Introduction
The development of chemical industry around the world has contributed to a higher risk of exposure to Toxic Industrial Chemicals (TICs) both for civilians and the military. Several chemical compounds are listed as TICs, both inorganic and organic in nature, with different classes of toxicity. Some of these substances are or were considered as Chemical Warfare Agents(CWA), e.g. hydrogen cyanide (AC, blood agent), phosgene (CG, pulmonary agent) or chlorine.
So far activated carbon is the most common medium used for removal of toxic chemicals frombreathable air, and it is usually applied as an adsorbent for breathable air filtration. Even though there are thousands of toxic chemicals used in theindustry, filter challenge tests for practical reasons are performed using few chosen chemicalsubstances with properties representing different classes of poisonous chemicals.
In Europe theRespiratory Protective Equipment (EPR) undergoes standardized testing and certificationprocedures according to European Committee for Standardization (CEN). Examples ofcommercially available filters suitable for different contaminants and their color code inaccordance with European regulations are given in Table 1.1 .
Adsorption of activated carbon
Activated (also called active) carbon is a form of porous carbon processed to yield a high surface area, which for commercially available carbons is usually between 500 – 1500 m2/g . Possible applications of activated carbon are almost countless, especially in the field of air and water filtration technologies, where a remarkably high surface area of activated carbon is utilized for adsorption of contaminants or undesired biological and chemical components.
Additionally, once the activated carbon becomes exhausted it is possible to regain its adsorptive powers by regenerative processes, e.g. heating or washing with chemicals, which makes the entire filtering process economically beneficial.
For these reasons activated carbons are utilized worldwide as adsorbents in various filtration systems for industrial, personal and military applications.
Pore size affects the adsorption of activated carbon
Activated carbon owes much of its sorptive powers to its porosity (Figure 2.7). The porosity in activated carbons is due to spaces and voids between defected and crumpled graphitic lamellas with low packing density. Pores with various shapes and sizes are formed during carbonization and activation processes, and their distribution depends on the source materials and the procedures included in carbon preparation.
According to IUPAC (International Union of Pure and Applied Chemistry) recommendations the pores can be divided into the following categories depending on their size:
- Macropores:r> 50 nm; responsible for adsorption of large organics (viruses, bacteria)
- Mesopores (transitional pores)50 nm>r>2 nm; monomolecular and polymolecular adsorption
- Microporesr<2 nm; size order corresponds to the size of molecules
Despite their minute size micropores contribute to the 95% of the specific area of the activated carbon. As can be seen from Figure 2.8, macropores open directly to the surface of carbon grain, and they are the connected with meso- and micropores in the bulk of the grain. Macropores serve mostly as transportation channels for the molecules which enter the micropore network.
In general the pore size corresponds to the size of the species adsorbed by activated carbon (Figure 2.8) . Nevertheless, the adsorptive powers of activated carbon cannot be explained entirely by its enormous porosity which provides the extended surface area. Activated carbon contains numerous imperfections – the leftovers of inorganic matter including metal ions, as well as the unsaturated carbon bonds from the partially burnt off layers of graphite. Those imperfections affect the electronic density on the surface, thus influencing the adsorptive properties towards polar or polarizable substances.
Impregnation enhances the adsorption of activated carbon
Despite the extended pore network and high surface area the adsorption capacity of activated carbon towards several chemical compounds is insufficient for many applications, such as filter canisters for personal respiratory protection in military and civil services. The performance of the filtering agent is greatly enhanced when chemisorption and/or catalytic decomposition occurs on its surface.
For this reason activated carbon, such as BPL, is impregnated to improve the adsorptive properties towards one or more particular contaminants present in the air stream. In principle, the impregnation blocks part of the porosity, but in return it makes the filter more capable in terms of chemical adsorption, necessary for trapping several highly volatile contaminants with low molecular weight, such as cyanides. Chemisorption is more selective than physical adsorption, where the size of molecules is crucial for effective trapping process.
In case of chemisorption certain chemical bonds have to be formed between the surface of activated carbon and the adsorbed substance, therefore the choice of the impregnating salts will depend on the application. Table 3.1 lists some of the possible combinations of adsorbates (adsorbed substances) and impregnating agents .
A wide variety of impregnated carbons is available commercially from several companies and the abbreviated names are given to those carbons depending on the chemicals used for impregnation. Those abbreviated names are to a large extent universal, even though some variations occur. Most common types of activated carbon include :
- AC-activated carbon
- ASC-AC impregnated with copper (Cu) and chromium (Cr) salts
- ASZ-AC impregnated by Cu (II), zinc (Zn) (II) and silver (Ag) (I) salts
- ASZM – AC impregnated by Cu (II), Zn (II), Ag (I) and molybdenum (Mo) (VI) salts
- ASZM-T (ASZM-TEDA) – ASZM impregnated by Cu, Zn, Ag, Mo salts and triethylenediamine (TEDA)
From military point of view, the type of activated carbon most important for personnel protection is ASZM-TEDA.
ASZM-TEDA is type of impregnated activated carbon-based filter effective towards TICs and CWAs. ASZM-TEDA has been in use by U.S. military as a part of NBC (Nuclear, Biological and Chemical) protection system since 1993 . TEDA is an amine which has been used successfully for removal of radioactive iodide and iodide-compounds. TEDA does not desorb easily from activated carbon.