JPH09234338A - Photolysis of organochlorine compound - Google Patents

Abstract

PROBLEM TO BE SOLVED: To efficiently decompose gas containing an organochlorine compd. in an ultraviolet reaction tank and to enable treatment by a compact reaction tank by returning a part of gas after the irradiation with ultraviolet rays to the ultraviolet reaction tank. SOLUTION: An ultraviolet reaction tank 1 is an internal irradation type cylindrical reaction container 3 made of pylex glass having a quartz pipe 2 arranged therein and a sterilizing lamp 4 is inserted into a quartz pipe 2 to be allowed to light. Thereafter, a part of gas after irradiation with ultraviolet rays is returned to the ultraviolet reaction tank by a pump. When gas containing chlorine molecules formed by decomposition reaction is returned to the ultraviolet reaction tank, chlorine molecules are optically excited without obstructing the light absorption of trichloroethylene or tetrachloroethylene. The optically excited chlorine molecules are dissociated to chlorine atoms to attack unreacted trichloroethylene or tetrachloroethylene molecules to oxidatively decompose them. Therefore, the photolysis of an organochlorine compd. is generated efficiently in the ultraviolet reaction tank to enable the treatment in the reaction tank.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a method for photodecomposing organic chlorine compounds. More specifically, the present invention is a gas obtained by vacuum extraction of soil contaminated with an organic chlorine compound, an organic chlorine compound contained in a gas obtained by air stripping groundwater, etc., decomposed by ultraviolet irradiation. The present invention relates to a photolysis method of an organochlorine compound which has a high decomposition efficiency and can be downsized in the reaction tank when it is removed.

[0002]

Organochlorine compounds have been used for many years as a solvent having an excellent dissolving power, but their carcinogenicity has recently been pointed out, and their release to the environment is limited. However, pollution of organic chlorine compounds in the environment has not been solved yet, and trichlorethylene, tetrachlorethylene, etc. are detected in wells nationwide, and pollution of groundwater and soil by organic chlorine compounds has become a major problem. Since organic chlorine compounds are relatively volatile, they readily diffuse into the atmosphere as a gas. As a method of removing organic chlorine compounds, which are substances that cause contamination of groundwater and soil, etc., currently, vacuum extraction from the soil or air stripping is performed, and then the waste is recovered by an adsorbent such as activated carbon. The method of processing is generally performed. However, the method using activated carbon adsorption is not a fundamental solution because it simply transfers contaminants unless the contaminants are collected and reused. Therefore, research on a method for decomposing an organic chlorine compound to render it harmless has been actively conducted so far, and various techniques have been proposed. For example, a method in which exhaust gas containing an organic halogen compound is irradiated with ultraviolet rays to make an acidic decomposition gas, which is then cleaned with an alkali to detoxify it (JP-A-62-191025), wastewater containing an organic halide. Aerating the gas, irradiating the discharged gas with ultraviolet rays and then cleaning with alkali (Japanese Patent Laid-Open No. 62-
No. 191095), an apparatus for decomposing a halogenated acyclic hydrocarbon compound by mixing a halogenated acyclic hydrocarbon compound and ozone and irradiating the mixture with ultraviolet rays (JP-A-1-236).
No. 925), an apparatus for aeration treatment of wastewater containing an organic halide while irradiating ultraviolet rays, and further irradiating the discharged gas with ultraviolet rays and then performing alkali cleaning (JP-A-2-
No. 75391) is proposed. Since organic chlorine compounds are relatively easily photochemically decomposed, many decomposition methods utilizing ultraviolet rays have been proposed.
However, it is not easy to completely decompose an organic chlorine compound only by a photoreaction by irradiation with ultraviolet rays, and a reaction intermediate having a chlorine atom in a molecule tends to remain. For this reason, volatile organic compounds contained in soil or water are extracted by aeration, and are irradiated with ultraviolet rays in the presence of oxygen to oxidize, and the gas containing the oxidation product is contacted with water, and then aerobic organisms are contacted. As disclosed in Japanese Patent Laid-Open No. 7-116467, a treatment method has been proposed in which a photoreaction by ultraviolet irradiation is performed and then a microbial reaction is further performed. However, when an organic chlorine compound is irradiated with ultraviolet rays, the organic chlorine compound is converted into a reaction intermediate that is easily decomposed by microorganisms, and at the same time, a reaction intermediate that adversely affects the microorganisms is also produced, which prolongs the treatment of microorganisms. It is difficult to do it stably for a period of time. On the other hand, in Japan, sites that are contaminated with organic chlorine compounds and require purification often have a small site area, and it is not easy to bring in a large-scale purification device.
Therefore, there is a demand for a more compact and high-performance purification device. In the case of gas extracted from soil in vacuum, the concentration of organochlorine compounds present in the gas gradually decreases in the long term immediately after the start of purification. There is a demand for a purification device capable of performing stable treatment. However, a general method for optimizing an ultraviolet reaction tank for performing ultraviolet irradiation has not yet been developed. Therefore, a large reaction tank may be designed carelessly so as to cope with the fluctuation of the concentration of the organic chlorine compound. Therefore, the ultraviolet decomposition method, which should have high efficiency originally, has not been put into practical use for economical reasons.

[0003]

DISCLOSURE OF THE INVENTION The present invention provides a method for photodecomposing an organochlorine compound which efficiently decomposes a gas containing an organochlorine compound in an ultraviolet reaction vessel and enables treatment in a compact reaction vessel. It was made for the purpose of doing.

[0004]

Means for Solving the Problems As a result of intensive studies to solve the above problems, the present inventors have led a gas containing an organochlorine compound to an ultraviolet reaction tank and irradiating it with ultraviolet rays to decompose it. At that time, it was found that the decomposition efficiency was improved by returning a part of the gas after the ultraviolet irradiation to the ultraviolet reaction tank, and the present invention was completed based on this finding. That is, the present invention provides (1) a method of introducing a gas containing an organochlorine compound into an ultraviolet reaction tank and irradiating it with ultraviolet rays to decompose the organochlorine compound, wherein a part of the gas after ultraviolet irradiation is introduced into the ultraviolet reaction tank. The present invention provides a method for photolyzing an organic chlorine compound, which is characterized in that it is returned. Furthermore, as a preferred embodiment of the present invention, (2) the photolysis method for an organochlorine compound according to (1), wherein the organochlorine compound is extracted in a gas state from contaminated soil or groundwater. ) Gas return rate after UV irradiation is 0.1-70%
The method for photolyzing an organochlorine compound according to item (1) or (2) is

[0005]

BEST MODE FOR CARRYING OUT THE INVENTION The method of the present invention can be applied to a gas containing an organic chlorine compound. The target organic chlorine compound is not particularly limited, but the method of the present invention is suitable for decomposing an aliphatic chlorine compound, and particularly trichlorethylene and tetrachloroethylene can be efficiently decomposed. There is no particular limitation on the gas containing the target organochlorine compound, and there is no particular limitation on exhaust gas from painting plants, exhaust gas from dry cleaning plants, vacuum extraction gas from contaminated soil,
It can be applied to gas generated during air stripping of groundwater. Among these, the method of the present invention is
In particular, it is suitable for treating gas obtained by extracting organic chlorine compounds from soil or groundwater. Organochlorine contamination of soil and groundwater is often mainly caused by trichloroethylene or tetrachloroethylene. When the contamination is soil, the organic chlorine compound is mainly taken out by vacuum extraction method, and in the case of groundwater, the organic chlorine compound is taken out in a state of being once mixed with air by pumping and aeration method. The concentration of the organic chlorine compound in the air may reach several thousands to 10,000 ppm in the case of the vacuum extraction method. In the method of the present invention, in the ultraviolet reaction tank,
The gas containing the organic chlorine compound is irradiated with ultraviolet rays. Since the energy of ultraviolet rays is inversely proportional to the wavelength, it is preferable that the ultraviolet rays include those having a wavelength of 300 nm or less, because sufficient energy is supplied to break the carbon-chlorine bond of the organic chlorine compound. When the organic chlorine compound is irradiated with ultraviolet light, non-bonded n electrons of a chlorine atom are excited,
The carbon-chlorine bond is radically broken. When the organic chlorine compound has a carbon-carbon double bond, such as trichloroethylene and tetrachloroethylene, and a chlorine atom is bonded to a carbon atom, it has a maximum absorption peak near a wavelength of about 200 nm, and thus has a wavelength of 300 nm. Ultraviolet rays having the following wavelengths are absorbed particularly efficiently, the π electron of the carbon-carbon double bond is excited, and radical cleavage of the carbon-chlorine bond occurs.

[0006] As described above, it is preferable that the ultraviolet rays include those having a wavelength of 300 nm or less. At the same time, light having a wavelength exceeding 300 nm is effectively absorbed by chlorine molecules generated by the decomposition of the organic chlorine compound. Examples of the light source that can be used for irradiation with ultraviolet light include a low-pressure mercury lamp, a medium-pressure mercury lamp, a high-pressure mercury lamp, a xenon lamp, a deuterium lamp, and a metal halide lamp. Among these, a low-pressure mercury lamp having high ultraviolet irradiation efficiency can be particularly preferably used. When a lamp protection tube is provided to protect the light source, quartz glass or transparent Teflon is preferable, and Pyrex glass is not suitable because it cuts wavelengths of 300 nm or less. In the method of the present invention, the organic chlorine compound undergoes a dechlorination reaction when it becomes unstable when it is excited by absorbing ultraviolet light, and when an oxygen molecule is present in the system, it reacts with the oxygen molecule to bond with an oxygen atom. It becomes a reaction intermediate having a chlorine atom. Two of the released chlorine radicals react to form chlorine molecules, and the remaining radicals attack other organic chlorine compound molecules radically to generate carbon-centered radicals. When an oxygen molecule is present in the system, the carbon-centered radical reacts with the oxygen molecule to form a reaction intermediate having a chlorine atom also bonded to an oxygen atom. As a result of such a radical reaction, carbon dioxide, hydrogen chloride, acid chloride, phosgene, chlorine molecules and the like are finally produced as photolysis products. Since the reaction proceeds in a chain, the quantum yield of this reaction is generally high, and the concentration of the organochlorine compound can be efficiently reduced.

Chlorine molecules produced by the photodecomposition reaction have a maximum absorption peak at a wavelength of about 330 nm, and the absorbance decreases sharply on the shorter wavelength side, but on the longer wavelength side, the decrease of the absorbance up to the visible region is compared. Since it is relatively small, it absorbs little of the wavelength of 254 nm, which is absorbed by trichlorethylene or tetrachloroethylene, but absorbs part of the relatively long wavelength ultraviolet and visible light that is not absorbed by trichlorethylene or tetrachloroethylene. To do. Therefore, when the gas containing chlorine molecules generated by the decomposition reaction is returned to the ultraviolet reaction tank, the chlorine molecules are photoexcited without interfering with the light absorption of trichlorethylene or tetrachloroethylene. The photoexcited chlorine molecule is cleaved into chlorine atoms, attacking unreacted trichlorethylene molecule and tetrachloroethylene molecule,
Subsequently, these are oxidatively decomposed. Therefore, the photolysis of the organic chlorine compound occurs efficiently in the ultraviolet reaction tank, and the treatment in the compact reaction tank becomes possible. In the method of the present invention, the gas return rate after irradiation with ultraviolet rays is preferably 0.1 to 70%, more preferably 5 to 60%, and further preferably 10 to 50%. In the method of the present invention, by returning a small amount of gas containing chlorine molecules after irradiation of ultraviolet rays to the ultraviolet reaction tank, the decomposition rate of the organic chlorine compound is dramatically improved, but the gas return rate is less than 0.1%. If so, the effect of the method of the present invention may not be sufficiently obtained. Further, even if the gas return rate exceeds 70%, the decomposition rate of the organochlorine compound does not improve with the increase in the gas return rate, and the energy consumption for returning the gas increases, which causes The pressure loss may increase.

[0008]

EXAMPLES The present invention will be described in more detail with reference to the following Examples, which should not be construed as limiting the present invention. Example 1 Using air containing about 500 ppm of trichlorethylene, the relationship between the gas return rate after ultraviolet irradiation and the trichlorethylene decomposition rate was investigated. FIG. 1 is a cross-sectional view of the ultraviolet reaction tank used in this example, and FIG. 2 is an explanatory view of the apparatus used in this example. The ultraviolet reaction tank 1 has a diameter of 2 inside.
An internal irradiation type cylindrical reaction vessel 3 made of Pyrex glass having a diameter of 10 cm and a height of 20 cm equipped with a quartz tube 2 of cm, and a sterilization lamp 4 [made by Toshiba Corporation, GL 6 (6
W)] was inserted and turned on. 1 liter / min of air containing trichlorethylene was introduced from a gas inlet 5 provided in the lower part of the ultraviolet reaction tank 1, and a bypass was provided in a gas outlet 6 after ultraviolet irradiation provided in an upper part of the ultraviolet reaction tank, and a pump 7
Then, a part of the gas after ultraviolet irradiation was returned to the ultraviolet reaction tank. The amount of gas returned after ultraviolet irradiation was measured by the flow meter 8 to calculate the return rate. When the gas is sampled at the points A and B in FIG. 2 and the reaction reaches a steady state,
The trichloroethylene concentration of the gas at the inlet and the outlet of the reaction vessel was measured by a gas chromatograph equipped with an FID detector.
When the gas was not returned after UV irradiation, the decomposition rate of trichlorethylene was 87.5%, but if the gas return rate was 5%, the decomposition rate of trichlorethylene was 9 at a stretch.
It improved to 7.4%. Furthermore, the gas return rate is 10%, 3
At 0% and 50%, the decomposition rates of trichlorethylene were 98.6%, 99.0% and 98.8%, respectively.
The results are shown in Table 1 and FIG.

[0009]

[Table 1]

From the results shown in Table 1 and FIG. 3, the decomposition rate of trichlorethylene was drastically improved by returning a very small amount of the gas after irradiation with ultraviolet rays to the ultraviolet reaction tank, and when the gas returning rate exceeded 10%. It can be seen that the decomposition rate of trichlorethylene becomes almost constant.

[0011]

EFFECTS OF THE INVENTION According to the method of the present invention, chlorine molecules contained in a gas after being irradiated with ultraviolet rays effectively absorb ultraviolet rays having a long wavelength which are not absorbed by the organic chlorine compounds in the ultraviolet reaction tank, and the chlorine compounds are Since it contributes to the decomposition of, the organic chlorine compound in the gas can be efficiently decomposed and removed. The method of the present invention is particularly effective for purification of groundwater and soil contaminated with organic chlorine compounds, and can economically purify soil and groundwater contaminated with organic chlorine compounds using a compact reaction tank.

[Brief description of drawings]

FIG. 1 is a sectional view of an ultraviolet reaction tank used in Examples.

FIG. 2 is an explanatory diagram of an apparatus used in Examples.

FIG. 3 is a graph showing the relationship between the gas return rate and the decomposition rate of trichlorethylene.

[Explanation of symbols]

 1 UV reaction tank 2 Quartz tube 3 Internal irradiation type cylindrical reaction vessel 4 Sterilization lamp 5 Gas inlet 6 Gas outlet 7 Pump 8 Flow meter

Claims (1)

[Claims] 1. A method of introducing a gas containing an organic chlorine compound into an ultraviolet reaction tank and irradiating it with ultraviolet rays to decompose the organic chlorine compound, wherein a part of the gas after ultraviolet irradiation is returned to the ultraviolet reaction tank. Photolysis method for organic chlorine compounds.