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Effect of AC Electromagnetic Field on Zeta Potential of Calcium Carbonate

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The influence of an AC electromagnetic field on the interface potentials (zeta potential) of calcium carbonate in an aqueous solution was investigated. By using a suspension of calcium carbonate microparticles in an electrolytic solution, we investigated the changes in the zeta potential of the particles by treating them to a weak AC electromagnetic field at different frequencies; the frequencies ranged from 1 to 10 kHz and the magnetic field intensity did not exceed approximately 150 μT. The method adopted for the measurement of the zeta potential is a simple technique that involves observing migrating particles in a handmade electrophoretic cell under a microscope and calculating the potential from their mobility. Consequently, we found that treatment at a specific frequency drastically changes the zeta potential of the particles in addition to causing an inversion of polarity. From this result, we concluded that the drastic change observed at the solid‐liquid interface was due to the AC electromagnetic treatment at a specific frequency (several kilohertz or more). Since the interaction of water, ions, and calcium carbonate with a magnetic field is considerably small, the abovementioned drastic change could be due to an induced electric field accompanying the magnetic field variation. This change is considered to be due to the specific adsorption of anions in the solution on the particle interface.
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Effect of AC Electromagnetic Field on Zeta Potential of
Calcium Carbonate
Senshin Umeki1, 2, Haruki Shimabukuro2, Takashi Watanabe2, Takuya Kato2,
Shoji Taniguchi2 and Kazuyuki Tohji2
1Institute of Fluid Science, Tohoku University
2Graduate School of Environmental Studies, Tohoku University
(6-6-02, Aoba, Aramaki, Aoba-ku, Sendai, Miyagi 980-8579, JAPAN)
Abstract. The influence of an AC electromagnetic field on the interface potentials (zeta potential) of calcium carbonate
in an aqueous solution was investigated. By using a suspension of calcium carbonate microparticles in an electrolytic
solution, we investigated the changes in the zeta potential of the particles by treating them to a weak AC electromagnetic
field at different frequencies; the frequencies ranged from 1 to 10 kHz and the magnetic field intensity did not exceed
approximately 150 T. The method adopted for the measurement of the zeta potential is a simple technique that involves
observing migrating particles in a handmade electrophoretic cell under a microscope and calculating the potential from
their mobility. Consequently, we found that treatment at a specific frequency drastically changes the zeta potential of the
particles in addition to causing an inversion of polarity. From this result, we concluded that the drastic change observed
at the solid-liquid interface was due to the AC electromagnetic treatment at a specific frequency (several kilohertz or
more). Since the interaction of water, ions, and calcium carbonate with a magnetic field is considerably small, the
abovementioned drastic change could be due to an induced electric field accompanying the magnetic field variation.
This change is considered to be due to the specific adsorption of anions in the solution on the particle interface.
Keywords: zeta potential, calcium carbonate, water, electromagnetic treatment
PACS: 82.70.Dd
INTRODUCTION
In the last half century, several experiments have
been conducted to study the effects of magnetic (or
electromagnetic) fields on aqueous solutions. For
example, the magnetic field treatment of an aqueous
solution was shown to suppress scale formation in
water pipes1–5 and to affect the morphological changes
during crystallization6–12 and the solid-liquid
interface.13–16 However, these experiments were not
always reproducible and showed mutually conflicting
results.1 These studies were performed on aqueous
solutions that generally do not contain magnetic
substances. The effects persist even after the removal
of the magnetic field and the aqueous solutions behave
as though the water possesses a memory to remember
the magnetic field. Water is a representative
diamagnetic material and its magnetic susceptibility is
–9.07 10–9 m
3 kg–1 (20°C).17 Therefore, the
interaction between water and a magnetic field is so
small that it can be disregarded. Since water does not
have magnetic memory, such magnetic field effects
have been considered to be mysterious phenomena.
We investigated previous reports that provide
details on the effects of magnetic fields on aqueous
solutions. The effects of magnetic fields were found to
be remarkable when an alternating magnetic field was
applied to an aqueous solution containing ions,
particles, and other impurities. Several papers have
reported that an alternating magnetic field treatment
produced a magnetic field effect even for a weak
magnetic field intensity that is not considerably
different from the geomagnetic field intensity (50
T).5,10,18 According to recent reports, the magnetic
treatment of water that contains particles of another
substance changes the thickness of the adsorption layer
and the interface potential at the solid-liquid
interface.13–16 One report indicated that treatments
employing an alternating magnetic field were
effective.14 Studies on the effects of magnetic fields on
aqueous solutions include those dealing with
morphological changes in calcium carbonate during
crystallization.6–12 In such studies, the solid-liquid
interface status can be considered to be important. One
of the major issues in studies on magnetic field effects
is the prevention of scale adhesion in water pipes and
scale exchangers.1–5 The interface potential of scale
particles (mainly calcium carbonate particles) in water
is an important parameter in these studies. Therefore,
in the present study, we have investigated the
influence of a weak AC electromagnetic field on the
interface potential of CaCO3 particles in a liquid.
EXPERIMENT
By using a suspension of CaCO3 particles in an
electrolytic solution, we investigated the changes in
the zeta potential of the particles by AC
electromagnetic field treatment at a specific frequency.
An electrolyte was added to commercial ion-exchange
water (measurement: approximately 2 M cm).
Immediately before each measurement, the solution
was used to suspend CaCO3 powder, which was
thoroughly ground in a mortar. The pH of the
suspension was almost stable at 9.7 to 9.9 for 10-mM
KCl.
The AC electromagnetic field was generated by
using a “Function generator” (NF Co., Ltd.,
WF1944A) with a handmade coil (: 88 mm, l: 91 mm,
460 windings). The magnetic field intensity was
approximately 80 to 150 T at the center of the coil.
The experimental results in this paper are based on a
square-wave electromagnetic field of 1 to 10 kHz. If a
square-wave electromagnetic field is applied to the
abovementioned coil, a waveform disorder tends to
generate an electromagnetic field with an unknown
frequency. Therefore, the voltage applied to the coil
was appropriately adjusted to minimize any
unintended electromagnetic fields caused by disorders
at each frequency, and the target electromagnetic field
intensity was set to approximately 100 T. The
electromagnetic field was applied for three minutes to
the specimen, which was maintained still at room
temperature (22 to 24°C).
The method adopted for the measurement of zeta
potential is a simple technique that involves observing
migrating particles in a handmade electrophoretic cell
under a microscope and calculating the potential from
their mobility.19 The 40 (length) 20 (width) 1
(thickness) mm electrophoretic cell was fabricated by
affixing three pieces of appropriately cut Pyrex object
glass (Fig. 1) and a Pd wire (: 1 mm) was connected
to each end. The specimen solution was gently poured
into this cell. The cell was then placed horizontally in
an optical microscope and a voltage of 10 V was
applied between the electrodes. Particle
electrophoresis was carried out and recorded by a
CCD camera through the optical microscope. (Fig. 2)
An untreated specimen is measured first and is then
treated after exposure to an AC electromagnetic field
of a specific frequency. Subsequently, a similar
measurement is performed immediately.
FIGURE 1. Schematic illustration of the handmade
electrophoretic cell
FIGURE 2. Experimental procedure for observing
electrophoresis (“FG” denotes “Function generator”)
We chose 10 arbitrary particles from the image
obtained by the CCD camera and calculated the
mobility u from their average speeds. As the particles
have a radius large enough for the thickness of the
electric double layer, similar to colloidal particles,
Smoluchowski's formula given below is known to be
applicable to particles of arbitrary forms.20
u=
r
0
(1)
Here, 0, r, and denote the permittivity in
vacuum, relative permittivity, and viscosity of the
solution, respectively. By using this formula, the zeta
potential of the particles can be obtained.
RESULTS AND DISCUSSIONS
Figure 3 shows the changes in the zeta potential at
different treatment frequencies. It can be observed that
the treatment at 6 kHz or at a higher frequency
drastically changes the zeta potential of the particles
toward a negative value. The variation in the potential
is 10 mV or more, including the reversal in the polarity.
This is a very interesting phenomenon and clearly
depends on the varying frequency; it was reproduced
10 times in this study for each case.
FIGURE 3. Dependence of the zeta potential of CaCO3
particles in 10-mM KCl aqueous solution on the treatment
frequency
The charging of CaCO3 in the liquid can be
attributed mainly to the concentration of potential-
determining ions (Ca2+, CO32–, HCO3, H+, and OH)
in the solution.21 Since the pH of the solution used in
this experiment did not change after treatment, the
changes in the interface potential could be attributed to
a different charging mechanism. The specific
adsorption of anions on the particle interface may be
the charging mechanism producing such changes. The
electrolytic solution used in this study is 10-mM KCl,
and therefore most of the ions in the solution are Cl
and K+ ions. Cl ions in the liquid are weak in
hydration because of their large radius and they are
known to be more easily adsorbed onto a solid
interface as compared to the K+ ions.22 Therefore, the
changes in the zeta potential measured in this
experiment can be attributed to the specific adsorption
of Cl ions on the particle interface.
Figure 4 shows the zeta potential changes for the
untreated solution and the solution after treatment at 7
kHz for different densities of the KCl solution. This
graph indicates that the density of the electrolyte must
be above a certain level for the effect to occur. When a
similar experiment was conducted with KF (instead of
KCl) as the supporting electrolyte, the zeta potential
did not change significantly. Compared with Cl ions,
F ions are smaller in radius. Further, because of their
strong hydration structure, F ions are hardly
adsorbed.22 This also indicates that the changes in the
interface potential resulting from the alternating
electromagnetic field treatment can be attributed to the
adsorption of Cl ions.
The pH of the 10-mM KCl solution was adjusted
by adding HCl or NaOH. Figure 5 shows the zeta
potential at each pH in the absence of magnetic field
treatment and after treatment at 7 kHz. Over almost
the entire pH range, the zeta potential shifts toward
negative values and the isoelectric point (IEP) moves.
This convincingly proves the specific adsorption on
the particle interface.23
FIGURE 4. Variation in the zeta potential between the
untreated solution and the solution after treatment at 7 kHz
for different densities of the KCl electrolyte
FIGURE 5. Relationship between zeta potential and pH
(IEP: isoelectric point)
CONCLUSION
The above results confirmed the occurrence of a
drastic change in the interface potential of particles
due to AC electromagnetic field treatment at a specific
frequency (several kilohertz). This change was
attributed to the specific adsorption of anions, which
were easily adsorbed, in the solution. These results
indicate the essence of the mysterious magnetic field
effects on aqueous solutions. However, the mechanism
by which the treatment produces specific adsorption
on the particle interface continues to remain unknown.
Since the interaction of water, ions, and calcium
carbonate particles with a magnetic field is
considerably small, this could be due to an induced
electric field (rotE = –dB/dt) accompanying the
magnetic field variation. Apparently, this phenomenon
depends on the frequency. Therefore, an
electromagnetic field of several kilohertz was found to
affect the hydrated water structure of the solid-liquid
interface.
The results obtained in this study not only clarify
the effects of magnetic fields on aqueous solutions but
can also lead to completely new applications.
ACKNOWLEDGMENTS
This study was supported by the 21st Century COE
Program of Tohoku University (“International COE of
Flow Dynamics”), Tohoku Steel Co. Ltd., and the
“ARECS” program of the Dept. of Materials Science
and Engineering, Tohoku University. We also thank
Dr. S. Usui (Prof. Emeritus of Tohoku University), Dr.
I. Mogi (IMR, Tohoku University), Dr. N. Yoshikawa
and Dr. K. Matsumoto (Graduate School of
Environmental Studies, Tohoku University), Mr. K.
Okita (Tohoku Steel Co. Ltd.), Mr. H. Otani, and Mr.
T. Fujino (Techno Labo Ltd., Saitama, Japan) for
helpful discussions.
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IEP
... However, the mechanisms and effects of electromagnetic fields as they relate to scale deposition prevention have not been identified. Umeki et al. have investigated the effect of applying a weak alternating current electromagnetic field (magnetic flux density: ~150 µT) on the zeta potential of CaCO 3 particles in water 13,14 . The zeta potential of CaCO 3 particles dispersed in water changed as a result of the application of an electromagnetic field, and the magnitude of the change depended on the frequency of the electromagnetic field. ...
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