Alkylammonium Montmorillonites as Adsorbents for Organic Vapors from Air Martin Harper” and Colin J. Purnell Department of Occupational Health, London School of Hygiene and Tropical Medicine, Keppel Street, London WCIE 7HT, U.K.
Montmorillonite clays may be modified by the exchange of the inorganic interlayer cations with alkylammonium ions, resulting in a fixed internal porosity. The pore size and shape depend on the nature of the alkylammonium ion. A number of different ions were used to prepare adsorbents with varying properties, and these were examined for their potential application to sampling organic vapors in air. Characterization involved determination of nitrogen and water contents, surface area, interlayer spacing, thermal stability, and breakthrough volumes of organic vapors. The adsorbent that showed the most promise [tetramethylammonium montmorillonite (TMA)] was further evaluated for use as an adsorbent in both thermal- and solvent-desorbable sampling systems.
Introduction Montmorillonite clay minerals consist of a planar three-layer aluminosilicate lattice, stacked vertically ( I ) . Through the isomorphic replacement of aluminum for silicon and magnesium for aluminum there is an excess net negative charge on the lattice, which is satisfied by the presence of inorganic cations (Ca, Na, K, etc.) situated within the layers, together with water molecules, which form a hydration sphere around the cations. It is well-known that these cations are readily exchangeable and that exchange is also possible with organic cations such as alkylammonium ions (2). Previous work on the resulting materials by Barrer and co-workers (3-6) was originally concentrated on their application as packings for gas chromatographic columns. Although a few such compounds are still available (Bentones), interest has largely shifted toward porous polymers and molecular sieves. However, sampling of organic vapors in air for occupational hygiene applications requires a wide range of adsorbents, in order to be able to choose the most appropriate for each situation (7). Sampling of gases in the workplace is now a legislated requirement in many countries, and the sensitivity, accuracy, and precision of these methods is under continual review. In general, sampling is normally by adsorption of the contaminants onto an inert medium with subsequent desorption and analysis in the laboratory. Air may be drawn through the adsorbent by a pump, or the contaminant may simply be allowed to migrate to the adsorbent by diffusion (8). Desorption of the collected material may be by solvent displacement or thermal desorption (9). In any case, it is necessary to choose the sorbent taking into consideration the sampling
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and analytical method, the concentration of the contaminant and duration of exposure, the environmental conditions, and the presence of interfering species. Available data on many such sorbents are summarized in Table I. Porous polymers are very useful for thermal desorption systems as the van der Waals forces binding the analyte to the adsorbent are sufficiently weak that the analyte can be driven off by an input of thermal energy. However, such weak binding may result in a substantial vapor pressure of a low-boiling analyte above the adsorbent surface, and this may lead to problems with the sampling efficiency (IO). Because of the enhanced adsorption potential in micropores, active carbons may sample more efficiently ( I I ) , but they require solvent desorption, which involves toxic chemicals (e.g., CS2),and the continual determination of desorption efficiencies. The aim of this study was to examine other adsorbents that might fill the gap between these two extremes.
Methods and Materials From previous studies it was thought likely that tetramethylammonium and tetraethylammonium montmorillonite would have good potential as adsorbents, but it was also thought necessary to examine other possible compounds of this type. The tert-octyl-(2,4,4-trimethylpentyl-) ammonium ions are a rather different shape from normal straight-chain molecules. It was thought that this molecule, in pushing further out from the interlayer surface, might confer an increased adsorption volume on the montmorillonite structure. From the available evidence (2),myristyltrimethylammonium should form a doublelayer complex, although there could still be large gaps between the molecules. As the methyl groups would surround these gaps, the adsorption volume would be highly hydrophobic. A number of montmorillonite clays are available (12). Wyoming bentonite (a sodium montmorillonite) is frequently used for experimental purposes. It has a high cation-exchange capacity (CEC) and forms a stable thixotropic gel in water. The high CEC was thought to be a possible disadvantage, since the more cations adsorbed the smaller the remaining volume for adsorption of other species. Fuller’s earth (a calcium montmorillonite) has a rather lower CEC, and this can be further reduced by acid activation with dilute hydrochloric acid (producing a hydrogen montmorillonite). The X-ray diffraction picture of the acid-washed material is not significantly different from that of the precurser material, but both have substantially broader peaks than the sodium clay. There are many analytical methods available whose results would aid in elucidating the structure and properties of these materials. Surface area measurements and
0 1989 American Chemical Society
Environ. Sci. Technol., Vol. 24,
No. 1, 1990 55
Table I. Properties of Some Solid Adsorbents Used in Organic Vapor Sampling" adsorbent active charcoals coconut shell Carbosieve B silica gels alumina Acta1 A U.G.1 molecular sieves GC packings Carbopack C-HT Tenax GC Chromosorb 101 102 103 104 105 106 107 108 Porapak P
Q
R T N Amberlite XAD2 XAD4 XAD7
specific surface area, m2/g
upper limit T,"C
800-1000 1100-1400 1000 300-800 340-670
pore typeb
polymer typec
I
I I 1-11 1-11
275 175 600-700
I1 I1
I
14 19
500 375
I I11
DPPPO
