Geoscience Reference
In-Depth Information
another (Bennett and Larter
1997
). The decreasing concentration of alkylphenols
with increases in water salinity is explained by the well-known salting-out effect
(
Sect. 6.5
).
Another example of transformation in the composition of an industrial petro-
leum product, due to dissolution-partitioning processes, is reported by Yaron et al.
(
1998
) and Dror et al. (
2000
). Figure
16.25
shows the gas chromatograms of a neat
kerosene and the kerosene extracted from an aqueous electrolyte solution (0.01 M
NaCl) after 100 h of contact at an ambient temperature of 22 C. From analysis of
the chromatograms, it is evident that aliphatic and branched aliphatic hydrocar-
bons in the range of C
9
-C
16
constitute the major group of components in neat
kerosene; there is only a minor set of aromatic compounds. In the aqueous elec-
trolyte solution, on the other hand, aromatic compounds, especially the branched
benzenes and naphthalene, make up the majority of compounds in the aqueous
phase. The C
9
and C
10
aliphatic and branched aliphatic components do not appear
in the chromatogram of aqueous kerosene. Aromatic components are several
orders of magnitude more soluble than aliphatic components. As such, aromatic
compounds that are minor constituents of neat kerosene dissolve in water much
more readily than in the major group of aliphatic compounds.
Broholm et al. (
2005
) discuss a field experiment at Borden, Ontario, where the
transformation of a mixture of solvents forming a contamination source is reflected
in the presence of solvents in an aquifer. A chlorinated solvent mixture containing
2.0 L of trichloroethylene (TCE), 0.5 L chloroform (TCM), and 2.5 L of tetra-
chloroethylene (PCE) was released into a sandy aquifer, and the development of a
dissolved-phase plume was studied over one year. Using a multiple-component
dissolution model, the authors estimated the mass of multiple-component dense
nonaqueous-phase liquid (DNAPL) source in the groundwater and the changes in
the source composition over time. When a multiple-component DNAPL is dis-
solved in an aqueous phase, the most soluble compounds are depleted fastest and
the DNAPL composition changes. Estimation of the mass of residual solvent is
based on a theory of dissolution from a multiple-component mixture of organic
compounds, for which the dissolved concentrations are described by Raoult's law:
C
i
¼ x
i
S
i
ð
16
:
4
Þ
where C
i
is the aqueous equilibrium concentration of compound i (called the
effective solubility), x
i
is the mole fraction of compound i in organic phase, and S
i
is the aqueous solubility of compound i.
Based on this theory, Broholm et al. (
2005
) present the theoretical dissolution
curve for a DNAPL mixture consisting of 10 % TCM, 40 % TCE, and 50 % PCE
by volume (see Fig.
16.26
a). TCM, which is the most soluble compound, depletes
quickly, while PCE, the less soluble fraction, is depleted slowly. Based on these
curves, concentration ratios between TCE and TCM, PCE and TCM, and PCE and
TCE were determined as a function of the source depletion (Fig.
16.26
b).
