Method for treating soil-contaminating water using photocatalytic material

Abstract

The present invention provides a novel method for treating soil-contaminated water, the method using a photocatalytic material capable of efficiently removing, by light irradiation alone, volatile organic compounds and heavy metals that give rise to soil contamination. The present invention provides a method for treating soil-contaminated water that detoxifies volatile organic compounds contained in soil-contaminated water using a photocatalytic material, the method being characterized by including the steps of (1) subjecting the soil-contaminated water to a gas-liquid separation to obtain a gas phase, and (2) decomposing the volatile organic compounds contained in the gas phase obtained in step (1) using the photocatalytic material. The present invention further provides a method for treating soil-contaminated water using a photocatalytic material to remove heavy metals contained in the soil-contaminated water, the method being characterized, by including the steps of (1) subjecting the soil-contaminated water to a gas-liquid separation to obtain a liquid phase, and (2) removing the heavy metals contained in the liquid phase obtained in step (1) using the photocatalytic material.

Claims

1. A method for treating soil-contaminated water using a photocatalytic material to detoxify volatile organic compounds contained in the soil-contaminated water, the method comprising the steps of: (1) producing the photocatalytic material, wherein the photocatalytic material has a crystalline anatase titanium oxide film, by: (i) heating titanium or a titanium alloy under a nitrogen gas atmosphere having a nitrogen gas pressure of 0.1 MPa to 10 MPa at a temperature of 750° C. or higher for 1 to 12 hours to form a titanium nitride layer on the surface of the titanium or the titanium alloy, wherein the titanium nitride layer has a thickness of 0.5 μm to 50 μm, (ii) immersing the titanium or titanium alloy obtained in step (i) in an electrolyte solution containing at least one acid selected from the group consisting of inorganic acids and organic acids having an etching effect on titanium, and (iii) anodizing the titanium nitride layer by controlling the current so that a voltage equal to or higher than the spark-discharge-generating voltage is applied to obtain the photocatalytic material having the crystalline anatase titanium oxide film, wherein the current density ranges from 1 A/dm.sup.2 to 10 A/dm.sup.2, (2) subjecting the soil-contaminated water to a gas-liquid separation to obtain a gas phase containing volatile organic compounds, wherein water content is reduced using a mist separator, (3) combining the gas phase obtained in step (2) with the photocatalytic material, and (4) decomposing the volatile organic compounds contained in the gas phase obtained in step (2) by irradiating the photocatalytic material with light of 400 nm or less.

2. The method according to claim 1, wherein when undecomposed volatile organic compounds remain in the gas phase after step (4), the method further comprises a step of: (5) subjecting the volatile organic compounds in the gas phase to a secondary treatment by using at least one method selected from the group consisting of a method of removal by adsorption with activated carbon and a method of removal by thermal oxidation.

3. A soil-contaminated water treatment device for detoxifying volatile organic compounds contained in soil-contaminated water using a photocatalytic material, the device comprising: a gas phase-production chamber for subjecting the soil-contaminated water to a gas-liquid separation to obtain a gas phase, a photocatalytic device for decomposing the volatile organic compounds contained in the gas phase by irradiating the photocatalytic material with light of 400 nm or less, wherein the photocatalytic material has a crystalline anatase titanium oxide formed on the surface of a titanium metal or a titanium alloy; and a mist separator in the gas-phase production chamber to reduce water content.

4. The soil-contaminated water treatment device according to claim 3, comprising a secondary treatment chamber for treating the volatile organic compounds in the gas phase by using at least one method selected from the group consisting of a method of removal by adsorption with activated carbon and a method of removal by thermal oxidation.

5. The soil-contaminated water treatment device according to claim 3, wherein the crystalline anatase titanium oxide film is obtained by a method comprising: (i) heating titanium or a titanium alloy under a nitrogen gas atmosphere having a nitrogen gas pressure of 0.1 MPa to 10 MPa at a temperature of 750° C. or higher for 1 to 12 hours to form a titanium nitride layer on the surface of the titanium or the titanium alloy, wherein the titanium nitride layer has a thickness of 0.5 μm to 50 μm, (ii) immersing the titanium or titanium alloy obtained in step (i) in an electrolyte solution containing at least one acid selected from the group consisting of inorganic acids and organic acids having an etching effect on titanium, and (iii) anodizing the titanium nitride layer by controlling the current so that a voltage equal to or higher than the spark-discharge-generating voltage is applied to obtain the crystalline anatase titanium oxide film, wherein the current density ranges from 1 A/dm.sup.2 to 10 A/dm.sup.2.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a graph showing the decomposition of trichloroethylene gas in Example 1.

DESCRIPTION OF EMBODIMENTS

Example 1

(2) Titanium metal was maintained in a nitrogen gas atmosphere at 950° C. for 3 hours to form titanium nitride on the surface of the titanium metal. The titanium metal with titanium nitride thereon was anodized for 30 minutes at a current density of 4 A/dm.sup.2 using an electrolyte solution of 1.5 M sulfuric acid, 0.1 M phosphoric acid, and 0.3 M hydrogen peroxide, thereby obtaining a photocatalytic material.

(3) Using 50 mm squares of the photocatalytic material, 100 ml of 100 ppmV trichloroethylene gas was introduced into a glass container and subjected to light irradiation using fluorescent light (black light: Toshiba Lighting & Technology Corporation) that emits near-ultraviolet of 400 nm or less from the above with the light intensity being adjusted to 1.1 mW/cm.sup.2.

(4) FIG. 1 shows the results of measuring the trichloroethylene concentration over time.

(5) It was found that when using a photocatalytic material comprising titanium metal and a large amount of anatase titanium oxide formed thereon by subjecting titanium metal to gas nitriding and then performing anodization, trichloroethylene gas can be highly efficiently decomposed.

Example 2

(6) Titanium metal was maintained in a nitrogen gas atmosphere, at 950° C. for 1 hour to form titanium nitride on the surface of the titanium metal. The titanium metal with titanium nitride thereon, was anodized for 30 minutes at a current density of 4 A/dm.sup.2 using an electrolyte solution of 1.5 M sulfuric acid, 0.1 M phosphoric acid, and 0.3 M hydrogen peroxide, thereby obtaining a photocatalytic material (total surface area: 52 m.sup.2).

(7) The contaminated groundwater (soil-contaminated water) was pumped and aerated (subjected to a gas-liquid separation) to obtain a gas phase containing trichloroethylene.

(8) A photocatalytic device comprising the photocatalytic material and thirty-two UV lamps with an output of 40 W was aerated with trichloroethylene gas obtained from the soil-contaminated water and subjected to light irradiation using fluorescent light that emits ultraviolet (254 nm).

(9) The concentrations of trichloroethylene at the inlet and outlet of the device were measured by gas chromatography. The flow rate of the gas containing trichloroethylene obtained by pumping and aerating groundwater was set to about 5 to 9 m.sup.3/min.

(10) Table 1 shows the results.

(11) This result also shows that when using a photocatalytic material, comprising titanium metal and a large amount of anatase titanium oxide formed thereon by subjecting the titanium metal to gas nitriding and then performing anodization, trichloroethylene gas in the gas obtained by pumping and aerating contaminated groundwater can be decomposed in a highly efficient manner.

(12) TABLE-US-00001 TABLE 1 Trichloroethylene Concentration (ppmV) Inlet Outlet Decomposition Flow rate concentration concentration rate (%) (m.sup.3/min) 78 12 84.4 5.5 136 42 68.9 6.7 196 65 67.0 8.0

(13) After the reaction of the decomposition treatment using the photocatalytic material, undecomposed trichloroethylene remained in the gas phase. Accordingly, removal by adsorption with activated carbon can be further performed as a secondary treatment to completely remove the trichloroethylene.