Geoscience Reference
In-Depth Information
catalysis process. In the subsurface, mainly in the near surface (soil layer) region,
conditions may favor microbial populations that act as a catalyst in redox reactions.
6.2 Aqueous Solubility of Organic Contaminants
Aqueous solubility of organic contaminants from a gaseous, liquid, or solid source
is of major interest in contaminant geochemistry. To define the solubilities and
activity coefficients of organic gases, the partial pressure of a compound in the gas
phase at equilibrium above a liquid solution is considered identical to the fugacity
(see
Sect. 7.3
) of the compound in the solution. Figure
6.2
conceptualizes the
fugacity of a compound in a nonideal liquid mixture (e.g., water solution) where
the gas and liquid phases are in equilibrium. Fugacity is a measure of chemical
potential that indicates the tendency of a substance to move from one phase to
another or from one site to another.
It is known that water is an associated liquid, in which molecules are hydrogen-
bonded forming packets of a number of H
2
O molecules. In this situation, the
organic solutes are not in direct contact with individual water molecules. During
the transfer of organic contaminant from its pure liquid phase into a pure aqueous
phase, a number of water molecules surround each organic molecule. The water
molecules adjacent to the organic solute are different than the bulk water mole-
cules, because of the presence of hydrogen bonds (Schwarzenbach et al.
2003
).
Meng and Kollman (
1996
) suggest that water surrounding a nonpolar (organic)
molecule maintains but does not enhance its H-bonding network. At ambient
temperature, water molecules adjacent to a nonpolar organic molecule lose only a
very small proportion of their total hydrogen bonds and, consequently, are able to
host a nonpolar solute of limited size (Blokzijl and Engberts
1993
).
Organic compounds exhibit, as a function of their molecular properties, a
spectrum of solubility values starting from almost zero (for compounds completely
immiscible with water, mainly nonpolar organic compounds) and reaching a large
value for polar organic compounds. Because of their very low solubility in water,
nonpolar organic compounds often are referred to as nonaqueous phase liquids
(NAPLs). When an NAPL comes into contact with a water source, some organic
molecules leave the organic phase and dissolve into the water, while some water
molecules enter the organic liquid. When the flux of molecules into and out of the
organic phase reaches an equilibrium, the amount of organic matter in the water is
constant.
The dissolution of composite nonaqueous phase liquids (CNAPLs) in water is
of major importance when considering the fate of organic contaminants in the
subsurface. In spite of their limited aqueous solubility, these organic contaminants
generally are toxic even in relatively minute amounts. Based on a laboratory
experiment examining the dissolution in water of a mixture of seven structurally
similar polycyclic aromatic hydrocarbons in toluene, Weber et al. (
1998
) con-
cluded that variations from predictions using Raoult's law by up to a factor of 2
