Surfactant Effects on Environmental Behavior of Pesticides - Reviews of Environmental Contamination and Toxicology - page 121

phenanthrene from the contaminated soil was much larger than that from the crys-
talline phenanthrene. Therefore, sorption of surfactant, its penetration into soil, and
swelling of soil matrix followed by matrix diffusion of a chemical were considered
to be the important processes in enhanced desorption. These effects are considered
to be highly dependent on the chemical class of surfactant. Jones-Hughes and
Turner (2005) reported an inhibitory effect of SDS on the sorption of phenanthrene
to estuarine sediment but a significant enhancement of sorption in the presence of
HDTMA Br. It was considered that nonspecific adsorption of SDS to negatively
charged sediment competed with phenanthrene for access to hydrophobic sites of
the sediment surface but that the electrostatic adsorption of HDTMA Br gave addi-
tional sorption sites for phenanthrene. This sorption process of PAHs to the
adsorbed surfactants has been examined for SDS and Tween 80 by using kaolinite
clay (Ko et al. 1998). The distribution coefficients ( K D ) of phenanthrene and naph-
thalene between clay and water gradually increased with the SDS concentration up
to its cmc, but decreased afterward for reasons of more favorable partition to
micelles in a water phase. Similar profiles were observed for Tween 80, but the
maximum K D values were obtained much above its cmc. These differences are con-
sidered to stem from the different adsorption characteristics of two surfactants.
Because the sorption of a chemical to soil or sediment proceeds via not only parti-
tion to their organic matter fractions but also by adsorption by van der Waals or
electrostatic forces, the effect of surfactant on adsorption to clay and minerals has
also been investigated. Theng et al. (1998) examined the partition of phenanthrene
to the complex of TDTMA Br and montomotillonite (nearly 1:1, w/w) where the
surfactant molecules were intercalated into the clay as they formed bilayers with
the (CH 2 ) 2 chains lying parallel, as evidenced by X-ray diffraction patterns and
13 C-NMR. Magic-angle spinning NMR showed that phenanthrene and TDTMA Br
were intimately associated in the clay interlayer.
The adsorption of small organic molecules such as benzene and polychloroeth-
ylene derivatives to soils has been studied in the presence of alkyltrimethylammo-
nium (Lee et al. 1989). Significant enhancement of adsorption following the linear
isotherm was observed. The longer the alkyl chain of the surfactant, the higher
adsorption to soils, indicating partition of the organic chemicals to the surfactants
sorbed on soil. When the linear isotherm is assumed for adsorption of a chemical
to soil with the coefficient of K d , the apparent adsorption coefficient ( K d * ) at the
surfactant concentration above cmc is expressed below (Lee et al. 2000):

K d * = K d [1+f sf ( K sf / K d )] / (1 + X mon . K mon + X mic . K m ), X mic = X − cmc

X is the total concentration of surfactant in a mass/mass unit, and the suffices
“mon” and “mic” mean monomer and micelle, respectively. K sf and f sf are the parti-
tion coefficient of surfactant on soil and mass fraction of the sorbed surfactant in
soil, respectively. The K d * / K d values in the presence of nonionic surfactants includ-
ing Triton X-100 were larger than 1 for benzene derivatives and gradually
increased to plateau (Lee et al. 2000, 2006). When SDBS and domiphen bromide
[CH 3 (CH 2 ) 10 CH 2 (CH 3 ) 2 N + (CH 2 ) 2 OPh Br − ] were used, the K d * / K d values showed