Magnetic Separation of Metal Ions

Article abstract of DOI:10.1021/jp030688c

The magnetic separation was investigated for Co²⁺ (9500 ?? 10^-6 cm³ mol^-1) and Fe^3- (14600 ?? 10^-6 cm³ mol^-1) ions and for Cr ³⁺ (6200 ?? 10^-6 cm³ mol^-1) and Al³⁺ (-2 ?? 10^-6 cm³ mol ^-1) ions. The metal ion solutions were spotted on a silica gel support, and exposed to a magnetic field of 410 kOe² cm^-1 intensity ?? gradient. The Co²⁺ ions move farther toward the maximum field than the Fe³⁺ ions. The result is explained by the fact that the Fe³⁺ ions are adsorbed more strongly on the silica gel surface than the Co²⁺ ions. The Cr³⁺ ions move farther toward the field center than the Al³⁺ ions. This occurs because the Cr³⁺ ions are attracted more strongly by the magnetic force than the Al³⁺ ions. It is demonstrated that the separation makes effective use of the adsorption activities as well as the magnetic susceptibilities.

Full text of DOI:10.1021/jp030688c

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14374
J. Phys. Chem. B 2003, 107, 14374-14377
Magnetic Separation of Metal Ions
K. Chie,† M. Fujiwara,*,‡ Y. Fujiwara,† and Y. Tanimoto*,‡
Graduate School of Science, Hiroshima UniVersity, Kagamiyama, Higashi-Hiroshima 739-8526, Japan, and
Institute for Molecular Science, Myodaiji, Okazaki 444-8585, Japan
ReceiVed: May 30, 2003
The magnetic separation was investigated for Co2+ (9500 × 10-6 cm3 mol-1) and Fe3+ (14600 × 10-6 cm3
mol-1) ions and for Cr3+ (6200 × 10-6 cm3 mol-1) and Al3+ (-2 × 10-6 cm3 mol-1) ions. The metal ion
solutions were spotted on a silica gel support, and exposed to a magnetic field of 410 kOe2 cm-1 intensity ×
gradient. The Co2+ ions move farther toward the maximum field than the Fe3+ ions. The result is explained
by the fact that the Fe3+ ions are adsorbed more strongly on the silica gel surface than the Co2+ ions. The
Cr3+ ions move farther toward the field center than the Al3+ ions. This occurs because the Cr3+ ions are
attracted more strongly by the magnetic force than the Al3+ ions. It is demonstrated that the separation makes
effective use of the adsorption activities as well as the magnetic susceptibilities.
1. Introduction
separation experiment, the test solutions were made by mixing
(1/1) the Al3+ and Cr3+ ion solutions and by mixing (1/1) the
In order for particles to be separated by a magnetic field, it
is necessary that the magnetic force acting on the particles is
more effective than the thermal diffusion induced by a solvent
atmosphere. Therefore, the separation has been believed to be
possible, when the particles possess larger size so that the
magnetic energy is larger than the thermal energy. Many studies
have been focused on the separation of paramagnetic particles
(>1.0 µm size).
Fe3+ and Co2+ ion solutions. The test solutions (50 mm3) were
spotted on the silica gel support at (100 mm from the field
center. The glass vessel was placed in the magnet bore (374 ×
ø 50.4 mm), allowed to stand at 295 K for 4 h, and then taken
out of the magnet bore. For the adsorption experiment, the test
solutions were made by diluting (1/1) the metal ion solutions
with deionized water. The test solutions (50 mm3) were spotted
on the silica gel support, and allowed to stand under zero field
at 295 K for 4 h.
Recently, we have succeeded in the separation of Cu2+ (1500
× 10-6 cm3 mol-1)1 and Ag+ (-24 × 10-6 cm3 mol-1)2 ions
in a magnetic field.3 It may be useful to develop the method of
separating ions by the magnetic force, since ions have a wide
range of paramagnetic (<20000 × 10-6 cm3 mol-1) and
diamagnetic (> -40 × 10-6 cm3 mol-1) susceptibilities.
The preset paper deals with the separation of Co2+ (9500 ×
10-6 cm3 mol-1)1 and Fe3+ (14600 × 10-6 cm3 mol-1)1 ions
and of Cr3+ (6200 × 10-6 cm3 mol-1)1 and Al3+ (-2 × 10-6
cm3 mol-1)2 ions in a magnetic field. The effect of adsorption
is discussed for a silica gel support on which the metal ion
solutions are spotted.
The Al3+ ions were colored red (Al(C22H13O9)3) by a spray
of 0.1% ammonium aurintricarboxylate (Wako) solution, and
the Fe3+ ions brown (Fe(C9H6ON)3) by a spray of 0.5%
8-quinolinol (Wako, >99%) ethanol solution. The Cr3+ ions
were colored green (Cr(OH)3), and the Co2+ and Cu2+ ions blue
([Co(NH3)6]2+ and [Cu(NH3)4]2+) by a spray of ammonia
solution (Kanto, 28-30%). The Ag+ ions were colored gray
(Ag metal) by irradiation of an ultrahigh-pressure mercury lamp
(Ushio UI-501C, 500 W). The distributions of the metal ions
were recorded with a camera (Nikon Nikomat FTN) on photo-
graphs, which were copied with a scanner (Epson GT-5000ART)
into a personal computer (Apple 7600/132). The color-density
profiles of the metal ions were analyzed with an image-
processing program (NIH Image 1.55).
2. Experiment
The magnetic field was applied with a superconducting
magnet (Oxford Spectromag 1000). The field direction was
horizontal. The field intensity was 80 kOe at the field center,
and the field intensity × gradient was 410 kOe2 cm-1 at (65
mm from the field center.
As the metal salts, AlCl3‚6H2O (Wako, >98%), CrCl3‚6H2O
(Wako, >99%), FeCl3‚6H2O (Wako, >99%), CoCl2‚6H2O
(Wako, >99%), Cu(NO3)2‚3H2O (Wako, >99%), and AgNO3
(Wako, >99%) were used. The metal ion solutions (1.0 mol
dm-3) were prepared by dissolving the corresponding metal
salts in deionized water. The silica gel support (Wako C-200,
75-150 µm, 27 g) was saturated with deionized water (36 cm3),
and laid in a glass vessel (390 × 40 × 10 mm). For the
3. Results and Discussion
3.1. Separation of Co2+ and Fe3+ Ions and of Cr3+ and
Al3+ Ions. The separation in the magnetic field was studied for
Co2+ (9500 × 10-6 cm3 mol-1)1 and Fe3+ (14600 × 10-6 cm3
mol-1)1 ions and for Cr3+ (6200 × 10-6 cm3 mol-1)1 and Al3+
(-2 × 10-6 cm3 mol-1)2 ions. The metal ion solutions were
spotted on the silica gel support at 100 mm from the field center,
and exposed to the magnetic field of 410 kOe2 cm-1 intensity
× gradient.
3.1.1. The Distributions. The distributions [cM(z)] of the Co2+
and Fe3+ ions measured in the field direction [z] from the spot
position are shown in Figures 1a and 2a. The Co2+ ions move
to a larger distance toward the field center than the Fe3+ ions.
* Corresponding authors. E-mail: fujiwara@sci.hiroshima-u.ac.jp;
tanimoto@ims.ac.jp.
† Graduate School of Science, Hiroshima University.
‡ Institute for Molecular Science.
∞
∞
The average moving distances [∫-∞ zcM(z) dz/∫-∞ cM(z) dz]
10.1021/jp030688c CCC: $25.00 © 2003 American Chemical Society
Published on Web 12/02/2003

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Magnetic Separation of Metal Ions
J. Phys. Chem. B, Vol. 107, No. 51, 2003 14375
are 58 mm for the Co2+ ions and 27 mm for the Fe3+ ions.
When the whole area is divided with a line at 45 mm, the purity
of the Co2+ ions is ∼82 mol % in the region of >45 mm, and
the purity of the Fe3+ ions is ∼82 mol % in the region of <45
mm. It is noted that the separation of the Co2+ and Fe3+ ions is
good.
The distributions of the Cr3+ and Al3+ ions are given in
Figures 1b and 2b. The Cr3+ ions move with sharp width toward
the field center, while the Al3+ ions follow them with tailing.
The average moving distances are 57 mm for the Cr3+ ions and
34 mm for the Al3+ ions. The purity of the Cr3+ ions is ∼77
mol % in the region of >51 mm, and the purity of the Al3+
ions ∼77 mol % in the region of <51 mm. The separation of
the Cr3+ and Al3+ ions is possible.
The separation of the Cu2+ (1500 × 10-6 cm3 mol-1)1 and
Ag+ (-24 × 10-6 cm3 mol-1)2 ions has been reported recently.3
The Cu2+ ions move by ∼48 mm toward the field center, but
the Ag+ ions stay at the spot position. The purity of the Cu2+
ions is ∼90 mol % in the region of >10 mm, and the purity of
the Ag+ ions ∼90 mol % in the region of <10 mm.
3.1.2. Magnetic MoVement. The movement of the Co2+ (9500
× 10-6 cm3 mol-1)1, Cr3+ (6200 × 10-6 cm3 mol-1)1, and Cu2+
(1500 × 10-6 cm3 mol-1)1 ions is understood by the magnetic
force acting on these paramagnetic metal ions. Since the
magnetic force works on the paramagnetic metal ions in the
direction where the field intensity increases, they are attracted
toward the field center.
Figure 1. Separation of (a) Co2+ and Fe3+ ions and of (b) Cr3+ and
Al3+ ions on silica gel support. The metal ion solutions were spotted
at the 0 mm position which was 100 mm from the field center.
The observation that the Fe3+ (14600 × 10-6 cm3 mol-1 1
)
for the Ag+ ions and 12.2 mm for the Cu2+ ions. The finding
is consistent with the fact that the Ag+ ions stay at the spot
position, when the Cu2+ ions move toward the maximum field.3
The Fe3+ ions are adsorbed more strongly on the silica gel
support and diffuse less easily by the thermal fluctuation than
the Co2+ ions. The average diffusion distances are 11.1 mm
for the Fe3+ ions and 14.9 mm for the Co2+ ions. The result
accounts for the observation that the Fe3+ ions move to a smaller
distance in the magnetic field than the Co2+ ions (Figure 2),
though the Fe3+ ions have a larger paramagnetic susceptibility
than the Co2+ ions. It should be mentioned that, in the separation
of the Co2+ and Fe3+ ions, the adsorption on the silica gel sup-
port is more effective than the attraction by the magnetic field.
The Al3+ ions are not adsorbed strongly on the silica gel
support, and diffuse most easily in Figure 4. The average
diffusion distances are 16.2 mm for the Al3+ ions and 11.5 mm
for the Cr3+ ions. This explains why the diamagnetic Al3+ ions
follow the Cr3+ ions easily in the magnetic field (Figure 2).
3.2.2. Adsorption ActiVities and Magnetic Susceptibilities. The
smaller diffusion distances [Ag+ (6.4 mm) < Fe3+ (11.1 mm)
< Al3+ (16.2 mm)] by the stronger adsorption in Figure 4
correspond to the smaller moving distances [Ag+ (∼0 mm) <
Fe3+ (27 mm) < Al3+ (34 mm)] by the magnetic force in Figure
2 and ref 3. Surprisingly, the paramagnetic Fe3+ ions move to
a smaller distance in the magnetic field than the diamagnetic
Al3+ ions, though the Co2+ ions (which are followed by the
Fe3+ ions) move to a larger distance than the Cr3+ ions (followed
by the Al3+ ions). The adsorption pulls the Fe3+ ions back from
the Co2+ ions. The result would be extended to general applica-
tions; the silica gel support is effective to separate paramagnetic
ions with different adsorption activities in the magnetic field.
Although the Al3+ ions are adsorbed less strongly on the silica
gel support than the Cr3+ ions in Figure 4, the diamagnetic Al3+
ions move to a smaller distance in the magnetic field than the
paramagnetic Cr3+ ions in Figure 2. As a result, the Al3+ ions
are separated from the Cr3+ ions by the magnetic force. The
observation makes a marked difference from the one in the usual
ions move to a smaller distance in the magnetic field than the
Co2+ ions is not explained by the fact that the Fe3+ ions have
a larger paramagnetic susceptibility than the Co2+ ions. The
Fe3+ ions must be attracted more strongly by the magnetic force
than the Co2+ ions. The discrepancy between the movement
and susceptibility for the Fe3+ ions is resolved, if the effect of
the adsorption of the silica gel support is taken into account
(section 3.2). The Fe3+ ions are adsorbed more strongly on the
support, and cannot move to a larger distance in the field than
the Co2+ ions.
The movement of the Al3+ (-2 × 10-6 cm3 mol-1)2 ions in
the magnetic field seems to be curious, because they are
diamagnetic. When only the Al3+ ions are exposed to the field,
they do not move. However, it has been shown that ions with
a paramagnetic susceptibility move by the magnetic force, not
as a single particle, but as a large group composed of the para-
magnetic ions and water molecules.4 When the group is formed
out of the Cr3+ ions, Al3+ ions, and water molecules, it becomes
paramagnetic as a whole and is attracted toward the field center.
The result that the Ag+ (-24 × 10-6 cm3 mol-1)2 ions stay
at the spot position in the magnetic field is understandable for
two reasons. First, the Ag+ ions have a small diamagnetic
susceptibility, and are not affected by a weak magnetic force.3
Second, the Ag+ ions are adsorbed most strongly on the silica
gel support, and do not move in any direction (section 3.2).
3.2. Adsorption on Silica Gel Support. The adsorption on
the silica gel support was studied for the Cu2+, Ag+, Co2+, Fe3+
,
Cr3+, and Al3+ ions. The metal ion solutions were spotted on
the silica gel support, and allowed to stand at zero field. The
distributions [cD(r)] of the Cu2+, Ag+, Co2+, Fe3+, Cr3+, and
Al3+ ions measured in the radial direction [r] are shown in
Figures 3 and 4.
3.2.1. Thermal Diffusion and Magnetic MoVement. The Ag+
ions are adsorbed most strongly in Figure 4 on the silica gel
support, and do not leave the spot position. The average diffu-
∞
∞
sion distances [∫0 rcD(r) 2πr dr/∫0 cD(r) 2πr dr] are 6.4 mm

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14376 J. Phys. Chem. B, Vol. 107, No. 51, 2003
Chie et al.
Figure 3. Adsorption of Cu2+, Ag+, Co2+, Fe3+, Cr3+, and Al3+ ions
on silica gel support. The metal ion solutions were spotted at the 0
mm position.
Figure 2. Distributions of (a) Co2+ and Fe3+ ions and of (b) Cr3+ and
Al3+ ions on silica gel support.
adsorption chromatography, where the less-adsorbed molecules
go farther than the more-adsorbed molecules. Generally, it is
demonstrated that the magnetic field is effective to separate
diamagnetic and paramagnetic ions, when the thermal diffusion
of ions is suppressed by contact with the silica gel support.
3.3. Relationship between Magnetic Movement and Ther-
mal Diffusion. The separation of the Co2+ and Fe3+ ions may
be understood by the relationship between the magnetically
moving distance and thermal-diffusion distance. The different
mechanisms of motions are that the magnetic movement takes
place as the motion of a group composed of metal ions and
water molecules,4 but that the thermal diffusion occurs as the
motion of a single metal ion.
3.3.1. Expressions for the Distances. For a group composed
of metal ions and water molecules, the distance z(t) of movement
at time t in a magnetic field H(z) measures the frictional
coefficient fG (of metal ion-water molecule group) and magnetic
force F(z), the latter of which is a function of a mole number
n and molar susceptibility ø of the metal ions.4
Figure 4. Distributions of Cu2+, Ag+, Co2+, Fe3+, Cr3+, and Al3+ ions
on silica gel support.
z(t) ) t [F(z)/f ] dt
∫
G
0
Approximately, the frictional coefficient fG of a metal ion-
water molecule group is inversely proportional to the distance
z(t) of movement.
For a single metal ion, the mean distance r(t) of diffusion
at time t in two-dimensional space is a measure of the diffusion
)
t(nø/f ) [H(z)∂H(z)/∂z] dt
(1)
∫
G
0
The parameter fG/nø is calculated by numerical integration.

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Magnetic Separation of Metal Ions
J. Phys. Chem. B, Vol. 107, No. 51, 2003 14377
coefficient D, which is related to the frictional coefficient fS
(of single metal ion) at temperature T.
The magnetic susceptibility nø for the group is assumed to
be the sum n1ø1 + n2ø2 for the component ions 1 and 2, since
the group is attracted as a whole by the magnetic force. The
frictional coefficient fG for the part of the surface is expected
to be proportional to the frictional coefficient fS for the single
ion, since the adsorption activity of the surface is similar to the
adsorption activity of the single ion. Then, the expectation is
in agreement with the above estimation of the ratios of the
frictional coefficients for the Co2+ and Fe3+ ions. This indicates
that the separation of the Co2+ and Fe3+ ions in a magnetic
field is achieved by the adsorption on silica gel particles.
The separation of the Cr3+ and Al3+ ions may not be
explained by the adsorption activities of the metal ions. The
observation is that, though the Cr3+ ions are adsorbed more
strongly on silica gel particles than the Al3+ ions, the paramag-
netic Cr3+ ions move to a larger distance in a magnetic field
than the diamagnetic Al3+ ions. When the Al3+ ions are adsorbed
on the silica particles, they may not be attracted by the magnetic
force and may not follow the magnetic movement of the Cr3+
ions.
r(t) 2 ) πDt
) πkTt/fS
(2)
where k is the Boltzmann constant. The frictional coefficient fS
of a single metal ion is inversely proportional to the square of
the mean distance r(t) of diffusion.
In the magnetic movement of the Co2+ and Fe3+ ions, the
parameter fG/nø is estimated from the average moving distances
in Figure 2. The denominator nø is assumed to be the same
value for the Co2+ and Fe3+ ions, since they move as a group
in the magnetic field. The ratio of the frictional coefficients
between the Co2+ and Fe3+ ions is
fG,Co/fG,Fe ) (fG,Co/nø)/(fG,Fe/nø)
) (0.84 × 1015 m-3 kg s-1)/(1.66 × 1015 m-3 kg s-1)
) 0.51
4. Conclusion
In the magnetic separation of metal ions, the adsorption
activities play an effective role as well as the magnetic
susceptibilities. The Co2+ (9500 × 10-6 cm3 mol-1) ions are
separated from the Fe3+ (14600 × 10-6 cm3 mol-1) ions, since
the Fe3+ ions are adsorbed more strongly on the silica gel
In the thermal diffusion of the Co2+ and Fe3+ ions, the
parameter fS/πkTt is given from the average diffusion distances
in Figure 4. The ratio of the frictional coefficients between the
Co2+ and Fe3+ ions is
support than the Co2+ ions. The Cr3+ (6200 × 10-6 cm3 mol-1
)
fS,Co/fS,Fe ) (fS,Co/πkTt)/(fS,Fe/πkTt)
ions are separated from the Al3+ (-2 × 10-6 cm3 mol-1) ions,
because the Cr3+ ions are attracted more strongly by the
magnetic field than the Al3+ ions.
) (14.9 mm)-2/(11.1 mm)-2
) 0.55
Acknowledgment. The work was supported partly by a
Grant-in-Aid for Scientific Research (C14540536) from Japan
Society for the Promotion of Science (JSPS).
3.3.2. Separation of More- and Less-Adsorbed Ions. Suppose
that a group (or sphere) contains more-adsorbed metal ions, less-
adsorbed metal ions, and water molecules. When the group is
moving in a magnetic field, it is attracted as a whole by the
magnetic force.4 The “friction” is considered to arise from the
adsorption on silica gel particles.5 Some part of the surface of
the group is covered with the more-adsorbed ions. If the more-
adsorbed surface is in contact with the silica particles, the friction
is caused by the more-adsorbed ions. The more-adsorbed surface
goes slowly and falls out from the group during the movement
in the magnetic field. After the magnetic movement, the resultant
distribution of the more-adsorbed ions reflects the friction of
the more-adsorbed surface. The reverse holds also for the less-
adsorbed ions.
References and Notes
(1) Koenig, E. In Landolt-Boernstein; Hellwege, K.-H., Hellwege, A.
M., Eds.; Springer-Verlag: Berlin, 1966; New Series, Vol. II/2, Chapter 2.
(2) Koenig, E.; Koenig, G. In Landolt-Boernstein; Hellwege, K.-H.,
Hellwege, A. M., Eds.; Springer-Verlag: Berlin, 1984; New Series, Vol.
II/12a, Chapter 1.1.6.
(3) Fujiwara, M.; Kodoi, D.; Duan, W.; Tanimoto, Y. J. Phys. Chem.
B 2001, 105, 3343.
(4) Fujiwara, M.; Chie, K.; Sawai, J.; Shimizu, D.; Tanimoto, Y. J.
Phys. Chem. B, submitted.
(5) The saying comes from the thought that, because the sphere (∼2.4
µm size)4 of metal ion-water molecule group makes a motion around the
particle (75-150 µm) of silica gel, the particle makes relatively the opposite
motion around the sphere.