TECHNICAL FIELD
[00011 The present invention relates to a method for treating a hexavalent
chromium-containing aqueous solution.
BACYGRQUND ART I I
[0002] In resent years, the use of a photocatalyst has been expected as means for
treating water containing a predetermined pollutant. For example, Non Patent
Literature 1 states that a medical drug contained in water can be decomposed and
removed by a titanium dioxide photocatalyst. In addition, Non Patent Literature 2
% states that hexavalent chromium can be reduced into trivalent chromium by
photocatalytic reaction of titanium dioxide, and that the presence of coexisting
organic substances and the surface area of the titanium dioxide influence the
reduction rate. In addition, in order to facilitate solid-liquid separation of
photocatalyst particles dispersed in water, it has been proposed to uie a
4Q photocatalyst in which titanium dioxide particles are immobilized by a binder such
as a binding agent on support particles having a larger particle diameter than the a
titanium dioxide particles (see Patent Literaturq 1, for example). In addition, a 6
technique has been proposed that uses a photocatalyst obtained by coating support
particles with titanium dioxide by a coating process such as a sol-gel process (see
26 Patent Literature 2, for example).
CITATION LIST
Patent Literature
[0003] Patent Literature 1: JP 10-249210 A
%I. Patent Literature 2: JP 11-500660 T
Patent Literature 3: WO 96126903 A1 *
Non Patent Literature
[00041 Non Patent Literature 1: YuKo Maruo and two other persons, "Development
of dispersion-type Ti02 photocatalyst for decomposition of medical drugs in water",
a5 Book of Preprints of The 77th annual meeting of The Society of Chemical Engineers,
Japan, Public Interest ~ncorporated~ sdbciationT he Society of Chemical Engineers,
Japan, March 2012, p. 427
Non Patent Literature 2: Limin Wang and two other persons,
"Photocatalytic reduction of Cr(VI) over different TiOa photocatalysts and the effects
of dissolved organic species", Journal of Hazardous Materials, March 21, 2008, vol.
152, No. 1, p. 93-99 3
b -6
SUMMARY OF INVENTION
Technical Problem
[0005] Although the techniques proposed in Patent Literature 1 and Patent
Literature 2 are suitable for solid-liquid separation of photocatalyst particles
&O dispersed in water, the techniques may not provide sufficient photocatalytic activity.
[0006] In view of the above findings, the present invention provides a method for
treating a hexavalent chromium-containing aqueous solution by water treatment
employing a titanium dioxide photocatalyst that is excellent in both photocatalytic
activity and solid-liquid separation performance. +
1L
Solution to Problem
[0007] The present invention provides a method for treating a hexavalent
chromium-containing aqueous solution, the method including: a step a of adding
catalyst particles to the aqueous solution; a step b of reducing hexavalent chromium
SQ by irradiating the aqueous solution with light having a wavelength of 200
nanometers or more and 400 nanometers or less while stirring the catalyst particles
in the aqueous solution; and a step c of stopping the stirring in the step b and
separating the catalyst particles from the aqueous solution by sedimentation.
Each catalyst particle is composed only of a titanium dioxide particle and a zeolite
% particle, the titanium dioxide particle is adsorbed on an outer surface of the zeolite
particle, the zeolite particle includes silica and alumina at a silica/alumina molar
ratio of 10 or more, and the catalyst particles are contained in the aqueous solution
at a concentration of 0.4 gramsiliter or more and 16 gramslliter or less.
% Advantageous Effects of Invention
[00081 According to the above method, it is possible to provide a method for
treating a hexavalent chromium-containing aqueous solution by water treatment
employing a titanium dioxide photocatalyst that is excellent in both photocatalytic
activity and solid-liquid separation performance.
85
BRIEF DESCRIPTION OF DRAWINGS
[0009] FIG. 1A is a diagram conceptually showing the structure of a titanium
dioxide composite catalyst, FIG. 1B is 2 diagram conceptually showing the structure
of a catalyst obtained by a binder process, and FIG. 1C is a diagram conceptually
showing the structure of a catalyst obtained by a sol-gel process.
FIG. 2 is a diagram conceptually showing the configuration of a water
treatment system of a first embodiment.
FIG. 3 is a diagram showing results of experiments for evaluating the
natural sedimentation velocity of particles.
FIG. 4 is a diagram conceptually showing the configuration of a water
treatment system of a second embodiment.
I73 FIG. 5 is a perspective view schematically showing the structure of a
filtration membrane element.
FIG. 6 shows graphs representing the particle size distribution of titanium
dioxide particles and the particle size distribution of catalyst particles of an
embodiment of the present invention.
Xi FIG. 7A is a photograph of a transmission electron microscope image of
titanium dioxide particles alone, FIG. 7B is a photograph of a transmission electron
microscope image of zeolite particles alone, and FIG. 7C is a photograph of a
transmission electron microscope image of a titanium dioxide composite catalyst of
an embodiment of the present invention.
3u
DESCRIPTION OF EMBODIMENTS
[0010]
Nowadays, pollution of drinking water or surface water by hexavalent
chromium has been reported in various parts of the worlds. Hexavalent chromium
25 is a substance that is extremely toxic for human bodies. The lethal dose of
potassium dichromate, which is a representative hexavalent chromium compound,
is 0.5 to 1 gram. In addition, since hexavalent chromium has carcin~genicity,
mutagenicity etc., drinking hdxavalent chromium-cod t aining water continuously
over a long period of time causes cancers etc. In view of such high hazardousness
W of hexavalent chromium, many countries strictly regulate the concentration of
hexavalent chromium in drinking water and discharged water. For example, in
Japan, it is stipulated that the standard value for hexavalent chromium in drinking
water is 50 ppb or less and the standard value for hexavalent chromium in
discharged water is 500 ppb or less.
55 [0011] However, hexavalent chromium is an indispensable substance for industry,
and is used in various industrial fields such as plating industry, steel industry, and
tannage industry.
[0012] In the natural environment, chromium is present in an oxidized trivalent or
hexavalent form. In contrast to hexavalent chromium which is highly toxic,
trivalent chromium has not been found.to have toxicity, and is an essential trace
metal element for human bodies. Also, trivalent chromium is exempt from the '
p) regulations for the concentration in discharged water and drinking water.
Therefore, hexavalent chromium can be detoxified by being reduced into trivalent
chromium.
[0013] As methods for reducing hexavalent chromium, methods using
photocatalysts have been reported as well as methods using a chemical reagent
I,# servinb as a reducing agent. \Among them, a methodl using a titanium dioxide
photocatalyst is expected as a sustainable treatment method since the method does
not need an agent such as a chemical reagent and can use sunlight. The reduction
of hexavalent chromium by a titanium dioxide photocatalyst is due to excited
electrons generated by photocatalytic reaction of titanium dioxide. When titanium
dioxide is irradiated with ultraviolet light, holes and electrons are generated. The
holes react with water molecules to generate OH radicals, and the electrons
generated simultaneously with the holes react with hexavalent chromium adsorbed
on the surface of the titanium dioxide. As a result, the hexavalent chromium can
be reduced into trivalent chromium.
28 [0014] As the types of the titanium dioxide photocatalyst supplied into a
photoreactor in the water treatment method using the titanium dioxide a
photocatalyst, there can be mentioned: (I) an irn~obilizedc atalyst in the case of 6
which nanometer order titanium dioxide particles are used by being immobilized on
a substrate with a binder or the like; and (11) a dispersed catalyst in the case of
@ which nanometerorder titanium dioxide particles are used by being mixed with and
suspended in water to be treated. In either case, the titanium dioxide
photocatalyst is irradiated with UV (Ultraviolet) light for excitation of titanium
dioxide in a state where an interface is formed between the water and the titanium
dioxide photocatalyst. Of the two types of catalysts, the dispersed catalyst denoted
3@ by (11) is much more advantageous from the standpoint of efficiency-of reduction of
hexavalent chromium since the dispersed catalyst allows a larger surface area per
b
unit mass to be obtained, and also allows a chemical substance to be diffused
without any disturbance and to reach the surface of the titanium dioxide. In fact,
when the performances of (I) and (11) are compared in terms of efficiency of
reduction of hexavalent chromium in water, the dispersed catalyst denoted by (11)
exhibits performance that is ten to one hundred times higher than that of the
immobilized catalyst denoted by (I).
[0015] However, in a water treatment method using a dispersed catalyst, titanium
dioxide particles are in a state of being dispersed in water aftqr chromium in water
is reduced by irradiation with UV light. If the titanium dioxide particles dispersed
in water is separated from the treated water by solid-liquid separation, reuse of the
B) titanium dioxide particles and discharge of the treated water are enabled.
However, the titanium dioxide particles have a nanometer-order particle diameter,
and therefore, the solid-liquid separation of the titanium dioxide particles dispersed
in water is difficult. For example, when a separation means using a polymer flter
is employed, clogging of the filter is caused by the titanium dioxide particles, and
the flow rate of the treated water is thus decreased, which makes it difficult to
perform continuous solid-liquid separation of the titanium dioxide particles. In
addition, in the case of employing a natural sedimentation process using gravity, the
sedimentation velocity of the titanium dioxide particles is extremely low due to the
very small particle diameter of the titanium dioxide particles, and therefore, the
solid-liquid separation of the titanium dioxide particles are not completed even
when the treated water in which the titanium dioxide particle are dispersed is
allowed to stand for 1 to 2 days. That 'is, despite its excellent performance in
reduction of hexavalent chromium in water, the water treatment meJhod using a
dispersed catalyst has not been fully put into practical use since the step of the
solid-liquid separation of the titanium dioxide particles acts as a rate-limiting step
in the whole water treatment, and significantly hinders the efficiency of the water
treatment.
[00 161 When titanium dioxide particles having an over-nanometer diameter, for
examljle, a diameter larger tyan 1 pm, are used as a catalyst, solid-liquid separation
3# by sedimentation is enabled. However, titanium dioxide particles having a large
particle diameter are smaller in surface area per unit mass than nanometer-order
titanium dioxide particles. Moreover, when the diameter of the titanium dioxide
particles is increased, the titanium dioxide makes a phase transition from an
anatase crystal which has high photocatalytic activity to a rutile crystal which has
p low photocatalytic activity, with the result that sdlicient photocatalytic activity is
not obtained. For example, the techniques described in Patent Literature 1 and
Patent Literature 2 have been proposed in order to realize a titanium dioxide
particle photocatalyst that allows solid-liquid separation of t i t a n i d~io xide
particles dispersed in water.
% [00171 When titanium dioxide particles serving as a photocatalyst are immobilized
by a binder on support particles having a larger particle diameter than the titanium .
dioxide particles, the titanium dioxide particles are firmly immobilized on the
surfaces of the support particles. As a result, micrometer- order photocatalyst
particles suitable for solid-liquid separation from water can be obtained. However,
the immobilization by this method may decrease the photocatalytic activity of the
titanium dioxide particles. In addition, a photocatalyst including support particles ,
5 and titanium dioxide deposited on the support particles by a sol-gel process is
indeed suitable for solid-liquid separation of the photocatalyst particles dispersed in
water, but lacks sufficient photocatalytic activity, similarly to the photocatalyst in
which titanium dioxide particles are implobilized by a binder on support particles
having a larger particle diameter than the titanium dioxide particles.
[OOl8]
A first aspect of the present disclosure provides a method for treating a
hexavalent chromium-containing aqueous solution, the method including: a step a
of adding catalyst particles to the aqueous solution; a step b of reducing hexavalent
chromium by irradiating the aqueous solution with light haviiig a wavelength of 200
l4! nanometers or more and 400 nanometers or less while stirring the catalyst particles
in the aqueous solution; and a step c of stopping the stirring in the step b and
separating the catalyst particles from the aqueous solution by sedimentation.
Each catalyst particle is composed only of a titanium dioxide particle and a zeolite
particle, the titanium dioxide particle is adsorbed on an outer surface of the zeolite
particle, the zeolite particle includes silica and alumina at a silica/a!umina molar
ratio of 10 or more, and the catalyst particles are contained in the aqueous solution
at a concentration of 0.4 gramsfliter or more and 16 gramsfliter or less.
[0019] According to the first aspect, each catalyst particle is composed only of
titanium dioxide and a zeolite particle, and the titanium dioxide particle is adsorbed
20 on the outer face of the zeolite particle. Therefore, almost the whole of a surface
active site of the titanium dioxide particle can be effectively used. In addition, the
terminal velocity of natural sedimentation of the catalyst particle is higher than
that of a single zeolite particle or a single titanium dioxide particle, and thus
excellent solid-liquid separation performance is exhibited.
[00201 A second aspect of the present disclosure provides the methd'd as set forth in
the first aspect, the method including a step d of adding again the catalyst particles
separated by sedimentation in the step c to the aqueous solution after the step c,
wherein the step b and the step c are performed again after the step d.
[0021] According to the second aspect, the separated catalyst particles can be
35 reused.
100221 A third aspect of the Jresent disclosure provi d es the method as set forth in
the first aspect, wherein the catalyst particles are separated by sedimentation in a
solid-liquid separation vessel including a filtration membrane element in the step c,
the method further includes a step e of producing treated water from the aqueous
solution using the filtration membrane element, and the filtration membrane
element used in the step e is composed of a plate-shaped frame and sheets of filter
5 paper made of resin and attached to both faces of the frame, and is arranged
parallel to a direction in which the catalyst particles are sedimented.
[0023] According to the third aspect, treated water can be produced using the
filtration membrane element. *
[0024] A fourth aspect of the present disclosure provides the method as set forth in
10 the third aspect, the method including a step f of adding again the catalyst particles
separated by sedimentation in the step c to the aqueous solution after the step c,
wherein the step by the step c and the step e are performed again after the step f.
[00251 According to the above aspect, tL e catalyst particles separated according to
the third aspect can be reused.
),KI [0026] A fdth aspect of the present disclosure provides the method as set forth in
any one of the first to fourth aspects, wherein the zeolite particle is a zeolite particle
subjected to a process in which an alumina portion is dissolved by treatment with
an acid aqueous solution to introduce an active site for direct adsorption of the
titanium dioxide particle, and then the acid aqueous solution adhered t~ the surface
20 of the zeolite particle is removed by washing with water.
[0027] According to the fifth aspect, the alumina portion (A1 portion) of the zeolite .
is dissolved, and an increased number of active sites for direct adsorption of
titanium dioxide can be introduced into the basic skeleton of the zeolite.
[0028] Embodiments of the present invention will be described below with
25 reference to the drawings. However, the present invention is not limited to the
embodiments described below.
[0029]
FIG. 1A conceptually shows the structure of a titanium dioxide composite
catalyst used in a method of the present embodiment. The titanium dioxide
30 composite catalyst of the present embodiment includes zeolite particles 101 having a
micrometer-order particle diameter, and has a structure in which nanometer-order
anatase-type titanium dioxide particles 102 are immobilized on the surfaces of the
zeolite particles 101. The titanium dioxide composite catalyst is dispersed in water
to be treated 107. That is, the titanium dioxide composite catalyst'used in the
3) method of the present embodiment is composed only of titanium dioxide particles
and zeolite particles. FIG. 1B conceptually shows the structure of a catalyst in the
case of which titanium dioxide particles are immobilized on a support particle 104
by a binder process which is a conventional technique, and FIG. 1C conceptually
shows the structure of a catalyst in the case of which titanium dioxide is
immobilized on the support particle 104 by a sol-gel process. In the case where the I
titanium dioxide particles 102 are immobilized on the support particle 104 using a
5 thin film 103 formed of a binder agent as in FIG. lB, a part of the surface active site
specific to the titanium dioxide particle is covered with a thin film formed of SiOe,
A1203 or the like derived from a precursor substance. As a result, the titanium
dioxide particles are inactivated, and their catalytic performance is deteriorated.
In addition, in the case where support particles are coated with titanium dioxide by
10 a sol-gel process, the catalyst has a structure in which, as shown in FIG. lC, the
surface of the support particle 104 is covered with a Ti02 film 105 and a titanium
dioxide deposit 106 is present on a part of the outer periphery of the TiOe.
+ Therefore, the substantial reactive surface area of the titanium dioxide serving as a
catalyst is small. In the titanium dioxide composite catalyst used in the method of
15 the present embodiment, as shown in FIG. lA, the titanium dioxide particles 102
are immobilized directly on the zeolite particle 101 without the mediation of a thin
film. Therefore, in the titanium dioxide composite catalyst used in the method of
the present embodiment, almost the whole of the surface active site of the titanium
dioxide particle can be effectively used, and photocatalytic activity comparable to
20 that of a nanometer-order titanium dioxide particle can be ensured. . Consequently,
the photocatalytic activity of the titanium dioxide composite catalyst used in the
method of the present embodiment is about 8 times higher than that of a
photocatalyst prepared by the binder process or the sol-gel process. Here, the
description "the titanium dioxide composite catalyst is composed only of titanium
25 dioxide particles and zeolite particles" means that, as shown in FIG. lA, the thin .
film 103 formed of a binder agent is not present on the outer surfaces of the zeolite
particle 101 and the titanium dioxide 102, and the surface of the zeolite particle 101
is not covered with the T i 0 2 thin film 105.
[0030] For example, zeolite particles and titanium dioxide particles' are mixed at a
30 predetermined weight ratio in pure water or nearly-pure water, the mixed liquid is
then immediately subjected to ultrasonic dispersion treatment to allow the titanium
dioxide particles to be adsorbed on the surfaces of the zeolite particles, and thus the
titanium dioxide particles are immobilized directly on the surfaces of the zeolite
particles. The purpose of the ultrasonic treatment is to forcibly disperse the
35 titanium dioxide particles intrinsically forming aggregations each consisting of
several hundred particles in water, and thereby to facilitate the immobilization of
the titanium dioxide particles on the surfaces of the zeolite particles. The time for
the ultrasonic dispersion treatment is desirably about 1 hour. Once the titanium
dioxide particles have been adsorbed and immobilized on the surfaces of the zeolite
particles, the titanium dioxide particles cannot easily been separated from the
surfaces of the zeolite particles in water because of the electrostatic attractive force
5 between the titanium dioxide particles and the zeolite particles.
[0031] The above titanium dioxide composite catalyst may be synthesized in
hexavalent chromium-containing water to be treated. However, synthesizing the p
titanium dioxide composite catalyst in pure watqr or nearly-pure water in advance A
yields more reproducible results. In order to immobilize the titanium dioxide
10 particles on the surfaces of the zeolite particles, it is desirable to preliminarily
subject the zeolite particles to activation treatment with an acid aqueous solution
before mixing the zeolite particles and the titanium dioxide particles. Zeolite has a
basic skeleton composed of silica and alumina. From another standpoint, it can be
said that zeolite includes (Si04)k and (A104)5- as basic units. The aforementioned
15 treatment with an acid aqueous solution dissolves only the alumina portions (A1
portions) of the zeolite, and accordingly, an increased number of active sites for
direct adsorption of titanium dioxide can be introduced into the basic skeleton of the ,
zeolite. The zeolite particles having such active sites can adsorb and immobilize an
increased number of the titanium dioxide particles on the surfaces thereof.
20 However, an acid solvent weakens the electrostatic attractive force between the
titanium dioxide particles and the zeolite particles. Therefore, the activation
treatment of the zeolite particles with an acid aqueous solution is desirably
performed before synthesis of the titanium dioxide composite catalyst.
[00321 FIG. 2 schematically shows an embodiment of a water treatment system 2 of
25 the present embodiment. The water treatment system 2 includes aepre-filtration
vessel 201, a slurry tank 202, a photoreactor 203, a light source 204 provided inside
the photoreactor 203, a solid-liquid separation vessel 205, and'a returning part 206.
The method of the present embodiment includes a catalyst adding step (step al), a
photoreaction step (step bl), and a sedimentation separation step (step cl). The
30 method includes a re-adding step (step dl) as necessary. Hereinafter, each of the .
steps will be described.
[00331
The most distinctive feature of the method of the present embodiment is
that the titanium dioxide composite catalyst is irradiated with excitation light in a
35 state where the titanium dioxide composite catalyst is uniformly dispersed in water
to be treated.
Lo0341 A slurry liquid containing the titanium dioxide composite catalyst is
supplied from the slurry tank 202. That is, the titanium dioxide composite catalyst
is added to a hexavalent chromium-containing aqueous solution held in the
photoreactor 203. As a result, the titanium dioxide composite catalyst can be
dispersed in the hexavalent chromium-containing aqueous solution in the
5 photoreactor 203. In order to prevent precipitation of the titanium dioxide
composite catalyst, it is desirable to stir gently the aqueous solution in the
photoreactor 203.
[0035] The concentration of the titanium dioxide composite catalyst in the water in
the photoreactor 203 is desirably 0.4 g/L or more and 16 g/L or less. ' When the
10 concentration of the titanium dioxide composite catalyst in the water is more than
16 g/L, the entry of the below-described UV light into the water is significantly B
disturbed, and the hexavalent chromium reductjon ratio is accordingly decreased. 1
On the other hand, when the concentration of the titanium dioxide composite
catalyst in the water is less than 0.1 g/L, the amount of the titanium dioxide
15 composite catalyst is insufficient with respect to the amount of UV light, which
decreases the hexavalent chromium reduction ratio.
[00361
UV light is applied from the light source 204. Upon the irradiation with
the UV light, hexavalent chromium contained in the aqueous solution is reduced by
20 the catalytic action of the titanium dioxide composite catalyst. The wavelength of
the light from the light source 204 is 200 nm or more and 400 nm or less. The UV ,
light from the light source 204 may be monochromatic light or continuous light as
long as its wavelength is within the above range. The shorter the wavelength of
the light applied to the titanium dioxide photocatalyst is, the higher the efficiency of
25 generation of excited electrons is. Therefore, a shorter wavelength of the light from
the light source 204 is desirable from t i e standpoint of the reduction ratio of
hexavalent chromium contained in the aqueous solution. Examples of the light
source that can be used in the photoreaction step include a low pressure mercury
lamp, a medium pressure mercury lamp, a high pressure mercury lamp, an excimer
30 lamp, a xenon lamp, sunlight, black light, and an LED. Also in the present step, it
is desirable to stir gently the aqueous solution held in the photoreactor 203.
[0037] The photoreactor 203 is either a batch-type reactor or a continuous-type
reactor. Examples of the batch-type reactor include a batch reactor and a batch
recirculation reactor. Examples of the continuous-type reactor include a stirred
35 tank reactor and a tubular reactor. When an inorganic compound such as mud and
sand which is derived from silicas is contained in the water to be treated, it is
desirable to preliminarily separate and remove the inorganic compo,und in the
pre-filtration vessel 201.
[0038]
After the photoreaction step, the aqueous solution containing the.dispersed
titaniqm dioxide composite cqtalyst is transferred frop the photoreactor 203 to the
5 solid-liquid separation vessel 205. Accordingly, the stirring of the aqueous solution
in the step bl is stopped. In the solid-liquid separation vessel 205, the titanium
dioxide composite catalyst in the aqueous solution is separated from the aqueous
solution by sedimentation, and a concentrate of the titanium dioxide composite
catalyst and treated water are produced. By making use of the fact that the
10 titanium dioxide composite catalyst readily sediments, the titanium dioxide
composite catalyst is separated by sedimentation in the aqueous solution.
Examples of the process for sedimentation separation include a gravitational
sedimentation process, a centrifugal sedimentation process, and a li'quid cyclone
process. When hexavalent chromium is reduced, trivalent chromium is produced.
15 The solubility of trivalent chromium is lower than the solubility of hexavalent a
chromium. Therefore, trivalent chromium can be removed from treated water a
relatively ea~ilyby being precipitated in the treated water.
[0039] In the case of separation using a gravitational sedimentation process, the
aqueous solution containing the titanium dioxide composite catalyst is allowed to
20 stand in the solid-liquid separation vessel 205. The titanium dioxide composite
catalyst naturally sediments under the action of gravity. Therefore, a concentrate
(slurry liquid) containing the concentrated titanium dioxide composite catalyst can
be obtained in the lower portion of the solid-liquid separation vessel 205, while
treated water that is a supernatant liquid free from the titanium dioxide composite
25 catalyst can be obtained in the upper portion of the solid-liquid separation vessel *
205. The time for which the aqueous solution is allowed to stand is, for example,
10 minutes or longer. The concentrate (slurry liquid) of the titanium dioxide
composite catalyst is returned to the photoreactor 203 via the returning part 206,
and can be reused in the photoreaction step.
30 [0040] In the case of separation using centrifugal sedimentation process, the
treated water containing the titanium dioxide composite catalyst is whirled in a
holeless rotating container to separate the titanium dioxide composite catalyst.
Only the titanium dioxide composite catalyst is moved toward the wall of the
container by a centrifugal force, and is concentrated. Thus, the treaded water and
35 the concentrate of the catalyst are separated from each other. * The centrifugal force
acting on the rotating container is, for example, 60 G or more.
[0041]
The concentrate of the titanium dioxide composite catalyst, which has been
produced in the sedimentation separation step, is returned to the photoreactor 203
through the returning part 206. That is, the titanium dioxide composite catalyst
separated in the sedimentation separation step is added again to thg hexavalent
5 chromium-containing aqueous solution. In order to reliably reduce hexavalent
chromium in the photoreactor 203, the concentrate of the titanium dioxide
composite catalyst needs to be continuously added to the to-be-treated water
continuously flowing into the photoreactor through the pre-filtration vessel 201.
As th concentrate of the tita ium dioxide composite atalyst, "1 ? Fa talyst, the concentrate
10 produced in the solid-liquid separation vessel 205 can be reused. That is, in the
present embodiment, the step b l and the step cl may be further performed after the
step dl. The titanium dioxide composite cataly.st can be reused a8 long as the
surface active sites of the titanium dioxide are not covered with scale or
hardly-decomposable thin films derived from organic or inorganic substances.
15 [0042]
In the sedimentation separation step, the catalytic particles are required to
have the capability to be separated, for example, by a gravitational sedimentation
process in a short time. This capability can be evaluated, for example, by a light
transmission method. The light transmission method is an evaluation technique in
20 which the change over time in light transmittance is monitored for a suspension of a
catalyst by continuously measuring the transmittance of laser light with which the
suspension is irradiated. For a suspension of a catalyst that has a high
sedimentation velocity, a significant change in transmittance is observed within a
short time since the catalyst sediments in a short time. By contrast, for a
25 substance that has a low sedimentation velocity, almost no change in transmittance
is observed even after a lapse of time.
Lo0431 To exemplify the excellent sedimentation separation performance of the
titanium dioxide composite catalyst particles used in the present embodiment, FIG.
3 shows how natural sedimentation proceeded for the cases of the titanium dioxide
30 composite catalyst particles, titanium dioxide particles (P25 manufactured by
Degussa AG), and zeolite particles (HY type). In FIG. 3, the horizontal axis
represents the elapsed time from injection of a specimen liquid into a sample, and
the vertical axis represents the light transmittance. The concentration of the
titanium dioxide composite catalyst particles in a specimen liquid was 3.6 g/L. The
35 concentration of the titanium dioxide phrticles in a specimen liquid was 0.9 g/L.
The concentration of the zeolite particles in a specimen liquid was 2.7 g/L. The
concentration of the titanium dioxide particles and the concentration of the zeolite
particles were respectively set equal to the concentrations of the titanium dioxide
particles and the zeolite particles included in the titanium dioxide composite
catalyst particles. As the titanium dioxide composite catalyst particles, the
following two types of catalysts were prepared: a catalyst A for which zeolite
5 particles (having a silicalalumina molar ratio of 30 and a Si1A.l molar ratio of 15)
were used that had been treated by being immersed in a 0.1 mol/L hydrochloric acid
aqueous solution and then stirred with an ultrasonic washer for 60 minutes; and a
catalyst B for which zeolite particles were used that had been treated by being
stirred with an ultrasonic washer for 60 minutes without immersion in a
10 hydrochloric acid aqueous solution.
[00441 In FIG. 3, the numeral 301 denotes the result for the catalyst A, the
numeral 302 denotes the result for the catalyst B, the numeral 303 denotes the
result for the titanium dioxidy particles alone, and numeral 304 denotes the
result for the zeolite particles alone. The progress sedimentation of the
15 titanium dioxide composite catalyst was confirmed from the increase over time in
transmittance. For comparison of sedimentation performance, the amount of
change in transmittance during the se&mentation time of 30 minutes was
calculated for each specimen. The transmittance change in the titanium dioxide
particle specimen was 0%, and the transmittance change in the zeolite particle
20 specimen was 0.026%. That is, there was almost no increase in transmittance even
after 30 minutes elapsed from injection of the specimen liquid into the sample cell.
It can be said that these particles hardly sediment in 30 minutes. On the other
hand, the transmittance change in the catalyst A specimen was 56%, and the
transmittance change in the catalyst B specimen was 36%. This indicates that,
a
25 regardless of whether or not the zeolite particles are subjected to the acid treatment, ,
both the catalyst A and the catalyst B can be separated by sedimentation from the
aqueous solution in a practical time of about 30 minutes to an extent sufficient to
allow water discharge (transmittance change of 20% or more).
[0045] The proportion (sedimentation amount) of the catalyst particles that
30 sedimented in 30 minutes after injection of the specimen liquid into the sample was
calculated from the transmittance change. In the catalyst A specimen, 92% of the
particles sedimented. In the catalyst B specimen, 86% of the particles sedimented.
That is, both the catalyst A and the catalyst B can be separated by eedimentation in
a practical time of about 30 minutes. By contrast, only 1.6% of the particles
35 sedimented in the zeolite particle specimen, and the particles did not sedimented at b
all in the titanium dioxide particle specimen.
[0046] When a spherical particle sediments freely in a fluid under the action of
gravity, the sedimentation velocity of the spherical particle is proportional to the
difference in specific gravity between the particle and water, and to 'the square value
of the particle diameter. In the titanium dioxide composite catalyst of the present
embodiment, the zeolite particles have'a micrometer-order particle diameter (of 1
5 pm to 10 pm, for example), and the titanium dioxide particles whose specific gravity
differs fiom water by an amount that is 100 or more times larger than the difference
in specific gravity between the zeolite particle and water are densely immobilized on
the outer surfaces of the zeolite particles. Therefore, the particle diameter of the
titanium dioxide composite catalyst is about three orders of magnitude larger than
10 that of a nanometer-order titanium dioxide particle, and the difference in specific
gravity between the titanium dioxide composite catalyst and water is about two
orders of magnitude larger than the difference in specific gravity between the zeolite
particle and water. These two facts are thought of as reasons why the titanium
dioxide composite catalyst exhibits a sedimentation velocity that is considerably
15 higher than those of the zeolite particles and the titanium dioxide particles.
Coo471 The zeolite particle used in the present embodiment is a porous inorganic
compdund having a basic skekton composed of silica bnd alumina. From another
standpoint, it can be said that the zeolite particle used in the present embodiment is
a porous inorganic compound that includes (SiOJ4- and 0110435- as basic units. The
20 sedimentation performance of the catalyst particles, which is key to the
sedimentation separation step of the present embodiment, is influenced by the ratio
between silica and alumina which compose the zeolite. Here, the silicdalumina
molar ratio between silica and alumina which compose the zeolite is twice the ratio
between (Si04)k and (AZ04)5- in the zeolite, that is, the SilAl molar ratio. The
25 sedimentation performance of the titanium dioxide composite catalyst in the
sedimentation separation step of the present embodiment is influenied by the ratio
between silica and alumina which compose the zeolite. Table 1 shows the
transmittance change during the sedimentation time of 30 minutes and the b
proportion (sedimentation amount) of the sedimented catalyst particles for titanium
30 dioxide composite catalysts synthesized using zeolite particles having different
silicalalumina molar ratios.
[0048] [Table 11
[0049] As shown in the table, when the zeolite particles have a silicdalumina
molar ratio of 10 or more (a SUA1 molar ratio of 5 or more), the titanium dioxide
composite catalyst can be sedimented in a practical time of about 30 minutes to an
extent sufficient to allow water discharge (transmittance change of 20% or more).
The titanium dioxide particles can be stably immobilized only when zeolite particles
having a silicdalumina ratio of 10 or more are used as a support material. The
reason is that the below-described adhesion is more likely to occur between the
770
385
61
93.2
titanium dioxide particles and the zeolite. Therefore, the titanium dioxide
particles can be used in water for a long period of time without desorption from the
zeolite/ particles. The crystal system of the zeolite p4rticles serving as a support
material is not particularly limited. For example, zeolite particles of a common
30
15
36
85.9
10
5
20
80.0
Silicalalumina
molar ratio
SUA1 molar ratio
Transmittance
change [%I
Sedimentation
amount [%I
type, such as faujasite-type particles and MFI-type particles, can be used.
[0050] In the titanium dioxide composite catalyst of the present embodiment,
strong adhesions derived from electrostatic attractive force exist between the
titanium dioxide particles and the zeolite particles. Therefore, the titanium
60
30
46
88.9
5
2.5
1.6
43.6
dioxide composite catalyst of the present embodiment has high durability, and many
of the titanium dioxide particles remain immobilized on the ieolite particles even
after, for example, a lapse of several months. On the other hand, for example,
when an inorganic material such as silica particles and alumina pafticles, or an
inorganic porous body such as brick and concrete is used as a support material, no
strong adhesion occurs between the support material and the titanium dioxide
particles. In such a case, ,when the catalyst is gtirred together with the aqueous
solution in a photoreactor, many of the titanium dioxide particles are
disadvantageously desorbed from the support material in a short time.
[0051] In the present embodiment, particles of a nanometer-order size that have an
anatase crystal system and a photocatalytic function, such as P25 manufactured by
I
Degussa AG, Germany, can be used as the titanium dioxide particles immobilized on
the zeolite particles. Particularly when the titanium dioxide particles have an
average particle diameter in the range of 1 nm to 100 nm, a suitable.titanium
dioxide composite catalyst can be formed. The average particle diameter is defined
as an average value of the long diameter and the short diameter of the titanium
dioxide particle. When the average particle diameter of the titanium dioxide
particle is less than 1 nm, the catalyst activity is decreased due to quantum size
effect. In addition, when the average particle diameter of the titanium dioxide .
particle is more than 100 nm, the gravity acting on the titanium dioxide particle is
larger than the force acting between the titanium dioxide particle and the zeolite
particle. For this reason, immobilization of the titanium dioxide particle on the
zeolite particle is unstable, and the titanium dioxide particle can easily desorb from
the zeolite particle. Accordingly, the reproducibility of the titanium dioxide
composite catalyst is deteriorated. The particle diameter distribution of the
titanium dioxide particles of the titanium dioxide composite catalyst of the present
embodiment was measured using a TEM (transmission electron microscope) image.
As a result, it was confirmed that the particle diameters of the titanium dioxide
particles of the present embodiment were distributed in the range of 25.8 k 24.6 nm,
and were within the aforementioned limits. The error is a standard deviation at
99% confidence limit.
Coo521 In the method of the present embodiment, electrons generated on the
surface of the titanium dioxide under irradiation with UV light serve to reduce
hexavalent chromium. When hexavalent chromium is reduced, the valence is
decreased one by one due to electrons generated by photochemical reaction of
titanium dioxide. That is, hexavalent chromium is reduced first into pentavalent
form, then into tetravalent form, and finally into trivalent form.
COO531
The photoreaction step (step b2) of the present embodiment is performed in
10 the same manner as the photoreaction step (step bl) of the firit embodiment.
Therefore, a detailed description thereof is omitted.
Lo0571
The aqueous solution treated in the step b2 is fed to the solid-liquid
separation vessel 405, and is allowed to stand. Accordingly, the stirring of the
15 aqueous solution in the step b2 is stopped. The titanium dioxide composite catalyst
sediments in the aqueous solution being allowed to stand, and a layer 410 of the
precipitated titanium dioxide composite catalyst is formed in the lower portion of
the solid-liquid separation vessel 405. . The layer 410 of the precipitated titanium
dioxide composite catalyst is separated from a mixed liquid 408 of the water to be
20 treated and the titanium di~xideco mposite catalyst, and thus a concentrated slurry
is obtained. The titanium dioxide composite catalyst in the concentrated slurry
includes the titanium dioxide composite catalyst originally dispersed in the aqueous
solution and the catalyst particles that have not passed through the filtration
membrane element 406 in the filtration step (step f'2) described below. The
25 proportion of the catalyst recovered in the form of the concentrated slurry is, for
example, 99.99% or more. The performance evaluation and theoret'ical
consideration for the sedimentation of the titanium dioxide composite catalyst of the
present embodiment are the same as described for the first embodiment.
[0058]
30 Microfiltration using the filtration membrane element 406 is performed
simult aneously with the sedi9 " entation of the catalys particles in the step c2, so as
to produce treated water 413 from the mixed liquid 4 E 8 of the water to be treated
and the catalyst particles. The concentration of the titanium dioxide composite
catalyst remaining in the treated water produced is, for example, 10 ppm or less.
35 [0059] FIG. 5 is a perspective view schematically showing the structure of the
filtration membrane element 406. The filtration membrane element 406 includes a
plate-shaped frame 501 and sheets of filter paper 502 made of synthetic resin and
19
attached fixedly to both faces of the frame 501. Filtration is performed by drawing
water through a filtered water extraction port 503 using a pump (not shown). The
water to be filtered passes through the filter paper 502 made of synthetic resin,
enters the inside of the frame 501, and is discharged as the treated water 413 +
5 through the filtered water extraction port 503.
[0060] As shown in FIG. 4, the filtration membrane element 406 is arranged
parallel to a sedimentation direction 409 in which the titanium dioxide composite
catalyst is sedimented. A layer of depqsited catalyst particles, which is called a
cake layer, is usually formed on the surface of the filtration membrane element 406
10 along with the progress of the filtration. This cake layer peels off from the surface
of the filtration membrane element 406 due to its own weight, and sediments to the
lower portion of the solid-liquid separation vessel 405. The performance evaluation
in terms of microfiltration for the titanium dioxide composite catalyst used in the
present embodiment will be described later.
15 LO06 11 As described above, when hexavalent chromium is reduced, trivalent
chromium is produced. The solubility of trivalent chromium is lower than the
solubility of hexavalent chromium. For this reason, trivalent chromium is more
readily precipitated in the treated water. Therefore, trivalent chromium can be
removed using the filtration membrane element 406. ,
20 [0062]
The re-adding step (step f2) of the second embodiment is performed in the
same manner as the re-adding step (step dl) of the first embodiment. The
concentrated slurry of the titanium dioxide composite catalyst separated in the step
c2 is returned to the photoreactor 403 through the returning part 407. That is, the
25 titanium dioxide composite catalyst separated in the sedimentation separation step
is added again to the hexavalent chromium-containing aqueous solution. The
titanium dioxide composite catalyst particles can be repeatedly reused as long as
the surface active sites of the titanium dioxide are not covered with scale or
hardly- decomposable thin films derived from organic or inorganic substances.
30 That is, in the present embodiment, the step b2, the step c2 and the"step e2 may be
further performed after the step f2.
[0063I The performance of the'titanium dioxide composite catalyst of the present
embodiment in terms of microfiltration will be described. In order to separate the
titanium dioxide composite catalyst by microfiltration, the particle diameter of the
35 titani m dioxide composite c talyst needs to be suffic ently larger than Y 7 i the pore diameter of the filtration membrane element. A larger difference between the
particle diameter of the titanium dioxide composite catalyst and the pore diameter
of the filtration membrane element allows the titanium dioxide composite catalyst
to be separated in a shorter time with a lower probability of clogging of the filtration
membrane element. FIG. 6 shows the results of particle size distribution
measurements performed on titanium dioxide particles and on the titanium dioxide
5 composite catalyst of the present embodiment. The numeral 601 denotes the result
of the particle size distribution measurement on the titanium dioxide particles, and
the numeral 602 denotes the result of the particle size distribution measurement on
the titanium dioxide composite catalyst of the present embodiment. The average 9
particle diameter of the titanium dioxide composite catalyst was 5.5 p.m. The
10 average pore diameter of the resin filtration membrane element used for the
microfiltration in the above filtration step (step f2) was 0.42 pm. The average
particle diameter of the titanium dioxide particles was 0.2 pm.
Examples
15 LO0641 (Example 1)
First, a titanium dioxide composite catalyst was produced by the procedure
described below. The zeolite particles used were HY-type zeolite particles
(faujasite-type particles manufactured by Zeolyst) having an average particle
diameter of 5.0 pm and a silica/alumina molar ratio of 30 (SiIAl molar ratio of 15).
20 The zeolite particles were immersed in a 0.1 mom hydrochloric acid aqueous
solution, and stirred in an ultrasonic washer for 60 minutes. Subsequently, only
the zeolite particles were separated and recovered from the water by suction
filtration. The resultant powder was sufficiently rinsed with water three times to
wash off the acid, and was then dried. An amount of 0.9 g/L of titahium dioxide
25 particles (P25 manufactured by Degussa AG) and 2.7 g/L of the HY-type zeolite
particles treated with the hydrochloric-acid aqueous solution were added to pure
water prepared by all ultrapure water production apparatus. This solution was
subjected to ultrasonic treatment using an ultrasonic generator for 1 hour.
Thereafter, the solution was stirred with a magnetic stirrer at a number of
30 revolutions of 300 rpm for 60 minutes to obtain a slurry liquid containing a titaniwa
dioxide composite catalyst. A transmission electron microscope (TEM) image of the
thus-fabricated titanium dioxide composite catalyst is shown in FIG. 7C. For
comparison, a TEM image of the titanium dioxide particles alone is ~hownin FIG.
7A, and a TEM image of the zeolite particles alone is shown in FIG. 7B. It can be
35 understood that, in the titanium dioxide composite catalyst 703, the titanium
dioxide particles 701 are immobilized directly on the zeolite particle 702 without the
mediation of a thin film.
[0065] The treatment system shown in FIG. 2 was constructed using the slurry
a
liquid of the titanium dioxide composite catalyst fabricated as described above. #
The slurry of the titanium dioxide composite catalyst was loaded into the slurry
tank 202, and was supplied to the photoreactor 203. An aqueous ~olutionh aving
5 dissolved therein Kr2Cr207, which is a hexavalent chromium compound, was used
as the water to be treated. The hexavalent chromium compound was dissolved in
pure water at an aqueous solution concentration of 1000 pg/L, and the resultant
solution was then introduced into the photoreactor 203. The slurry liquid of the
titanium dioxide composite catalyst was supplied to the photoreactor so that the
10 concentration of the catalyst was 0.4 g/L. Thereafter, UV light irradiation was
performed in conjunction with stirring at 200 rpm. The light source 204 was
composed of a combination of a xenon light source (MAX 302 manufactured by Asahi b
Spectra Co., Ltd.) and a band-pass filter. The light had a wavelength X of 350 nm,
a bandwidth of about 10 nm, and an intensity of 1 mWJcm2.
15 [0066] The solution having been subjected to the light irradiation was collected,
and the concentrations of substances in1 the solution were quantitatively analyzed
by HPLCIICP MS. The hexavalent chromium reduction ratio achieved by
irradiation for 8 minutes was 78.0%. Thus, it was confirmed that the titslnium
dioxide composite catalyst is effective for reduction of hexavalent chromium, that is,
20 detoxification of hexavalent chromium.
[0067] The aqueous solution having undergone the photoreadion step and
containing the suspended titanium dioxide composite catalyst was introduced to the
solid-liquid separation vessel 205, and evaluation of gravitational sedimentation
was performed by a light transmission method. A HeNe laser (632.8 nm, 3 mW, .
25 nonpolarized) was used as the llght source. A fiber coupler including an objective
lens was used, laser light was introduced into the fiber, and thus the solid*liquid
separation vessel 205 containing the suspension was irradiated with the light. The
light having transmitted through the solid-liquid separation vessel 205 was
introduced again into the fiber, and finally, a light-receiving surface of a photodiode
30 (C10439-03 manufactured by Hamamatsu Photonics K.K.) was irradiated with the
light to measure the transmittance. The transmittance change after the titanium
dioxide composite catalyst was sedimented for 30 minutes was 30 St 5%. This value
of the transmittance change is beyond 20% which is a baseline for determining that
the treated water is slllowed to be discharged. Thus, it was confirmed that
35 solid-liquid separation of the titanium dioxide composite catalyst from an aqueous
solution in which the titanium dioxide composite catalyst is suspended can be
achieved in a short time by sedimentation separation. The titaniurp dioxide
composite catalyst separated and recovered was able to be continuously reused by
being introduced again to the photoreactor 203 through the returning part 206.
[00681 As described above, according to Example 1, a water treatment method
excellent in both photocatalytic activity and solid-liquid separation performance was
5 achieved. *
[00691 (Example 2) b
A water treatment system for treating a hexavalent chromium-containing
aqueous solution was constructed by the same way as in Example 1. A 1000 pg/L
hexavalent chromium aqueous solution was used as the water to be treated. The
10 amount of the titanium dioxide composite catalyst slurry supplied was adjusted to
prepare five aqueous solutions containing the titanium dioxide composite catalyst at
different concentrations, and the hexavalent chromium reduction ratio and the
sedimentation performance were evaluated for each solution. The concentration of
the titanium dioxide composite catalyst was set to 0.04 g/L for a solution Al, 0.4 g/L
15 for a solution B1, 3.6 g/L for a solution C1, 16 g/L for a solution Dl, and 40 g/L for a
solution El. UV light irradiation was performed under the same conditions as in b
Example 1, and the hexavalent chromium reduction ratio achieved after 8 minutes
was evaluated. Furthermore, as in Example 1, the aqueous solution having
undergone the photoreaction step and containing the suspended titanium dioxide
20 composite catalyst was introduced to tde solid-liquid separation vessel 205, and the
light transmittance change after 30 minutes was evaluated as the sedimentation
performance. The hexavalent chromium reduction ratios in the solutions having
catalyst concentrations of 0.4 g/L, 3.6 g/L, and 16 g/L were 78.0%, 77.8%, and 74.3%,
respectively, which revealed that high reduction ratios are achieved in these
25 solutions. However, the hexavalent chromium reduction ratios in the solutions
having catalyst concentrations of 0.04 g/L and 40 g/L were 11.2% and 13.0%,
respectively, and were smaller than 115 of the reduction ratio in the solution having
a catalyst concentration of 3.6 g/L. That is, in the treatment method of the present.
example, the suitable concentration of the titanium dioxide composite catalyst was
30 in the range of 0.4 g/L to 16 g/L.
[00701 (Comparative ~xamples)
For comparison, a catalyst composed of quarts beads and titanium dioxide
particles immobilized by a binder on the quarts beads was fabricated, and was
evaluated for the hexavalent chromium reduction ratio and the sedimentation
35 performance. Amorphous silica, which is commonly used, was used as the binder.
An amount of 10 g of P25 which is titanium dioxide manufactured by Degussa AG,
8.7 g of TEOS (tetraethoxysilane) as silica alkoxide, 20 g of ethanol,. and 50 g of a
hydrochloric acid aqueous solution with a concentration of 1 mom, were mixed in a
beaker, and then the mixture was immediately cooled in an ice bath while being
stirred with a magnetic stirrer for 30 minutes. Evaluation was performed using
this mixed liquid within 30 minutes from the start of stirring. Quartz beads
5 having a particle diameter of 5 pm were immersed in this m&ed liquid, and the
quartz beads were coated with photocatalyst layers. The quartz beads were taken
out from the mixed liquid by filtration, dried in a draft chamber for about 1 hour,
and then dried further in an oven at 80°C. Through these steps, a photocatalyst
composed of quartz beads and titanium dioxide particles immobilized on the quartz r
10 beads by TEOS-derived amorphous silica was obtained. It was confirmed by SEM '
observation that the titanium dioxide particles were immobilized on the surfaces of
the quartz beads by the TEOS-derived amorphous silica. A slurry of the
photocatalyst was supplied to the photoreactor similarly to the above, and a
comparative solution 1 was thus prepared. The concentration of the titanium
15 dioxide particles in the comparative solution 1 was equal to the concentration in the
solution C1 (3.6 g/L).
[0071] In addition, a solution in which only nanometer-order titanium dioxide
particles were dispersed was also tested as a comparative solution 2. P25
manufactured by Degussa AG was used as the titanium dioxide particles, and the a
20 concentration of the titanium dioxide particles was set equal to the titanium dioxide
particle concentration in the solution C1 (3.6 glL). The hexavalent chromium
reduction ratio and the sedimentation performance were evaluated using the same
hexavalent chromium aqueous solution, and the same conditions as employed in
Example 2.
25 [0072] Table 2 shows the hexavalent chromium reduction ratio and the
sedimentation performance in each of the titanium dioxide composite catalyst
solutions having different concentrations and the comparative solutions.
100731 [Table 21
Cr(VI) reduction ratio
[%I
Sedimentation performance
(Change in light
transmittance)
Titanium dioxide composite catalyst solution Comparative
(concentration) I solution 1
Comparative
solution 2
(titanium
dioxide
particles)
(binder
A1 process)
(0.04 gL)
B1
(0.4 g/L)
C1
(3.6 gL)
Dl
(16 gL)
E 1
(40 gL)
[00741 In the comparative solution 1 for which a binder was used, the
sedimentation performance oq the catalyst was at a sIpilar level to that in the
composite catalyst solutions A1 to El of Example 2. However, the hexavalent
chromium reduction ratio was very low, and specifically was 9.8% which was
smaller than 118 of that in the solution C1. In the comparative solution 2 which
was a dispersion liquid of titanium dioxide particles, the hexavalent chromium
reduction ratio was 83.0%, which means that the highest decomposing performance
was exhibited. However, the sedimentation performance of the catalyst was 0%;
namely, the catalyst was not separated by sedimentation at all. On the other hand,
for the solutions B1 toDl used in the present example, it was confirmed that both
the hexavalent chromium reduction performance and the sedimentation
performance were so high that practical water treatment can be achieved.
[00751 (Example 3) N
The water treatment system shown in FIG. 4 was constructed using a b
concentrated slurry liquid of a titanium dioxide composite catalyst fabricated in the
same manner as in Example 1. The concentrated slurry of the titanium dioxide
composite catalyst was loaded into the slurry tank 402, and was supplied to the
photoreactor 403. An aqueous solution having dissolved therein &2Cr207, which is
a hexavalent chromium compound, was used as the water to be treated. The
hexavalent chromium compound was dissolved at an aqueous solution concentration
of 1000 pg/L, and the resultant solution was then introduced into the photoreactor
403. The concentrated slurry liquid of the catalyst particles was supplied to the
photoreactor so that the concentration of the catalyst was 3.6 g/L. Thereafter, UV
b
light irradiation was performed in conjunction with stirring at 200 rpm. The light
source 404 was composed of a combination of a xenon light source (MAX 302
manufactured by Asahi Spectra Co., Ltd.) and a band-pass filter. The light had a
wavelength X of 350 nm, a bandwidth of about 10 nm, and an intensity of 1 mWlcm2.
[00761 The solution having been subjedted to the light irradiation was collected,
and the concentration of hexavalent chromium in the solution was quantitatively
analyzed by HPLCIICP MS (manufactured by Agilent). The hexavalent chromium
reduction ratio achieved by irradiation for 8 minutes was 77.8%, which revealed
that most of the hexavalent chromium was reduced by the photocatalyst. Thus, it
was confirmed that the titanium dioxide composite catalyst is effective for reduction,
i.e., detoxification of hexavalent chromium which is actually observed in water
environments such as groundwater.
[0077l The suspension solution having undergone the photoreaction step was
introduced to the solid-liquid separation vessel 405. Then, evaluation of
gravitational sedimentation was performed by a light transmission method, and
evaluation of separation by microfiltration was performed. A HeNe laser (632.8 nm, .
3 mW, nonpolarized) was used as the light source for the light trans;hission method.
A fiber coupler including an objective lens was used, laser light was introduced into
5 the fiber, and thus the solid-liquid separation vessel containing the suspension was
irradiated with the light. The light having transmitted through the solid-liquid
separation vessel 405 was introduced again into the fiber, and finally, a
light-receiving surface of a ph todiode ((310439-03 m nufactured P "in ufactured by ~amamatsu Photonics K.K.) was irradiated with the light to measure the transmittance. A
10 sedimentation amount was calculated from the transmittance. The sedimentation
amount in 30 minutes was 85 A 5%. This value was beyond 80% which is a
baseline for solid-liquid separation of catalyst particles. Thus, it was confirmed
that solid-liquid separation in the suspension was achieved in a short time by
sedimentation separation.
15 [0078] Sheets of filter paper made of resin and having an average pore diameter of
0.42 pm were used in a filtration membrane element for the microfiltration. The
output flow rate of the treated water in the microfiltration was monitored. The
output flow rate was about 100 mumin, and was stable for 24 hours. The
concentration of the catalyst remaining in the treated water was 10 ppm or less.
20 By contrast, in the case of titanium dioxide particles alone, the flow rate which was
*
*
initially 100 mLlmin was decreased to 1.2 mUmin in 8 hours, and clogging of the
filtration membrane element was caused.
[0079] As described above, Example 3 verified that a water treatment method
excellent in both photocatalytic activity and solid-liquid separation performance was
achieved using the titanium dioxide composite catalyst.
[0080] (Example 4)
A water treatment system for treating a hexavalent chromium-containing
aqueous solution was constructed by the same way as in Example 3: A 1000 pg/L
hexavalent chromium aqueous solution was used as the water to be treated. The
30 amount of the titanium dioxide composite catalyst-concentrated slurry supplied was '
adjusted to prepare five aqueous solutions containing the titanium dioxide
composite catalyst at different concentrations, and the hexavalent chromium
reduction ratio, the sedimentation amount, and the extraction flow rate of the
treated water were evaluated for each dolution. The concentration of the titanium
35 dioxide composite catalyst was set to 0.04 g/L for a solutionA2, 0.4 g/L for a solution
B2, 3.6 g/L for a solution C2, 16 glL for a solution D2, and 40 g/L for a solution E2.
UV light irradiation was performed under the same conditions as in-Example 3, and
the hexavalent chromium reduction ratio achieved by 8-minute irradiation was
evaluated. Furthermore, as in Example 1, a suspension having undergone
photodecomposition treatment was introduced to the solid-liquid separation vessel,
and the sedimentation amount after 30 minutes and the extraction flow rate of the
5 treated water were evaluated. The hexavalent chromium reduction ratios in the
solutions containing the titanium dioxide composite catalyst at concentrations of'0.4
g/L, 3.6 g/L, and 16 g/L were 78.0%) 77.8%) and 74.3%) respectively, which revealed
that high reduction performance is exhibited in these solutions. However, the
hexavalent chromium reduction ratios in the solutions having catalyst
10 concentrations of 0.04 g/L and 40 g/L were 11.2% and 13.0%) respectively, and were
smaller than 115 of the hexavalent chromium reduction ratio in the solution having
a catalyst concentration of 3.d g/L. That is, in the trgatment method of the present
example, the suitable concentration of the catalyst in the solution was 0.4 g/L or
more and 16 g/L or less.
15 [0081] (Comparative Examples)
A photocatalyst composed of quarts beads and titanium dioxide particles
immobilized on the quarts beads by TEOS-derived amorphous silica was obtained in
the same manner as for the above comparative solution 1. It was confirmed by
SEM observation that the titanium dioxide particles were immobilized on the
20 surfaces of the quartz beads by the TEOS-derived amorphous silica. As in
Example 4, a concentrated slurry of the photocatalyst was supplied 'to the
photoreactor to prepare a comparative solution 3. The titanium dioxide particle
concentration in the comparative solution 3 was equal to the concentration in the 3
solution C2 (3.6 g/L). a
25 [0082] In addition, a solution in which only nanometer-order titanium dioxide
particles were dispersed as in the comparative solution 2 was tested as a
comparative solution 4 in the same manner as in Example 4. P25 manufactured by
Degussa AG was used as the titanium dioxide particles, and the concentration of the
titanium dioxide particles was set equal to the titanium dioxide particle
30 concentration in the solution C1 (3.6 g/L). The hexavalent chromium reduction
ratio, the sedimentation amount, and the extraction flow rate of the treated water
were evaluated using the same hexavalent chromium aqueous solution and the
same conditions as employed in Example 4. *
[00831 Table 3 shows the results for each of the catalyst particle solutions having
35 different concentrations and the comparative solutions. In the comparative
solution 3 for which a binder was used, the sedimentation amount and the
extraction flow rate were at similar levels to those of the catalyst particle solutions
I
A2 to E2 of Example 4; however, the hexavalent chromium reduction ratio was very
low, and specifically was 9.8% which was smaller than 115 of that in,the solution C2.
In the comparative solution 4 which was a dispersion liquid of titanium dioxide
particles, the hexavalent chromium reduction ratio achieved was 83.0% which was
5 the highest reduction ratio. However, the sedimentation amount was 0%; namely,
the catalyst was not separated by sedimentation at all. Moreover, the extraction
flow rate was 0 mLlmin; namely, the treated water was not able to be separated.
On the other hand, for the solutions B2 toD2 used in the present example, it was
confirmed that all of the hexavalent chromium reduction ratio, the sedimentation
10 amount, and the extraction flow rate were at such good levels that practical water
treatment can be achieved.
[00841 [Table 31
15 INDUSTRIAL APPLICABILITY
[0085] The present invention relates to detoxification of hexavalent chromium
Comparative
solution 4
(titanium
dioxide
particles)
83.0
0
0
contained in drinking water, discharged water, continental rivers, lakes, etc., and B
can provide a method and system that can treat water continuously in a practical 6
Comparative
solution 3
(binder
process)
9.8
89
90
1
Cr(V1) reduction ratio
[%I
Sedimentation amount
[%I
Extraction flow rate
[mltminut el
time. The method according to the present invention can be used for household
20 clean water systems and public clean water systems. Furthermore, the method
can be used as one of the steps of effluent treatment in factories or of sewage
treatment process.
Titanium dioxide composite catalyst solution
(concentration)
A2
(0.04 g+)
11.2
81.0
110
B2
(0.4 glL)
78.0
82.6
100
E2
(40 g&)
13.0
89.3
100
C2
(3.6 g&)
77.8
85.9
105
D2
(16 gly
74.3
88.5
110

We Claim
1. A method for treating a hexavalent chromium-containing aqueous solution,
comprising:
a step a of adding catalyst particles to the aqueous solution;
a step b of reducing hexavalent chromium by irradiating the aqueous
solution with light having a wavelength of 200 nanometers or more and 400
nanometers or less while stirring the catalyst particles in the aqueous solution; and
a step c of stopping the stirring in the step b and separating the catalyst
particles from the aqueous solution by sedimentation,
wherein each catalyst particle is composed only of a titanium dioxide
particle and a zeolite particle, the titanium dioxide particle is adsorbed on an outer
surface of the zeolite particle, the zeolite particle has a silica/alumina molar ratio of
10 or more, and the catalyst particles are contained in the aqueous solution at a
concentration of 0.4 grams/liter or more and 16 grams/liter or less.
2. The method according to claim 1, comprising a step d of adding again the
catalyst particles separated by sedimentation in the step c to the aqueous solution
after the step c,
wherein the step b and the step c are performed again after the step d.
3. The method according to claim 1, wherein
the catalyst particles are separated by sedimentation in a solid-liquid
separation vessel including a filtration membrane element in the step c,
the method further comprises a step e of producing treated water from the
aqueous solution using the filtration membrane element, and
the filtration membrane element used in the step e is composed of a
plate-shaped frame and sheets of filter paper made of resin and attached to both
faces of the frame, and is arranged parallel to a direction in which the catalyst
particles are sedimented.
4. The method according to claim 3, comprising a step f of adding again the
catalyst particles separated by sedimentation in the step c to the aqueous solution
after the step c, wherein the step b, the step c and the step e are performed again
after the step f.
5. The method according to claim 1, wherein the zeolite particle is a zeolite
particle subjected to a process in which an alumina portion is dissolved by
treatment with an acid aqueous solution to introduce an active site for direct
adsorption of the titanium dioxide particle, and then the acid aqueous solution
adhered to the surface of the zeolite particle is removed by
Dated this on 29th day of January 2014
ATTORNEY F