Page 99 - Partition & Adsorption of Organic Contaminants in Environmental Systems

90 ADSORPTION OF VAPORS ON MINERALS AND OTHER SOLIDS

their preexisting pore sizes, as depicted here for several mineral oxides in
accordance with their N 2-isotherm shapes.
The N 2 monolayer capacities [i.e., Q m(N 2)] and BET surface areas for the
solids, as determined by Eq. (4.7), are presented in Table 6.1. For a solid with
a relatively smooth surface, on which the BET model is based, the completion
of N 2 monolayer occurs usually at P/P° = 0.05 to 0.30.The data for such mineral
samples as silica, alumina, goethite, and kaolinite fall closely into this range.
By contrast, the P/P° at Q m(N 2) for Ca-SAz-1 (0.015), K-SAz-1 (0.018), and
activated carbon (0.020) are noticeably less than the common lower limit of
0.05. Results for the latter solids, especially the activated carbon with a huge
Q m(N 2), signify the massive N 2 vapor condensation into highly adsorbing pre-
existing micropores of the solids, on which the exactness of the measured
surface areas become somewhat meager (because the BET model was for-
mulated for relatively open surfaces). For the former solids, there exists pre-
sumably no significant amount of preexisting micropores.

6.3 MICROPORE VOLUME

The noted differences in BET surface for the solids reflect their subdivision,
or more exactly their porosities, because the apparent sizes of the particles
make a relatively minor contribution to the surface area. For example, a non-
porous solid with a geometric particle size of 1mm has a surface area of about
2
1m /g. The pores contributing most to the measured surface area are those
called micropores, which have a diameter or a slit width less than about 20
angstroms (Å), where 1Å = 0.1nm. Pores with dimensions between 20 and
500Å are called mesopores, and those larger than 500Å, macropores (Gregg
and Sing, 1982). The micropore volume associated with the micropores may
be estimated by some developed analytical methods. The t-method of de Boer
et al. (1966) and the a s-method of Sing (1970) are frequently used in this
respect to determine the micropore volume of a solid by use of a set of suit-
able inert-vapor adsorption data. The method consists of plotting the adsorp-
tion data of a (nonpolar) vapor on a test solid against that of the same vapor
on a nonporous standard solid, as detailed below.
In the t-plot of de Boer et al., the adsorbed liquid N 2 volume (V) on a test
solid is plotted against the statistical thickness (t) of the adsorbed N 2 layer on
a nonporous standard solid to yield the micropore volume and nonporous
(open) surface area of the test solid. The relation between V and t is given by
Remy and Poncelet (1995) as
V = V m + 10 -4 S t (6.2)
0

where V is the adsorbed volume of the condensed liquid N 2 on the test solid
(mL/g), V m the volume of N 2 adsorbed onto the solid’s micropores (mL/g), S 0
the nonporous surface area (i.e., the area associated with the nonporous struc-