Patent Application
20230225327
The present invention relates to a sterilizing liquid and a method of producing the same that can be widely used for sterilization in fields of agriculture, food, hygiene, medical treatment, and the like.
Conventionally, there has been proposed a sterilizing method which involves causing cavitation in a liquid substance, generating plasma in air bubbles generated by the cavitation to produce reactive oxygen species such as hydroxyl radicals (OH) in the liquid substance, and making the liquid substance free from microorganisms by oxidizability of the reactive oxygen species (see Patent Documents 1 to 3, for example).
The method of sterilizing a liquid substance described in Patent Documents 1 to 3 can obtain a strong sterilizing power because this method makes the liquid substance free from microorganisms by oxidizability of reactive oxygen species. However, this method has a problem that application methods and usage are restricted because it is necessary to put the liquid substance to be sterilized directly into a sterilization device.
In view of the problem of the above-described conventional method of sterilizing a liquid substance, it is an object of the present invention to provide a sterilizing liquid and a method of producing the same that have no restrictions on application methods and usage and can be widely used for sterilization.
In order to achieve the above object, a sterilizing liquid of the present invention is characterized by including water containing reactive oxygen species and a nanoparticle catalyst (including secondary particles formed by aggregated nanoparticles) in a stationary state.
In this case, the sterilizing liquid is characterized in that a concentration of hydrogen peroxide in the reactive oxygen species immediately after production of the sterilizing liquid is 30 to 2000 ppm, and a concentration of the nanoparticle catalyst is 16 ppm or more.
Here, “immediately after production of the sterilizing liquid” means within a few minutes after production of the sterilizing liquid.
Further, the sterilizing liquid is characterized in that a sterilizing effect lasts for more than a few hours.
A method of producing a sterilizing liquid of the present invention for producing the above-described sterilizing liquid is characterized by causing cavitation in water having a conductivity of 2000 μS/cm or less and COD of 2000 ppm or less while causing the water to flow, and generating plasma by a plasma generation mechanism in which a pulse voltage is applied across electrodes in the water including air bubbles mainly containing water vapor generated by the cavitation, thereby producing a sterilizing liquid including water containing reactive oxygen species and a nanoparticle catalyst in a stationary state.
According to the sterilizing liquid and the method of producing the same of the present invention, it is not necessary to put a liquid substance to be sterilized directly into a sterilization device, unlike the liquid substance sterilizing method described in Patent Documents 1 to 3. In particular, the sterilizing liquid of the present invention is secure and safe because it uses water as its raw material, and the reactive oxygen species that are components exerting a sterilizing action finally turn back to water. Therefore, the present invention can provide the sterilizing liquid and the method of producing the same that have no restrictions on application methods and usage and can be widely used for sterilization.
An embodiment of a sterilizing liquid and a method of producing the sterilizing liquid according to the present invention are described below.
The sterilizing liquid of the present invention includes water containing reactive oxygen species and a nanoparticle catalyst in a stationary state. This sterilizing liquid can be produced by causing cavitation in water having a conductivity of 2000 μS/cm or less and COD of 2000 ppm or less while causing the water to flow, and generating plasma by a plasma generation mechanism in which a pulse voltage is applied across electrodes in the water including air bubbles mainly containing water vapor generated by the cavitation.
A device of producing the sterilizing liquid can be a conventionally known device, that is, a mechanism of causing cavitation in water and generating plasma using a plasma generation mechanism in which a pulse voltage is applied across electrodes in the water including air bubbles mainly containing water vapor generated by the cavitation. More specifically, such a device of producing the sterilizing liquid can be a device obtained by combining a cavitation generation mechanism for causing cavitation in water by changing the flow-passage cross-sectional area with a nozzle, an obstacle, etc., and the plasma generation mechanism with each other as described in Patent Documents 1 and 2, a device obtained by combining a cavitation generation mechanism for causing cavitation in water by rotating a rotor and the plasma generation mechanism with each other as described in Patent Document 3, or the like.
Here, an example is described which uses the device obtained by combining the cavitation generation mechanism that causes cavitation in water by rotating the rotor and the plasma generation mechanism with each other as the sterilizing liquid producing device.
This sterilizing liquid producing device is configured to include a tank 1 that stores water therein, a stirrer 2 as a cavitation generation mechanism that stirs the water supplied from the tank 1, a plasma generation mechanism 3 that causes cavitation by the stirrer 2 and causes generation of plasma in the water including air bubbles (cavitation bubbles) mainly containing water vapor generated by the cavitation, and a conduit line 4 that connects these mechanisms and causes the water to circulate, as shown in
Causing the water to circulate is not essential. A configuration for one-path processing may be used, as long as processing efficiency is increased more, for example, by providing a plurality of plasma generation mechanisms and/or installing a plurality of pairs of electrodes.
The stirrer 2 is configured to include a rotor 22 that is provided in a casing 21 to be rotatable and is concentric with the casing 21, a motor 23 that drives the rotor 22 to rotate, and the like.
The plasma generation mechanism 3 is configured to include electrodes 31 formed by a conductor, a pulse power supply 32 that applies, for example, a voltage equal to or higher than a discharge inception voltage and a pulse voltage with a pulse width of 1.5 μs or less and a repetition frequency of 100 kHz or more across the electrodes 31, and the like. By the plasma generation mechanism 3, vapor is ionized (changed to plasma) through high voltage breakdown discharge caused by the pulse voltage in an insulating air-bubble region, whereby in-liquid plasma (cavitation plasma, which may be referred to as “CBP (Cavitation bubble plasma)” in the present specification) is generated.
The form of discharge caused by the pulse voltage is preferably glow discharge (low-temperature plasma produced by glow discharge). The glow discharge can synthesize a nanoparticle catalyst in the crystal form, the quasicrystal form, the amorphous form, and the like at low temperature in an energy-efficient manner; the nanoparticle catalyst is made of, for example, an inorganic compound such as a metal or metal oxide arising from a wear component of the electrode 31, or a sulfide or chloride arising from impurities contained in the water produced by the in-liquid plasma. That is, the nanoparticle catalyst is nanoparticles of a component of the electrode 31. In the case where the electrodes are made of a metal, the nanoparticles are metal nanoparticles at the moment when being generated. Depending on the type of the metal, the nanoparticles are oxidized or, when chlorine or sulfur is contained in the water, chlorinated or sulfurized (the nanoparticles may be turned into a compound with a substance contained in impurities).
The flow rate of water near the electrodes 31 in the plasma generation mechanism 3 is about 10 m/s and desirably 5 m/s or more.
The electrodes 31 are preferably disposed to be opposed to each other in the direction perpendicular to the flow of water. However, other arrangements including the inverted V-shape arrangement can be employed, as long as plasma can be produced.
As the material for the electrodes 31, any of conductor materials including: metals such as tungsten, copper, iron, silver, gold, platinum, aluminum, scandium, titanium, vanadium, chromium, manganese, cobalt, nickel, zinc, gallium, germanium, yttrium, zirconium, molybdenum, technetium, ruthenium, rhodium, palladium, cadmium, indium, tin, antimony, lanthanoid, hafnium, tantalum, rhenium, osmium, iridium, thallium, bismuth, and polonium; carbon; conductive diamond; alloys of these materials; composite materials of these materials (including a member covered with a thin film formed by plating, dry coating, or the like); and oxides (including a resultant material obtained by a reaction of the surface of the electrodes 31 with water) can be selected depending on the usage. The electrodes 31 disposed to be opposed to each other can differ in the material to be used and/or the size. For example, the electrodes 31 are made of gold and silver, respectively.
The electrodes 31 may have a shape of a cylinder, a prism, an elliptical cylinder, a cone, or a pyramid. Although one pair of electrodes 31 is enough, two or more pairs of electrodes 31 may be provided in order to increase processing efficiency more. Regarding the plasma generation mechanism, one set is enough. However, two or more sets may be provided in order to increase the processing efficiency more.
One of the electrodes 31 may be grounded or not grounded. The configuration in which one of the electrodes 31 is not grounded is safer because a discharge path is limited between the electrodes.
Further, water can be processed at a temperature of 50° C. or less by operating a cooler provided in the sterilizing liquid producing device, for example, a jacket cooling unit (not shown) provided in the stirrer 2, as necessary.
In the water thus produced by in-liquid plasma, hydrogen peroxide or the like as reactive oxygen species is stably present, in addition to the nanoparticle catalyst, even in a stationary state in which the operation of the sterilizing liquid producing device is stopped. Therefore, this water can be used as a sterilizing liquid having a sterilizing action due to the oxidizability of hydrogen peroxide.
This sterilizing liquid continuously produces reactive oxygen species, for example, hydroxyl radicals (·OH) that have the highest oxidizability among the reactive oxygen species due to a sterilizing action by long-lived reactive oxygen species (superoxide anion radicals (·O2−), hydroperoxy radicals (HOO·), and hydrogen peroxide (H2O2)) present in the sterilizing liquid and a catalyst action of the nanoparticle catalyst present in the sterilizing liquid in which hydrogen peroxide (H2O2) is present, whereby the sterilizing effect is further enhanced, and the effect lasts a long time.
Therefore, regarding the sterilizing liquid, the amounts of hydrogen peroxide and the nanoparticle catalyst that are present in the sterilizing liquid are important. According to the present invention, a large amount of sterilizing liquid can be produced only from water as a raw material in a simple, fast, and simultaneous manner.
As is apparent from
Next, a test is described which was conducted using a sterilizing liquid producing device (specifically, a “cavitation plasma device” manufactured by Nihon Spindle Manufacturing Co., Ltd.).
A processing condition (preferable ranges) for a sterilizing liquid producing device is shown in Table 1, and relations between processing time and a pH value, a conductivity, a hydrogen peroxide concentration, and the amount of electrode wear for each electrode material are shown in
As for Table 1 and
Next, a residual sterilizing effect test was conducted using Escherichia coli as a model microorganism in accordance with the following test method.
Escherichia coli NBRC 3301
SCDLP agar medium (manufactured by Nihon Pharmaceutical Co., Ltd.), pour plate cultural method, 35° C.±1° C., for 2 days
The test bacteria were cultured on a nutrient agar medium (manufactured by Eiken Chemical Co., Ltd.) at 35° C.±1° C. for 18 to 24 hours, then suspended in purified water, and adjusted to set the number of bacteria to about 107 mL. A test bacteria solution was thus prepared.
A sodium hypochlorite solution prepared to have a concentration of about 0.02% was put into the sterilizing liquid producing device, and the device was operated for 15 minutes under a condition without plasma processing. After drainage, the inside of the sterilizing liquid producing device was rinsed with tap water, tap water containing 0.002% sodium thiosulfate, and distilled water in this order, followed by drainage.
Into the sterilizing liquid producing device after pre-processing, 250 mL of purified water (ion-exchanged water) was put. The device was then operated for 5 minutes under a processing condition shown in Table 2. Sample water was thus obtained. The sample water was collected in a sterile container made of synthetic resin and left to stand for a predetermined time. Then, 0.1 mL of the test bacteria solution was inoculated into 10 mL of the sample water after standing for the predetermined time, to obtain a test solution. Thereafter, the test solution was diluted 100 times with an SCDLP medium (manufactured by Nihon Pharmaceutical Co., Ltd.), and the number of viable bacteria in the test solution was measured using a medium for measuring the number of bacteria.
As a control, purified water (ion-exchanged water) with the test bacteria solution inoculated thereto was also tested in the same manner, and the number of viable bacteria in the test solution was measured using the medium for measuring the number of bacteria.
The residual sterilizing effect test shown in
Next, a sterilizing effect test (1) was conducted using an aqueous solution of organic matter (methylene blue (which may be referred to as “MB” in the present specification) in place of microorganisms in the following procedure.
The reason why the aqueous solution of organic matter (methylene blue) was used in place of microorganisms is that a factor of a sterilizing effect and a factor of an effect of decomposing of the organic matter (methylene blue) are both reactive oxygen species, and therefore methylene blue, which can be evaluated more easily, is used (the same applies to a sterilizing effect test (2) described later).
Tables 3 and 4 show processing conditions for a sterilizing liquid producing device.
First, a test was conducted in order to confirm that an organic matter decomposition effect may be greatly affected by residues such as organic matter in the sterilizing liquid producing device.
The change with time was greatly different depending on the pre-processing immediately before the processing. In particular, an MB decomposition speed was greatly reduced after the MB solution was put before the processing. Thus, it was confirmed that the organic matter decomposition effect was greatly affected by residues (contamination) such as organic matter in the sterilizing liquid producing device.
Further, based on the above findings, the inside of the CBP device was washed five times with ion-exchanged water before the test, and glass cells, glass beakers, and a processed liquid discharge port of the CBP device were subjected to ultrasonic cleaning with ion-exchanged water for 5 minutes, whereby contaminants were removed.
In addition, a container (a quartz glass cell (for an ultraviolet-visible absorption spectrum) or a beaker (container)) from which a sterilizing liquid is taken out is also cleaned. It is desirable that a container for storing the sterilizing liquid therein be non-organic and uncontaminated.
Therefore, in the following test, Table 4 (tp=5 min) was set as a standard condition, and a method including the above-described pre-processing was adopted.
These show that OH radicals and O radicals are produced in plasma-processed water processed under the processing condition for the sterilizing liquid producing device.
Next, an organic matter decomposition effect test was conducted in the following procedure.
Immediately after processing under the processing condition for a sterilizing liquid producing device shown in Table 4, 260 μL of an MB solution having a concentration of 8600 ppm was put into the device. The change with time of a peak intensity at a wavelength of 664 nm in an ultraviolet-visible absorption spectrum of an MB solution after the MB solution was put was measured using an ultraviolet-visible spectrophotometer (V-730BIO, JASCO Corporation).
As shown in
Meanwhile, when tp=2 min, the absorption peak value decreases exponentially in a range of te=2 to 20 min with increase in the elapsed time. After te=20 min, the slope becomes gentle. At te=30 min, the absorption peak value decreases to 1.8. The decreased speed is the maximum at tp=5 min, and decreases to 1.4 when the absorption peak value te is 30 min. After tp=5 min, the decrease speed becomes smaller, and when tp=20 min, the absorption peak value decreases only up to 1.82. From these results, it can be said that the decomposition effect remains more in the plasma-processed water in the case of tp=5 min.
As shown in
Here, the hydrogen peroxide concentration is a measurement value immediately after production of a sterilizing liquid (within a few minutes after production of the sterilizing liquid).
As shown in
As shown in
Next, a sterilizing effect test assuming contaminated water was conducted using an MB solution in place of ion-exchanged water.
This test assumes a case where water to be used is contaminated before CBP processing (a case where COD is high), uses methylene blue in place of contamination, and uses an MB solution obtained by dissolving methylene blue in water before the CBP processing. Here, a test in which plasma processing time is shortened is a test assuming that contamination remains even after the CBP processing.
Table 5 shows a processing condition for a sterilizing liquid producing device.
The following was revealed from the test for an organic matter decomposition effect shown in
Next, a sterilizing effect test (2) was conducted using an aqueous solution of organic matter (methylene blue) in place of microorganisms in the following procedure.
This sterilizing effect test (2) was conducted to examine a duration of a sterilizing effect of a sterilizing liquid.
Table 6 shows a processing condition for a sterilizing liquid producing device.
In the following test, Table 6 (tp=5 min) was set as a standard condition, and a method including the same pre-processing as that in the sterilizing effect test (1) was adopted.
Next, an organic matter decomposition effect test was conducted in the following procedure.
Immediately after processing under the processing condition for a sterilizing liquid producing device shown in Table 6, 260 μL of an MB solution having a concentration of 8600 ppm was put into the device, as shown in
From
That is, as shown in
Here, it is considered that hydrogen peroxide (H2O2) is produced by performing CBP processing for water, as shown in
Further, a part of tungsten (W) nanoparticles (a nanoparticle catalyst) is present as tungsten (W) ions under the presence of hydrogen peroxide (H2O2), as shown in
Furthermore, by an action of the tungsten (W) nanoparticles (the nanoparticle catalyst) and the tungsten (W) ions, hydrogen peroxide (H2O2) dissociates to produce hydroxyl radicals (·OH) that are OH radicals, as shown in
Consequently, a sterilizing effect of a sterilizing liquid lasts a long time (specifically, more than a few hours (16 hours or more from the test result in
Here, it is apparent from the result of a comparative test for a sterilizing liquid that is CBP-processed water (CBPTW) and a hydrogen peroxide solution, shown in
A relation between a processing condition for a sterilizing liquid producing device in the sterilizing effect test (2), the amount of electrode wear, and the water quality (with no MB solution added) is described here, with reference to
As shown in
When tp=1 min, the pH value is 4.4, and remains substantially constant irrespective of increase in the elapsed time. It is found that the pH value decreases with increase in the plasma processing time, but is substantially constant at any elapsed time after processing except for the case of tp=30 min. In the case of tp=30 min, the pH value is 3.3 at te=0.03 h, then increases until te=1 h, and thereafter is substantially constant to be 3.6. It is considered that the cause of decrease in the pH value with increase in the processing time is that tungsten reacts with water or hydrogen peroxide to produce H+.
When tp=1 min, the conductivity is about 16 μS/cm at te=0.1 h, increases until te=1.5 h, and tends to be saturated after reaching about 18.5 μS/cm. Thereafter, the conductivity decreases little by little and is about 18.3 μS/cm at te=24 h. When tp=5 min or less, change of the conductivity with time has the same tendency as that when tp=1 min. However, when tp=7 min, the conductivity decreases as the elapsed time after processing increases. When tp=30 min, the decrease is particularly remarkable. The conductivity rapidly decreases until te=1 h, that is, the conductivity that is 200 μS/cm at te=0.1 h decreases to 140 μS/cm at te=1 h. Thereafter, the conductivity tends to be saturated and is 138 μS/cm at te=24 h. The cause of the decrease in conductivity is considered to be that W dissolves in water having a high hydrogen peroxide concentration. It is considered that W is precipitated in the CBP-processed water to cause decrease of W ions in a solution and to lower the conductivity.
When tp=1 min, the ORP is about 588 mV at te=0.1 h, increases little by little with increase in elapsed time after processing, and increases to about 592 mV at te=24 h. Further, when tp=7 min or less, the ORP increases little by little with increase in the elapsed time after processing, as with the tendency when tp=1 min. However, when tp=10 min or more, the ORP decreases with increase in the elapsed time after processing. It is considered that hydrogen peroxide is involved in this tendency. Hydrogen peroxide is decomposed into OH radicals by being added to metal powder. OH radicals are an oxidant and are involved in the increase in the ORP. From this, it is considered that the ORP value is increased by decomposition of hydrogen peroxide. Therefore, it is considered that the ORP decreased due to decrease in hydrogen peroxide and decrease in produced OH radicals.
Hydrogen peroxide is produced by a reaction of water during plasma production. When tp=1 min, the hydrogen peroxide concentration is about 360 ppm at te=0.03 h, decreases exponentially with increase in elapsed time after processing, and is about 60 ppm at te=24 h. Although the hydrogen peroxide concentration decreased exponentially with regard to all processing times, the same level of hydrogen peroxide concentration was shown at te=0.03 h (about 30 ppm when tp=30 min). When tp=5, 7, and 10 min, the hydrogen peroxide concentration at te=30 h became lower as the processing time was longer. It is known that hydrogen peroxide dissociates into OH radicals in an environment of metal ions. As for the hydrogen peroxide concentration, it is considered that due to the above factors, a dissociation speed increases as the processing time increases. However, considering the relation between the elapsed time after production of a sterilizing liquid and the MB decomposition rate for each processing condition (tp=1 min to 30 min) for a sterilizing liquid producing device shown in
From the experimental results shown in
The in-liquid plasma (cavitation plasma) is produced by application of high-repetition and high-voltage pulses.
As shown in
The reason for this is considered to be that a conductivity of processed water increases as CBP processing proceeds, a voltage applied across electrodes decreases in relation to the output impedance of a power supply, the probability that the applied voltage falls below a discharge inception voltage increases, and the number of times that plasma is not produced increases.
As described above, when the plasma generation rate decreases, reactive oxygen species present in an initial stage decrease. Therefore, in the case of using a sterilizing liquid within 2 hours after production of the sterilizing liquid, a preferable CBP processing time is tp=about 5 min.
Next, a relation between an electrode material, a sterilizing effect, the amount of electrode wear, and the water quality (with no MB solution added) as for a sterilizing liquid (CBPTW) produced under the processing condition (Table 6 (tp=5 min)) for a sterilizing liquid producing device in the sterilizing effect test (2), using W, Fe, Cu, and Ag as the electrode material, is described with reference to
In the case of a W electrode, the decomposition rate exponentially increases to 20% when elapsed time after processing te=0.03 to 3 h. Thereafter, the slope becomes gentle, and the decomposition rate increases to 58% at te=30 h. In the case where CBPTW was prepared with other electrodes and MB was added, the decomposition rate rapidly increased at te=0.1 to 3 h, and then showed a saturation tendency as with the W electrode. However, in the case of Cu and Fe electrodes, the decomposition rate at te=30 h increased to about 100%. In the case of the Cu and Fe electrodes, a manner of increase of the decomposition rate was also different. At te=3 h, the decomposition rate for the Cu electrode increased by about 76%, whereas the decomposition rate for the Fe electrode increased by only about 65%. These results show that an effect of decomposing MB is higher in the case where the electrode material is Cu than in the case where the electrode material is Fe. In the case where the electrode material is Ag, the decomposition rate at te=30 h is about 10%, i.e., almost no decomposition occurs. These results show that the MB decomposition effect is higher in the order of Ag<W<Fe<Cu as the electrode material.
As shown in
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A sterilizing liquid and a method of producing the same according to the present invention have been described by way of an embodiment in the above description. However, the present invention is not limited thereto, and the configuration can be changed as appropriate without departing from the gist of the present invention.
A sterilizing liquid and a method of producing the same according to the present invention are safe and secure, have no restrictions on application methods and usage, and can be widely used for sterilization because a raw material of the sterilizing material of the present invention is water, and reactive oxygen species as a component exerting a sterilizing action return to water finally. Accordingly, the sterilizing liquid and the method of producing the same according to the present invention can be widely used for sterilization (for example, for killing pathogens in farms by spraying the sterilizing liquid, for killing pathogens in plant factories, as a post-harvest sterilizing liquid for putrefactive bacteria, and for prevention of virus infection in dairy farming) in fields of agriculture, food, hygiene, medical treatment, and the like.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2020-096108 | Jun 2020 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2021/020982 | 6/2/2021 | WO |
