Page 100 - Partition & Adsorption of Organic Contaminants in Environmental Systems

MICROPORE VOLUME 91
2
ture) of the solid (m /g), t the statistical thickness of the adsorbed N 2 layer in
-4
angstroms on a reference nonporous solid, and 10 is a conversion factor. To
convert the mass of N 2 adsorbed into the liquid volume at 77K, a liquid N 2
density of 0.808g/mL is generally assumed. The value of t as a function of P/P°
is calculated from the adsorption data of N 2 on the reference solid. A univer-
sal t-curve of N 2 on nonporous solids has been developed (de Boer et al.,1966),
which gives
05
.
t = {13 99 [0 034. - log ( P P∞)]} (6.3)
.
where t is in angstroms. If the plot of V versus t gives a straight line passing
through the origin, the test solid is considered to be free of micropores. For a
microporous test solid, the t-plot yields a straight line at high t and a concave-
down curve at low t; the extrapolation of the upper linear line to t = 0 gives a
slope of S 0 and an intercept of V m .
The a s plot of Sing (1970) is an alternative method of the t-plot. In this
method, the amount of a (nonpolar) vapor adsorbed at some fixed P/P° on a
reference solid is first normalized to the amount at P/P° = 0.4 (i.e., a s = Q/Q 0.4 )
to produce a standard a s -curve rather than a t-curve. The a s -curve is then used
to construct an a s -plot from the isotherm of the vapor on a test solid, just as
is the t-curve used to construct a t-plot. The reference solid is chosen to be a
nonporous solid having a chemical composition similar to that of the test solid.
Similar to t-plot, the a s -plot gives a straight line passing through the origin if
the test solid is free of micropores; for a microporous solid, the a s -plot yields
a straight line at high a s and a curve at low a s , and the extrapolated intercept
from the upper linear line gives the micropore volume. If the test sample con-
tains a large number of mesopores, an upward deviation from a straight line
will occur at relatively high t and a s . In general, if the N 2 data are used together
with an appropriate reference solid, the micropore volume of the test solid
determined by the a s -plot should be the same as that obtained by the t-plot.
Unlike the t-plot, the applicability of the a s -plot is not restricted to the N 2
adsorption data. This enables separate a s -plots to be constructed from the
data of N 2 and other suitable vapors for the test solid. The a s -plot offers an
advantage in elucidating the sizes of various fine pores of a solid of interest
by adopting appropriate reference nonpolar vapors of specified molecular
sizes (Rutherford et al., 1997). When molecular sieving is observed by use of
a large reference adsorbate but not with a small adsorbate, the microporosity
detected based on the former should then be lower.
The calculated micropore volumes and the open surface areas of the solids
by t-plot with N 2 data are shown in Table 6.1. As noted, the three mineral
oxides and kaolinite (KGa-2) are virtually free of microporosity, since the total
BET surface areas are practically the same as the open surface areas derived
from the t-plot. In the absence of micropores, the surface areas of solids should
generally be relatively small in magnitude. On the other hand, certain solids,
such as K-SAz-1, Ca-SAz-1, and especially activated carbon, display very