Degassing

DEGASSING (degassing, desinfection, oczyszczanie z gazow), a term adopted in military chemical practice to denote the means and methods of neutralizing poisonous substances (P.S.) in cases of contamination ('gassing') of soil, air, water, buildings, clothing, and other objects. At present all P.S. can be divided into 3 groups according to their gassing properties and the methods used for their degassing: the first group includes P.S. that are only adsorbed by objects of gassing, the second includes those that are not only adsorbed but also enter into chemical interaction, and the third includes substances that produce persistent gassing. Typical representatives of P.S. of the first group are benzyl bromide C6H5CH2Br, chloropicrin CCl3NO2 and chloroacetophenone C6H5COCH2Cl; the gassing properties of these substances follow the general rule of adsorption: the greater the surface of the objects being gassed, the more substance is adsorbed. Therefore, woolen fabrics, for example, are gassed by them (all other conditions being equal) more than cotton fabrics; loose bodies more than solid ones. Being strong irritants, P.S. of this group, even in small concentrations, are easily detected due to irritation of the organs of sight or smell (detection of P.S. by organoleptic means). Being in an unstable physical bond with adsorbents, they are easily and comparatively quickly removed from them by simple ventilation. When absorbed by soil, they are not retained in it for long due to their high volatility, especially in warm weather. For degassing water, simple boiling is sufficient. From enclosed spaces they are removed by creating a draft, from trenches by waving or bonfires. To the second group of poisonous substances belong substances such as chlorine and phosgene (Cl2 and COCl2), which are not only adsorbed but also act chemically, due to which they are not more firmly retained on the objects of gassing but may cause their damage. The chemical changes of these P.S. consist mainly in hydrolysis, with the formation of strong inorganic acids (COCl2 + H2O → CO2 + 2HCl). The latter cause damage, and besides, having lower volatility than the P.S. itself, they are retained longer. Mainly metallic items are damaged, with thin mechanisms becoming inoperative. Fabrics change their color and strength, products their taste. The degree of these changes is directly dependent on the concentration and duration of action; at small concentrations and brief exposure the changes are practically imperceptible. Possessing to some extent irritating properties, substances of this group are detected organoleptically at concentrations that do not have significant toxicity. For degassing small amounts of these P.S., ventilation enhanced by artificial draft is also sufficient. At high concentrations and prolonged exposure, chemical degassing is resorted to, based on the neutralization of both the P.S. itself and the products of its hydrolysis. Neutralizers are 10-20% solutions of soda, 2-5% solutions of hyposulfite or saturated solutions of lye (ash). They are sprayed from sprayers (hydro-pults) or used to wipe the objects being degassed. Metallic objects and fabrics especially need chemical degassing. The latter, if they are not too moist, can also retain the P.S. itself for a long time and thus serve as a source of poisoning. There are known cases when people, after staying in a phosgene cloud protected by gas masks, subsequently did not degas their clothing and were poisoned during their stay in enclosed premises. It is necessary to protect mainly metallic objects from gassing by substances of this group. Thin mechanisms are either placed in special premises (gas shelters) or tightly covered with specially adapted covers. Weapons, ordnance, and large machines are heavily lubricated with lubricating oil, which is removed after a chemical attack and replaced with fresh. It is recommended during an attack to operate the mechanisms from time to time, and after the attack, if possible, to disassemble and thoroughly wipe them. Particular importance in relation to degassing is attached to substances of the third group, a typical representative of which is mustard gas >S. (dichlorodiethylsulfide): ClCH2CH2-S-CH2CH2Cl. A characteristic feature of mustard gas (see Mustard Gas for details) is its ability to gasify all objects with which it comes into contact, to penetrate them and to be persistently retained. Three methods of gassing with mustard gas are distinguished: by vapors, by fog, and in liquid form, with the gassing properties being different for each of these methods. For degassing mustard gas (demustardizing) both chemical and physical methods are used, as well as, depending on the circumstances, combinations of both. - The application of chemical methods is based on the ability of mustard gas under the influence of strong oxidizing agents and chlorination to give products less toxic than itself. Oxidation proceeds by the addition of oxygen to the sulfur atom, resulting in dichlorodiethylsulfone (ClC2H4)2SO and dichlorodiethylsulfoxide (ClC2H4)SO2, of which the first is almost non-toxic, the second is somewhat more toxic. Chlorination is carried out by the replacement of hydrogens of ethyl groups by chlorine atoms with the formation of low-toxic products, with highly chlorinated products: ClCH2CH2SO2CH2CH2Cl losing their toxicity almost completely. The widest application for chemical degassing is found for bleaching powder (CaOCl2), which simultaneously oxidizes and chlorinates mustard gas. It is used either in dry form or in aqueous solution. In the first case, a vigorous exothermic reaction occurs, up to ignition, with the evolution of dense white smoke. When moistened with water, bleaching powder reacts with mustard gas less vigorously and more slowly than when dry. The amount of bleaching powder required to neutralize a given amount of mustard gas theoretically slightly exceeds the latter, but in practice it is taken in 10-20 times the amount. Bleaching powder with a chlorine content of not less than 30% is used, because with lower chlorine activity it acts worse, and with activity below 15% it is almost unsuitable for degassing. Loss of chlorine occurs more easily in the presence of high humidity, which is why it is necessary that bleaching powder be stored dry and in good packaging. For the same reason, when degassing with solutions of bleaching powder, water cannot be added to it in advance; the solution is prepared just before the moment of degassing. As a strong oxidizing agent for degassing, potassium permanganate (KMnO4) is recommended, but due to the high cost of this product, its application is very limited. For chlorination of mustard gas, gaseous chlorine is used, the degassing with which is carried out in special chambers or adapted (hermetically sealed) premises. Since chlorine itself can cause damage to the objects being degassed, they must be ventilated after degassing. Those objects that are particularly susceptible to its action are not degassed with it at all. To some extent, the destruction of mustard gas in the objects being degassed by boiling them in water and under the influence of high temperatures (burning) can be attributed to chemical methods. Degassing by boiling is based on the fact that water hydrolyzes mustard gas with the formation of hydrochloric acid and thioglycol: ClCH2CH2-S-CH2CH2Cl + H2O → 2HCl + HSCH2CH2OH. Hydrolysis is significantly accelerated by boiling. By exposing to fire those objects that allow it, evaporation and decomposition of mustard gas occur. It should be remembered that the smoke formed in this case has high toxicity. Among the physical methods of degassing are the removal of mustard gas from gassed surfaces mechanically and ventilation both under ordinary conditions and at elevated temperature with enhanced draft. The first is performed simply by wiping the gassed surfaces with rags or with powders that absorb them, for example ash, as well as by washing with water. The second, carried out under ordinary conditions, is most applicable to objects gassed by gaseous mustard gas, but it requires quite a long time. To accelerate ventilation and for degassing liquid mustard gas, ventilation with enhanced draft at t° 70-80° or with steam treatment is used. The application of this method is possible only in special chambers that allow degassing to be carried out under the specified conditions. The essence of the process is that at high t° the evaporation of mustard gas is accelerated (under the action of steam hydrolysis also occurs), which is carried out of the chamber by enhanced draft (0.25-0.5 volume per minute) without having time to be re-adsorbed by the object being degassed. To the combined method of degassing can be attributed the mentioned washing with water, especially hot water, since along with mechanical removal, hydrolysis also takes place. To this method also belongs the degassing of mustard gas with the help of solvents: alcohol, gasoline, kerosene, etc. They are used for moistening rags in mechanical removal, which makes the removal more complete, as well as for extracting mustard gas that has penetrated from the surface into the depth of the gassed object. For this purpose, the surface is abundantly moistened, for example with gasoline, which is then allowed to evaporate freely.

The extracted mustard gas will evaporate along with the solvent. Extraction from small items is done by immersing them in a bath with the solvent. For degassing soil, the most widespread method has become covering it with bleaching powder. The amount of the latter depends on both the density of gas contamination and the quality of the soil, with an average norm of 500 g per 1 m² of soil, with variations from 200 to 800 g. The bleaching powder should be spread in an even layer, but in the presence of puddles of mustard gas, it is sprinkled until the smoke emission ceases. The spreading can be done with shovels or special devices. For degassing with solutions of bleaching powder, these solutions are sprayed from sprayers of the garden sprinkler type or from watering cans. Given that enormous amounts of bleaching powder are required for soil degassing, attempts are made to degas it by burning, which should generally be done before sprinkling with bleaching powder if the soil is covered with dense grass. For burning, petroleum, kerosene, hay, and other readily available combustible materials are used, which are quite sufficient for dry ground, especially covered with dried grass. If only safe passage through the contaminated area is required, then a strip (road) sufficient for passage is dug up. For this, a layer of earth is removed to the depth of a half-shovel (mustard gas penetrates the soil no deeper than 10-15 cm), and the removed earth is piled up in the downwind direction. Along such a strip, one can safely pass 1-2 km while wearing a gas mask, especially in not very hot weather or at night. For degassing dense soil (highways, asphalt, etc.) for the purpose of saving bleaching powder, the French recommend first covering it with a layer of peat, sawdust, etc. This layer, which absorbs most of the mustard gas, is then burned or buried in the ground. Water, even with complete hydrolysis of mustard gas, does not lose its toxicity until the smell disappears and there is no reaction to it. For this reason, main attention is paid to protecting it from contamination and, first of all, from the entry of liquid mustard gas into it, since contamination by vapors has no practical significance. Therefore, covering vessels with water and wells with metal or wooden shields or with dense, thick material is recommended. Air is degassed by the same methods as for non-persistent chemical agents, but its purification can only be carried out after the removal or degassing of all objects that could contaminate it. Products made of cotton fabric are best and most simply degassed by boiling for 30-45 minutes in a soap-alkaline solution, followed by rinsing in cold, running water. Woolen fabrics, in case of contamination by mustard gas vapors, can be degassed with chlorine for 10-15 minutes, at a concentration of about 0.5-1%, which is recommended by the English instructions. The best degassing is treatment with hot air or steam, which is quite sufficient even when small amounts of liquid mustard gas are present. However, when contaminated with large amounts of liquid mustard gas, fabrics are not degassed but must be destroyed. Leather goods, if degassing is carried out immediately after contamination, are degassed with a paste of bleaching powder; for delayed degassing, 2-3 treatments with solvents are recommended. After degassing, the leather is smeared with some kind of fat, since solvents extract fats, which leads to rapid deterioration of leather goods. The technical difficulty of degassing large quantities of leather and textile products forces special attention to be paid to implementing measures to protect them from contamination (see Chemical Warfare Agents). Metal objects are degassed by mechanical or combined methods. For wooden objects, a paste of bleaching powder is applied, which is spread in a thin layer and periodically washed off with water, followed by replacement with a fresh layer. For very delayed degassing, treatment with solvents is recommended. Food and feed products cannot be degassed, for which reason protection is of special importance for them. Dense covering with tarpaulin provides protection from vaporized mustard gas for 2-3 hours. Complete protection from fog-like and liquid mustard gas is provided by metal sealing or airtight wooden containers. The latter must consist of three-ply plywood or boards not thinner than 5 mm. When carrying out any kind of degassing, strict implementation of precautionary measures is required: degasators work in gas masks and protective suits (for agents that act on the skin), and the degassing itself is carried out in such a way that the removed chemical agents or their decomposition products cannot cause injury. This is achieved by selecting an appropriate place for degassing or removing people and animals from it, paying attention to the direction of the wind, burning materials used in degassing if they are flammable, or burying them. All degassing work must be provided with sanitary supervision and service.