Environmental Engineering Reference
In-Depth Information
6.4.4 PCB-surfactant solubilization experiments
Surfactants have the capacity to greatly increase the overall aqueous solu-
bility of hydrophobic organic compounds, including chlorinated biphenyls,
at surfactant concentrations above the critical micelle concentration (CMC),
the point at which individual surfactant monomers aggregate to form
micelles in solution (e.g., Pennell et al., 1997). The aqueous solubility of
4-CBP in solutions of Tween 80 and Tergitol NP-15 was determined from
batch experiments conducted in 26-ml glass serum tubes. The lower portions
of the tubes were coated with 4-CBP that was transferred in a solution of
hexane. The hexane was allowed to evaporate, leaving a thin film of 4-CBP
of sufficient mass to exceed the solubility limit. Surfactant solutions were
prepared over a concentration range of 50 to 2000 mg/l. Approximately 20
ml of surfactant solution was added to each serum tube; these tubes were
placed on an orbital shaker and mixed at 150 rpm for 3 weeks. The solubility
experiments were conducted at 22˚C (room temperature) and 30˚C (temper-
ature of microbial growth studies).
After equilibrium was reached, 0.4-ml samples were taken from the
serum tubes and mixed with 1.2 ml of isopropanol in 1.6-ml glass autosam-
pler vials. The concentration of 4-CBP was determined using an Agilent 6890
Gas Chromatograph equipped with an autosampler and electron capture
detector. To improve detection for lower concentrations of 4-CBP, samples
from the serum tubes were transferred as 10- to 15-ml aliquots to clean serum
tubes, into which 3 to 5 ml of hexane was added. These serum tubes were
mixed by hand and centrifuged at 3000 rpm for 20 min to separate the
aqueous and organic phases. Samples were collected from the hexane phase
and transferred to autosample vials for subsequent GC/ECD (electron cap-
ture detection) analysis.
Results of representative 4-CBP solubility experiments, conducted for
Tween 80 and Tergitol NP-15, are presented in Figure 6.22. The aqueous
solubility of 4-CBP is approximately 2 mg/l (0.011 m M ) at 22 ˚ C. A linear
enhancement in 4-CBP solubility was observed for both surfactants above
the CMC, which is consistent with the conceptual model of HOC partitioning
into the hydrophobic core of surfactant micelles (Rosen, 1992). The micellar
solubilization capacity of a surfactant for a particular HOC can be described
in terms of a molar solubilization ratio (MSR):
CC
CC
o
o c
,
c
MSR
=
(6.1)
s
s c
,
c
where C o is the molar concentration of the organic species, C o,cmc is the molar
concentration of the organic species at the CMC (i.e., the aqueous solubility),
C s is the molar concentration of surfactant in solution, and C s,cmc is the molar
concentration of surfactant at the CMC. The resulting MSR values for 4-CBP
in micellar solutions of Tween 80 and Tergitol NP-15 at 22 ˚ C were 0.33 and
Search WWH ::




Custom Search