Page 23 - 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

2 Fundamentals of the

Solution Theory

2.1 INTRODUCTION

In natural systems, the solubilities of organic contaminants in water and other
phases play a crucial role in the behavior and fate of the compounds. The solu-
bility affects not only the limit to which a substance can be solubilized by a
solvent or a phase, but also dictates the distribution pattern of the substance
between any two solvents or phases of interest. Water is apparently the most
important natural solvent,not only because it is a huge medium to hold various
contaminants, but also because it is a common medium through which con-
taminants are transported to other media. Depending on specific local envi-
ronments, natural organic substances such as mineral oils, biological lipids,
soil organic matter, and plant organic matter play vital roles in extracting and
sequestering these contaminants, thereby mediating their environmental
impact and fate.
As recognized, the water solubilities of organic compounds vary much more
widely with their structures and compositions than do their corresponding
solubilities in an organic-solvent phase. For liquid substances (i.e., solutes),
the solubility in a solvent (or medium) is determined by the degree of
solute–solvent compatibility. For solid substances, the solubility is also affected
by the energy required to overcome the solid-to-liquid transition (called the
melting-point effect). These features suggest immediately that both the poten-
tial level of contamination and the distribution pattern may vary widely for
the various sources or types of organic compounds. To understand the solu-
bility and partition behavior of organic compounds in natural systems, it is
essential that one capture the essentials of the relevant solution theory.

2.2 RAOULT’S LAW

Raoult (1887, 1888) recognized that the addition of a small amount of solutes
to a solvent does not radically change any extensive property of the solvent,
because it changes the solvent mole fraction only slightly. On the other hand,
the properties of a solute may change much more substantially as it goes from
a pure substance to one in dilute solution. One would therefore expect the
ideal-solution approximation to apply more widely and closely to the solvent
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