Industrial Ecology and the Geoenvironment - Sustainable Practices in Geoenvironmental Engineering - page 135

the waste pile. The relative abundance of a given dissolved component is a function of the
composition of the principal liquid. Neutral nonpolar organic liquids will have large car-
rying capacities, and will easily carry other neutral nonpolar organic chemicals. Aqueous
liquids have very limited carrying capacity and will not be capable of carrying nonpolar
organics in its dissolved phase. Water, meanwhile, has a relatively large carrying capacity
for polar organic chemicals (they may be miscible in each other in all proportions) and for
inorganic acids, bases, and salts.
4.9.1 Physicochemical Properties and Processes
Water is the carrier for contaminants in the subsoil. The movement and distribution of the
various contaminants in the soil depend not only on the hydrogeological setting, but also
on the interactions between the contaminants carried in the liquid phase and the soil frac-
tions. Many of these interactions have been described in Chapter 2 as transport processes.
In this section, we will look at the physicochemical properties and processes involved
when the kinds of contaminants listed in Table 4.2 are introduced to the geoenvironment.
4.9.1.1 Solubility
The amount of solutes needed to reach a saturated state in a given quantity of solvent at a
speciic temperature is deined as the solubility of the given solvent. For considerations of
ionic equilibrium in aqueous solutions, the solutes are those that fall into the class of spar-
ingly or slightly soluble ionic solids. We deine the solubility product k sp as the equilibrium
constant for the equilibrium that exists between the sparingly soluble ionic solid and its
ions in a saturated solution. Because of the signiicant electrostatic attraction between ions,
crystals composed of small ions packed closely together are generally harder to pull apart
than crystals made up of large ions. For example, luorides (F − ) and hydroxides (OH − ) are
less soluble than nitrates ( NO 3 − ) and perchlorates ( ClO 4 − ).
Solubility equilibria are useful in predicting whether a precipitate will form under spec-
iied conditions, and in choosing conditions under which two chemical substances in solu-
tion can be separated by selective precipitation. Substances that are more soluble are more
likely to desorb from soils and less likely to volatilize from water. Meanwhile, substances
with no hydrogen bonding groups or little polar character, such as hydrocarbons or halo-
genated hydrocarbons, usually have very low solubilities compared with compounds
such as alcohols, which are capable of interaction with water. The solubility of an organic
compound depends primarily upon the sorption-desorption characteristics of the sorbate
(organic compound) in association with the sorbent (soil and sediment).
4.9.1.2 Partition Coeficients
Partition coeficients were described in Chapter 2. They provide a measure of the dis-
tribution of a given inorganic contaminant—between sorption onto the soil solids and
the porewater. For organic chemicals, the octanol-water partition coeficient is used to
describe partitioning. The octanol-water partitioning coeficient ( k ow ), which has been deined
in Chapter 2 as the ratio of the amount of a solute dissolved in octanol and water in
octanol-water immiscible mix. k ow is well correlated with the solubility of several organic
chemicals. Log k ow values normally range from −3 to 7. Highly water-soluble compounds
such as ethanol, have values of log k ow < 1, and hydrophobic compounds, such as certain
PCBs and chlorinated dioxin congeners, have values of 6 to 7.