Environmental Engineering Reference
In-Depth Information
colloids reduce the value of air-water partition constant, it is probably negligible
for most of the soluble organics at typical environmental concentrations of DOCs.
Callaway et al. (1984) reported a reduction in
K
aw
for two low-molecular-weight
organic compounds, namely, chloroform and trichloroethene (TCE) in the presence
of humic acid.The experiments involved measuring the concentrations of theVOCs in
closed vessels after equilibration of a known volume of the aqueous phase containing
humic acid (10 wt%). It was then compared with a system without humic acid. The
ratio
K
aw
:
K
aw
was 0.39 and 0.79 for chloroform, the different values corresponding
to the pretreatment procedure used for preparing the humic acid solution. The ratio
for TCE was 0.28. Yurteri et al. (1987) observed that the ratio of air-water partition
constants decreased to 0.94 for toluene in the presence of 10 mg/L humic acid in
water, whereas it was 0.99 for 5 mg/L humic acid in water. A significant observation
was that the combined effects of inorganic salts, surfactants, and humic acids on the
partition constant were far more significant than the effect of any single component
in wastewater samples.
Municipal wastewaters in many regions of the world contain high concentrations
of detergents (e.g., see Takada and Ishiwatari, 1987). Linear alkyl benzene sulfonates
(LABs) and linear alkyl sulfates (LAS) are common constituents of synthetic deter-
gents. These and other surfactants behave in a manner similar to the DOCs described
above. They tend to increase the aqueous solubility of many organic and inorganic
species.
Valsaraj and Thibodeaux (1988) studied the effects of surfactants (both anionic
and cationic) on the air-water partition constants of low-molecular-weight hydro-
carbons of environmental interest. The effects of two anionic surfactants (sodium
dodecylbenzene sulfate, DDS, and sodium dodecylbenzenesulfonate, DDBS) and
one cationic surfactant (hexadecyltrimethyl ammonium bromide, HTAB) were stud-
ied. The reduction in air-water partition constant from that in pure water was
deduced. It was observed that the partition constant for chloroform decreased to
0.77 times its value in pure water in the presence of 42 mM of DDBS, and it fur-
ther decreased to 0.66 times its pure water value in the presence of 82 mM of
DDBS.There was no decrease till the critical micellar concentration (CMC) of DDBS
was exceeded. The results with DDS and HTAB micelles were similar to that with
DDBS.
E
XAMPLE
4.7
K
aw
AND
D
OC
For anthracene, what will be the concentration in water in the presence of 1000 mg/L
of a DOC?
From Table 4.5,
K
cw
for anthracene is 10
4.92
L/kg. Given
C
C
is 1000 mg/L
=
10
−
3
kg/L, the increase in aqueous-phase concentration is
C
app
i
/C
i
=
1
+
C
C
K
cw
=
16.8. Since
C
i
=
3.3
×
10
−
5
mol/L,
C
app
=
5.6
×
10
−
4
mol/L. Moreover, note that
i
since
C
app
i
/C
i
=
x
app
app
i
/x
i
= γ
i
/
γ
, the activity coefficient is 16.8 times lower in the
i
presence of DOC.
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