Page 61 - Partition & Adsorption of Organic Contaminants in Environmental Systems

52 FUNDAMENTALS OF THE ADSORPTION THEORY

small. When adsorption is weak, the isotherm usually assumes a relatively
linear shape over the entire range. For strong adsorption, which normally
involves adsorbate condensation, the isotherm develops a marked concave-
downward shape at low P or C e.
Because the sorption of organic compounds to many natural solids may be
dictated by processes other than adsorption (e.g., by a partition interaction),
the isosteric plot of the isotherms provides useful heat data for the undergo-
ing process. For example, in a typical partition process of an organic solute
from water to a partially miscible organic phase, the isotherm is usually highly
linear over a wide concentration range, and therefore the molar isosteric heat
of sorption is largely constant, independent of solute concentrations. This
unique characteristic enables one to distinguish an uptake by partition from
that by adsorption for a contaminant of interest. As we will find out later,
ordinary soils act as a dual sorbent in uptake of organic compounds, where
either adsorption on soil minerals or partition into soil organic matter may
predominate the soil uptake, depending on the system condition. The detected
isosteric heat for the system helps to pinpoint the dominant mechanism.