TECHNICAL FIELD
[0001] The present invention relates to a method for treating an arsenic-containing
aqueous solution.
I
BACKGROUND ART
10 [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, Nqn Patent Literature 2
states that highly toxic trivalent arsenic contained in water can be oxidized into less
15 toxic pentavalent arsenic by photocatalytic reaction of titanium dioxide. In
addition, in order to facilitate solid-liquid separation of photocatalyst particles
dispersed in water, it has been proposed to use a 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 titanium dioxide particles (see
20 Patent Literature 1, for example). In addition, a technique has beeh proposed that
uses a photocatalyst obtained by coating support particles with titanium dioxide by
a coating process such as a sol-gel process (see Patent Literature 2, for example).
CITATION LIST
25 Patent Literature
[0003] Patent Literature 1: JP 10-249210A
Patent Literature 2: JP 11-500660 T
Patent Literature 3: WO 96126903 A1
Non Patent Literature
30 [0004] Non Patent Literature 1: Yuko Maruo and two other persons, "Development
of dispersion-type Ti02 photocatalyst for decomposition of medical drugs in water",
Book of Preprints of The 77th annual meeting of The Society of Chemical Engineers,
Japan, Public Interest Incorporated Association The Society of Chemical Engineers,
Japan, March 2012, p. 427
35 1 Non Patent Literaturq 2: Paritam K. Dutta ayd three other persons,
"Photocatalytic Oxidation of Arsenic (111): Evidence of Hydroxyl Radicals",
Environmental Science and Technology, March 15, 2005, vol. 39, No. 6, p. 1827-1834
SUMMARY OF INVENTION
Technical Problem
[0005] Although the techniques proposed in Patent Literature 1 and Patent
5 Literature 2 are suitable for solid-liquid separation of photocatalyst particles
dispersed in water, the techniques may not provide sufficient photocatalytic activity.
[0006] In view of the above circumstances, the present invention provides a method
for treating an arsenic-containing aqueous solution by water treatment employing a
titanium dioxide photocatalyst that is excellent in both photocatalytic activity and
10 solid-liquid separation performance.
Solution to Problem
[0007] The present invention provides a method for treating an arsenic-containing
aqueous solution, the method including: a step a of adding catalyst particles to the
15 aqueous solution; a step b of oxidizing trivalent arsenic 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
I a steplc of stopping the stirridg in the step b and separating the catalyst particles
from the aqueous solution by sedimentation. Each catalyst particle is composed
20 only of a titanium dioxide particle and a zeolite particle, the titanium dioxide
particle is adsorbed on an outer surface of the zeblite particle, the zeolite particle
has a silicalalumina molar ratio of 10 or more, and the catalyst particles are
contained in the aqueous solution at a concentration of 0.4 gramslliter or more and
16 gramsfliter or less.
25
Advantageous Effects of Invention
[0008] According to the above method, it is possible to provide a method for
treating an arsenic-containing aqueous solution by water treatment employing a
titanium dioxide photocatalyst that is excellent in both photocatalytic activity and
30 solid-liquid separation performance.
BRIEF DESCRIPTION OF DRAWINGS
[0009] FIG. 1A is a diagram conceptually showing the structure of a titanium
dioxide composite catalyst, FIG. 1B is a diagram conceptually showing the structure
35 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
5 treatment system of a second embodiment.
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 particfes of an
10 embodiment of the present invention.
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
15 an embodiment of the present invention.
DESCRIPTION OF EMBODIMENTS
[0010]
Nowadays, pollution of groundwater by arsenic has been reported in various
20 parts of the world. Arsenic is a substance that is highly toxic for human bodies.
Therefore, drinking arsenic-polluted water continuously over a long period of time
causes various chronic intoxication symptoms such as cutaneous disorders and
cancers, even when the arsenic concentration in the water is small. Since arsenic
pollution has such hazardousness, the WHO (World Health Organization) has
25 admonished countries to limit the arseqic concentration in drinking water to 10 ppb
or less. In the world, however, there aie many people who have no choice but to
use arsenic-polluted groundwater as drinking water for economical or regional
reasons despite the groundwater being polluted by arsenic in excess of the standard
value. Nowadays, pollution by arsenic is exerting a significant influence on the
30 health and life of such people.
C00lll As conventional examples of the process for removing Brsenic, there are an
adsorption process, a coprecipitation process, and a reverse osmosis membrane
process. The coprecipitation process is the most common process for removing
arsenic. In this process, a ferric salt such as FeCl3 is added to an
35 arsenic-containing aqueous solution, a precipitate is formed from the added iron
ions and arsenic, and thus arsenic is removed.
[00121 Examples of the features of these conventional removal processes include
large dependence of arsenic removal efficiency on the chemical form of arsenic. In
water, arsenic is usually present in the form of trivalent or pentavalent inorganic
arsenic. Trivalent arsenic as typified by arsenious acid is highly .toxic, and is
difficult to remove by the above conventional removal processes. By contrast,
*
5 pentavalent arsenic as typified by arsenic acid has toxicity that is 1/50 of the
toxicity of trivalent arsenic, and is easy to remove by the conventional removal
processes. Trivalent arsenic, which is difficult to remove, is predominant as the
chemical form of arsenic in groundwatdr, since groundwater is in oxygen-free
conditions or reducing conditions. Therefore, detoxification of trivalent arsenic
10 contained in groundwater essentially requires a pretreatment step of oxidizing the
trivalent arsenic into pentavalent arsenic.
[0013] The difference in removal efficiency depending on the valences of trivalent
arsenic and pentavalent arsenic is due to the nature of arsenic which is insoluble in
water in a pH range of 2 to 8 when in the form of trivalent arsenic but which is
15 soluble in water in the above pH range when in the form of pentavalent arsenic.
[0014] As methods for oxidizing trivalent arsenic, methods using photocatalysts .
have been reported as well as methods using chemical reagents such as potassium
permanganate, hydrogen peroxide solution, and ozone. Among them, a method
using a titanium dioxide photocatalyst is expected as a sustainable treatment
20 method since the method does not need an agent such as a chemicaf reagent and
can use sunlight. Oxidation of trivalent arsenic by a titanium dioxide
photocatalyst is due to holes and OH radicals generated by photocatalytic reaction
of titanium dioxide. When titanium dioxide is irradiated with ultraviolet light,
holes are generated in the titanium dioxide. Some of the holes generated diffuse to
25 the surface of the titanium dioxide, and react with water molecules adsorbed on the
surface to generate OH radicals. These holes and OH radicals react with trivalent
arsenic adsorbed on the surface of the titanium dioxide. As a result, the trivalent
arsenic can be oxidized into pentavalent arsenic.
[00151 As the types of the titanium dioxide photocatalyst supplied into a
30 photoreactor in the water treatment method using the titanium dioxide
photocatalyst, there can be mentioned: (I) an immobilized catalyst in the case of
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 nanometer-order titanium dioxide particles are used by being mixed with and
35 suspepded in water to be treated. In either case, thd 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
by (11) is much more advantageous fiom the standpoint of efficiency of oxidation of
trivalent arsenic since the dispersed catalyst allows a larger surface area per unit
mass to be obtained, and also allows a chemical substance to be diffused without
5 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 oxidation of
trivalent arsenic 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).
10 [0016] However, in a water treatment method using a dispersed catalyst, titanium
dioxide particles are in a state of being dispersed in water after arsenic in water is
oxidized 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
titanium dioxide particles and discharge of the treated water are enabled.
15 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 filter
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 'difticult to
20 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
25 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
oxidation of arsenic in water, the water treatment method 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
30 treatment, and significantly hinders the efficiency of the watei treatment.
[0017] When titanium dioxide particles having an over-nanometer diameter, for
example, a diameter larger than 1 pm, are used as a catalyst, solid-liquid separation
by sedimentation is enabled. However, titanium dioxide particles having a large
particle diameter are smaller in surface area per unit mass than nanometer-order
35 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
low photocatalytic activity, with the result that sufficient photocatalytic activity is
not obtained. For example, the techniques described in Patent Litgrature 1 and
Patent Literature 2 have been proposed in order to realize a titanium dioxide
particle photocatalyst that allows solid-liquid separation of titanium dioxide
5 particles dispersed in water.
[0018] When titanium dioxide particles serving; as a photocatalyst are immobilized
by a binder on support particles having a larger particle diameter than the titanium
dioxiqe particles, the titaniunp dioxide particles are firmly immobilized on the
surfaces of the support particles. As a result, micrometer-order photocatalyst
10 particles suitable for solid-liquid separation of the photocatalyst particles dispersed
in 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 and titanium dioxide deposited on the
support particles by a sol-gel process is indeed suitable for solid-liquid separation of
15 the photocatalyst particles dispersed in water, but lacks sufficient pliotocatalytic
activity, similarly to the photocatalyst in which titanium dioxide particles are
immobilized by a binder on support particles having a larger particle diameter than
the titanium dioxide particles.
[0019]
N
20 A first aspect of the present disclosure provides a method for treating an e
arsenic-containing aqueous solution, the method including: a step a of adding
catalyst particles to the aqueous solution; a step b of oxidizing trivalent arsenic 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
25 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 has a silica/alumina molar ratio of 10 or more, and the catalyst
t 30 particles are contained in the aqueous solution at a concentration of 0.4 gramslliter
or more and 16 gramslliter or less.
[00201 According to the first aspect, each catalyst particle is composed only of
titanium dioxide and a particle, and the titanium dioxide particle is adsorbed on the
outer face of the zeolite particle. Therdfore, almost the whole of a surface active
35 site of the titanium dioxide particle can be effectively used, and thus excellent
photocatalytic activity is exhibited. 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.
[0021] A second aspect of the present disclosure provides the method 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 pkrformed again after the step d.
LO0221 According to the second aspect, the separated catalyst partides can be
reused.
Lo0231 A third aspect of the present disclosure provides 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
solutidn using the filtration rdembrane element, and &he 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.
LO0241 According to the third aspect, treated water can be produced using the
filtration membrane element.
LO0251 A fourth aspect of the present disclosure provides the method as set forth in
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 b, the step c and the step e are performed again after the step f.
LO0261 According to the fourth aspect, the catalyst particles separated according to
the third aspect can be reused. *
Lo0271 A fifth aspect of the present disclosure provides the method as set forth in
*
any one of the first to fourth aspects, wherein pentavalent arsenic produced in the
step b is removed by a coagulation-sedimentation process, an adsorption process, or
a reverse osmosis membrane process.
LO0281 According to the fifth aspect, arsenic converted into the pentavalent form
can be separated and removed from water.
[00291 A sixth aspect of the present disclosure provides the method as set forth in
any one of the first to fifth 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 to the surface
of the zeolite particle is removed by washing with water.
[00301 According to the sixth aspect, the alumina portion (Al 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.
[0031] Embodiments of the present invention will be described belgw with
reference to the drawings. However, the present invention is not limited to the
embodiments described below.
[0032]
FIG. 1A conceptually shows the structure of a titanium dioxide composite
catalyst used in a method of the present embodiment. The titanium dioxide
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 hsed in the
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 catal st in the case of which titanium dioxide is ' immodilized on the support p iir ticle 104 by a sol-gel p 4o cess. In the case' where the
titanium dioxide particles 102 are immobilized on the support particle 104 using a
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 SiOz,
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
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 TiO2.
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
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
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
dioxide particles and zeolite particles" means that, as shown in FIG. lA, the thin
5 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 Ti02 thin film 105.
Lo0331 For example, zeolite particles and titarlium dioxide particles are mixed at a
predetermined weight ratio in pure water or nearly-pure water, the mixed liquid is
10 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
titanium dioxide particles intrinsically forming aggregations each consisting of
15 several hundred particles in water, and thereby to facilitate the immobilization of
the titanium dioxide particles on the surfaces of the zeolite particlee. 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
20 surfaces of the zeolite particles in water because of the electrostatic attractive force
between the titanium dioxide particles and the zeolite particles.
[003411 The above titanium dioxide composite catalys$ may be synthesized in
arsenic-containing water to be treated. However, synthesizing the titanium dioxide
composite catalyst in pure water or nearly-pure water in advance yields more
25 reproducible results. In order to immobilize the titanium dioxide 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
30 zeolite includes (Si04)4- and (A104)5- as basic units. The aforementioned treatment
with an acid aqueous solution dissolves only the alumina portions (h 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 a
zeolite particles having such active sites can adsorb and immobilize an increased a
35 number of the titanium dioxide particles on the surfaces thereof. 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.
[0035] FIG. 2 schematically shows an embodiment of a water treatment system 2 of
the present embodiment. The water treatment system 2 includes a pre-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 all, a
photoreaction step (step bl), and a sedimentation separation step (step cl). The
method includes a re-adding step (step dl) and an arsenic removing step (step gl) as
necessary. Hereinafter, each of the steps will be described.
[0036]
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
state where the titanium dioxide composite catalyst is uniformly dispersed in water
to be treated.
[0037] 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 an arsenic-containing aqueous solution held in the photoreactor 203.
As a result, the titanium dioxide composite catalyst can be dispersed in the
arsenic-containing aqueous solution in the 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.
[0038] 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
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
disturbed, and the efficiency of oxidation of arsenic is accordingly decreased. 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 composite
catalyst is insufficient with respect to the amount of UV light, which decreases the
efficiency of oxidation of arsenic.
[00391
UV light is applied 6om the light source 204. Upon the irradiation with
the UV light, arsenic contained in the aqueous solution is oxidized by the catalytic
action of the titanium dioxide composite catalyst. The wavelength bf 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 a
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
generation of holes and OH radicals is., Therefore, a shorter wavelength of the
light from the light source 204 is desirable from the standpoint of the efficiency of
oxidation of arsenic 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
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 pho'toreactor 203.
[0040] 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
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 compound in the
pre-filtration vessel 201.
[00411
After the photoreaction step, the aqueous solution containing the dispersed
titanium dioxide composite catalyst is transferred from the photoreactor 203 to the
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
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 liquid cyclone
process.
[00421 In the case of separat'on using a gravitations sedimentation process, the aqueous solution containing tX e titanium dioxide comtp osite catalyst is allowedt htoe
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
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.
5 [0043] In the case of separation using a 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 caialyst is moved toward the wall of the
container by a centrifugal force, and is concentrated. Thus, the treaded water and
10 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.
LO0441
The concentrate of the titanium dioxide composite catalyst, which has been
produced in the sedimentation separation step, is returned to the photoreactor 203
15 through the returning part 206. That is, the titanium dioxide composite catalyst
separated in the sedimentation separation step is added again to the
arsenic-containing aqueous solution. In order to reliably oxidize trivalent arsenic
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
20 the photoreactor through the pre-filtration vessel 201. As the concentrate of the
titanium dioxide composite catalyst, the concentrate produced in the solid-liquid
separation vessel 205 can be reused. That is, in the present embodiment, the step
bl and the step cl may be further performed after the step dl. The titanium
dioxide composite catalyst can be reused as long as the surface active sites of the
25 titanium dioxide are not covered with scale or hardly-decomposable thin films
derived from organic or inorganic substances.
[0045]
Pentavalent arsenic has a relatively high solubility. Therefore, pentavalent
arsenic contained in the treated water separated in the step cl is removed by a
30 coagulation-sedimentation process such as a coprecipitation process, by an
adsorption process, or by a reverse osmosis membrane process. According to the
water quality standard regardmg arsenic, the total concentration of trivalent
arsenic and pentavalent arsenic should be equal to or less than a standard value
(e.g., 10 ppb). Therefore, when arsenic contained in the treated water is.
35 pentaJalent arsenic but the ahount of the pentavaledt arsenic is more than the
standard value, the pentavalent arsenic has to be removed from the treated water.
The pentavalent arsenic in the treated water is removed, for example, by a
coprecipitation process using alum or an adsorption process using activated
alumina.
COO461
In the sedimentation separation step, the catalytic particles are required to
5 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
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
10 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 sedimentstin a short time. By contrast, for a
substance that has a low sedimentation velocity, almost no change in transmittance
is observed even after a lapse of time.
15 Lo0471 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
composite catalyst particles, titanium dioxide particles (P25 manufactured by
Degussa AG), and zeolite particles (HY type). In FIG. 3, the horizontal axis
20 represents the elapsed time from injection of a specimen liquid into a sample cell,
and the vertical axis represents the light transmittance. The concentration of the
titanium &oxide composite catalyst particles in a specimen liquid was 3.6 g/L. The
concentration of the titanium dioxide particles 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
25 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
30 particles (having a silica/alumina molar ratio of 30 and a SiM mola; 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
35 hydrochloric acid aqueous solution.
[00481 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 dioxide particles alone, and the numeral 304 denotes the
result for the zeolite particles alone. The progress of the sedimentation of the P
titanium dioxide composite catalyst was confirded from the increase over time in
6
transmittance. For comparison of sedimentation performance, the amount of
change in transmittance during the sedimentation 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
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, +
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 fiom the
aqueous solution in a practical time of about 30 minutes to an extent sufficient to
allow water discharge (transmittance cbange of 20% or more).
[0049] The proportion (sedimentation amount) of the catalyst particles that
sedimented in 30 minutes after injection of the specimen liquid into the sample cell
was calculated fiom 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
sedimentation in a practical time of about 30 minutes. By contrast, only 1.6% of
the particles sedimented in the zeolite particle specimen, and the particles did not
sedimented at all in the titanium dioxide particle specimen.
[0050] 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
ym to 10 ym, for example), and the titanium dioxide particles whose specific gravity
differs &.om 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
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
higher than those of the zeolite particles and the titanium dioxide particles.
[0051] The zeolite particle used in the present embodiment is a inorganic
5 compound having a basic skeleton composed of silica and alumina. From another
standpoint, it can be said that the zeolite particle used in the present embodiment is *
a porous inorganic compound that includes (SiOd4- and (AlOd5. as basic units. The 4
sedimentation performance of the catalyst particles, which is key to the
sedimentation separation step of the present embodiment, is influenced by the ratio
10 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 (Al04)5. in the zeolite, that is, the SiM molar ratio. The
sedimentation performance of the titanium dioxide composite catalyst in the
sedimentation separation step of the present embodiment is influenced by the ratio
15 between silica and alumina which compose the zeolite. Table 1 shows the
transmittance change during the sedimentation time of 30 minutes and the @
proportion (sedimentation amount) of the sedimented catalyst particles for titanium
dioxide composite catalysts synthesized using zeolite particles having different
silicdalumina molar ratios.
20 Lo0521 [Tablel] I
silicdalumina
molar ratio 1 5 1 10 1 30 1 60 7 7 0 1
1 SiiAl molar ratio 1 2.5 1 5 1 15 1 30 3851 '
Transmittance
change [%I 1 1 . 6 / 20 1 36 1 4 6 1 6 1 1
Sedimentation
amount [%I 1 43.6 1 80.0 1 85.9 1 88.9 / 93.2 1
LO0531 As shown in the table, when the zeolite particles have a silicdalumina
molar ratio of 10 or more (a SiM molar ratio of 5 or more), the titanium dioxide
25 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 silica/alumina 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
I
titanium dioxide particles and the zeolite. Therefor!, 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 particles serving as a support
material is not particularly limited. For example, zeolite particles of a common
5 type, such as faujasite-type particles and MFI-type particles, can be used.
[00541 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
dioxide composite catalyst of the present embodiment has high durability, and many
10 of the titanium dioxide particles remain immobilized on the zeolite 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 particles, or an b
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
15 particles. In such a case, when the catalyst is stirred 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.
[0055] 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
20 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
25 titanium dioxide particle. When the average particle diameter of the titanium
dioxide particle is less than 1 nm, the batalyst 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
30 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.
[00561 In the method of the present embodiment, holes and OH radicals.generated
on the surface of the titanium1 dioxide under irradiatipn with W light serve to
5 oxidize trivalent arsenic. When trivalent arsenic reacts with the generated holes
or OH radicals, electrons of the trivalent arsenic are transferred to the holes or OH
radicals. As a result, the trivalent arsenic releases two electrons and is thus
oxidized into pentavalent arsenic. Metal ion species, as well as arsenic, can also be
oxidized by titanium dioxide. For example, divalent lead, which is highly toxic, can
10 also be oxidized by titanium dioxide.
[00571
A method of a second embodiment will be described with reference to FIG. 4.
The second embodiment is the same as the first embodiment, except' for the matters
particularly described below.
15 [0058] FIG. 4 conceptually shows a water treatment system for implementing the
method of the present embodiment. The water treatment system 4 of the present
embodiment includes a pre-filtration vessel 401, a slurry tank 402, a photoreactor
403, a light source 404 provided inside the photoreactor 403, a solid-liquid
separation vessel 405, a filtration membrane element 406 provided inside the
20 solid-liquid separation vessel 405, and a returning part 407. The method of the
present embodiment includes a catalyst adding step (step a2), a photoreaction step
(step b2), and a sedimentation separation step (step c2). The method includes a
re-adding step (step f2) and an arsenic removing step (step g2) as necessary, and
further includes a filtration step (step e2). Hereinafter, each of the-steps will be
25 described.
LO0591
The catalyst adding step (step a2) of the present embodiment is performed
in the same manner as the catalyst adding step (step al) of the first embodiment.
Therefore, a detailed description thereof is omitted.
30 [00601 '
The photoreaction step (step b2) of the present embodiment is performed in
the same manner as the photoreaction step (step bl) of the first embodiment.
Therefore, a detailed description thereof is omitted.
[0061]
35 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
aqueous solution in the step b2 is stopped. The titanium dioxide composite catalyst
sediments in the aqueous solution beiag 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
5 treated and the titanium dioxide composite 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
solutibn and the catalyst partjicles that have not passkd through the filtration
membrane element 406 in the filtration step (step e2) described below. The
10 proportion of the catalyst recovered in the form of the concentrated slurry is, for
example, 99.99% or more. The performance evaluation and theoretical
consideration for the sedimentation of the titanium dioxide composite catalyst of the
present embodiment are the same as described for the first embodiment.
[0062]
15 Microfiltration using the filtration membrane elemedt 406 is performed
simultaneously with the sedimentation of the catalyst particles in the step c2, so as
to produce treated water 413 fkom the mixed liquid 408 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.
20 [0063] 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
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
25 water to be filtered passes through the filter paper 502 made of synthetic resin,
enters the inside of the Erame 501, and is discharged as the treated water 413
through the filtered water extraction port 503.
LO0641 As shown in FIG. 4, the filtration membrane element 406 is-arranged
parallel to a sedimentation direction 409 in which the titanium dioxide composite
30 catalyst is sedimented. A layer of deposited catalyst particles, which is called a
cake layer, is usually formed on the surface of the filtration membrane element 406
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
35 in terms of microfiltration for the titanium dioxide composite catalyst used in the
present embodiment will be described later.
Lo0651
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
5 titanium dioxide composite catalyst separated in the sedimentation separation step
is added again to the arsenic-containing aqueous solution. The titanium dioxide
composite catalyst particles can be repeatedly reused as long as the'burface active
sites of the titanium dioxide are not covered with scale or hardly-decomposable thin
films derived from organic or inorganic substances. That is, in the present
10 embodiment, the step b2, the step c2 and the step e2 may be further performed after
the step f2.
[0066] The performance oft e titanium dioxide com osite catalyst of 4 P 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
15 titanium dioxide composite catalyst needs to be sufficiently larger than the pore
diameter of the filtration membrane element. A larger hfference 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
20 membrane element. FIG. 6 shows the results of particle size distribution
measurements performed on titanium dioxide particles and on the titanium dioxide
composite catalyst of the present embodiment. The numeral 601 denotes the result
of the particle size distribution measurement on the titanium dioxide particles, and
1 the numeral 602 denotes the result of the particle size distribution measurement on ,
25 the titanium dioxide compbsite catalyst of the present embodiment. The average
particle diameter of the titanium dioxide composite catalyst was 5.5 pm. The
average pore diameter of the resin filtration membrane element used for the
microfiltration in the above filtration step (step e2) was 0.42 pm. The average
particle diameter of the titanium dioxide particles was 0.2 pm.
30 [0067]
The arsenic removing step of the present embodiment (step g2) is performed
in the same manner as the arsenic removing step (step gl) of the firpt embodiment.
Therefore, a detailed description thereof is omitted.
4
35 Examples
[0068] (Example 1)
First, a titanium dioxide composite catalyst was produced by the procedure
I
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 (SM molar ratio of 15).
The zeolite particles were immersed in a 0.1 mol/L hydrochloric acid aqueous
5 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 glL of titanium dioxide
particles (P25 manufactured by Degussa AG) and 2.7 gL of the HY-type zeolite
10 particles treated with the hydrochloric acid aqueous solution were added to pure
water prepared by an 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
revolutions of 300 rpm for 60 minutes to obtain a slurry liquid containing a titanium
15 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 shown in FIG.
7A, add a TEM image of the deolite particles alone is Lhown in FIG. 7B. It can be
understood that, in the titanium dioxide composite catalyst 703, the titanium
20 dioxide particles 701 are immobilized directly on the zeolite particle 702 without the
mediation of a thin film.
[0069] The water treatment system shown in FIG. 2 was constructed using the
slurry liquid of the titanium dioxide composite catalyst fabricated as described
above. The slurry of the titanium dioxide composite catalyst was l~adedin to the
25 slurry tank 202, and was supplied to the photoreactor 203. Trivalent arsenic was
used as a pollutant. An aqueous solution having dissolved therein As203, which is
a trivalent arsenic compound, was used as the water to be treated. As203 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 *
30 of the titanium dioxide composite catalyst was sbpplied to the photoreactor so that L
the concentration of the catalyst was 0.4 gL. 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
Spectra Co., Ltd.) and a band-pass filter. The light had a wavelength X of 350 nm,
35 a bandwidth of about 10 nm, and an intensity of 1 mWlcm2.
[00701 The solution having been subjected to the light irradiation was collected,
and the concentration of trivalent arsenic in the solution was quantitatively
analyzed by HPLC/ICP MS (6130 manufactured by Agdent). The trivalent arsenic
oxidation ratio achieved by irradiation for 8 minutes was 95.0%, which revealed
that most of the trivalent arsenic was oxidized by the titanium dioxide composite
catalyst of the present example. Thus, it was confirmed that the titanium dioxide
composite catalyst is effective for oxidation of trivalent arsenic.
[0071] The aqueous solution having undergone the photoreaction 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,
nonpolarized) was used as the light 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 irrahated 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
(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 sehmented for 30 minutes was 30 Lt 5%. This value
of the transmittance change is beyond 20% which is a baseline for determining that
the treated water is allowed to be discharged. Thus, it was confirmed that
solid-liquid separation of the titanium dioxide composite catalyst from an aqueous
soluti n in which the titaniunp dioxide composite cat lyst is suspended 9 7 can be
achieved in a short time by sedimentation separation. The titanium dioxide
composite catalyst separated and recovered was able to be continuously reused by
being introduced again to the photoreactor 203 though the returning part 206.
[0072] Furthermore, pentavalent arsenic contained in the treated water separated
in the solid-liquid separation vessel 205 was removed by a coprecipitation process
using iron (111) chloride. An iron (111) chloride aqueous solution having a
concentration of 10 mglL was added to the treated water, andAs(V) contained in a
supernatant of the treated water after the addition was quantified by
HPLC/ICP MS (6130 manufactured by Agdent). It was found that. the
concentration of As(V) was 10 ppb or less.
[00731 As described above, according to Example 1, a water treatment method
excellent in both photocatalytic activity and solid-liquid separation performance was
achieved.
Coo741 (Example 2)
A water treatment system for treating an arsenic-containing aqueous
solution was constructed by the same way as in Example 1. A 1000 pg/L trivalent
I
arsenic aqueous solution was used as a hazardous substance to be treated. The
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 trivalent arsenic oxidation ratio and the
5 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
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. W light irradiation was performed under the same conditions as in
Example 1, and the trivalent arsenic oxidation ratio achieved after 8 minutes was
10 evaluated. Furthermore, as in Example 1, the aqueous solution having undergone
the photoreaction step and containing the suspended titanium dioxide composite
catalyst was introduced to the solid-liquid separation vessel 205, and the light
transmittance change after 30 minutes was evaluated as the sedimentation
performance. The trivalent arsenic oxidation ratios in the solutions having
15 catalyst concentrations of 0.4 g/L, 3.6 glL, and 16 g/L were 95.0%, 94.5%, and 92.4%,
respectively, which revealed that high trivalent arsenic oxidation ratios are achieved
in these solutions. However, the trivalent arsenic oxidation ratios in the solutions
having catalyst concentrations of 0.04 g/L and 40 g/L were 13.4% a d 15.9%,
respectively, and were smaller than 115 of the trivalent arsenic oxidation ratio in the
20 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 in the range of 0.4 g/L to 16 g/L.
Lo0751 (Comparative Examples)
For comparison, a catalyst composed of quarts beads and titanium dioxide
25 particles immobilized by a binder on the quarts beads was fabricated, and was
evaluated for the trivalent arsenic oxidation ratio and the sedimentation
perfor ance. Amorphous s i p , which is commonly sed, was used as pl r" 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
30 hydrochloric acid aqueous solution with a conceqtration of 1 mol/L, 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
having a particle diameter of 5 pm were immersed in this mixed liq~Ada, nd the
35 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
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 gmorphous silica. A slurry of the
5 photocatalyst was supplied to the photoreactor similarly to the above, and a
comparative solution 1 was thus prepared. The concentration of the titanium
dioxide particles in the comparative solution 1 was equal to the concentration in the
solution C1 (3.6 g/L).
[0076] In addition, a solution in which only nanometer-order titanium dioxide
10 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
concentration of the titanium dioxide particles was set equal to the titanium dioxide
particle concentration in the solution C1 (3.6 g/L). The trivalent arsenic oxidation
ratio and the sedimentation were evaluated using the same trivalent
15 arsenic aqueous solution and the same conditions as employed in Example 2.
[(I0771 Table 2 shows the trivalent arsenic oxidation ratio and the sedimentation
performance in each of the titanium dioxide composite catalyst solutions having
different concentrations and the comparative solutions.
[0078] [Table 21
[00791 In the comparative solution 1 for which a binder was used, the
As(II1) oxidation ratio
[%I
Sedimentation performance
(Change in light
transmittance )
sedimentation performance of the catalyst was at a similar level to that in the
comp site catalyst solutions 1 to E l of Example 2. However, the 9 4 trivalent arsenic 25 oxidation ratio was very low, and specifically was 10. 6 % 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 trivalent arsenic oxidation ratio was 99.8%,
which means that the highest decomposing perhrmance was exhibited. However,
the sedimentation performance of the catalyst was 0%; namely, the catalyst was not
30 separated by sedimentation at all. On the other hand, for the solutions B1 toDl
Titanium dioxide composite catalyst solution
(concentration)
Comparative
solution 1
(binder
~rgiess)
10.6
35 %
A 1
(0.04 g5)
13.4
33%
Comparative
solution 2
(titanium
dioxide
particles)
99.8
0%
B1
(0.4 g/L)
95.0
32%
C 1
(3.6 g/L)
, 94.5
36%
D 1
(16 g5)
92.4
38%
El
(40 g5)
15.9
35%
used in the present example, it was confirmed that both the trivalent arsenic
oxidation performance and the sedimentation performance were so high that
practical water treatment can be achieved.
[0080] (Example 3)
5 The water treatment system shown in FIG. 4 was constructed using a
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 blurry tank 402, and was supplied to the
photoreactor 403. An aqueous solution having dissolved therein Asz03, which is a
10 trivalent arsenic compound, was used as the water to be treated. Asz03, which is a
trivalent arsenic compound, was dissolved in pure water at an aqueous solution
concentration of 1000 pgL, 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 gL.
15 Thereafter, UV 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 A of 350 nm, a bandwidth of about 10 nm, and an intensity
of 1 mWlcm2.
20 [00811 The solution having been subjected to the light irradiation Gas collected,
and the concentration of trivalent arsenic in the solution was quantitatively
analyzed by HPLCIICP MS (manufactured by Agilent). The trivalent arsenic
oxidation ratio achieved by irradiation for 8 minutes was 94.5%) which revealed
that most of the trivalent arsenic was oxidized by the photocatalyst. Thus, it was
25 confirmed that the titanium dioxide composite catalyst is effective for oxidation of
trivalent arsenic, which is actually observed in water environments such as
groundwater, into pentavalent arsenic.
[0082] The suspension solution having undergone the photoreaction step was
introduced to the solid-liquid separation vessel 405. Then, evaluation of
30 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 transmission method.
A fiber coupler including an objective lens was used, laser light was introduced into
the fiber, and thus the solid-liquid separation vessel containing the suspension was
35 irradihted with the light. Tde light having transmitted through the solid-liquid
separation vessel 405 was introduced again into the fiber, and finally, a
light-receiving surface of a photodiode (C10439-03 manufactured by Hamamatsu
Photonics K.K.) was irradiated with the light to measure the transmittance. A
sedimentation amount was calculated from the transmittance. The sedimentation
amount in 30 minutes was 85 f 5%. This value was beyond 80% which is a
baseline for solid-liquid separation of catalyst particles. Thus, it was confirmed
5 that solid-liquid separation in the suspension was achieved in a short time by
sedimentation separation.
[00831 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
10 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.
By contrast, in the case of titanium diohde particles alone, the flow rate which was
initially 100 mL/min was decreased to 1.2 mL/min in 8 hours, and clogging of the
filtration membrane element was caused.
15 [00841 Furthermore, pentavalent arsenic contained in the treated water obtained
through the filtered water extraction port 503 was removed by a coprecipitation
process using iron (111) chloride. An iron (111) chloride aqueous solution having a
concentration of 10 mg/L was added to the treated water, andAs(V) contained in a
supernatant of the treated water after the addition was quantified by
20 HPLC/ICP0MS (6130 manufactured byAgilent). It was found that the
concentration ofAs(~w) as 10 ppb or less.
[0085] As described above, according to Example 3, a water treatment method
excellent in both photocatalytic activity and solid-liquid separation performance was
achieved using the titanium dioxide composite catalyst.
25 [00861 (Example 4)
A water treatment system for treating an arsenic-containing aqueous
solution was constructed by the same way as in Example 3. A 1000 pg/L trivalent
arsenic aqueous solution was used. The amount of the titanium dioxide composite
catalyst-concentrated slurry supplied was adjusted to prepare five a-queous
30 solutions containing the titanium dioxide composite catalyst at different
concentrations, and the trivalent arsenic oxidation ratio, the sedimentation amount,
and the extraction flow rate of the treated water were evaluated for each solution.
The concentration of the titanium dioxide composite catalyst was set to 0.04 g/L for
a solution A2, 0.4 g/L for a solution B2, 3.6 g/L for a solution C2, 16 g/L for a solution
35 D2, and 40 g/L for a solution E2. W light irradiation was performed under the
same conditions as in Example 3, and the trivalent arsenic oxidation ratio achieved
by $-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 I
b
the extraction flow rate of the treated water were evaluated. The trivalent arsenic
oxidation 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 95.0%, 94.5%, and 92.4%,
respectively, which revealed that high trivalent arsenic oxidation ratios are achieved
in these solutions. However, the trivalent arsenic oxidation ratios in the solutions
having catalyst concentrations of 0.04 g/L and 40 g/L were 13.4% and 15.9%,
respectively, and were smaller than 115 of the trivalent arsenic oxidation ratio in the
solution having a catalyst concentration of 3.6 g/L. That is, ih the treatment
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. b
E00871 (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 comgarative solution 1. 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. 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
solution C2 (3.6 g/L).
E00881 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
concentration in the solution C1 (3.6 g/L). The trivalent arsenic oxidation ratio, the
sehmentation amount, and the extraction flow rate of the treated water were
evaluated using the same trivalent arsenic aqueous solution and the same
conditions as employed in Example 4.
E00891 Table 3 shows the results for each of the catalyst particle solutions having
different concentrations and the comparative solutions. In the comparative
solution 3 for which a binder was used, the sedimentation amount A d the
extraction flow rate were at similar levels to those of the catalyst particle solutions
A2 to E2 of Example 4; however, the trivalent arsenic oxidation ratio was very low,
and specifically was 10.6% 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 trivalent arsenic oxidation ratio achieved was 99.8% which was the
highest oxidation ratio. However, the sedimentation amount was 0%; namely, the
catalyst was not separated by sedimentation at all. Moreover, the extraction flow
5 rate was 0 mL1min; 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 a
1
confirmed that all of the trivalent arsenic oxidation ratio, the sedimentation amount,
and the extraction flow rate were at such good levels that practical water treatment
can be achieved.
10 [0090] [Table 31
INDUSTRIAL APPLICABILITY
[0091] The present invention relates to treatment of arsenic contained in drinking
15 water, discharged water, continental rivers, lakes, etc., and provides a method and
system that can treat water continuously in a practical time. The method and
system according to the present invention can be used for household clean water
systems and public clean water systems.
*
As(II1) oxidation ratio
[%I
Sedimentation amount
[%I
Extraction flow rate
[mllminutel
Titanium dioxide composite catalyst solution
(concentration)
Comparative
solution 3
(binder
process)
10.6
89
90
A2
(0.04 glL)
13.4
81.0
110
Comparative
solution 4
(titanium
dioxide
particles)
99.8
0
0
B2
(0.4 glL)
95.0
82.6
100
C2
(3.6 g/L)
94.5
85.9
105
D2
(16 g/L)
92.4
88.5
110
E2
(40 glL)
15.9
89.3
100

We Claim:
1. A method for treating an arsenic-containing aqueous solution, comprising:
a step a of adding catalyst particles to the aqueous solution;
a step b of oxidizing trivalent arsenic 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 gramsfliter or more and 16 gramsfliter 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 pentavalent arsenic produced in
the step b is removed by a coagulation-sedimentation process, an adsorption process,
or a reverse osmosis membrane process.
6. 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