Page 116 - Partition & Adsorption of Organic Contaminants in Environmental Systems

BACKGROUND IN SORPTION STUDIES 107

in soil and the effect of water on their individual functions enables us to make
reasonable predictions of the sorption behavior of a wide variety of organic
compounds under different system conditions.
Sorption data are most often analyzed at equilibrium (or near-equilibrium)
conditions. Although concentrations of contaminants in soil, water, and other
phases in natural systems frequently deviate from those at equilibrium, the
equilibrium data serve as an essential guide to the direction of contaminant
movement at a particular point in time and to the likely consequence of an
earlier contamination event. A comparison of the field data with equilibrium
values also enables one to elucidate whether a compartment (such as soil or
sediment) functions as a sink (to receive a given contaminant) or as a source
(to release a given contaminant) under specified conditions. Such information
is often valued in the characterization of a contamination site.
In the description of the roles of soil or sediment organic matter (SOM)
and minerals in uptake of organic compounds and pesticides, select terminol-
ogy is often used to refer to the mechanism involved. The term sorption is used
to denote the uptake of a contaminant (solute or vapor) by soil or sediment
without reference to a specific mechanism (i.e., by adsorption and/or parti-
tion). The terms partition and adsorption are used to refer to specific processes
involved. As one recalls, the term partition refers to a process in which the
sorbed material penetrates into the entire network of an organic phase
by forces common to solution, whereas the term adsorption refers to con-
densation of vapors or solutes on the surfaces or interior pores of a solid
(adsorbent).
The term organic matter is used to refer to the bulk of the organic content
in soil (or sediment). While variations in composition between humic and non-
humic organic matter in soil can influence the overall behavior of the organic
matter, it is generally not possible to separate their effects because practically
all sorption studies have been carried out with intact soils. In general, except
for top surficial soils where a significant amount of undecomposed (or poorly
decomposed) plant litter may exist, the SOM normally contains mainly well-
humified organic material.

7.2 BACKGROUND IN SORPTION STUDIES

7.2.1 Influences of Mineral Matter, Organic Matter, and Water

To better understand the sorption of an organic compound to a soil or a
natural solid under a particular system condition, it is helpful to have a brief
overview of important and unique sorption characteristics in relation to the
soil or solid composition and the water content associated with it. The highly
heterogeneous nature of soil samples from different geographic sources
greatly complicates the resulting sorption of organic contaminants. This made
it a formidable challenge for scientists to interpret the wide range of soil