Activated Carbon Surfaces in Environmental Remediation
T.J. Bandosz (editor)
9 2006 Elsevier Ltd. All rights reserved.
Adsorption of chemical warfare agents P. Lodewyckx Royal Military Academy, Department of Chemistry, Renaissancelaan 30, B-1000 Brussels, Belgium 1. INTRODUCTION The amount of carbon used in military applications is negligible when compared to the total annual consumption. Even when only considering the market of activated carbon used in gas phase adsorption applications, military purchases represent only a small volume. Quite contradictory, much of the research in this last domain has been performed either by military scientists or by academic researchers under government contracts. This is a direct result of the very particular and stringent requirements imposed on carbons for protection against warfare gases. The spectrum of gases that might be used is vast and the particular threat is, a priori, unknown. The same being true for the environmental and human circumstances, activated carbons for military applications have to be as versatile as possible: they must be a real "Jack of all trades". Herein also lies the continuing success of activated carbon as the prime adsorbent for warfare gases: none of the other well known adsorbents (eg. zeolites, pillared clays, silicagels) is capable of protection against such a vast range of toxic compounds. Added to that, even carbons that have been specifically tailored for military applications, e.g. by introducing metal salts into the carbon matrix, come at a price that is significantly lower than all other commercially available and viable adsorbents. These two (versatility and price) are the main reasons for the continuing interest of defence specialists in activated carbon. 2. CHEMICAL WARFARE AGENTS
2.1. Definition The definition of chemical warfare agents (or CWAs) is certainly not straightforward. There is no unique view on exactly what property makes a gas a CWA. It is certainly not its lethality, as many very d~ngerous gases (e.g. dioxins) are not considered to be CWAs. Nor is there a direct link with the production process, as some gases (e.g. CNC1 and HCN) are commonly classed as CWAs, but have a widespread use in industry. Even the actual use as a chemical weapon does not suffice. For example, chlorine (see section 2.2) is no longer considered a CWA. One possibility is to use the definition given in Article II of the Chemical Weapons Convention  that describes chemical warfare agents as: "Toxic chemicals and their precursors, except when intended for purposes not prohibited under this Convention, as
long as the types and quantities are consistent with such purposes". In paragraph 2.3 the most common of these chemicals are treated in more detail. A toxic chemical is further defined as "Any chemical which through its chemical action on life processes can cause death, temporary incapacitation or permanent harm to humans or animals. This includes all such chemicals, regardless of their origin or of their method of production, and regardless of whether they are produced in facilities, in munitions or elsewhere." For the purpose of implementing this convention, i.e. the verifications carried out by the OPCW (Organisation for the Prohibition of Chemical Weapons), all products considered to be CWAs or precursors have been listed in three Schedules. Effectively, these lists can therefore be considered to "define" CWAs. Some supplementary information on these chemicals is given in 2.3. 2.2. History No doubt the origins of chemical warfare are to be found long before any written records were kept. Even nowadays, jungle tribes still use poisoned arrows for bows or blowpipes. Contact with these toxins, be it directly in the blood or through the skin, will incapacitate, and in some instances kill, any adversary. But the true birth of chemical warfare is situated in World War I. From the start France, quickly followed by Germany, employed tear gases. On April 22, 1915 the German army went one step beyond and released a cloud of chlorine gas from containers in the sector between the small villages of Steenstrate and Langemark, in the vicinity of Ieper (Ypres), on the Belgian front. They were however too surprised by the success of this attack to exploit it to its full extent. From that moment on a new arms race was born. Much like the struggle between penetrating power and ballistic protection, both sides actively sought new and deadlier agents, more effective ways to spread these agents (bombs, artillery shells, see Fig.l,2) and more effective protective measures for their troops. By the end of the war, CWAs had made a name for themselves. Even though the actual number of victims was rather small in comparison with the monstruous losses from conventional arms such as artillery shells, machine guns and grenades, their psychological impact had been huge. According to most analysts, this mutual psychological deterrence was the main reason for the fact they were not used in the Second World War despite the large quantities of highly toxic compounds stockpiled by both sides. There were alleged uses in the Chinese and Abyssinian theatres of war prior to WWII and against countries not equipped with offensive, and hardly any defensive means. Since the Second World War instances of proven use, and even alleged use, have been rare. Over the last decade, there has been much speculation on the use of CWAs in asymmetric warfare involving a small and poorly equipped country attacked by overwhelming forces, and by terrorists. Fortunately, the first case has not yet presented itself, but the use by terrorists was sadly affirmed during the attack with Sarin gas by the Aum Shinrikyo sect in Matsumoto and later, on March 20, 1995, in the Tokyo subway [2,3]. These attacks have demonstrated the possibly horrendous effect of CWAs on an unprotected civilian population, and have reaffirmed the necessity for protective measures.
Adsorption of Chemical Warfare Agents
2.3. Types of warfare agents 2.3.1. General Chemical warfare agents are divided in two major categories: incapacitating and lethal agents. The goal of the first group is to incapacitate people, i.e. diminishing their fighting abilities, usually without any lasting effects. The agents of the second group aim at killing people, or to put them out of action for a longer period of time (in order to "overload" the medical
treatment system). Many agents are known under non-IUPAC names. Some have been given a multitude of different names, often as a result of historic developments and national preferences. There is also a NATO (North Atlantic Treaty Organisation) code for most of the CWAs. Some of these codes are well known and are commonly used by specialists outside NATO. In the following sections the agents are mentioned by their most commonly used names (which, in some cases, e.g. CS, is the NATO code), and where possible with the official IUPAC name or formula, and the NATO code [1,6,7].
Fig. 1. Picture taken by a Russian aviator of a release of toxic agents from containers on the Eastern Front (1916?) 
Fig.2. French artillery crew wearing crude gas masks, but no other protection (1915?) 
2.3.2. Incapacitating agents Here we find tear gases (also used in riot control), vomiting agents and psychedelic agents. Most of these agents are in fact liquid or solid aerosols. Examples are the vomiting agent Adamsite (10-chloro-5,10-dihydrophenarsazine), the tear gases CN (chloroacetophenone) and CS (ortho-chlorobenzalmalonitrile), and the psychedelic agents LSD25 (lysergic acid
diethylamide) and Quinuclidinol. 2.3.3. Vesiccants or blister agents The first group of lethal agents attacks both via the respiratory system and the skin. These substances cause severe internal (lungs) and external (eyes, skin) burns. Furthermore, most of these agents are either confirmed or suspected carcinogens. The best known agent of this
group is Mustard gas (bis(2-chloroethyl)sulfide, HD or Yperite). The origins of this agent lie
in the First World War, where this gas became the symbol of chemical warfare. Most of the other vesiccants such as the nitrogen mustards (HN-1 or 2-chloroethylamine, HN-2 and HN-3) and Lewisites (L1 or 2-chlorovinyldichlorarsine, L2 and L3) were also developed during WWI. These chemicals present a rather low volatility, giving them a certain persistence. When used they will contaminate persons, materials and the environment, resulting in a prolonged health threat.
2.3.4. Choking agents These agents act directly on the pulmonary system (causing oedema). Generally they are highly volatile, presenting only a very limited persistency. Most of them are substances with a widespread use in the chemical industry, such as phosgene (COC12 - CG), chlorine and chloropicrin (CC13NO2 - PS). 2.3.5. Blood agents These agents interact negatively with the oxygen transport by blood. Similarly to choking agents they are generally highly volatile and many of them can be readily found in the chemical industry. Examples are hydrogen cyanide (HCN - AC), cyanogen chloride (CNCI CK) and arsine (AsH3- SA). 2.3.6. Neurotoxic agents It is usually this group that the general public thinks of when CWAs are mentioned. These extremely toxic agents inhibit the action of the cholinesterase enzyme, resulting in a collapse of the central nervous system. This group includes, amongst others, VX (O-ethyl S-2diisopropylaminoethyl methyl phosphonothiolate), Sarin (O-isopropyl methylphosphonofluoridate - GB), Soman (O-pinacolyl methylphosphonofluoridate - GD) and Tabun (O-ethyl N,N-dimethyl methylphosphonofluoridate - GA). Except for Sarin, these "gases" are in fact liquids with a very low volatility (Example: P~at298x(VX) = 0.00084 hPa). As such, they pose a much lesser immediate risk for action via the respiratory system, and the prime danger of these agents lies in their percutaneous action (entering the body via the skin) after dissemination as droplets. Their prime use is not to make immediate casualties but to use their persistency to deny access to certain zones or materials for a prolonged period of time. The notable exception to this rule being Sarin (Psat298x(Sarin) = 3.9 hPa) which poses a severe threat of dissemination as a gas and, consequently, requires respiratory protection (see also section 2.2). 2.3. 7. Industrial chemicals Strictly speaking, these compounds are not chemical warfare agents. However, over the last few years, there has been a growing concern among most governments regarding the occupational health and safety of their troops in "operations other than war" (OOTW). These are low-intensity conflicts (peace-keeping, humanitarian aid, etc), with little or no threat of classic chemical warfare. However the problems at hand in that country have usually resulted
Adsorption of Chemical WarfareAgents
in a total lack of government control over industry and, in many cases, degradation and damage of chemical plants and other industrial sites. In such circumstances, one cannot neglect the risk of an accidental or deliberate release of toxic industrial chemicals. Consequently, military commanders want their troops to be protected up to a certain level against these chemicals and want to know precisely for how long this protection will last. 2.3.8. Radioactive gases In general, radiological dangers are related to radioactive dust, i.e. radioactive aerosols posing an inhalation and/or a skin contamination risk. These particles are retained by protective clothing and the aerosol filter (see section 3.1.4). However, one has to be warned against a prolonged use of the filter. As soon as the cloud of radioactive particles is passed, the filter should be exchanged for a new one. Indeed, as the radioactive dust is settling on the aerosol filter it is concentrating and the canister itself is becoming a radioactive source. As thi~ source is situated near one of the radiologically most vulnerable parts of the human body (the thyroid gland) it must be removed as quickly as possible without exposing the respiratory tract to further radioactive dust. Apart from these aerosols, nuclear and radiological incidents, be it industrial accidents or terrorist attacks, will usually generate radioactive gases. Several gases are radioactive (e.g. 85Kr), but are not considered to pose a health danger as they are too volatile. They do not build up to a dangerous concentration in open air and they do not, or only to a very limited extent, metabolise inside the lungs. In other words, they do not absorb in the human body. Radioactive isotopes of iodine, on the other hand, are of particular concern. They are found in irradiated nuclear fuel, but are also used as radioactive tracers, e.g. in medical imaging. They are also liberated by nuclear bombs, so they are likely to be released in a wide variety of incidents. Contrarily to other radioactive species, iodine (and other halogen) isotopes are quite commonly released as vapours, e.g. radioactive iodine (13112), methyl radioiodide (CH3131I) and methyl radiobromide (CH382Br). Methyl radioiodide and -bromide in particular pose an inhalation risk as these vapours are rather volatile (see section 4.1.1). Furthermore, radioactive elements are known to be better metabolised (i.e. absorbed in the human body) when linked to organic molecules.
3. THE USE OF ACTIVATED CARBONS IN THE PROTECTION AGAINST CWAs 3.1. General Efforts to protect soldiers from the adverse effects of CWAs started during the First World
War, immediately after their initial use. At first, trials were made with holding simple handkerchiefs or bags over mouth and nose, sometimes impregnated with basic solutions, such as the urea from urine, to neutralise the "acid" gases. But it was quickly realised one should protect the whole face, especially the respiratory tract (nose and mouth) and eyes. This has lead to the development of gas masks. However, in order to assure a sufficient flow of clean respiratory air, the environmental air has to be filtered, i.e. one has to integrate a filtering device into the gas mask. The gas mask itself has seen a huge development since its
early beginnings (see Fig.3), but for the filter, scientists had quickly developed the "ideal" model: a canister filled with activated carbon. In fact, it is a bit surprising this still took so much time. In 1850, John Stenhouse of Glasgow (Scotland) had designed a mask that used wood charcoal as filtering material. During the 1870s several other inventors put masks on the market with all features of a complete respirator - flexible face-piece, goggles (eye-pieces), activated carbon filter and aerosol filter (usually cotton wool) [ 10].
Fig.3. Example of the evolution of the gas mask (France: approx. 1915-'16-'17-'31-'51-'90) [8,9] 3.2. Individual filters
Hence the first military filters were individual filters fitted on gas masks. During WWI they were not always used exclusively by soldiers...(see Fig.4,5). This type of filter is basically a bed of activated carbon, arranged in such a way that one gets a linear flow of contaminated air through the bed (see Fig.6). However, the actual use of this type of filter involves some problems.
Adsorption of Chemical WarfareAgents
Fig.4. Hand to hand combat during a chemical attack in the village of Montauban (France, 1915)[ 11]
Fig.5. Protective measures for horses (WWI)
In reality, the flow is not constant. The inhaled air passes through the filter, but the exhaled air is expelled from the gas mask by means of an outlet valve (see Fig.7). The resulting flow pattern is a half-sine function, as the inhaled air follows the normal sinus pattern, and there is no flow over the carbon bed during the exhalation part (negative part) of this sinus (see Fig.8). The effects of this rather peculiar flow pattern will be discussed later. The main limitation on protection capacity for an individual filter is the amount of carbon that can be put into the canister. Too much carbon will result in too cumbersome a canister, and, above all, in an excessive mass. This will not only put an extra burden on the wearer, but can also result in leakage of the gas mask (the rubber of the mask deforming due to the local force exercised by the mass of the filter). The maximum mass depends on the placement of the filter, either on the side, i.e. on the cheek, or centrally underneath the chin, and the overall characteristics (e.g. elongation) of the rubber. For centrally placed canisters, a maximum of 350 to 380g is commonly accepted , even though most countries prefer lower masses. A major problem for individual canisters is the evaluation of the residual protection time while the filter is in use (see section 3.7). This is largely determined by the filter capacity and the bed packing. Due to the limited amount of carbon present in the filters, any irregularities in the packing of the bed will result in a dramatic loss of protection efficiency The gas will follow the preferential channels through the carbon bed and will break through the filter much earlier than expected. To overcome this problem military filters, and most industrial ones, are filled using a so-called "snowfall" apparatus. Here carbon granules are first distributed uniformly on a mass per granule basis, and then dropped vertically through a tube with internal sieves to ensure a parallel "snow-like" falling of the particles into a filter body. In some cases the filters are subsequently vibrated, placing a weight on top of the carbon bed, before closing. This as a final means of ensuring the best possible packing of the bed.
Fig.7. Air flow through a gas mask
Fig.8. Real vs mean inhaled flow rate (individual filter)
3.3. Collective filtration
Collective filters differ from the individual ones by the fact that they are not connected to ones gas mask. They can provide clean air to either a closed and sealed room such as a shelter, vehicle or helicopter, or to a set of individual gas masks linked to a central filter in an Armed Infantry Fighting Vehicle. In the first case, the most common one, the room will normally be placed in overpressure, to ensure all air entering the room will pass through the filtration system. Many of the problems encountered with individual canisters do not exist with collective filters: 9 The airflow is not regulated by the breathing rhythm of the wearer. It is possible to have a steady, constant flow through the filter, eliminating any flow pattern related problems. 9
Neither mass nor available space are major limitations for collective filters. Even though in some circumstances they will have to be taken into account, for instance on vehicles or
Adsorption of Chemical WarfareAgents
helicopters, these constraints will be less stringent than for individual protection. It is possible to calculate the minimum amount of carbon necessary for a given protection time against different, well-defined threats, and to increase this amount by a considerable safety margin. As the collective filters are overdimensioned, the risk of a premature breakthrough is much lower than in the case of individual filters. It is also much more feasible to install an end-of-service indicator. A popular set-up is two filters, placed in series, with a dedicated detection device placed in between them. In this way the first filter can be replaced as soon as it is saturated and the agent is detected, while the second filter assures the continued supply of clean air. As there is more than enough carbon present in the filters, any irregularities in the packing of the bed still result in a loss of protection efficiency, but this loss will not be as catastrophic as in the case of individual filters. Given the bigger dimensions of the filter, it is also easier to obtain a good packing density. New military technology for collective filtration is focusing on PTSA-systems (Pressure and Temperature Swing Adsorption). Basically these use two filters alternatively, one providing protection while the other one is being "cleaned", i.e. the toxic compounds desorbed by a combined action of reduced pressure and temperature. Due to the nature of the adsorption processes, this is very difficult to achieve with activated carbon filters as changing temperature and pressure will reverse physical adsorption, but will have a negligible effect on chemisorption (see section 4). 3.4. Aerosol filters Aerosol filters are not activated carbon filters, but they are an integrated part of all military
individual canisters (see Fig.6) and collective filtration systems. They usually comprise two parts: a coarse filter (a metallic "sieve") and the actual aerosol filter, a folded cellulose, glassfibre or other type of sheet. The first will stop larger physical objects, such as dust particles, to enter the filter. In this way it prevents these particles damaging the second filter, and lessens the possibility of clogging of the filter inlet. It also keeps out any radioactive dust in the event of a radiological or nuclear incident. The second filter will retain smaller airborne particles (i.e. aerosols) typically in the diameter range of around 0.5-1~tm. These can be liquid droplets from chemical agents, or biological agents such as spores and bacteria. Military aerosol filters are very efficient and are usually tested with aerosols of 0.31am diameter. Whereas industrial HEPA (High Efficiency Particulate) filters typically present a protection factor (quantity of aerosol retained)  of 99.95 to 99.97%, military filters go quite commonly up to 99.997% [ 12]. There is no real interaction between the aerosol filter and the activated carbon, except as a barrier to prevent solid and liquid particles penetrating into and being deposited on the carbon bed. Similarly, the carbon bed will adsorb any toxic vapours that might evaporate from the liquid droplets retained by the aerosol filter.
3.5. Protective clothing The role of activated carbon in modem protective garments is rather similar to its interaction with the aerosol filter, as it is shown in Fig.9 . Military protective clothing is usually of the (semi-)permeable type. This is different from the chemical industry, where most protective suits are of the impermeable type. This is due to their apparent advantage: as long as impermeable clothes are not degraded by the chemicals (e.g. strong acids), nothing will come through, be it liquid or vapour. For military applications there is another constraint, the minimisation of heat stress (see section 3.6.1), as military personnel can be compelled to wear these garments for several hours, even days. This represents a situation that is quite different from the one in industry where workers put on their protective suits immediately before entering a contaminated area, leaving it rather quickly, constrained by occupational health laws.
Fig.9. The role of activated carbon in protective garments Permeable garments usually consist of a minimum of three layers [ 15]: 9 An inner layer, that does not contribute to the protection but only acts as a support and increases the comfort of the wearer, avoiding contact with the activated carbon layer. 9 An outer layer that provides the necessary protection against liquids and aerosols. It is usually a plastic membrane with excellent liquid repellant capabilities, but permeable to vapours (hence the term "semi-permeable"), so as to allow the evaporation of transpiration (i.e. water vapour) in order to limit heat stress [ 16]. 9 The activated carbon layer that adsorbs toxic vapours. These can be present in the atmospheric environment, or they can be liberated by the droplets and aerosol particles retained by the outer layer. There are several different types of activated carbon layers [17,18], including activated carbon (nano)particles, activated carbon impregnated foams,
Adsorption of Chemical WarfareAgents
activated carbon microbeads and activated carbon cloth (woven or non-woven activated carbon fibres). Activated carbon particles have also been used to limit the permeability of the outer layer, while still having good water vapour exchange capacity. This has been realised by embedding carbon particles in an impermeable plastic film. In this way, the open spaces in the membrane are replaced by activated carbon "windows", retaining toxic vapours but allowing water vapour to pass . Recently much research has been focussed on increasing the reactivity of the carbon. Because the nature of clothing, and the constraints on weight and heat exchange, it is only possible to include a very thin carbon layer. So, even with the low concentrations passing the membrane, it is difficult to obtain complete adsorption / destruction of the toxic compounds. One way to overcome this problem is the introduction of so-called reactive carbon nano-particles. These are small carbon particles, impregnated with reactive substances (mostly metal oxides) that will react with the most common toxic compounds [20,21]. Usually the destruction process will involve hydrolysis, followed by adsorption. Interestingly, the impregnation products, such as TiO2, A1203 and MgO, are quite different from the on.ts used in filters (i.e. Whetlerites, see section 220.127.116.11).
3.6. Operational effects of using chemical protection 3.6.1. Heat stress The main problem related to wearing a chemical protective garment is the heat stress [ 19,2225]. As the protective layers are preventing the toxic chemicals from reaching the skin, they also prevent transpired water evaporating. Even with special materials, specifically designed to form a barrier against the permeation of toxics but to let the sweat out, the rate of transpiration will go down. Chemical protection suits will also increase the body temperature by isolation as they present a heat resistance (expressed as K.mZ.W -1) against any heat exchange between the skin and the environment. As a result, skin and body temperatures can experience a significant increase over a short period of time. This will also have a direct, negative effect on the heart rate (cardiovascular strain) and the blood pressure. As a rule of thumb, continuing operations wearing protective clothing becomes dangerous if at any one of the following conditions is met: 9 Body temperature (calculated as 0.8Trace.t+ 0.2Tskin) becomes higher than 311.6K (38.5~ 9 Heart rate exceeds 80% of the maximum rate for that person 9 Blood pressure gets higher than 22/11
A supplementary indication is the difference between the rectal and the skin temperature, which should not exceed 1K, as it is a direct indication of a heat loss problem.
3.6. 2. Breathing resistance Theoretically, the deeper the filter bed, the better the respiratory protection. However, the size and depth of an individual canister are limited. Size (and mass) are obviously limited by the fact that the filter has to be worn in connection to a gas mask. Too big dimensions will hamper head movement, will cause neck fatigue and even neck pain, and can limit the protection factor of the gas mask by deforming the rubber during head movements, creating leakages between the face and the mask, thus jeopardising the facial seal. This problem can be avoided by not connecting the canister directly to the mask. In some systems the filter unit is located elsewhere, e.g. in a pouch on the persons waist or hip, and connected to the mask by means of a flexible hose. It is clear this solution is not well suited for heavy duty in the field as it involves extra equipment, limitation of movement, and possible entanglement of the tube. In other cases, e.g. aircraft crews, it constitutes a viable solution. The height of the carbon layer is not only limited by the dimensions, but also by the pressure drop over the bed. This pressure drop results in a breathing resistance, making it more difficult for the wearer to breathe. As this resistance increases with the flow rate, it will hinder most when the wearer is performing heavy duty work and really needs high quantities of clean air. One way of reducing flow resistance is the use of bigger activated carbon particles, as flow resistance around a particle is known to be directly proportional to its specific surface (Darcy's law), which in turn is inversely proportional to its diameter. 120
E 0.05 |
= O,04x ~
YR2 = 0,999
,- 80 .E_ ~
Mean particle diameter (ram)
Fig. 10. Flow resistance vs mean particle diameter
Vapour pressure at 293K (mmHg)
Fig. 11. Breakthrough time vs vapour pressure for monochlorinated hydrocarbons
This is illustrated in Fig.10  where the flow resistance per unit of bed depth and per unit of flow velocity is plotted against the mean diameter of the particles. Unfortunately, the kinetics of adsorption are inversely proportional to the mean diameter of the activated carbon particles. As a result, smaller particles exhibit better adsorption characteristics but cause a higher breathing resistance. This can be overcome by using assisted filtration in which
Adsorption of Chemical WarfareAgents
filters are equipped with a powered fan (usually carried on the hip and connected to the mask via a hose) to overcome the breathing resistance. But as this device needs a power supply, the total breathing "package" consisting of mask, filter, power supply and fan becomes rather voluminous and heavy, making it unsuitable for most military applications. Consequently, the size of the particles in an individual military canister is the result of a compromise, resulting in the use of a mean particle diameter of around 1 mm (usually expressed as fractions passing/not passing the ASTM sieves = 12x30 Mesh size). 3.6.3. Psychological strain Wearing a chemical protective suit, including gas mask and filter, leads to a significant amount of psychological strain [23,24]. Not only is the wearer confronted with heat stress (3.6.1), leading to a general discomfort of the body, and increased breathing resistance (3.6.2), but there is also the limitation in the field of vision as many military gas masks have a pair of goggles. Even the ones with a panoramic visor induce limitations on the peripherical field of vision, leading to a diminished sense of equilibrium, loss of dexterity and even claustrophobia. Dexterity is also hampered by the gloves and the rigidity of the protective suit as a whole. The gas mask will also have negative effects on communications. Even with the help of special membranes, voice recognition and speech intelligibility are reduced. Communication is also more difficult as it is harder to recognise people and eye contact is nearly impossible. These diminished capabilities frustrate the wearer and increase his stress levels which are usually already rather high given the particular circumstances that made it necessary to put on the chemical protective suit in the first place... 3.7. End-of-service indicators Both in military and in industrial applications it is very important to know when a filter has reached the end of its service life, i.e. when a toxic compound is on the verge of breaking through the filter. In an industrial environment it is a question of economics. As the employer must provide adequate protection to the employees, filters must be changed regularly. This can be done either by a one-use policy, which is, of course, a very expensive way, or by relying on predictive models for estimating service life. These models always present an inherent uncertainty and they are developed for one continuous use, not for the intermittent use that is customarily in industrial applications. As a result, one has to take into account rather large safety margins, increasing again the total annual cost of safety equipment. For military applications the situation is quite different as there is normally no intermittent use, the filters being disposed of after use. Military personnel will usually have to stay in the contaminated environment for longer periods and if they cannot leave this area, changing the filters risks exposing themselves. In short, End of Service Life Indicators (ESLIs) are primarily of economic interest in industry but can be life-saving in military applications. There are several types of ESLIs. The bigger ones are real chemical detectors based on infrared, photo-ionisation, acoustic wave, etc., that use (micro)probes to measure gas
concentrations inside the activated carbon bed. As such, they can only be used in large filters, i.e. filters for collective protection . In individual filters there exist three promising techniques, chemiresistor microsensors, signal compounds and colorimetric indicator films . Microwatt chemiresistor microsensors  give a signal (audio or LED) triggered by a chemically induced change in a local resistance when the adsorption front reaches a certain point in the filter, e.g. 80% of the total depth. The advantage of this technique is its general response as "all" gases will provoke a change in resistance, giving raise to a signal. However, there are some major problems, specifically the sensitivity and response time, the fact the sensor has to be embedded in the carbon bed and the fact it needs a power supply source however small it may be. Signal compounds are pre-adsorbed chemical compounds . They are situated in the original carbon, or in a supplementary layer of a different carbon. When the toxic compound reaches this depth of the filter, the pre-adsorbed compound will be desorbed (expelled) by the toxic material and can be detected at the filter outlet, either by the human nose or by an instrumental detector inside the gas mask. This is a very clean and straightforward method, suffering however from one major drawback. In order to give a warning against a wide range of different compounds the pre-adsorbed compound must be very weakly bound by the carbon. As a result, as will be described further on (see section 4.1.1), this signal compound will be easily desorbed from the carbon by the simple action of (clean) air flowing through the filter. While this is not really a problem in an industrial environment, where gas masks and filters are usually only worn when there is a toxic concentration present, it poses severe problems in a military context. Prior to an attack, the mask can be worn for quite a long time, flowing air through the filter and causing premature alerts from desorbing the signal compound. Finally, the most promising route seems to be the use of colorimetric indicator films [31,32]. The compounds with which these films are impregnated react with certain gases, giving differently coloured products. When part of the filter body is transparent, these films can be placed along the perimeter of the carbon bed, and the saturation of the bed can be followed, in real time, by the colour change along the film. The downside of this method is its specificity. While it can be extremely useful in an industrial environment, where one usually knows exactly which toxic compounds to expect, it becomes rather tedious in military applications. Either the film should react with all chemical warfare agents and toxic industrial chemicals, which is not feasible, or one needs to place a significant number of these films in parallel in the filter body. While in theory this is a viable option, it would almost certainly lead to confusion in operational circumstances.
Adsorption of Chemical WarfareAgents
4. ADSORPTION MECHANISMS 4.1. Physisorption
4.1.1. General Physisorption of gases and vapours by Van der Waals forces is the basis of the protection afforded by activated carbons. Even for weakly adsorbed gases, where one needs special impregnants on the carbon in order to stimulate neutralisation reactions, it plays two important roles. First of all, it is nearly impossible to get chemisorption (see section 4.2) without prior physisorption. If the different components of the reaction, especially the chemical warfare agent, are not physisorbed on the carbon even for a very small period of time, this reaction will not take place. There is a need to the temporally immobilisation of the molecules, to give them enough contact time with the impregnates. The same is true for the hydrolysis reactions by which many compounds are destroyed. These reactions take place between the physically adsorbed vapour and the physically adsorbed ambient water vapour. Added to that, it has been shown that for several reactions the carbon surface itself is not inert, but plays a significant role in the actual reaction, for instance as an electron donor or receptor . Secondly, the by-products of the neutralisation reaction can be harmful themselves. Fortunately they are usually retained by the activated carbon as they are more strongly physically adsorbed than the initial CWA. How well a CWA, and a vapour in general, is retained by physisorption on a bed of activated carbon is determined by two parameters, the affinity of the carbon for this vapour, and the volatility (i.e. the saturated vapour pressure) of the CWA. This can be illustrated by the use of the Dubinin-Radushkevich equation :
I-BT We = WodL exp ~2 log 2
Where We is the equilibrium adsorption capacity (g.gcarbon-1), mo the micropore volume (cm3.gcarbon-1), dL the liquid density of the organic vapour (g.cm-3), B a structural constant of the carbon (K-2), T the air temperature (K),/3 the affinity coefficient of the organic vapour, Cs the concentration of the contaminant corresponding to the saturation vapour pressure of the contaminant Ps (g.cm -3) and Co the contaminant concentration in air (g.cm -3) This equation gives the equilibrium adsorption capacity of a given carbon (described by the parameters Wo and B) for a given concentration, Co, of a vapour (determined by ,/3and C~). The affinity coefficient fl can be calculated through the use of the molar polarisation Pe and a reference vapour, usually benzene, with a value of P~, of 26.2 cm3.mo1-1 . The molar polarisation can be calculated from the tabulated values of the refractive index at the sodium-D line, no, the liquid density dL (g.cm -3) and the molecular weight Mw (g.moll): j~ = peVap~ Re Benzene
2_1/M riD w 2 +2 d L n D
It is clear that a lower value of C~ will yield a higher value of We, which means that less volatile compounds are more strongly adsorbed. As the molar mass plays an important role in the calculation of fl (through Pe) and, generally speaking, larger and heavier molecules tend to be less volatile, it is clear that for most vapours /3 reinforces the effect of Cs. As a general rule of thumb we can say that in one group of compounds (e.g. alcohols) the larger the molecule, the better it will be adsorbed. When comparing different groups (e.g. halogenated and non-halogenated alkenes) this is not strictly true, but the main principle stays valid in that bigger, less volatile compounds will be strongly adsorbed and smaller, more volatile compounds will be weakly adsorbed. This is illustrated in Fig. 11, where breakthrough time of a given carbon bed is plotted against the vapour pressure of different organic compounds of a same group (monochlorinated hydrocarbons) . For highly volatile compounds the carbon hardly retains them, and pure physisorption alone does not suffice to provide adequate protection.
4.1.2. Types of activated carbon The "ideal" carbon for pure physisorption can be derived directly from the DubininRadushkevich equation. The two carbon-related parameters are the structural constant B and the micropore volume Wo. Usually these values are obtained from N2-isotherms at 77K. A lower value of B will yield a higher value of We, i.e. a higher adsorption capacity. In the original DR-equation B is related to the width of the Gaussian micropore distribution but it is also well known that B [K 2] is directly related to the adsorption energy Eo [kJ.mo1-1] by Eq.4, the latter being in turn related to the mean micropore half-width L [nm] by Eq.5 . Eo=
0.01914 ~ 10.8
E o = - - + 11.4 L
Combining these two equations demonstrates the relationship between B and the mean micropore size of the carbon, a lower B corresponding to smaller pores. This result is not surprising. The adsorption forces in smaller pores are much stronger due to the overlapping potential of the opposite pore walls, so these pores adsorb the vapour more strongly. In other words, a lower value of B is a sign of a carbon with a higher adsorption potential. Typically, B-values for commercially available carbons for gas phase adsorption applications range between 5.10 -7and 5.10 6 K -2. But even more important is the value of the micropore volume Wo. Whereas B is a measure of the adsorption force, Wo gives the available adsorption space. It is clear that a
Adsorption of Chemical WarfareAgents
higher micropore volume has a direct, positive, impact on the adsorption capacity We. Typically, Wo values for commercially available carbons for gas phase adsorption applications range between 0.4 and 0.6 cm3.g-1. Carbons for military use will normally present slightly lower values, typically between 0.3 and 0.4 cm3.g-1. This is due to the impregnation (see section 4.2.2.) .
4.2. Chemisorption 4.2.1. General From the previous sections one can see there is a real problem for activated carbon to adsorb highly volatile compounds, i.e. gases. Most of the gases are small inorganic molecules. This problem has also been recognised in industry, where many of these gases are used in a wide variety of processes. In order to retain these gases on a carbon filter there has to be a reaction of some sort between the chemical compound and the filter. As the "normal" carbon surface (see section 4.1.1) contains only a limited number of weakly reactive groups, it behaves almost as a perfect, inert, physisorption medium. Therefore it is necessary to "impregnate" the carbon with other, more reactive, substances. The nature of the impregnation-gas interaction can be twofold. The simplest way is the establishment of a real chemical bond. This way the gas molecules are bound to the impregnant, that is, in turn, attached to the carbon. It is clear that in this case the capacity of the carbon is directly proportional to the amount of impregnant. Another possibility is the impregnant acting as a catalyst in order to provoke a reaction or polymerisation of the gas molecules. In this way they can become larger and less volatile (e.g. by hydrolysis), and they will, subsequently, be removed from the gas stream by pure physisorption. One has to bear in mind that these reactions can also produce other volatile molecules, sometimes as toxic as the original compound. Alternatively, the gas molecules can react giving products that are much less volatile than the original gas, or, if more volatile, that will not cause harm if they are not retained by the carbon filter. In these two cases where the impregnant behaves as a catalyst, the capacity of the carbon is not directly related to the amount of impregnant present. In the first case (reaction followed by physisorption) it depends on the micropore volume and distribution in much the same way as for pure physisorption (see section 4.1.1). The actual reaction pathways can be very different, depending on both the chemical compound and the nature of the impregnation, but it is clear that the character of the carbon-compound bonding for chemisorption is quite different from physisorption. The main distinction lies in the much higher bonding energies (chemical interactions versus Van der Waals forces). In most instances chemisorption also results in an irreversible adsorption. While this can be a problem in catalysis or gas separation applications, it is, of course, an advantage in filtration. In all cases (direct reaction, reaction to form physisorbed molecules, or reaction to produce non-toxic molecules) the reaction pathways differ from the reactions between the chemical compound and the impregnant molecules in their "free" state, i.e. in the absence of the carbon matrix. Not only does the carbon interfere actively (usually as a proton donor/receptor) but it also facilitates the solid-
gas interactions as the gas is physically adsorbed on the carbon (pore) surface, although very weakly. This, however, dramatically enhances the chances of interaction between the impregnant, either as a reactant or a catalyst, and the chemical compound. In fact, the chances of an interaction without prior physisorption are so slim that one can assert that chemisorption is impossible without a certain prior physisorption. This is also demonstrated when modelling chemisorption as in the majority of circumstances this prior, weak, physisorption is the rate controlling step in the chemisorption process (see section 5.2.3). Table 1 Commercially used impregnations against industrial hazards Impregnation with... Chemisorption of... Sulfuric acid Ammonia, amines, mercury Phosphoric acid Ammonia, amines Potassium carbonate Acid gases (HC1, HF, SO2, H2S, NO2), CS2 Iron oxide H2S, mercaptanes, COS Potassium iodide H2S, PH3, Hg, ASH3, radioactive I2 and CH3I Triethylene diamine Radioactive I2 and CH3I Sulfur Mercury Potassium permanganate H2S (in the absense of 02) Manganese(IV)oxide Aldehydes Silver PH3, ASH3, bacteria Copper H2S, HCN in cigarette filters Zinc oxide HCN A secondary result of chemisorption is the possible release of rather high amounts of energy in the form of heat. This is a direct result of the exothermicity of many of the involved reactions. This heat build-up has several negative implications on the adsorption process. First of all, as predicted by the principle of Le Chfitelier, it has a negative influence on the equilibrium of these reactions. Secondly it reduces the effectiveness of the physisorption process (see section 6.2). Both these effects clearly diminish the overall efficiency of the carbon filter. An even bigger danger lies in a spontaneous auto-combustion of the activated carbon. The heat build-up, and the poor thermal conductivity of the carbon bed, can indeed surpass locally the self-combustion temperature of the impregnated carbon (see section 18.104.22.168), resulting in a fire inside the filter. It is clear that this will lead to a total failure of the filter, and a direct threat to its user from fire, hazardous substances released from the combustion and, last but not least, its loss as an effective filter medium. Many types of impregnants have been used for many years, both against industrial gases and against chemical warfare agents, dating back to the use of urine on cloth during the First World War (see section 3.1.1). Table 1 gives an overview of some of the commercially used impregnants against a wide variety of industrial hazards [26,39,40]. As for typical military impregnants, they will be treated in the next section.
Adsorption of Chemical WarfareAgents
4.2.2. Types of activated carbon 22.214.171.124 Whetlerites At the end of the First World War, not long after the first use of carbon for protection against CWAs, a typical military carbon was developed. This was called "Whetlerite" after its inventors J.C. Whetzel and E.W. Fuller . In fact, the name "Whetlerite" is now used for a whole family of impregnated active carbons, to the point where it has become the synonym of "an activated carbon used for military protection". The main reason for the development of a specific carbon was the threat posed by highly volatile chemical compounds such as HCN and ASH3. Whetlerites have seen a continuous development since their first use. At first additional impregnants were added to counter new threats such as cyanogen chloride (CNC1). More recently, environmental and safety concerns have focussed on avoiding the use of toxic or eco-toxic heavy metals. This has already led to the development of a series of chromium-free whetlerites, and research in this area continues. Table 2 gives the main types of whetlerite and their impregnation agents. This impregnation results in a loss of porosity as some of the pores are filled with impregnants. It has also been shown [41,42] that the impregnants will block a significant number of pore entrances, reducing the available micropore volume well beyond their own volume. Consequently, the physisorption capacity of whetlerites is usually smaller than for non-impregnated carbons tailored for gas adsorption. Table 2 Some different types of whetlerite carbon Type A AS ASC ASCT ASV ASM ASZM ASZMT
Impregnation Cu Cu, Ag Cu, Cr, Ag Cu, Cr, Ag, TEDA Cu, Ag, V Cu, Ag, Mo Cu, Ag, Zn, Mo Cu, Ag, Zn, Mo, TEDA
Table 3 A possible recipe to produce ASC-whetlerite Compound Formula Basic copper carbonate CuCO3.Cu(OH)2 Aqueous ammonia NH3(aq) Ammonium carbonate (NH4)2CO3 Chromic anhydride CrO3 Silver nitrate AgNO3 Water H20 Total -
Amount 114 g 284 cm 3 142 g 34.6 g 3.2 g 390 cm 3 1000 g
This solution contains approximately 8% Cu, 2% Cr and 0.05% Ag. Ammonia and ammonium carbonate are added to enhance solubility. The carbon is added to this solution (at 308K). After a sufficient equilibration time the wet carbon is removed and mixed with dry impregnated carbon, in order to absorb excess impregnation solution. Then the carbon is dried at 443-448K in air. These reactions are also highly dependent on the exact nature of the impregnants, particularly their oxidation state and their exact location in the pore system or on the outer surface of the carbon particles. There are also many interactions between the carbon surface and the impregnants, as well as between the impregnants themselves. Even though the different metals are impregnated separately (see Table 3 for a possible ASC whetlerite "recipe") [40,43] they can act in concert. The way several metals can act together is surely related to the impregnation pathway. Experiments with vapour deposition of the metals have shown that carbons impregnated in this way are much less effective in adsorbing certain gases such as CNC1 (see section 126.96.36.199). The exact reaction mechanisms, as far as they are known, are described in the next section. In some cases, e.g. the adsorption of CNCI, they are very complex, due to the fact that the carbon surface is not only an inert matrix but plays a significant role in the oxidation/reduction reactions. 188.8.131.52 Metal impregnations The net surface composition in atom % has been established for an ASC-whetlerite by XPS (X-ray Photoelectron Spectroscopy): 78.9% C, 13% O, 4.1% Cr, 1.3% Cu and 2.7% N . Generally speaking, impregnation levels (in weight %) of commercial ASC-carbons vary from 6 to 8% Cu, 2.5 to 3.5% Cr and 0.02 to 0.05% Ag [44,45]. But as a result of the clear influence of the actual state of the impregnated metals, there has been much debate on their exact chemical structure and the possible formation of complexes. Silver has been the only impregnation metal to escape the discussion, basically because the very low Ag concentrations were below the detection limits of most analytical methods . Furthermore, there are no indications of complex formation between Ag and any of the other impregnation metals, so it is safe to assume silver will be present either as Ag § or in its reduced form as metallic silver. The situation is less clear for copper and chromium. Some authors have put forward very precise formulations such as 2[CuCO3.Cu(OH)2] and CuCrzO7.2H20 [44,46]. During the heating after impregnation (drying) 3CuCr207 will convert to CuCrOa.2CuO + 5CRO3 with a subsequent reduction, under the influence of the carbon matrix, to Cr203 . Other authors found a whole range of compounds, often with the aid of an impressive number of different analytical techniques. This list includes CuO, CuCO3, Cu(OH)2, CuCrO4.H20, CrzO3.4H20, Cu(NH3)4CO3, CuCr207, CuzO, CuCrO4.2CuO, Cu(NH3)4CrO4, Cu ~ (metallic copper), Cu(NH3)zCrO4, Cu4(OH)6CrO4, (NH4)2CrO4 and Cu(OH)NH4CrO4 [33,40,48,50]. The only thing most authors agree upon is the necessity to have both hexavalent chromium and bivalent copper present on the whetlerite. This seeming to be a prerequisite for CNC1 adsorption (see section 184.108.40.206), but up to now no impregnation mechanism is known to produce only Cu 2+ and Cr 6+. Some measurements give a ratio of Cr6+/Cr3+ of approximately 1.2 .
Adsorption of Chemical WarfareAgents
Many other metal impregnations have been tried in the search for a non-toxic, noncarcinogenic chromium replacement. Several metals are well-known for their catalytic activity. But they either were toxic (e.g. Ni, Co), did not present good HCN and CNC1 retention (e.g. A1, W), or they were subject to ageing (e.g. V) (see section 6.6). Finally the best overall results were obtained by a copper-zinc-molybdenum impregnation. This impregnation is carried out in order to obtain Cu 2§ Zn 2§ (usually as ZnO) and Mo 6+ (impregnated as (NH4)zMoO4) on the surface of the carbon. Usually impregnation levels are 4 to 6% Cu, 4 to 6% Zn, 1 to 3% Mo and 0.02 to 0.05% Ag (mass%) [51 ]. These whetlerites are only slightly less effective in HCN and CNC1 retention under dry circumstances, and are equivalent to the traditional ones under more severe circumstances of increased presence of water vapour and/or ageing of the carbon, providing an additional TEDA (tri-ethylene diamine) impregnation (see section 220.127.116.11). Finally one should note that a second disadvantage, apart from the loss of available micropore volume, is a noticeable reduction in self-ignition temperature. The presence of metal ions on the surface of the carbon will lower the temperature at which self combustion of the carbon can occur, bringing it down to around 500K.
18.104.22.168 Organic impregnations Apart from metal impregnations, most modern whetlerites contain organic species. The main reason for introducing them is the ageing of the metal salts under the influence of adsorbed water vapour (see section 6.6.2). This has a very negative influence on the protection capacity against volatile inorganic compounds in general, and CNC1 in particular. As early as during the First World War, organic amines such as pyridine have been used to retain cyanogen chloride on activated carbon filters. Apparently these compounds do not only react with CNCI but they also stabilize the metal salts, reducing the deleterious effects of water vapour. This is shown by comparing the CNC1 breakthrough times under dry and under humid conditions. Extensive test programs have been set up to determine the most effective organic amine to incorporate into whetlerite carbons. The compounds investigated include the already mentioned pyridine, use of which is excluded on the basis of its carcenogenicity, 4aminopyridine and triethylenediamine (or TEDA) [40,53-55]. From the results one can deduce that the essential parameters for the organic substance to be effective are a high basicity, low volatility, ensuring that the molecule is well-retained by physisorption on the carbon substrate, and a maximum of active amine groups for interaction with CNC1 (see section 22.214.171.124). Another important parameter is the optimum amount of organic amine. This has to be determined experimentally, as it is the case for the metal impregnants. The best results for CNC1 retention under both as received and aged conditions are obtained for TEDA impregnation levels of about 5 mass%. This amount is usually applied to the carbon by spraying after the metal impregnation process has been completed. The optimum quantity varies with the type of impregnated carbon, both precursor and pore size distribution seeming to have an influence. Generally speaking, carbon derived from "soft" precursors such as
wood, fruit shells and peat require higher levels of TEDA impregnation than carbon from coal. TEDA levels in commercial activated carbons for military applications vary between 2 and 10 mass%. If the amount of organic amine is higher than the experimentally determined optimum, there is no further gain in CNC1 retention, but there is a significant loss of physisorption capacity due to the partial blocking of the micropore system. In addition, TEDA will further reduce the self-ignition temperature for an activated carbon to about 470K. At this point, it is possible to have spontaneous combustion of the carbon filter when challenged by high concentrations of chemically adsorbable gases (see section 126.96.36.199). Finally, organic amine addition to military carbons has an additional advantage that was not foreseen, but is nowadays regarded as very important in the context of the broader use of military filters in an industrial environment and in response to terrorist attacks. It results in the filter having a higher ability to retain some of the more dangerous radioactive gases (see section 4.3.7). 4.3. Adsorption m e c h a n i s m s of various C W A s
4.3.1. Incapacitating agents (see section 2.3.2) These compounds are nearly always disseminated as aerosols. As such they do not interfere with the activated carbon, but are retained by the aerosol filter. It is possible that, due to reactions such as hydrolysis, or as part of the spraying system, organic vapours are released by the aerosol particles. If this is the case, these vapours will be retained by the activated carbon on the basis of pure physisorption. 4.3.2. Vesicants or blister agents (see section 2.3.3) All vesicants (Mustard gas, Nitrogen mustards and Lewisites) are rather large molecules with low vapour pressures. As a result, they are readily physisorbed by non-impregnated activated carbons. In the presence of moisture they are also known to hydrolyse, decomposing in other well-adsorbed, large molecules and acid compounds, specifically HC1. Only Lewisite-3 (tris(2-chlorovinyl)arsine) will not hydrolyse. As an example we can look at the hydrolysis of Lewisite-1 (or 2-chlorovinylarseneous dichloride) according to equations 6 to 8 . Subsequently, the adsorbed HC1 will react with the copper ions of the impregnation according to reaction 9. The other hydrolysis products are all large molecules, some being solids, and are very well retained on the carbon by physisorption. CI i
--'~'~'" ASc I 2H20
+ 2 HCI Ci J " ~ / A S . o m
2-chlorov inylarsonous acid OH I
Lewi site oxide CI/-~.~As - O
Adsorption of Chemical WarfareAgents
2 [HCl]ads + CuO --> CuCl2 + H20
4.3.3. Choking agents (see section 2.3.4) 188.8.131.52 Phosgene Phosgene (COC12) will hydrolyse when physisorbed  on activated carbon (Eq.10). [COCl2]ad s + [H2O]ads ~ CO 2 "~ + 2 HCI
As with vesicants, HC1 is released and is retained in the same way, the reaction with copper giving CuC12 and water. Thus the chlorine will be retained, and extra water will be available for reaction 10. The downside is the depletion of the copper catalyst. It has been shown that the overall reaction, especially the first step, is highly exothermic. This can result in a significant rise in temperature of the carbon bed, particularly at high flow rates. The build-up of heat can be considerable, resulting in a temperature rise in the carbon of up to 130K, reaching 425K after a phosgene challenge in an air stream of 295K . There even have been reports of incidents in testing laboratories where spontaneous ignition of the activated carbon has occurred when exposed to high concentrations of phosgene. This is not surprising, as the specific impregnation of the whetlerite carbons, especially the ones with TEDA, is known to lower the self-combustion temperature from over 650K to under 500K . An adsorption front with a measured mean temperature of 425K can easily contain hot spots with local temperatures over 500K, resulting in the spontaneous combustion of the carbon bed.
184.108.40.206 Chlorine The least one can say is that literature on the subject of chlorine gas adsorption is scarce and not very recent . The most probable reaction mechanism involves hydrolysis in either the gaseous or, more likely, the adsorbed state as shown in Eq.11. The process is similar to that occurring with phosgene:
[C12 ]ads + [H2O]ads ---->
2 0 2 1" + 2 HC1
with a subsequent chemisorption of HC1 on the copper impregnant. This mechanism, especially the hydrolysis in the physisorbed state, is suggested by the fact that the overall kinetics of this process seem to be essentially governed by surface diffusion (see section 5.)
220.127.116.11 Chloropicrin Although rather outdated as a CWA, chloropicrin (CC13NO2), one of the choking agents, is still widely used by (military) testing laboratories as a simulant for nerve agents, in particular for Sarin, the other major nerve agents (GA, GD and VX) being far less volatile than chloropicrin. Given the inherent toxicity danger of chloropicrin, that it is still considered to be a CWA by the Chemical Weapons Convention, and the fact that it is purely physisorbed on
whetlerite type carbons, this a poor choice. Even though nerve agents are essentially physisorbed, there are some interactions with the metal impregnants (see section 4.3.5). These are not accounted for by chloropicrin, which simply determines the physical adsorption capacity of the carbon. This task can be performed by any non-toxic, non-CWA, cheap and readily available organic vapour such as cyclohexane or n-heptane .
4.3.4. Blood agents (see section 2.3.5) 18.104.22.168 Hydrogen cyanide Hydrogen cyanide is one of the chemical warfare agents that are only poorly retained by physisorption, due to its high volatility. Several different reaction schemes for its removal have been put forward over the years. Experiments have however shown beyond doubt that cyanogen gas, (CN)2 is formed. In fact, it is cyanogen that first breaks through a filter challenged by HCN. Consequently, in most cases, breakthrough concentrations are measured by methods selective to cyanide ions rather than to either HCN or (CN)2. Experiments have also shown that the primary metal for HCN adsorption is copper, more precisely in its oxidised form. It is generally accepted that the first step in the process of HCN removal is dissolution in the moisture present on the carbon, followed by reaction 12 [40,61 ]: (12)
2 Cu 2+ + 4CN- ---~ 2 CuCN + (CN)2
Specific reactions will involve either CuO or CuCO3, both leading to the formation of H20 and the second evolving CO2. Water and cyanogen will normally react (hydrolysis), with a resulting regeneration o f H C N (Eq.13) (13)
(CN)2 + H20 ~ HOCN + HCN
whereas further hydrolysis of HOCN will yield CO2 and NH3. This hydrolysis could be catalysed by the presence of the Cu-Cr combination [61-63]. Another possible retention mechanism for cyanogen is a result of direct reaction with Cu 2+(Eq.14) : OCN / CuO + (CN)2 ~ CuO.CzN2
Cu \ CN
It has also been observed experimentally that there is an increase in HCN breakthrough time when using ASC carbon instead of A or AS. This suggests that chromium has a beneficial effect on HCN retention. Moreover, it has been shown that this is primarily an effect on (CN)2 retention, rather than on HCN itself [40,64,65]. The overall reaction seems to
Adsorption of Chemical Warfare Agents
be the formation of oxamide. This reaction is catalysed by Cr6+according to Eq. 15 (formation of cyanoformamide and oxamide):
(CN)2 + H20
CONH2 I CN
II II H2N-C-C-NH2
So, very generally, the complete chemisorption mechanism for HCN on an ASC whetlerite can be described by Eq. 16: CLI2+
C r 6+
In this process copper is actively involved, and will be reduced from Cu 2+ to Cu l+ and subsequently to Cu ~ Chromium on the other hand will act as a pure catalyst and will remain in its oxidised Cr 6+ state. This is confirmed by experimental data from ESCA (Electron Spectroscopy for Chemical Analysis)before and after HCN adsorption . Other sources report a reduction of chromium from the hexavalent to the trivalent state suggesting an oxidation reaction rather than a catalytic process, but no possible reaction schemes were given. However, this reduction to Cr 3+ could explain the fact that carbons loose their capacity against HCN before being saturated with oxamide. As the breakthrough is usually by cyanogen rather than HCN, the limiting factor seems to be the availability of hexavalent chromium. Of course, this could also be explained by a gradual shielding of the chromium salts by deposition of the reaction products, i.e. the oxamide . Clearly it seems that the overall reaction is HCN ~ Cyanogen --~ Oxamide
but the individual reactions are less obvious, especially in the presence of water vapour which can have an additional reducing effect on the hexavalent chromium. Probably all cited reactions will take place in real filters, their individual relevance in the overall process depending on many different factors such as HCN concentration, the moisture content of the air stream, temperature, carbon porosity and carbon surface chemistry. For the next generation ASZM whetlerites, in which chromium is replaced by zinc and molybdenum, the overall reaction scheme seems to hold true. The first step (reaction with copper) is identical, and the Zn2+/Mo 6+ combination seems to play a role equivalent to the Cr 6+ of ASC . Apparently, Z n 2+ c a n also react directly with HCN, however without the formation of (CN)2 that is responsible for early breakthrough in copper-containing whetlerites . Molybdenum would then just play a stabilising role, preventing the Cu 2+ from reacting with the HCN rather than the Zn 2+. Even though there are experimental data to support this mechanism, it does not explain the presence of oxamide on ASZM carbons after exposure to HCN [53!. It q r o , ~ ,1~, once again, the exact reaction scheme, and ~!~, ~ : '
any beneficial effect on HCN retention. On the contrary, most experimental results seem to indicate a slight loss in capacity . This can be explained by the loss in physisorption capacity due to pore blocking, and in chemisorption capacity due to a shielding of the copper and chromium ions by the TEDA deposition.
22.214.171.124 Cyanogen chloride Cyanogen chloride is one of the other chemical warfare agents that are only weakly physisorbed by unimpregnated carbons. Comparable to HCN, this is the result of its high volatility. As is the case for HCN, the reactions involved in the retention of CNC1 on whetlerite carbons are still subject to much debate. There is one thing all sources agree upon, which is that the chemisorption of CNC1 is the result of a combined effect of the different impregnated metals. Impregnation with only copper or chromium gives the carbon some capacity, but by far not enough to be considered as a protection against a CNC1 threat. One possible pathway is given by Eq.18 through 20 : CNC1 + whetlerite.H20 --~ HOCN + HC1
2 HC1 + (NH4)2CO3 ~ 2 NH4C1 + H2CO3
H2CO3 ~ CO2 -k- H20
This accounts for the experimentally determined liberation of C02 and the presence of NH4C1 on the carbon. The (]~qH4)2CO3 was present in the Cu/Cr impregnation solution used to obtain the ASC whetlerite (see Table 3). If this reaction scheme is correct, there should be no liberation of CO2 after altering the impregnation procedure, using Cu(NO3)2.3H20 as the impregnating solution rather than the carbonate (Eq.21-23) : CNCI + whetlerite.H20 ~ HOCN + HC1
2 HC1 + Cu('NO3)2.3H20 ~ 2 HNO3 + CuC12 + 3 H20
HNO3 ~ ? (Not CO2 !)
Experimentally, there was no significant difference between the amount of CO2 released in both cases, suggesting some other reaction scheme rather than the one represented by equations 18 through 20. Another possibility would be a direct catalytic oxidation according to Eq.24: 2 CNC1 + O2
CuZ+/fr6+ 9 C12 + CO2 + N2
Chlorine could be hydrolysed and follow the normal pathway for C12 adsorption (see section 126.96.36.199). This would also effectively account for the presence of CO2 but not for the experimentally determined presence of NH3 [61 ]. It seems highly unlikely that nitrogen would
Adsorption of Chemical WarfareAgents
convert into NH3 on the carbon surface. The advantage of this reaction is that it can take place in the absence of water vapour. As it has been experimentally determined that there is indeed a limited adsorption of CNC1 from a dry air stream on a dried activated carbon , this reaction cannot be excluded completely. It will probably take place, but only to a limited extend under normal circumstances of humidity and pre-wetting of the carbon bed. In the end, the most probable reaction path is given by the equations 25-27 [40,44,50,61,64,67,68]: CNC1 + H20 HOCN + H20
CuZ+/Cr 6+ CuZ+/Cr 6+
.- HOCN + HCI (with some formation of HOCI+HCN)
~ NH3 + C02
2 HCI + CuO --~ CuC12+ H20 2+
When the carbon runs out of available Cu , the HC1 could be destroyed according to [50,61 ]" 6 HC1 + 2 Cr6+--+ 3 C12 + 2 Cr 3++ 6 H +
This would account for the observed change in the ratio Cr6+/Cr 3+. It has been demonstrated by XPS [61,66] that when adsorbing CNC1 this ratio decreases, i.e. Cr 6+ is reduced to Cr 3+, but the same studies show there is still a significant amount of Cr 6+ left after breakthrough. This is explained by the shielding of the Cr 6+ by reaction products such as CuCl2 and local polymerisation products (e.g. the formation of C3N3C13). It seems strange however that chromium would only start to act as a reagent instead of a catalyst after all copper has been "used". Not the least because this would mean that the initial hydrolysis of the CNC1 which, according to the majority of the authors is to be attributed to the CuZ+/Cr 6+ combination, would then stop with the immediate consequence of halting any HC1 production. Neither is there any evidence of a decrease of the Cr6+/Cr 3+ ratio when directly adsorbing HCI or C12. Apparently this has never been investigated, although it could shed some light on the issue of chromium reduction due to CNC1 adsorption. Nor does this destruction of HC1 by Cr 6+ explain the huge gain in breakthrough time between ASC and AS carbon. Normally there should be sufficient copper present on the AS carbon to give a reasonable retention time, which is not the case. One really needs the chromium to obtain decent CNC1 breakthrough times. On the other hand there is evidence to show that the ageing of ASC whetlerite (see section 6.6.2), which is essentially a reduction of the Cr6+/Cr 3+ ratio, is the result of the interaction of the copper-chromium salts with water. So one could suggest that, in the adsorption of CNC1, Cr 6+ acts as a pure catalyst, working together with Cu 2+ in the hydrolysis of CNC1 to HOCN and subsequently to NH3 and CO2. The reduction of Cr 6+ to Cr 3+ could then be attributed to either secondary reactions, or to the direct influence of water, exactly as in the case in HCN adsorption (see section 188.8.131.52). As for chromium-free whetlerites, e.g. ASZM-TEDA, there is very little known about the actual processes involved in the CNC1 retention. As there seems to be very little difference in the capacity to remove CNC1 between ASZ (Cu 2+, Zn 2+) and ASZM (Cu 2+, Zn 2+, Mo 6+) carbons , it is safe to suggest that the Zn 2+ takes over the tasks of Cr 6+, as catalyst,
oxidising agent, or co-catalyst with copper, its most probable role. Molybdenum is only introduced into the impregnation to increase HCN retention times under humid/aged conditions (see section 184.108.40.206) and, apparently, does not interfere with CNC1 adsorption. All whetlerites in use today contain TEDA (triethylenediamine), essentially to enhance the CNC1 capacity under humid conditions. Apparently TEDA works in two different ways, preventing, at least partially, the ageing of the carbon (see section 6.6) and providing a supplementary chemisorption capacity. As opposed to the Cu2+/Cr6+ combination, this role of TEDA seems to be only marginally affected by the presence of water vapour. The exact interactions between TEDA and CNC1 are still not completely clear, but a reaction analogous to the one proposed for methyl radio iodide (see section 4.3.7) seems highly likely (Eq.29). F----X N}/-""x,4N+ 2 CNCI'-*C1- [CN+N ~ N +CN] C1-
220.127.116.11 Arsine Arsine (ASH3) is a gas so, like other highly volatile materials, it is hardly retained by unimpregnated carbon. Fortunately it is quite easily oxidised by the catalytic action of copper. According to some sources , however, this catalytic activity decreases with increasing air stream humidity. At high humidities silver takes over as the main catalyst. Different oxidation states of arsenic are possible, so the arsenic is mostly converted to a solid mixture of oxides and elemental As which tend to shield the catalysts and reduces their effectiveness. 4.3.5. Neurotoxic agents (see section 2.3.6) Nerve agents are large organic molecules, with high molar masses and very low (VX, Soman, Tabun) to low (Sarin) volatility. Their low values of Cs lead to a very strong physisorption. Therefore, from a purely respiratory protection point of view, nerve agents do not pose a severe threat. As long as one is wearing a well-fitted gas mask, most commercial activated carbon filters will provide plenty of protection. However, the same problem arises as with the vesicants in that the physically adsorbed compounds are hydrolysed under the influence of water vapour (see section 6.4). Except for VX, the decomposition products can be harmful and volatile. For example the hydrolysis of Soman (Eq.30) yielding organophosphorous hydroxide, which is large enough to be well retained by physisorption, and HF which, on the contrary, is very volatile and toxic, and could pass through the activated carbon bed rather rapidly. Fortunately, the standard impregnation of the military carbons (see section 4.2.2) includes Cu-ions and organic amines that will react with HF gas in order to retain it on the carbon. Direct chemical binding in the form of CuF2 will result in the retention of this compound by the carbon filter. (30) O II H3C~P
CH3 I ~C ~
CH3 I C~CH3
0 II +
CH3 I ~C ~
CH3 I C~CH3
Adsorption of Chemical WarfareAgents
In the case of VX, none of the hydrolysis products are volatile so that even under humid conditions physisorption will suffice for protection against VX. 4.3.6. Industrial gases and vapours (see section 2.3.7) 18.104.22.168 Organic vapours Most industrial organic vapours are purely physisorbed. It is however possible that in some cases hydrolysis in the adsorbed state will occur, and there may also be interactions of the vapour or its hydrolysis products with the metal impregnants. These will only enhance the retention on the carbon. One should remember that, in general, carbons for military use (whetlerites) do have a smaller micropore volume than industrial carbons dedicated to physisorption. Compared to "general purpose" industrial activated carbons designed for use against both organic vapours and acid gases, military carbons perform quite well, with a physisorption capacity that is more than equivalent, due to a larger amount of activated carbon per filter. There has been some concern about some highly volatile and toxic organic molecules, so-called "mask breakers", of which the most well known examples are PFIB (2trifluoromethyl-l,l,3,3,3-pentafluoropropene or perfluoroisobutene) and CPFP (or 3-chloropenta-fluoropropene). These vapours are only very weakly retained on unimpregnated carbons, but it has been shown that by a catalytic effect of Cu and/or TEDA, and, probably, some hydrolysis, these compounds are transformed on whetlerites into less volatile chemicals that are sufficiently retained by physisorption [69-74]. 22.214.171.124 Chlorine As in the past chlorine was considered to be a CWA, it has already been treated with the other choking agents (see section 126.96.36.199). 4.3.6. 3 Ammonia Ammonia is a major concern for people working with military carbons. Due to its volatility, it is only weakly bound by physisorption, and there is very little indication of any chemisorption or catalytic activity of the copper, chromium or silver salts towards this gas. Some sources  suggest a slightly higher uptake on ASC-whetlerite, compared to type AS. This would suggest a limited activity of the chromium. There is some retention of NH3 on military filters, probably as a result of dissolution of the gas into adsorbed water. This is shown in Fig.12, where the breakthrough time of ASCT carbons is plotted against the pre-wetting of the filter. This pre-wetting was obtained by equilibrating the filter with flowing air at a certain relative humidity prior to the exposure to ammonia. The filter was then challenged with 1600 ppm NH3 in an air stream at the same relative humidity to avoid any exchange of water between the carbon and the air stream. It is clear that the humidity of the filter causes an increase in ammonia adsorption capacity. The drop for very high humidities (> 80%), is probably the result of kinetic effects. At these
values water is filling the pore system, slowing down the physisorption of the ammonia which is a first, and necessary, step towards its dissolution in the adsorbed water . ~. 60
g 5o f
E 40 .m ~ 30 O '-r
% Relative Humidity
Fig.12. Breakthrough time of NH3 on ASC-T carbon as a function of filter pre-wetting. 4.3.6. 4 Sulphur dioxide Sulphur dioxide is adsorbed to some extent on non-metal impregnated carbons by physisorption [76-78]. A chemisorption capacity can also exist which depends essentially on the basicity of the surface. A possible reaction scheme is given by a combination of hydrolysis and oxidation (Eq.31 & 32) of the SO2 in the physisorbed state [79,80]. This effect will be even more pronounced when a high number of nitrogen atoms is present on the surface, i.e. when the surface basicity of the carbon increases . Given the impregnation process of whetlerites (see section 188.8.131.52), it is clear that the residual ammonia from the impregnation solution will enhance the capacity of the carbon for SO2 adsorption. The presence of the basic TEDA molecules will reinforce this effect. S O 2 nt-
89 0 2 ~
Apparently a direct reaction between the metal salts and 802 has, as yet, not been documented. However, a reaction similar to the one with other acid gases (see section 184.108.40.206) cannot be ruled out completely. The H2SO4, formed by a combination of hydrolysis and oxidation, is likely to react with the copper according to Eq.33: H2SO4 + CuO ~ CuSO4 + H20
220.127.116.11 Other acid gases Acid gases such as HC1, H2S and HF will react with the metal salts, especially copper, after an initial weak physisorption on the carbon. The reaction will proceed according to the general scheme of Eq.34 [40,82]:
Adsorption of Chemical WarfareAgents
2[HX]ads + CuO--~ CuX2 + H20
This reaction is very efficient, as illustrated by the protection time afforded by military filters compared to industrial, even specifically impregnated, carbons. For example, when testing with a concentration of 5000 ppm H2S, at a relative humidity of 70%, European law prescribes a minimum breakthrough time of an industrial filter of 40 minutes (EN 141 - C l a s s B2 ). The Belgian military filter, as an example, yields breakthrough times of more than 120 minutes, which is more than double the protection of the industrial filter. Even when taking into account differences in amount of carbon, breathing resistance, mass, etc., it illustrates the efficiency of whetlerites in retaining acid gases. 4.3.7. Radioactive gases (see section 2.3.8) Non-impregnated activated carbons exhibit a low to medium retention against volatile radioactive gases such as CH3I and CH3Br [84,85]. Most industrial filters eliminate radioactive iodine (primarily 131I, but also ~32Iand 123I) and bromine by ion exchange with non-radioactive I or Br. Usually the activated carbon has been impregnated with potassium iodide (KI, see Table 1). Another possibility is an impregnation with triethylenediamine (TEDA) [40,86-88]. There are several possible reaction schemes, but the most probable one is the formation of a quaternary ammonium salt (Eq.35)
/,N + 2 CH3I----* I- +CH3Nxx
//NCH3 + I-
As TEDA is present on all modern types of whetlerite, military filters exhibit a rather good retention of radioactive gases by direct chemisorption. However, one has to bear in mind this is only a retention, not a destruction. This means that the filter itself will become a radioactive source, much like when retaining radioactive aerosol particles (see section 2.3.8), and has to be replaced, and disposed of, as soon as possible. 4.4. Adsorption of water Of course, water is not a chemical warfare agent. However, it plays a major role in the various adsorption mechanisms, either directly, in a benevolent role (e.g. hydrolysis), or indirectly, usually influencing in a negative way the adsorption behaviour of the carbon (see sections 6.4 and 6.5). Therefore it is quite important to have a good understanding of the water adsorption behaviour of carbons in general, and military carbons in particular. This behaviour is characterised by two parameters, the water uptake and the time to reach this uptake. The water uptake depends on the structure and surface chemistry of the carbon. This is illustrated by the water adsorption isotherm (see Fig. 13), that presents a very distinctive shape, especially when compared to the isotherms of other vapours such as chloropicrin (see Fig.14). Even when
these vapours have a saturation vapour pressure very close to that of water (Ps (chloropicrin) = 27 hPa, Ps (H20) = 28 hPa). '~" 0.4
0 ,,a L_
9e~". a~ 0.13
o . ~ 0.2
Fig. 13. Example of a water isotherm (Type V) on activated carbon
Fig. 14. Example of organic vapour isotherm (Type I) on activated carbon
This is the result of the very weak interaction (physisorption) between water vapour and activated carbon . The value of the Dubinin-Radushkevich affinity parameter fl for water is approximately 0.06, much lower than the values for organic vapours which are roughly between 0.5 and 1.5. Basically, the water isotherm of military carbons such as whetlerites can be divided in three parts. The first one is the chemisorption of water vapour by surface groups. These can be oxygen groups (involuntarily introduced during the activation process) or other species, such as impregnated metal salts (copper, chromium, etc.) or organic molecules (TEDA). When sufficient molecules are adsorbed in this way, pore filling of the micropores will start. It is still not very clear how this transition takes place, but pore filling initiated by the formation of water clusters around the primary adsorption sites (hydrogen bonding), the clusters behaving as water in a semi-liquid state, seems a viable pathway [90,91]. Just as for organic vapours, the micropore filling results in a steep rise of the isotherm. Subsequently, after filling the total micropore volume, adsorption will start in the mesopores due to capillary condensation . Another typical feature of water isotherms is the large hysteresis when comparing adsorption and desorption. This is, probably, related to the very weak adsorption forces between water and carbon. But at this time, no satisfactory general explanation has been found for this phenomenon. The second parameter is the time to reach an equilibrium uptake for a given moisture level in the environment. Whereas there is still much debate regarding the exact nature of water adsorption and, consequently, the interpretation of the isotherm, there is even less known about the kinetics of water adsorption. Up to now the only existing models are purely based on experimental data and, as such, they are completely empirical [93-97].
Adsorption of Chemical WarfareAgents 4.5. Novel forms of carbon
4.5.1. Industrial-military carbons From the previous sections it is clear that military filters filled with whetlerite carbon retain most industrial vapours and gases at least as effectively as dedicated industrial filters, especially when the supplementary impregnation with TEDA is added. This is also illustrated in Fig. 15, where the breakthrough times for a number of gases of some typical military filters (filled with whetlerites) are compared with the minimum requirements of an industrial filter ABEK-2, tested conform the European Standard EN141 . This norm describes the testing conditions and requirements of the most common types of industrial filters. The data presented are for tests with a flow rate of 30 litres/min, an inlet concentration of 5000 ppmv and a relative humidity of the air stream of 70%.
Fig.15. Comparison between an industrial filter type ABEK-2 and some typical military filters regarding the protection against certain industrial compounds Fig.15 shows clearly the weak and strong sides of the whetlerite carbon. ASC carbon will protect rather well against acid gases, SO2, and organic compounds, CC14. The supplementary TEDA impregnation enhances this, but only very slightly. The weak points of the whetlerite are ammonia and highly volatile organic compounds such as ethylene oxide. The small capacity against highly volatile organic compounds is logical as these compounds are very weakly physisorbed (see section 4.1.1) and do not react in any way with the impregnants. On the other hand, when taking proper precautions, the risk from these vapours is minimal, the high volatility making the build up of a toxic concentration in free air very unlikely. Proper procedures for entering industrial buildings can therefore eliminate almost completely the risk from these compounds. The main problem is the very low capacity of whetlerites against NH3. Ammonia is, together with chlorine, the most commonly employed toxic chemical throughout the world . Consequently, a low protection against NH3 puts a severe burden on planning the deployment of troops in an industrialized environment, with a non-negligible risk to the health of these troops. Therefore several countries have sought to improve the capacity of the military carbons against ammonia. This way the conventional military filter, offering already
good protection against most industrial gases, will effectively become a combined C W A Industrial Gases filter (also known as ABEK-NBC). Some countries (e.g. France) have indeed developed this kind of filter. There is, however, a downside to this enhanced spectrum of protection. In order to retain a good physisorption capacity, and a good distribution of the impregnants throughout the filter, the amount of activated carbon has to be increased considerably. This will lead to a higher filter mass and an increased breathing resistance through the carbon bed. Both these will have a negative influence on the already high physiological burden of wearing respiratory protection. It has also been shown that in some cases an increase in the mass of the filter will reduce the overall protection factor of the mask filter ensemble by creating deformations of the rubber mask during use, inducing small leaks around the facial seal. However, many countries and manufacturers pursue this line of research, as the advantages of one single filter being able to protect against both "all" chemical warfare agents and as many industrial gases as possible are evident. Another approach is to incorporate two types of carbon in one filter, either by mixing them, or by putting the two different beds in series in the canister. The obvious advantage of this technique is the possibility to optimise the "tailoring" of each adsorbent for a given set of gases. In this way it is possible to introduce e.g. both basic sites for SO2 adsorption and acidic sides for NH3-adsorption, without negative interactions. The result are filters that are effective against a very wide range of dangerous substances, from very volatile organic compounds (e.g. ethylene oxide), and different industrial gases (SO2, H2S and even NH3), to Chemical Warfare Agents (CNC1, HCN, ASH3) . Of course, as it is the case for filters filled with "good for everything"-carbons, the main task of obtaining maximum protection for a minimum breathing resistance and a minimum total mass still remains a dilemma. -
4.5.2 Carbon monoliths
Monoliths are usually cylinders, with a high number of interior channels (see Fig.16) with a circular or square cross-section. They are either completely made of carbon, or from a skeleton material (such as ceramics), coated with carbon. The surface of this carbon is activated, either by physical or by chemical activation (Activated Carbon Monolith or ACM). The main advantages of this form of activated carbon are the high micropore volume, the very fast adsorption kinetics and the low breathing resistance. The first two are a result of the almost total absence of meso- and macroporosity. All micropores are directly connected to the inner surface of the channels. Indeed, the particular form of the monolith makes it easier to "tailor" the pore size distribution and favour the creation of micropores during the activation process. There is no need to create a complete pore network, as one has to inside a carbon particle. The channels are effectively playing the role of macropores and give an unobstructed access to the activation agents, and, later, to the contaminated air stream. This also results in fast adsorption kinetics as, compared to activated carbon granules, several steps in the diffusion process are absent (e.g. Knudsen diffusion). The low breathing resistance is due to the fact that, contrary to granular activated carbon, the path of the air flow is free of any tortuosity, resulting in an almost perfect laminar flow pattern. Indeed, the flow will essentially
Adsorption of Chemical WarfareAgents
pass through the large channels and will not have to "find its way" through a bed of granules. A final advantage of activated carbon monoliths is the possibility to eliminate the use of a canister. The monolith is usually cylindrical in shape, but could, theoretically, be manufactured in any given form. This way it could, for example, be integrated in a helmet or another rigid structure. But activated carbon monoliths do also present some major disadvantages which are a direct result of the specific geometry of the channel and pore system. The first of these is leakage. It has been reported on several occasions that, in spite of the fast adsorption kinetics and the high adsorption capacity, there is an immediate breakthrough of the activated carbon monolith filter, but only at a very low concentration of the chemical compound. After the amount of time that one could normally expect from breakthrough models, this is followed by a real breakthrough curve. This is illustrated in Fig.17. The effect is directly related to the flow pattern, i.e. to the effect of the channels. As the flow is (pseudo-)laminar, the carbon monolith can be compared to a cylindrical plug flow reactor. For these reactors it is well known that the residence time of a reagent molecule inside the reactor is a mean value. In reality, there is a spread of residence times, some molecules reacting immediately, others passing through the reactor without reacting at all.
Fig. 16. General appearance of a carbon monolith
Fig. 17. Breakthrough and leaking of an ACM
Here one can observe a similar effect. Some of the molecules of the chemical compound will get through the monolith, staying in the centre of a channel, without being adsorbed in the micropores on the channel walls. As in military applications one is generally confronted with very toxic chemicals, this is a highly undesirable effect. Usually the leakage concentration, i.e. the amount of the chemical that gets through the monolith without being adsorbed, is higher than the concentration that is deemed to present a health hazard. A possible solution can be to use several, shorter, cylinders placed in series with intervening void spaces in order to make the laminar flow become turbulent in the void spaces, enhancing mixing and reducing chances of molecules passing through the filter unadsorbed. This is shown in Fig. 18. Experiments show that increasing the number of cylinders for a given filter length by increasing the number of void spaces decreases filter leakage. As a result, this method yields better performance of monolith filters without, however, eliminating completely the problem of premature breakthrough . There seems to be an exponential decay relation between the number of voids and leakage, leading to a zero chance of leakage
for an infinite number of voids. This last solution, however, represents nothing else than a granular carbon bed, the granules being very small monoliths. It is clear this solution is not viable, as, apart from the technical feasibility, it eliminates most of the other advantages of monoliths. Another major problem lies in the impregnation procedures for monoliths. From the previous sections it is clear that the adsorption of chemical substances in general, and warfare agents in particular, is the result of a joint process of physisorption and chemisorption. Consequently, in the case of monoliths, the impregnants must be situated inside the channels. The pore entrances of the micropore system are also situated directly in the channels, as there is no meso- and macropore system between these micropores and the surface as it is the case in granular activated carbon. This leads to a disproportionate part of the micropore system being blocked by the impregnants, severely reducing the physisorption capacity of the monoliths, which, in turn, not only leads to a diminished capacity for organic molecules, but also has a negative effect on the chemisorption capacity itself . At this time no satisfactory solution has been found to this problem.
Fig. 18. Effect of void spaces in a carbon monolith filter 4.5.3. Carbon fibres
Another recent development is the use of activated carbon fibres (ACF). These look very promising for use in military applications since they couple a large micropore area to very fast kinetics, nearly all pores being micropores situated directly beneath the surface of the fibre. One would also expect a lower breathing resistance, as the fibres will present more void space for the same physisorption capacity when compared to conventional granular activated carbons. Also, the fibres could fulfill the role of an aerosol filter, eliminating another part of the canister, thereby reducing mass, volume and breathing resistance. However, at this point the future of ACF in military respiratory protection does not seem to be very promising. There are several reasons for this, the first one being the same as that encountered with monoliths. "Whetlerising" commercial ACF gives very poor results, both for the chemical and physical adsorption capacity. Apparently the metal salts do not behave in the same way as on granular carbon, resulting in much shorter breakthrough times for HCN and CNC1 than would be expected on the basis of micropore volume and impregnation level. The micropore capacity, which is initially very high, is dramatically reduced by the blocking of the pore entrances [ 102, 103]. However, the main problem lies in the void volume between the fibres. Unless the fibres are really pressed together, there is a problem of leakage as some of the gas molecules will find a pathway through the filter with a maximum of void spaces separated by thin, easily saturated volumes occupied by activated carbon fibre. This can be overcome by
Adsorption of Chemical WarfareAgents
compressing the fibres, reducing dramatically the voids inside the filter. One way of doing this is pressing the carbon fibres into a felt and placing several layers of felt on top of each other. This way, from a leakage point of view, an ACF filter becomes as good as a granular filter, and the necessary high military protection capacities can be obtained. However, the pressure drop across the filter increases dramatically, to a point where it becomes impossible to breath through it (see Fig.19) . This means that, at the current state of technology, activated carbon fibres are only efficient in respiratory protection when short protection times are required, and merely against organic vapours or very specifically known inorganic gases. One example of this is escape hoods for chemical incidents or fires in which case the fibres will also effectively block the solid and liquid aerosol particles, but for military applications the presently available activated carbon fibres do not provide adequate respiratory protection. An area in which fibres could prove very useful is in protective garments, i.e. body protection. As stated earlier (see section 3.5), protective clothing contains, amongst other things, a layer of activated carbon. Usually this layer consists of spherical particles of granular carbon embedded in a resin or another retaining substance. The use of woven or non-woven activated carbon fibres adds more flexibility to the garment and will reduce the chance of leakage. Indeed, when mechanically disturbed the carbon beads can be pushed apart, giving rise to a carbon-free "channel" towards the inner layers and, eventually, the skin. As the air flow through the garment is less important, and certainly a lot slower, than through a carbon filter leakage and pressure drop are much less of an issue than in the case of respiratory protection. 80
._= E 0
- t - Pressure drop
40 ~ |
Density fibres (g/cm3)
Fig.19. Effect of carbon fibre density on breathing resistance (pressure drop) and organic vapour breakthrough time.
5. M O D E L L I N G A D S O R P T I O N
5.1. Chemical engineering models The first group of models that is used to describe adsorption onto activated carbon filters are the so-called chemical engineering models. These consider the filter as a chemical reactor, using the conventional equations for mass and heat transfer to describe the transport processes
through the filter. Even with modem computational techniques, one has to make some assumptions and simplifications to the real phenomena [ 105-109]: 9 There is no radial dispersion, i.e. one uses a unidimensional model of a plug flow. This is generally true, as the (usually) cylindrical shape of the filter, in combination with a high flow rate, will result in a negligible radial dispersion. Axial dispersion, however, can not be neglected. 9 There is no pressure drop over the filter. Of course this is not true (see section 3.6.2), but the influence of pressure drop on the different equations (e.g. by changing the partial pressure values) is very small. 9 The heat capacities (Cp and ev) remain constant. In the range of the measured differences in pressure and temperature this assumption is correct. 9 The contaminant is passive. This means the contaminant will not change the macroscopic dynamic parameters of the carrier gas (usually air). The limited amount of the toxic compound will not change gas parameters such as viscosity and density, hence one can use the known parameters of dry or humid air to calculate the dynamics of the gas flow. 9 The filter is considered to be adiabatic, i.e. there is no heat exchange between the activated carbon bed and the environment. This is certainly not true, and some models do take the heat exchange (loss) with the environment into account. However, this is not a feature of the activated carbon bed, but is directly related to the filter housing or canister. 9 The temperature of the gas and the activated carbon, at a certain spot inside the filter, are always equal. This presumes an infinitely fast heat exchange between the gas (heated by the exothermic reactions) and the carbon. This would also mean that all the carbon downstream from the adsorption front would have the same temperature as the gas in the adsorption front. Experimental evidence shows clearly this is not the case . However, as this heat exchange is very case sensitive, it is in most cases impossible to calculate it correctly. With these provisos, one can rely on the following set of equations: 9 A mass balance over the filter (interparticle diffusion) 9 An energy (heat) balance over the filter 9 A mass balance over one carbon particle (intraparticle diffusion) 9 An energy (heat) balance over one carbon particle As one can see, this is a system of coupled differential equations. Some of these balances can even involve more than one equation, e.g. intraparticle diffusion that can be a combination of Knudsen diffusion and surface diffusion. A supplementary equation is needed to describe the adsorption itself. In the case of pure physisorption, this is simply the mathematical description of the adsorption isotherm, e.g. the Dubinin-Radushkevich equation (see section 4.1.1). In the case of chemisorption, one has to find an equation to describe the interaction between the carbon (and the impregnants) and the specific gas. In most cases equilibrium conditions can be fitted with a Langmuir-type isotherm. A combination of these equations, including a correct set of boundary conditions, will yield a time-concentration profile through the filter. These equations have been verified by measuring the breakthrough
Adsorption of Chemical WarfareAgents
curve (concentration at the filter outlet as a function of time). This curve is nothing more than the mirror image of the adsorption front (see Fig.20). Real time in situ measurements with computer tomography [ 110] have also been used to verify these equations. These models have two main advantages: they simulate the complete breakthrough curve and they are, within the limits of the assumptions and simplifications, physically correct. The first one is not really needed in the case of protection of military personnel since the most important parameter that has to be determined is the breakthrough time (see section 5.2). The complete breakthrough curve is of particular interest in gas separation applications and more complex systems such as PTSA (Pressure and Temperature Swing Adsorption). The main advantage, the physical correctness, of these models comes at a price, as one has to describe, in detail, every physical and chemical phenomenon inside the carbon, and one has to know all the interaction parameters. Some of these, such as the tortuosity, the different diffusion coefficients and any chemical interactions, are difficult to calculate or even to evaluate from a limited number of experiments. As a result, these models that are supposed to be very general can only be used for one very specific adsorbent-adsorbate couple. If either changes, one has to go back to the drawing board, estimating, guessing or determining experimentally the missing parameters. Even though this limits the practical use of these models, their importance should not be underestimated. A detailed comparison between the results of the model and experimental breakthrough curves can yield very important information about the adsorption and transport processes, especially if one is interested in the values and effects of adsorption heats and heat transport through the bed.
5.2. Breakthrough models
5.2.1. Different breakthrough equations For most military applications, and for protective purposes in general, there is no need to simulate the complete breakthrough curve. The point of interest is when the breakthrough curve reaches a certain concentration, the so-called breakthrough concentration. This is a predetermined value, different for each toxic compound. Logically, it is the concentration at which the person wearing the protective equipment starts to experience adverse effects. For industrial chemicals these are normally tabulated values, such as TLV (Threshold Limit Value) or MAK (Maksimale Arbeitsplatz Konzentration), that take into account long term effects on safety and health. For the chemical warfare agents these are typically IDLH-values (Immediate Danger to Life and Health). This is purely a practical approach, as, sadly enough, for many CWA these are the only thresholds that have been established experimentally. Once this value is reached at the filter outlet, the protection is considered compromised and the filter has to be exchanged for a new one. Nearly all these models are based on some sort of a mass balance over the filter. In other words, all other mass and energy balances (see section 5.1) are either ignored, or are not treated separately. Usually these equations, especially the ones related to intraparticle diffusion, only influence adsorption kinetics. The capacity of the activated carbon bed is then
determined by a combination of the mass balance over the filter, combined with an expression defining the maximum uptake per unit of carbon, i.e. the adsorption isotherm. All kinetic terms are usually combined into one or two parameters that "correct" the capacity of the filter in order to give the proper breakthrough time. A very good, and comprehensive, overview of these models and equations can be found in Ref. [111,112].
Fig.20. Adsorption front and breakthrough curve for an activated carbon bed
5.2.2. The Wheeler-Jonas equation One of the most commonly used models is the one proposed by Wheeler and Jonas in the early 70s [113,114]. This equation, also known as the Reaction Kinetic equation, can be expressed in several forms. The most explicit one is given in Eq.36:
tb =We'W Q.ci,
We'Pbln(Ci"--C~ I kv.C~, \ Co~,
The breakthrough time tb (min) is expressed as a function of the sorption capacity We (ggas per gearbon), the total mass of carbon in the filter W(gcarbon), the volumetric flow rate Q (cm3.minl), the inlet concentration cin (ggas.cm3), the bulk density of the carbon in the filterpb (gcarbon.Cm'3), the breakthrough concentration Cout(ggas.cm-3) and the overall mass transfer coefficient kv (min-'). There are some limitations to this equation: 9 The flow pattem has to be a perfect plug flow with axial, but no radial diffusion. This is normally satisfied when the diameter of the bed is not too small compared to the bed length. For filter systems it is commonly accepted that diameters have to exceed 2 cm to be considered plug flow. 9 The original equation was based on physisorption into micropores (see further). 9 The rate constant kv has to be of a first order with respect to the number of gas molecules (= Cin). For pure physisorption this is only true in the first, convex, part of the sigmoidal breakthrough curve, i.e. for values of Cout/cin < 4 %.
Adsorption of Chemical WarfareAgents
The specific arrangement of the different terms clearly shows the rationale of the model: the first term is the total capacity of the carbon (W. We) for a given vapour divided by the total amount of vapour entering the filter per unit of time (Q.ein). In other words, this is the time the filter would resist if the adsorption was instantaneous and the adsorption front (see Fig.20) would have zero depth. After t = tb the concentration at the outlet of the filter would jump from zero to cin. In reality, the adsorption front has a certain width, and there are vapour molecules in front of the saturated part of the filter (see Fig.20). Consequently, the breakthrough time will be shortened by the second term. This term is of course a function of the reduction factor R (ci~.Cou/1), i.e. the point chosen on the breakthrough curve to define the breakthrough time. But the most important parameter is the overall mass transfer coefficient kv. This factor accounts for all possible resistances against mass transport during the adsorption process. In this way it covers both the interparticle and intraparticle diffusion. The Wheeler-Jonas equation can be used as such to extrapolate experimental data to other circumstances, varying airflow, concentration, bed depth, etc. To do this, We and kv can be treated as fitting parameters and derived from a number of breakthrough experiments. Then, the obtained values can be used to make the necessary extrapolations. However, if one wants to predict breakthrough times for a given filter-toxic vapour system, there has to be a way to calculate We and kv without any prior breakthrough experiment. For We this problem is not too difficult to solve  as the capacity, in the case of physisorption, can be approximated by the static capacity as given by the adsorption isotherm. Accordingly, We can be calculated, rather straightforwardly, from the Dubinin-Radushkevich equation with Co = ci~ (see section 4.1.1):
We=Wod L e x p I - ~ T 2 log2/C~o/1
The only unknown (not tabulated) parameters are Wo and B. These can be derived from any known isotherm of the activated carbon (e.g. N2 at 77K). The affinity coefficient fl can be found in the literature or calculated . Estimating the overall mass transfer coefficient kv is more difficult. It is, a priori, impossible to differentiate between the different diffusion steps in the adsorption process. Therefore kv is estimated on the basis of semi-empirical equations. Up to now, the most complete one is given by Eq.37 [ 116]:
k v = 8 0 0 . / ~ 0"33
In this equation, kv is the overall mass transfer coefficient (minl), fl is the affinity coefficient of the Dubinin-Radushkevich equation, dp the mean diameter of the carbon particles (cm), vL the linear velocity of the air stream through the bed (cm.s-1), We the equilibrium adsorption capacity (gvapour.gearbon-1) and Mrv the molar mass of the toxic organic
vapour (g.mol-1). As such, this equation does not account explicitly for the influence of temperature on the adsorption rate, unlike the equation to calculate We. This is not that big a problem, as the influence of temperature on transport phenomena is known to be far less important than its influence on the adsorption capacity.
5.2.3. Extensions of the Wheeler-Jonas equation The Wheeler-Jonas equation has been used extensively in the case of pure physisorption on granular carbons. But recently, it has been demonstrated it can be equally well applied in a number of very divergent cases. The first extension is on the type of adsorbent, especially new types. The validity of the Wheeler-Jonas equation has been demonstrated [117,118] for both activated carbon fibres (ACF) and activated carbon monoliths (ACM). The equation itself and the calculation of the capacity We remain unchanged. As for the estimation of the overall mass transfer coefficient, the normal equation stays valid, providing a correct interpretation of the "equivalent diameter of the particles". For ACFs, dp has been calculated from the total external surface. Given their small diameter, dp is essentially related to the length of the fibres [ 119]. For ACMs, dp seems to be related to the internal diameter of the channels. A second effort has been directed towards chemisorbed gases [ 117]. Here the situation is reversed as in many cases kv can be calculated in the same way as for physisorbed vapours, especially at intermediate to high values of the inlet concentration c/n. This is due to the physisorption, which always precedes chemisorption (see section 4.2), being the rate controlling step in the overall adsorption process. Apparently, in many cases both the hydrolysis and the chemical reactions with surface complexes proceed at a higher rate than the diffusion through the bed and/or through the carbon particles. But it is clear the adsorption capacity We is no longer exclusively related to the available micropore volume of the carbon. In the case of chemisorption, We has to be experimentally determined for a given adsorbentadsorbate couple. As such, the Wheeler-Jonas equation can not be used to estimate breakthrough times without any prior experiments, but it can still be used to extrapolate experimental data towards other conditions of airflow, concentration, etc. [120,121 ], and to evaluate the effect of changing environmental conditions on the adsorption capacity and kinetics. 6. F A C T O R S INFLUENCING THE ADSORPTION OF CWAs 6.1. General Many factors influence adsorption of chemical warfare agents and toxic industrial chemicals by activated carbon. Some of them have already been mentioned in the previous sections. In this section the nature of this influence will be described and explained, where possible. In most cases this can be illustrated by looking at the effect of changing a particular parameter on breakthrough time predictions. Therefore, where possible, the Wheeler-Jonas equation (Eq.36) will be used to explain the effect of changing a given parameter.
Adsorption of Chemical WarfareAgents
6.2. Temperature The influence of temperature has been explained, partially, in section 4: an increase in temperature normally has a negative effect on the adsorption capacity of the carbon . This is true in all cases when dealing with pure physisorption (see Dubinin-Radushkevich equation in 4.1), and in most cases involving chemisorption. However, according to diffusion theories, the overall mass transfer coefficient kv should increase with increase in temperature, thus having a positive effect on the breakthrough time. k~ should be approximately linearly proportional to the temperature T. Up to now, no predictive equation for k~ takes this into account, even though it has been suggested . On the contrary, Eq.40 (see section 5.2.2) shows a proportional dependency of k~ on We, and thus an inverse proportionality with T. However, as We is exponentially proportional to T, and kv only linearly, the effect of higher temperatures on breakthrough time will almost always be negative. 6.3. Flow rate and flow pattern 6.3.1. Flow rate Flow rate will influence both the total capacity and the mass transfer kv, but to a different extent. An increased flow rate will, just as an increase in inlet concentration [123,124], result in a higher quantity of toxic vapour per unit of time being adsorbed by the activated carbon bed. As a result, an increase in flow rate will lower the first term of the Wheeler-Jonas equation. On the other hand, for a same filter geometry, a higher flow rate means a higher linear velocity vL through the bed , which, in turn, leads to a higher k~, faster kinetics, and an increased breakthrough time. Here the effect on both terms of the Wheeler-Jonas equation is more equal than in the case of temperature, the first term being proportional to vL-1 and the second term to vL-~ Overall, as the capacity term usually accounts for more than 75% of tb, an increase in flow rate will reduce breakthrough time. 6.3.2. Flow pattern As explained in section 3.2, real flow patterns in filters differ from the theoretical constant flow. They can be simulated by a half-sine function (see Fig.8). It has been experimentally determined that this flow pattern exerts a negative influence on both the capacity We and the mass transfer coefficient kv, resulting in shorter breakthrough times than in the case of a constant flow [126,127]. The first term, We, is only marginally reduced, but the second, kv, is reduced to a much larger extent, which accounts for the apparent absence of this phenomenon in filters with larger bed depths [128-131]. In shallow beds the breakthrough time is influenced by both terms of the Wheeler-Jonas equation, whereas for larger bed depths the high value of the capacity term, We, becomes predominant. The small loss of capacity can be attributed to the fact that, in this specific case, the dynamic capacity does not reach the theoretical equilibrium capacity. As yet, this influence on We can not be calculated or estimated theoretically.
The diminished mass transfer rate can be related directly to the flow pattern via Eq.37, more specifically via the linear velocity vL. When taking into account the real flow pattern, calculated values of kv are typically 20% lower than those calculated for a constant mean flow. Physically, this can be interpreted as a loss in driving force each time no air is flowing over the carbon bed, slowing down the adsorption process and broadening the adsorption front. If the exhaled air passes through the filter, as is the case in some half masks or escape hoods, breakthrough times for organic vapours actually increase . This is a result of (weakly) adsorbed vapours being partly desorbed by the air stream into the ambient air during exhalation. 6. 3.3. Intermittent use It has been shown that intermittent use will shorten breakthrough times. Intermittent use is defined as use of the filter for a given time interval, storing it, using it again, storing, etc. Apparently, in some cases, it has been demonstrated that vapours can desorb and re-adsorb inside the bed during storage, broadening the adsorption front . Theoretically, this could lead to a uniform level of contamination throughout the carbon bed, resulting in an immediate breakthrough once the filter is challenged again. This phenomenon is primarily observed with highly volatile (i.e. weakly bound) compounds and very rarely with chemisorbed species. It is also of lesser importance in military applications as in most armies filters are treated as oneuse items and are changed, and disposed of, after any type of real use. 6. 3.4. Bed geometry Influence of bed geometry on breakthrough times has also been reported . However, when taking care to keep all other parameters, particularly the amount of activated carbon, constant, the observed differences are very small. They can probably be attributed to local and/or gradual differences in the flow pattern throughout the filter, for example in a conical bed where there will be a depth-dependent linear velocity, resulting in a non-constant kvvalue. 6.4. Grain size The influence of grain size has already been discussed in points 3.2 (packing of the bed), 3.6.2 (breathing resistance) and 5.2.2 (kinetics of adsorption). In general, appropriate grain size will depend on filter dimensions. Adsorption kinetics are optimized by using the smallest particles as possible, without raising the breathing resistance of the activated carbon bed above an acceptable level. 6.5. Humidity 6.5.1. Effect o f humidity Humidity has a marked effect on breakthrough times. This is not in the least due to the abundance of moisture in the ambient air. A relative humidity of 60% at 296K, which is not at
Adsorption of Chemical Warfare Agents
all exaggerated and quite normal in most parts of the world, is equivalent to a concentration of approximately 16500 ppmv of water molecules. For a filter, this concentration comes into contact with the activated carbon through the inhaled air stream. For clothing, the transpiration going out will be an additional source of water "contamination" of the activated carbon. Thus even in the absence of any toxic compound, the activated carbon in filters and clothing will be exposed to water vapour. This is often referred to as pre-wetting, or prehumidification, of the carbon bed. When a chemical incident occurs, the toxic compound will be in competition with the water vapour in the air stream, and with the pre-adsorbed water, if any, for available adsorption space. This can be "real" space, i.e. micropore volume in the case of physisorption, or reactive/catalytic species on the surface of the carbon (e.g. Cu ions on whetlerite). On the other hand, one should not forget that in a number of cases the presence of water can reinforce adsorption and, sometimes, is even necessary as a prerequisite for hydrolysis or for certain types of chemisorption (see section 4.3, e.g. Fig.12). But even then, an abundance of water can have a negative overall effect as it will slow down adsorption kinetics and limit the adsorption space available to other adsorptives.
6.5.2. Influence on adsorption capacity In the case of physisorption, the loss of adsorption capacity can be modelled. The most successful way of doing this is by volume exclusion [134-136]. Where there is water, there cannot be any vapour, and if the vapour is to replace the water, it will have to displace it from the micropore volume it occupies. As can be seen from the theory of physisorption (see section 4.1), adsorption forces depend primarily on the volatility of the compounds. Hence the more volatile the toxic compound, the more it will be influenced by water adsorption as it is unable to replace the more strongly adsorbed water molecules. This can be expressed by equations 38 and 39 : m ' ~ = m o - m p r e -- A m a i r + A m
AW s <_O=> AW s = 0 The available micropore volume Wo' can be calculated from the dry micropore volume, Wo, the amount of pre-adsorbed water, Wpre, the amount of water ad- or desorbed by the contaminated air stream, AWait, and the amount of water replaced by the vapour AWs. The latter being function of the total amount of water on the carbon (Wpre + AWair), the ratio of the amounts of water and vapour in the air stream ([Cw+Co]/Co) and the ratio of their saturation vapour pressures ([Ps+Pw]/Pw). The value of Wo' thus obtained can be used in the DubininRadushkevich equation (see section 5.2.2) to calculate the capacity (We') under humid conditions.
6.5.3. Influence on adsorption kinetics The water adsorbed on the carbon will also influence adsorption kinetics. Various authors [137,138] have demonstrated this. In contradiction to the capacity which can be influenced positively or negatively (see section 6.5.1), mass transfer will always slow down. The overall mass transfer coefficient, kv, of the Wheeler-Jonas equation does not differentiate between the different types of diffusion. Consequently, every impact of water on the adsorption kinetics will be translated into a drop of kv values.
This is illustrated by equation 40 , which expresses the mass transfer coefficient under humid conditions (kv ') as a function of kv in the absence of water, the total pore volume (TPV) of the carbon and the total amount of water adsorbed. 250 -
9 9 9 
Carbon pre-humidifation = 0 % R H Carbon pre-humidifation = 9 0 % R H Model Model
200 ~ A
,,c o') O it_ ,,c
m O !._
Air stream humidity (%RH) Fig.21. Theoretical vs Experimental breakthrough times for CC14 on BPL-type carbon under varying humidity conditions. Clearly, it is not only the presence of water in the micropore system that plays a role (as it is the case for the physisorption capacity), but also in the rest of the pore system . It has been verified that even in cases where water has a beneficial effect on capacity (e.g. CNC1 ), kv will decrease for high water loading. Probably, the adsorbed water
Adsorption of Chemical Warfare Agents
hampers transport of the toxic compound to the adsorption sites (micropore volume or reactive sites) and, when there is replacement of water by the vapour, it will also impede this water from exiting the carbon bed. A model based on the loss of physisorption capacity and diminished mass transport can adequately describe the breakthrough of organic vapours, even under severe conditions of carbon pre-wetting and moist air. This is illustrated in Fig.21 .
6.6. Ageing 6. 6.1. Influence on physisorption capacity The prolonged exposure of activated carbon to moist air is known as ageing. It was first witnessed for whetlerite carbons as it can have a very marked effect on the retention of chemical warfare agents, especially CNC1 (see section 6.6.2). Evidence of changes in the physisorption capacity, i.e. in the pore structure of the carbons, is inconclusive. Some groups have found evidence of an increase in micropore volume after ageing, essentially due to a widening of the smallest pores, whereas others did not observe any pore structure related changes [142,143]. In any case these changes, if any, are small and of the same order of magnitude as the experimental error in measuring micropore volumes and pore size distributions. 6. 6.2. Influence on chemisorption capacity Contact with water vapour, especially over long periods of time, will change the composition of the surface of the carbon: it will oxidise this surface, introducing additional oxygencontaining complexes, such as carboxylic groups. Most of these groups are hydrophilic and will enhance water vapour adsorption [ 144]. This effect is permanent, even when the carbon is subsequently dried [142,143]. The result is an increased sensitivity of the carbon towards water vapour, i.e. a higher uptake for a given partial water pressure (i.e. air stream humidity) and faster water adsorption kinetics, which, in turn, will lead to a lower physisorption capacity under humid conditions (see section 6.5). Not only is the carbon surface affected, but also other chemical structures can be oxidised by ageing, specifically the copper and chromium ions on whetlerites. In this case, ageing reduces the individual metals from Cu 2+to Cu ~+and from Cr 6§ to Cr 3§ [49,145-152]. The exact reactions involved are still not completely understood, but the effects are reinforced by higher temperatures [ 147,153]. The reduction of copper makes the whetlerite carbons less effective in almost all chemisorption reactions (see section 4.3), and in conjunction with the reduction of chromium, it has a disastrous effect on both HCN and CNC1 adsorption mechanisms. In particular, breakthrough times for the latter component (CNC1) are drastically reduced, usually to values well below any acceptable threshold. New whetlerites such as ASZM also suffer from ageing effects, even though the exact mechanisms are even less well known than in the case of the conventional ASC type whetlerites . It is interesting to note the particular behaviour of TEDA on whetlerites. The presence of TEDA seems to reduce considerably the effects of ageing. It is commonly
assumed that TEDA, in some yet unknown way, shields the copper-chromium complexes from the degradation by water vapour. More importantly, the TEDA itself seems not to be affected at all by ageing. It keeps its full capacity to chemisorb CNC1 (see section 18.104.22.168) and takes over part of the task that is usually performed by the Cu-Cr complexes.
6.7. Multi-component adsorption Multi-component adsorption can be described by some of the more sophisticated simulation programs (see section 5.1) , but it is still a difficult task especially when more than two different vapours or gases are involved. In general, the effects of multi-component organic vapour adsorption are very similar to the case of vapour-water co-adsorption explained in section 6.5. More strongly adsorbed, i.e. less volatile, vapours will win the competition for the available sorption space from the more volatile ones. Taking the co-adsorption of two vapours as an example, experiments have shown the breakthrough time for the less volatile compound to remain more or less unchanged, usually a bit shorter than for the single vapour due to the slower kinetics. For the more volatile vapour, breakthrough will be much shorter than in the case of this vapour alone. There will also be the effect of "rollup", as the more strongly adsorbed vapour will replace the already adsorbed, weakly retained vapour, increasing the outlet concentration of the latter to well above its concentration in the contaminated air. This is illustrated in Fig.22. Of course, things get even more complicated when some of the compounds are physisorbed and others are primarily chemisorbed.
- - V a p o u r A = more volatile ---
less vo at e
._~15 Inlet concentration ~8
~ 8 ~~ -~
~ .......... o.O
T Time (min)
Single vapour breakthroughtime Fig.22. Multi-component organic vapour adsorption: rollup effect Whereas this phenomenon is quite common in industrial applications, it is not really relevant to the military use of activated carbons, except, of course, the case of water as explained in the previous sections. It is indeed unlikely that troops in operations will be subjected to mixtures of chemical warfare agents, and even more unlikely that in such
Adsorption of Chemical WarfareAgents
instances the CWAs will have very different volatilities. Sometimes toxic chemicals are mixed with tear or vomit agents, but as most of these are distributed as aerosols there is little or no interaction on the activated carbon. Only when confronted by industrial threats could this become a real problem. Even in this case, since most military canisters exhibit a high to very high physisorption capacity and the troops will normally be in the open, multicomponent adsorption and rollup phenomena are not really an issue. 7. C O N C L U S I O N S The protection against Chemical Warfare Agents is a very small, but highly specialised field amongst activated carbon applications. Even after several decades of intensive use and elaborate research, many aspects of the military carbons, most of which are known as whetlerites, remain unclear. Much as is the case for other applications, activated carbon provides an adequate answer to a wide variety of problems, but raises a lot of questions as to the how and why of its excellent performance. This will, however, not prevent it from continuing to play its role as the first line of defence against the threat of CWAs in the near and even in the not so near future. REFERENCES
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