in [14] and purified by oxidation in air at
550°C [15]. A complex-forming sorbent was prepared
from CNTs placed into a test tube with a ground glass
stopper, to which a weighed amount of L and an aque-
ous 3 M solution of HNO3 were added. The mixture
was stirred at a rate of 60 rpm for 1 h. The sorbent was
separated by filtration and dried in air until its weight
ceased to change. Aqueous solutions of Ln(III) nitrates
were prepared by dissolving the corresponding oxides
in nitric acid of os. ch. (special purity) grade. The dis-
tribution of microamounts ofY, La, Ce, Pr, Nd, Sm, Eu,
Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu nitrates in sorption
systems was studied using model variable-concentra-
tion solutions of nitric acid. The initial concentration of
Aryl substituted methylenediphosphine dioxides are
highly effective reagents for the extraction of rare-earth
metals from nitric acid solutions [8–10], which is
related to bidentate coordination of dioxide molecules
in complexes with Ln(III) accompanied by the forma-
tion of six-membered rings in extraction complexes [8].
Earlier, we showed the possibility of noncovalently all the metals was 2 × 10–5 mol/l.
2223

2224
TURANOV et al.
–
where [L] and [L ] are the equilibrium concentrations
[L], mol/kg
of L in the aqueous and solid phases, respectively;
[L ]max is the maximum concentration of L in the sor-
bent phase under the conditions of the formation of a
monolayer on the surface of CNTs; and K is the con-
stant characterizing sorbent–sorbate equilibrium. The
0.4
0.2
K and [L ]max values determined graphically after the
linearization of Eq. (1) in the [L]/[L ]–[L] coordinates
were 1.58 × 103 and 1.15 mol/kg sorbent, respectively.
A similar distribution of neutral bidentate organophos-
phorus compounds was observed in their sorption with
macroporous polymer sorbents [17]. According to [18],
the sorbed molecules were oriented toward the poly-
meric matrix by their hydrophobic parts and toward the
aqueous phase by polar groups. The same orientation is
likely characteristic of the sorption of tetraphenylmeth-
ylenediphosphine dioxide on the surface of CNTs.
0
2
4
6
[L] × 104, mol/l
An increase in the concentration of HNO3 in the
aqueous phase favors the transfer of L into the sorbent
Fig. 1. Distribution of L between a 3 M solution of HNO
phase. For instance, an increase in cHNO from 0.1 to
3
3
and multiwalled carbon nanotubes.
5 mol/l is accompanied by a monotonic increase in the
distribution coefficient of L (DL) from 610 to 2070 at a
0.3 mol/kg initial concentration of L in the sorbent
Experiments with the sorption of metal ions were
performed under static conditions at 20 2°C. A sor-
bent sample (10 mg) was mixed with an aqueous solu-
tion (3–5 ml) for 1 h. It was shown in preliminary
experiments that this was sufficient for the establish-
ment of equilibrium in the system. Special experiments
also showed that nanotubes did not sorb Ln(III) from
nitric acid solutions in the absence of L.
The concentration of metal ions in the initial and
equilibrium aqueous solutions was determined mass
spectrometrically with sample ionization in an induc-
tively coupled plasma (ICP-MS). A PlasmaQuad (VG,
UK) mass spectrometer and the procedure described in
[16] were used. The concentration of L in equilibrium
aqueous solutions was calculated from the content of
phosphorus determined by the ICP-MS method. The
concentrations of rare-earth metals and L in the sorbent
phase were found using the material balance equation,
and distribution coefficients D were calculated as the
ratios between concentrations in equilibrium phases.
The concentration of HNO3 in the equilibrium aqueous
phase was determined by potentiometric titration with
a solution of alkali.
phase. A similar dependence of DL on cHNO was
3
observed in [11] for the distribution of L between aque-
ous solutions of HNO3 and fullerene black; this can be
related to the interaction of L with HNO3 [19] resulting
in the blocking of hydrophilic PO groups of the tet-
raphenylmethylenediphosphine dioxide molecule with
HNO3. The high DL values in the CNT–L–aqueous
solution of HNO3 system determine insignificant loss
of L related to the transfer of L into the aqueous phase
during the sorption concentration of Ln(III) in nitric
acid media.
At a constant concentration of HNO3 in the aqueous
phase, the effectiveness of the extraction of Ln(III)
increases as the concentration of L in the sorbent phase
grows. The slope of the logDLn–log[L] dependences is
close to 3 (Fig. 2), which corresponds to the extraction
of Ln(III) with modified CNTs in the form of coordina-
tion-solvated nitrates [20] as a result of the interphase
reaction
Ln3+ + 3NO–3 + 3L
Ln(NO3)3L3,
(2)
where the bar labels the sorbent phase components.
Ln(III) is sorbed from nitric acid solutions with
fullerene black modified with L [11] and extracted with
solutions of L in chloroform [9] as a complex of the
same stoichiometry.
RESULTS AND DISCUSSION
It follows from the data on the distribution of L
between the equilibrium aqueous and solid phases
(Fig. 1) that CNTs effectively sorb L.At a constant con-
centration of HNO3 in the aqueous phase, equilibrium
between phases over the range of L concentration stud-
ied can be described by the Langmuir equation
At a constant L concentration in the sorbent phase,
the dependences of DLn on the concentration of HNO3
in the equilibrium aqueous phase are curves with max-
ima (Fig. 3), which is related to the shift of equilib-
rium (2) to the right as the concentration of NO–3 ions
[L] = K[L]max[L](1 + K[L])–1,
(1)
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 82 No. 13 2008

THE SORPTION PROPERTIES OF CARBON NANOTUBES MODIFIED
2225
logDLn
logDLn
La
Ce
4.0
Pr
La
Ce
Pr
4.0
3.2
2.4
1.6
Nd
Sm
Nd
Sm
Eu
Gd
Eu
Gd
Tb
Dy
Ho
3.0
2.0
Er
Dy
Tm
Yb
Lu
Tm
–0.25
0
0.25
0.50
0.75
log[HNO3]
Fig. 3. Dependences of the distribution coefficients of rare-
earth metals on the concentration of HNO in the equilib-
rium aqueous phase for sorption with modified CNTs. The
initial concentration of L in the sorbent phase is 0.5 mol/kg.
3
Ho
Lu
Tb
1.00
ylcarbamoylmethyl)phosphine oxide [21] and L [22],
from moderate-concentration solutions in nitric acid.
This was explained [21] by an increase in the hydration
energy of Ln3+ ions as their ionic radii decreased (Z
increased) in the series of rare-earth metals [23].
0.75
0.50
–
–log[L]
Fig. 2. Dependences of the distribution coefficients of rare-
earth metals on the equilibrium concentration of L in the
sorbent phase for sorption from a 3 M solution of HNO .
3
When brought in contact with solutions of HNO3,
solid tetraphenylmethylenediphosphine dioxide trans-
increases and a decrease in the concentration of free L forms into a liquid because of the formation of
in the sorbent phase as a result of the interaction of L (HNO3)nL(H2O)m complexes [24]. The degree of
with HNO3 as the concentration of HNO3 in the aque- extraction of Ln(III) with such a liquid extractant is
much higher than when solutions of L in organic sol-
vents are used [25]. We compared the degree of the
extraction of Ln(III) with the liquid extractant and mod-
ified CNTs. At equal molar ratios between L and the
sum of Ln(III), the degree of extraction of Ln(III)
increases in the system with CNTs, especially notice-
ably for the sorption of heavy rare-earth metals and
yttrium (Fig. 4). According to [26], the effectiveness of
the extraction of Ln(III) from solutions in HNO3 in
ous phase grows [19]. The maxima shift to higher
HNO3 concentrations as the atomic number (Z) of the
rare-earth metal increases, from 2 mol/l (La–Eu) to
3 mol/l (Gd, Tb) and 5 mol/l (Dy, Ho) (Fig. 3). In the
sorption of Er, Tm, Yb, and Lu, DLn values increase
over the whole range of cHNO variations. This can be
3
related to the extraction of Ln(III) with modified CNTs
in the form of acido complexes [Ln(NO3)3 + p] · pLH as
cHNO in the aqueous phase increases, as with the extraction and sorption systems is determined by the
3
stability of Ln(III) complexes with organic reagents
extraction of Ln(III) and Am(III) with solutions of L in
and their distribution constants between phases. An
dichloroethane [10]. The La/Lu separation coefficient
increase in the effectiveness of the extraction of Ln(III)
decreases from 155 at cHNO = 1 mol/l to 18 at cHNO
=
3
3
in the system with CNTs can therefore be related to an
increase in the distribution constants of Ln(NO3)3L3
complexes in this system because of the enhanced
hydrophobic properties of this carbon matrix.
7 mol as cHNO in the aqueous phase grows.
3
Over the whole range of cHNO variations, we
3
observe a tendency toward a decrease in DLn from La to
Lu (Fig. 3). A similar DLn–Z dependence was reported
The data presented above show that carbon nano-
tubes noncovalently modified with tetraphenylmethyl-
for the extraction of rare-earth metals with bidentate enediphosphine dioxide have high sorption ability with
neutral organophosphorus compounds, diphenyl(dieth- respect to Ln(III) in nitric acid media and can be used
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 82 No. 13 2008

2226
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