Page 95 - Partition & Adsorption of Organic Contaminants in Environmental Systems

Partition and Adsorption of Organic Contaminants in Environmental Systems. Cary T. Chiou
Copyright ¶ 2002 John Wiley & Sons, Inc.
ISBN: 0-471-23325-0

6 Adsorption of Vapors on

Minerals and Other Solids

6.1 INTRODUCTION

Before investigating the sorptive characteristics of organic contaminants
either as vapors or as solutes onto composite soil or sediment samples, it is
instructive that one first examine the adsorptive behavior of some vapors on
basic minerals (and other natural solids) that occur in common soils. In
Chapter 7 we will see that the principal fraction of the soil to adsorb a
nonionic compound is the mineral matter and, in some special cases, some car-
bonaceous material (e.g., charcoal-like substance) admitted into the soil. Since
the process of adsorption is competitive in nature, as pointed out earlier, the
amount of a contaminant adsorbed by a mineral matter would depend criti-
cally on the competitive power of other species coexisting in the system. In
natural environments, the most important competitive species is probably
water, because of its ubiquity and abundance.
In this chapter we examine first the adsorption data of N 2 vapor on a few
representative minerals and solids and then use the N 2 adsorption data to
calculate the surface areas and micropore volumes of the samples. We then
consider how the surface areas of some natural solids achieved by improper
analytical methods lead to serious discrepancies from the results by the stan-
dard BET-N 2 method. Subsequently,we compare the vapor uptakes of a model
organic compound (benzene) and water onto these samples to give one a good
sense of the relative adsorptive powers of these samples for a nonionic organic
compound and water. The comparative adsorption of an organic and water
vapors by minerals renders a crucial link to organic–solute sorption to com-
posite soils or sediments from water solution, where the solvent (water) com-
petes forcefully for mineral adsorption, thus strongly depressing the solute
adsorption. Similarly, the contrasting feature for organic vapor and water
adsorption on activated carbon sets the ground for the weak competition of
water against the organic–solute adsorption onto a carbonaceous substance,
such as natural charcoal.

6.2 NITROGEN ISOTHERM AND SOLID SURFACE AREA

The surface phenomena of solids are frequently related to their subdivision
or porosity, which in turn is evaluated in terms of their surface areas. The uni-
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