JPH09234339A - Photodecomposition of organochlorine compound - Google Patents

Abstract

PROBLEM TO BE SOLVED: To efficiently decompose and remove a hardly decomposable organochorine compd. by irradiating gas containing hardly decomposabie organochlorine compd. such as dichloromethane, carbon tetrachlorlde, vinyl chloride or dichloroethylene with ultraviolet rays by using a low pressure mercury lamp using relatively inexpensive natural quartz. SOLUTION: Trichloroethylene and/or tetrachloroethylene is mixed with gas containing at least one kind of a hardly decomposable organochlorine compd. among dichloromethane, carbon tetrachloride, vinyl chloride, dichloroethylene, dichloroethane, trichloroethane and dichloropropene in concn. equal to or more than that of the hardly decomposable organochlorine compd. and the resulting mixture is irradiated with ultraviolet rays.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a method for photodecomposing an organic chlorine compound. More specifically, the present invention contains a gas obtained by vacuum extraction of soil contaminated with an organochlorine compound, an organochlorine compound such as a gas obtained by air stripping groundwater contaminated with an organochlorine compound. Even when the gas contains persistent organic chlorine compounds such as dichloromethane, carbon tetrachloride, vinyl chloride, dichloroethylene, dichloroethane, trichloroethane, dichloropropene, etc., ultraviolet rays are emitted using a low-pressure mercury lamp using relatively inexpensive natural quartz. The present invention relates to a photolysis method of an organic chlorine compound that can be efficiently decomposed by irradiation.

[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 the environment with organochlorine compounds has not been solved. Organochlorine compounds used as a solvent, due to their properties such as solubility and flame retardancy for oils and fats, are often trichloroethylene and tetrachloroethylene, but when they are present in the soil, they become dichloroethylene, vinyl chloride, etc. by the action of anaerobic microorganisms Change. Therefore, dichloroethylene, vinyl chloride, etc. are often detected in soil or groundwater contaminated with trichlorethylene or tetrachlorethylene. In addition, as in the case of dichloromethane, carbon tetrachloride, etc., they may be released into the environment as a result of being used in large amounts in extraction operations, etc., and may be contained in exhaust gas in the compound manufacturing process and analysis process. . 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 the substances that cause contamination of groundwater and soil, currently, vacuum extraction from soil or air stripping of groundwater is performed, and then recovered with an adsorbent such as activated carbon and discarded. The method of treating as a thing 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 form an acidic decomposition gas, which is then washed with alkali to detoxify it (Japanese Patent Laid-Open No. 62-191025).
(Japanese Patent Laid-Open No. 62-191095), an apparatus for aeration-treating wastewater containing an organic halide, irradiating the discharged gas with ultraviolet rays, and then washing with an alkali (JP-A-62-191095), halogenated acyclic hydrocarbon compounds and ozone. A device for decomposing a halogenated acyclic hydrocarbon compound by irradiating with ultraviolet rays by mixing with (Japanese Patent Application Laid-Open No. 1-236925), aeration treatment of wastewater containing an organic halide while irradiating with ultraviolet rays,
Further, an apparatus (Japanese Patent Application Laid-Open No. 2-75391) for irradiating the discharged gas with ultraviolet rays and then washing with alkali has been proposed. Since many organic chlorine compounds are relatively easily photochemically decomposed, many decomposition methods using ultraviolet rays have been proposed. As a UV source,
Low-pressure mercury lamps, which have a high conversion efficiency of electric energy into light energy, are generally used. From the low-pressure mercury lamp, ultraviolet rays having wavelengths of 185 nm and 254 nm are mainly irradiated as short wavelength ultraviolet rays. In terms of price, lamps using natural quartz, which is relatively cheaper than synthetic quartz, are often used. However, a low-pressure mercury lamp that uses natural quartz absorbs ultraviolet rays with a wavelength of 185 nm and thus has a wavelength of 254 nm.
Only the above ultraviolet rays are irradiated. Trichlorethylene,
Easily decomposable organochlorine compounds such as tetrachloroethylene can be decomposed by irradiation with ultraviolet rays having a wavelength of 254 nm.
Dichloroethylene, dichloroethane, trichloroethane, dichloropropene and other persistent organic chlorine compounds are 2
It is not decomposed by irradiation with UV light of 54 nm. For this reason,
If these persistent organochlorine compounds are included,
It was necessary to perform photolysis using a low-pressure mercury lamp, a high-pressure mercury lamp, and a xenon lamp using expensive synthetic quartz.

[0003]

DISCLOSURE OF THE INVENTION The present invention uses a low-pressure mercury lamp that uses a relatively inexpensive natural quartz for a gas containing a hardly decomposable organic chlorine compound such as dichloromethane, carbon tetrachloride, vinyl chloride, and dichloroethylene. The present invention has been made for the purpose of providing a method for photodecomposing an organochlorine compound, which is capable of efficiently decomposing and removing a hardly decomposable organochlorine compound by irradiating it with ultraviolet rays.

[0004]

As a result of intensive studies to solve the above problems, the present inventors have mixed trichlorethylene and / or tetrachloroethylene with a gas containing a hardly decomposable organochlorine compound such as dichloroethylene. Then, it was found that the hardly decomposable organochlorine compound can be efficiently decomposed and removed by irradiating with ultraviolet rays, and the present invention has been completed based on this finding. That is, the present invention relates to (1) a gas containing at least one kind of hardly decomposable organic chlorine compound selected from dichloromethane, carbon tetrachloride, vinyl chloride, dichloroethylene, dichloroethane, trichloroethane and dichloropropene. Provided is a method for photolyzing an organic chlorine compound, which comprises mixing trichloroethylene and / or tetrachloroethylene in an amount equal to or higher than the total concentration of the compounds and irradiating with ultraviolet rays. Furthermore, as a preferred embodiment of the present invention, the organochlorine compound according to item (1), wherein (2) the hardly decomposable organochlorine compound is extracted in a gaseous state from soil or groundwater contaminated with the organochlorine compound. The method of photodegradation can be mentioned.

[0005]

BEST MODE FOR CARRYING OUT THE INVENTION
It can be applied to a gas containing a hardly decomposable organochlorine compound such as carbon tetrachloride, vinyl chloride, dichloroethylene, dichloroethane, trichloroethane, and dichloropropene, which is not easily decomposed by ultraviolet irradiation. From soil polluted with organic chlorine compounds, mainly by vacuum extraction method,
In the case of groundwater, once pumped up, the aeration method extracts the organochlorine compounds in a gaseous state. At this time, the situation varies depending on the pollution site, but anaerobic decomposition of trichlorethylene, tetrachloroethylene, etc. progresses and vinyl chloride, dichloroethylene, etc. are contained at relatively high concentrations, and dichloroethylene, vinyl chloride etc. do not progress anaerobic decomposition. In many cases, there are mixed locations that are rarely included. Trichlorethylene and tetrachlorethylene are
Since it absorbs ultraviolet rays having a wavelength of about 254 nm, it efficiently absorbs the ultraviolet rays emitted from the low-pressure mercury lamp using natural quartz, and the π electrons of the carbon-carbon double bond are excited to generate radicals of carbon-chlorine bond. Disconnection occurs. When a dechlorination reaction occurs, when an oxygen molecule exists in the system, the organochlorine compound reacts with the oxygen molecule and becomes a reaction intermediate having a chlorine atom bonded to the oxygen 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 reactions proceed in a chain, the quantum yield of this reaction is generally high,
The concentration of the organic chlorine compound can be efficiently reduced.

However, the gas taken out from the place where the anaerobic decomposition has progressed contains vinyl chloride, dichloroethylene and the like in addition to trichlorethylene and tetrachlorethylene. When irradiated with ultraviolet rays, trichlorethylene and tetrachloroethylene are excited to cause a dechlorination reaction, and by themselves react with an oxygen molecule existing in the system to generate a reaction intermediate having a chlorine atom bonded to an oxygen atom. Two of the released chlorine radicals react with each other to form a chlorine molecule, but the rest reacts with an organic chlorine compound to generate a carbon-centered radical. At this time, chlorine radicals are more likely to react with vinyl chloride, dichloroethylene, etc., which have less carbon-chlorine bonds and less steric hindrance than trichloroethylene or tetrachloroethylene. Chlorine atoms are less likely to be released from the reaction intermediate thus produced, and the reaction efficiency is markedly lower than that in the case where only trichlorethylene or tetrachloroethylene is present. Therefore, vinyl chloride, dichloroethylene, and the like are hardly decomposed by ultraviolet irradiation, and are considered as hardly decomposable organic chlorine compounds. Other hardly decomposable organic compounds include dichloromethane, carbon tetrachloride, dichloroethane, trichloroethane, dichloropropene and the like. In order to decompose such a hardly decomposable organochlorine compound, a low-pressure mercury lamp using expensive synthetic quartz capable of irradiating with ultraviolet light having a wavelength of 185 nm having a larger energy was required. Further, even if a low-pressure mercury lamp using synthetic quartz is used, it is difficult to enlarge the reaction tank because ultraviolet rays having a wavelength of 185 nm are easily absorbed by air.

If anaerobic decomposition has not progressed and trichloroethylene and tetrachloroethylene are present in high concentrations,
Even with a low-pressure mercury lamp that mainly uses natural quartz that irradiates ultraviolet rays having a wavelength of 254 nm, vinyl chloride, dichloroethylene, and the like mixed in a low concentration are easily decomposed. Furthermore, when anaerobic decomposition progresses and the concentration of vinyl chloride, dichloroethylene, etc. is higher than that of trichlorethylene or tetrachloroethylene, by mixing trichlorethylene or tetrachloroethylene with the gas, it is possible to remove vinyl chloride, dichloroethylene, etc. It becomes possible to decompose the hardly decomposable organochlorine compound by irradiation with ultraviolet rays having a wavelength of 254 nm. In the method of the present invention, the total amount of trichlorethylene and tetrachloroethylene in the gas irradiated with ultraviolet rays is the total amount of the concentration of persistent organic chlorine compounds such as dichloromethane, carbon tetrachloride, vinyl chloride, dichloroethylene, dichloroethane, trichloroethane, and dichloropropene. The above is 1 to 1 of the total amount of the concentration of the persistent chlorine compound.
It is preferably 5 times, and more preferably 1.5 to 3 times the total amount of the concentration of the persistent chlorine compound. Dichloromethane, carbon tetrachloride, vinyl chloride,
The concentrations of hardly decomposable organic chlorine compounds such as dichloroethylene, dichloroethane, trichloroethane, dichloropropene, etc., and the concentrations of trichloroethylene and tetrachloroethylene can be measured by gas chromatography.
A calibration curve by gas chromatography is prepared in advance for these organic chlorine compounds, the concentration of each organic chlorine compound in the gas to be treated is measured, and the required amount of trichlorethylene and / or tetrachloroethylene is mixed. The concentration here is the value indicated by the volume ratio, and is usually
Expressed in ppm. When the organochlorine compound is photolyzed by the method of the present invention, dichloroacetyl chloride, formyl chloride, formic acid, phosgene, carbon monoxide, carbon dioxide, hydrogen chloride, chlorine molecules and the like are produced as decomposition products. These decomposition products can be detoxified by a known method, for example, can be adsorbed and removed by an alkaline substance, or can be absorbed in water to be biologically decomposed.

[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. FIG. 1 is a sectional view of an ultraviolet reaction tank used in Examples and Comparative Examples. The UV reactor has a diameter of 10 cm and a height of 20 with a quartz tube 1 having a diameter of 2 cm.
Pyrex glass internal irradiation type cylindrical reaction vessel 2 cm
The quartz tube has a germicidal lamp 3 [Toshiba Corp., GL
6 (6W)] is inserted. The ultraviolet reaction tank is provided with a gas inlet 4 in the lower part and a gas outlet 5 after irradiation with ultraviolet light in the upper part. The gas was sampled at the gas inlet and the gas outlet and analyzed by a gas chromatograph equipped with an FID detector. Example 1 About 230 ppm of trichlorethylene was mixed with air containing about 100 ppm of cis-dichloroethylene, and the mixture was supplied to an ultraviolet reaction tank in which a sterilization lamp was turned on at a flow rate of 1 liter per minute. When the reaction reaches a steady state, ci at the gas inlet
The s-dichloroethylene concentration was 101 ppm, the trichlorethylene concentration was 233 ppm, the cis-dichloroethylene concentration at the gas outlet was 8 ppm, and the trichlorethylene concentration was 10 ppm. Example 2 Example 1 except that about 190 ppm of tetrachloroethylene was mixed instead of about 230 ppm of trichlorethylene.
The same operation was repeated. When the reaction reached a steady state, the cis-dichloroethylene concentration at the gas inlet was 9
9ppm, tetrachloroethylene concentration is 189ppm,
The cis-dichloroethylene concentration at the gas outlet is 7 ppm,
The tetrachloroethylene concentration was 9 ppm. Example 3 Instead of about 230 ppm of trichlorethylene, about 130 ppm of trichlorethylene and about 1 of tetrachlorethylene.
The same operation as in Example 1 was repeated except that 60 ppm was mixed. When the reaction reached a steady state, the cis-dichloroethylene concentration at the gas inlet was 100 ppm, the trichloroethylene concentration was 131 ppm, the tetrachloroethylene concentration was 163 ppm, the cis-dichloroethylene concentration at the gas outlet was 7 ppm, and the trichloroethylene concentration was 7 ppm.
The tetrachloroethylene concentration was 8 ppm. Comparative Example 1 The same operation as in Example 1 was repeated without mixing trichloroethylene or tetrachloroethylene with air containing about 100 ppm of cis-dichloroethylene. When the reaction reached a steady state, the cis-dichloroethylene concentration at the gas inlet was 98 ppm and the cis-dichloroethylene concentration at the gas outlet was 93 ppm. Comparative Example 2 The mixing amount of trichlorethylene was changed from about 230 ppm to about 3
The same operation as in Example 1 was repeated except that the amount was reduced to 0 ppm. When the reaction reaches a steady state, at the gas inlet
The cis-dichloroethylene concentration was 97 ppm, the trichloroethylene concentration was 31 ppm, the cis-dichloroethylene concentration at the gas outlet was 73 ppm, and the trichlorethylene concentration was 11 ppm. Example 4 About 270 ppm of trichlorethylene was mixed with air containing about 100 ppm of vinyl chloride, and the mixture was supplied at a flow rate of 1 liter / min to an ultraviolet reaction tank in which a sterilization lamp was turned on. When the reaction reached a steady state, the vinyl chloride concentration at the gas inlet was 97 ppm and the trichlorethylene concentration was 2
The concentration was 69 ppm, the vinyl chloride concentration at the gas outlet was 3 ppm, and the trichlorethylene concentration was 13 ppm. Comparative Example 3 The same operation as in Example 4 was repeated without mixing trichloroethylene with air containing about 100 ppm of vinyl chloride. When the reaction reached a steady state, the vinyl chloride concentration at the gas inlet was 97 ppm and the vinyl chloride concentration at the gas outlet was 94 ppm.
The results of Examples 1 to 4 and Comparative Examples 1 to 3 are summarized as the first
It is shown in the table.

[0009]

[Table 1]

In Example 1, Example 2 and Example 3 in which an equal amount or more of trichloroethylene and / or tetrachloroethylene was mixed with 100 ppm of cis-dichloroethylene and irradiated with ultraviolet rays, cis in the gas flowing out from the reaction tank was determined. -The amount of dichloroethylene has dropped to below 8 ppm. On the other hand, in Comparative Example 1 in which neither trichloroethylene nor tetrachloroethylene was mixed, cis-dichloroethylene was decomposed only slightly, and even in Comparative Example 2 in which the mixing amount of trichlorethylene was smaller than cis-dichloroethylene, the gas flowing out from the reaction tank Cis inside
-Dichloroethylene is only reduced to 73 ppm. Example 4 in which the same amount or more of trichlorethylene was mixed with 100 ppm of vinyl chloride and the mixture was irradiated with ultraviolet rays
In the above, the amount of vinyl chloride in the gas flowing out from the reaction tank has dropped to 3 ppm. On the other hand, in Comparative Example 3 in which neither trichloroethylene nor tetrachloroethylene was mixed, vinyl chloride was decomposed only slightly. From these results, it is understood that mixing trichloroethylene and / or tetrachloroethylene in the same amount or more has a remarkable effect on improving the decomposition rate of cis-dichloroethylene or vinyl chloride.

[0011]

According to the method of the present invention, it is possible to decompose a hardly decomposable organochlorine compound, which was photodecomposable only by using an expensive ultraviolet lamp, by using an inexpensive low pressure mercury lamp. Become. This makes it possible to economically purify soil and groundwater contaminated with organic chlorine compounds.

[Brief description of drawings]

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

[Explanation of symbols]

 1 Quartz tube 2 Internal irradiation type cylindrical reaction vessel 3 Sterilization lamp 4 Gas inlet 5 Gas outlet

Claims (1)

[Claims] 1. A gas containing at least one refractory organochlorine compound selected from the group consisting of dichloromethane, carbon tetrachloride, vinyl chloride, dichloroethylene, dichloroethane, trichloroethane and dichloropropene. A method for photolyzing an organic chlorine compound, which comprises irradiating with ultraviolet rays by mixing a total amount of trichloroethylene and / or tetrachloroethylene.