Adsorbent graphitized

Comparison of Tables III and IV shows a general correlation between adsorbent color (as defined by the Munsell indices) and optical absorbance. Note that the "darkest" adsorbents (graphite WK, IL, KA, NM, EA) ashes are those on which PAH photoreactivity tends to be low, while the "lighest" adsorbents (silica, alumina, glass, TX and AR ashes) are those on which adsorbed PAHs are most photosensitive. Again, the ET ash is an exception (being a light-colored ash but one on which adsorbed PAHs are not especially susceptible to phototransformation). 

Another class of adsorbents, graphitized carbon blacks (GCB), exhibit the same positive characteristics as the bonded silicas, with the difference of being more retentive, absolutely nonpolar and nonporous. With GCBs, quite good results have been obtained for classic chlorinated pesticides. Recoveries of selected OCPs from different adsorbents are reported in Table 22.4. 

Fig. 2. Logarithm of the three-dimensional pressure of xenon of the first and second layer adsorbed graphite vs. reciprocal temperature (from Ref 15).

Preparation of the columns is based on a two-step procedure, namely 1) preparation of a slurry where the adsorbent (graphitized carbon black) and the liquid phase (for instance, SP-1000) are mixed together in a suitable solvent and sonicated 2) coating of the capillary columns by means of a static method using the prepared slurry. 

Clearly, it is important that there be a large contact angle at the solid particle-solution-air interface. Some minerals, such as graphite and sulfur, are naturally hydrophobic, but even with these it has been advantageous to add materials to the system that will adsorb to give a hydrophobic film on the solid surface. (Effects can be complicated—sulfur notability oscillates with the number of preadsoibed monolayers of hydrocarbons such as n-heptane [76].) The use of surface modifiers or collectors is, of course, essential in the case of naturally hydrophilic minerals such as silica. 

The above methods for obtaining D, as well as other ones, are reviewed in Refs. 3-12, and Refs. 7-9 give tables of D values for various adsorbents. For example, D is close to 3 for the highly porous silica gels and close to 2 for nonporous fumed silica and for graphitized carbon black coconut charcoal and alumina were found to have D values of 2.67 and 2.79, respectively [7]. 

Some general points are the following. One precondition for a vertical step in an isotherm is presumably that the surface be sufficiently uniform that the transition does not occur at different pressures on different portions, with a resulting smearing out of the step feature. It is partly on this basis that graphitized carbon, BN, MgO, and certain other adsorbents have been considered to have rather uniform surfaces. Sharp LEED patterns are another indication. 

It is noted in Sections XVII-10 and 11 that phase transformations may occur, especially in the case of simple gases on uniform surfaces. Such transformations show up in q plots, as illustrated in Fig. XVU-22 for Kr adsorbed on a graphitized carbon black. The two plots are obtained from data just below and just above the limit of stability of a solid phase that is in registry with the graphite lattice [131]. 

Kjems J K, Passell L, Taub H, Dash J G and Novaco A D 1976 Neutron scattering study of nitrogen adsorbed on basal plane-oriented graphite Rhys. Rev. B 13 1446-62... 

Fig. 2.12 Plot of the calorimetric difTeFential enthalpy of adsorption A h) against amount adsorbed (n), for (u) n-pentane, (f)) /i-hexane, (c) n-heptane, d) n-octane, all adsorbed on graphitized car n black. The point corresponding to n = is marked on each curve. (Courtesy Kiselev.)...

An example of a stepped isotherm, for krypton at 90 K, is shown in Fig. 2.21(a), where the adsorbent is graphitized carbon black, which is known to possess a very uniform surface. Figure 2.21(h) shows the steps obtained, also with krypton, on cadmium bromide. 

Fig. 2.29 Comparison of nitrogen adsorption at 78 K on a carbon black (Sterling FT) before and after graphitization (a) The amount adsorbed on the ungraphitized sample plotted against the amount x, adsorbed on the graphitized sample, at the same pressure, b) The corresponding isotherms O, adsorption, , desorption on the ungraphitized sample (4 runs) A. adsorption A desorption, on the graphitized sample (4 runs).

Low-pressure hysteresis is not confined to Type I isotherms, however, and is frequently superimposed on the conventional hysteresis loop of the Type IV isotherm. In the region below the shoulder of the hysteresis loop the desorption branch runs parallel to the adsorption curve, as in Fig. 4.26, and in Fig. 4.2S(fi) and (d). It is usually found that the low-pressure hysteresis does not appear unless the desorption run commences from a relative pressure which is above some threshold value. In the study of butane adsorbed on powdered graphite referred to in Fig. 3.23, for example, the isotherm was reversible so long as the relative pressure was confined to the branch below the shoulder F. 

Fig. 5.13 Adsorption isotherms on graphitized and on ungraphitized charcoal, (a) Adsorption of water vapour (fc) adsorption of nitrogen at — 195°C. The adsorption values are expressed in cm of liquid adsorbate per gram of adsorbent. (Courtesy Kiselev. )...

---