Environmental Engineering Reference
In-Depth Information
Furthermore, the surfactant would be adsorbed onto the soil surface as a monomer,
hemimicelle, or admicelle. Adsorption of a chemical to soil would be reduced when
the chemical is solubilized to micelles in a water phase, whereas it increases by
solubilization to hemimicelles and admicelles on soil (Krogh et al. 2003; Pennell et al.
2003). Therefore, the complex interactions of a chemical with each species in both
the water and soil phase together with the concentration of surfactant should be
taken into account.
Simple Organic Chemicals
Many investigations have been conducted from the aspect of remediation of soil
contaminated with PAHs. Aronstein et al. (1991) reported the enhanced desorption
of biphenyl and phenanthrene from mineral soil by using two alcohol ethoxylates
having the HLB values around 11, but the effect was minimal for organic muck soil.
The desorption of phenanthrene and pyrene from loamy sand soil was found to
increase markedly by a factor of more than 10 when nonionic surfactants were
used, but SDS showed an insignificant effect, possibly because of its lower affinity
to soil surface (Cheng and Wong 2006a). The desorption
K
d
values of two PAHs
mostly decreased on the order of Tween 80 > Triton X-100 > Brij 35 > SDS in the
presence of 150 - 300 mg surfactant/L. The addition of dissolved organic matter
(DOM) enhanced desorption of these PAHs by Tween 80 several times (Cheng and
Wong 2006b). No additive effect of DOM in desorption may imply some interac-
tions between DOM and Tween 80.
In parallel with the experimental approach, the effect of surfactant on adsorption
and desorption of a chemical has been theoretically investigated. Edwards et al.
(1992) indicated that a higher concentration of surfactant in a soil-water system is
necessary to show the same surface tension in water. When the Freundlich isotherm
is assumed for adsorption of surfactant on soil, the apparent fraction of soil organic
carbon content can be expressed as f
oc
*
= f
oc
+ (
K
F
.
C
mon
1/n
)
.
f
c
, where f
oc
,
K
F
, C
mon
,
and f
c
are the organic carbon content of soil, Freundlich adsorption coefficient of
surfactant, monomer concentration, and weight fraction of C in surfactant, respec-
tively. As the solubility enhancement of a chemical at concentrations less than cmc
(sub-cmc) is proportional to the surfactant concentration (see Fig. 4a), the apparent
linear adsorption coefficient
K
d
*
of a chemical can be written as
K
d
*
=
K
d
[1 + (
K
F
.
C
mon
1/n
.
f
c
/ f
oc
) / [1 +
K
mon
.
C
mon
], where
K
d
and
K
mon
are the adsorption coefficient
in the absence of surfactant and the association constant between a chemical and
monomer, respectively. In the case of nonionic Triton X-100, the
K
d
*
value of
pyrene gradually decreases with an increase of surfactant concentration at sub-cmc
levels. At higher concentrations of surfactant above cmc, the similar process includ-
ing solubilization of a chemical to micelles greatly reduces the adsorption coeffi-
cient (Di Cesare and Smith 1994; Edwards et al. 1994).
The desorption process of PAHs from coal tar-contaminated soil has been exam-
ined by Yeom et al. (1996). The percentage of solubilization by nonionic surfactants
was well explained by partition of PAHs to micelles, but the dissolution rate of
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