Page 10 - 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

1 Important Thermodynamic

Properties

1.1 INTRODUCTION

In environmental systems, one is keenly interested in the transfer of a chem-
ical (contaminant) from one phase (or medium) to another and in the manner
it distributes itself between phases at equilibrium. In most cases, contaminants
are transported through mobile water or atmosphere into other natural biotic
or abiotic phases or media. Depending on the material properties of individ-
ual phases and on variable environmental factors, such as temperature and
humidity, the manner by which a contaminant is retained by individual natural
phases can vary widely. For most organic contaminants, particularly electri-
cally neutral species, the way a contaminant is retained by a biotic or abiotic
matter falls mainly into either or both of two categories: The contaminant
adheres only onto the surface of a natural material, or it dissolves into the
latter’s molecular network. Although these different modes of action are not
readily distinguishable to our eyes, they are consequential to the extent of con-
taminant uptake and to the activity and fate of the contaminant in its local
environment.
It is important to understand the terms system, phase, and medium as they
are referred to in the context above. A system is defined as a physical domain
enclosed by a real or imaginary boundary that separates it from its surround-
ings. The content of a system may be simple or complex, ranging from a single
vapor, liquid, or solid to a multicomponent and heterogeneous mixture of
considerable complexity. In heterogeneous systems, there exist molecularly
homogeneous regions, which we refer to as phases. Examples of phases in a
heterogeneous system are the organic solvent and water phases in their par-
tially miscible mixtures and the vapor and liquid phases of a volatile liquid in
a partially filled vessel or a subsurface space. The term medium is less precise
than the term phase, although they are sometimes used interchangeably. The
former refers to matter that is apparently uniform in its macroscopic appear-
ance but is not well characterized, such as a soil sample composed of many
finely divided mineral and organic phases or a plant-matter sample composed
of many constituents or phases (e.g., water, cellulose, and lipids) in its
composition.
Whether mass transfer occurs for any component across phases or the com-
ponent at the time is at equilibrium between phases at constant temperature
1