Geoscience Reference
In-Depth Information
log
ð
S
0
=
S
Þ
¼kM
T
¼
X
i
N
i
k
i
M
T
;
ð
6
:
9
Þ
where M
T
is the total molar concentration of the salts and N
i
is the mole fraction of
salt i(M
T
=
P
i
M
i
.); S
0
and S are solubilities of the organic compound measured
in water and in salt solution, respectively; and k is the salting-out constant. This
calculated solubility was validated experimentally for seawater. Millero (
2000
)
used the Pitzer parameters for ions, which are related to the interaction of an ion
with a nonelectrolyte, to estimate the activity coefficients and the solubility of
nonelectrolytes in a water solution containing an electrolyte mixture.
A salting-in effect is promoted when there is a simultaneous presence of large
organic molecules (e.g., tetramethyl-ammonium) and electrolytes in a water
solution. In contrast to the salting-out process, salting in leads to an increase in
organic compound solubility or a decrease in activity coefficient (Table
6.3
).
Almeida et al. (
1983
) observed a similar salting-in effect for very polar com-
pounds, which may interact strongly with certain ions.
Despite the possibility of a salting-in route in some particular cases, the salting-
out effect—and the decrease in organic compound solubility—in a saline envi-
ronment is the main process controlling organic contaminant solubility in sub-
surface water solutions. Whitehouse (
1984
) studied the effect of salinity (from 0 to
36.7 %) on the aqueous solubility of polycyclic aromatic hydrocarbons (PAHs)
and found that phenanthrene, anthracene, 2-methylanthracene, 2-ethylantracene,
and benzo(a)pyrene experienced salting out, while only 1,2-benzanthracene
exhibits a salting-in effect. Where salting out was observed, fairly large changes in
salinity were required to cause significant changes in the solubility; such was not
the case for the compound exhibiting a salting-in effect.
A simple correlation was determined for estimating the Setschenow constants
for a variety of organic solutes in seawater, which yields an overall reduction in
solubility by a factor 1.36 (Xie et al.
1997
). The hydrophobicity of organic solutes
increases by this factor, but the salting-out effect must be quantified when com-
paring the behavior of specific organic contaminants in freshwater and in sub-
surface aqueous solutions.
Under certain environmental conditions, when subsurface water contains
potential organic ligands, trace metals also may be subject to salting out following
their organic complexation. Turner et al. (
2002
) demonstrated that sediment-water
partitioning of Hg(II) in estuaries characterized by high salinity may be controlled
by a salting-out process. This partitioning applies to the aqueous solubility of
nonelectrolytes and suggests that the organic complexes of mercury are removed
from the aqueous phase via a coupled sorption-salting-out mechanism. Evidence
of salting out of other metal complexes (Cd, Cr, Cu, Ni, Pb, Zn) also was reported
(Turner et al.
2002
). In highly contaminated, organic-rich estuaries, an increase in
the sediment-water distribution of metal complexes was observed as salinity
increased, except for Cd. Such behavior contradicts conventional speciation and
partitioning processes and may be explained only by a salting-out effect. Turner
et al. (
2002
) propose a mechanism by which trace metals are complexed and
