Page 179 - Gas Adsorption Equilibria

3. Gravimetry 165

From this figure it clearly can be seen that the AC is selective for as
both the as well as the concentration in the adsorbed phase are
considerably reduced compared to their values in the sorptive gas phase.
Information of this kind is very valuable for the selection of adsorbents in gas
purification and separation processes [3.37].

Example 2

Adsorption of dye from an aqueous solution onto activated carbon.

The adsorption of dye Levafix ® Brilliant Red E-4BA from an aqueous
solution onto activated carbon F300 (Chemviron Carbon) at 24 °C and
ambient pressure has been investigated using a magnetic suspension balance
(MSB) (Rubotherm GmbH, Bochum, Germany) [3.46, 3.47]. The
experimental setup is shown in Figure 3.28. The dark region within the
adsorption vessel of the MSB indicates the volume filled with liquid solution.
More details of design and materials are given in [3.47] and literature cited
there. Results of gravimetric measurements are given in the next Figure 3.29.
The lower curve refers to the sorption process of pure water into the activated
carbon. There are experimental indications that the density of the water in the
pores is about the same as the density of ice at 0 °C i. e. lower
than the density of water in its liquid state. Indeed, it is known that pore
liquids can exhibit three dimensional structures [3.48] and that adsorption in
micropores often is accompanied by kind of freezing process which also can
be simulated by statistical molecular models [3.49]. The upper curve shows
the increase of mass of an activated carbon sample due to the combined
adsorption of water and dye. Seemingly both processes are fairly slow and did
not reach equilibrium after a period of obeservation of 30 hours and more.
Gravimetric measurements were complemented by traditional beaker
experiments allowing thus to calculate separately the masses of water and dye
adsorbed in the activated carbon. The procedure is similar to that described in
the next Chapter 4 (Volumetric-Gravimetric Measurements). Details are given
[3.47].

Though these measurements certainly are of preliminary character and
need more detailed discussion, it should be emphasized that they demonstrate
the possibility to explore adsorption processes of complex molecules like dye
Levafix ® Brilliant Red E-4BA (whose chemical formulae is classified) on
microporous sorbents. This certainly is of interest not only for textile
engineering but also for waste water management and for environmental
protection.