Surfactant Effects on Environmental Behavior of Pesticides - Reviews of Environmental Contamination and Toxicology - page 82

(x/m) max is the maximum adsorbate concentration in an adsorbent phase. Irrespective
of the isotherm, the presence of soil reduces the aqueous concentration of surfactant
because of its adsorption, which results in apparently a larger cmc value in soil-
water suspension. In most cases, the Freundlich isotherm is applicable at lower
concentrations of surfactant, but the adsorption profiles of nonionic surfactants at a
wider concentration range usually obey the Langmuir isotherm, as observed for
Triton X-100 and rhamnolipids (Mata-Sandoval et al. 2002).
Liu et al. (1992) examined the adsorption of three nonionic micelle-forming
alkylphenoxy ethoxylates and a lamella-forming alcohol ethoxylate below and
above their cmc values. Their adsorption followed the Freundlich isotherm below
cmc, whereas the maximum plateau values on adsorption of micelle-forming sur-
factants were obtained above cmc. In the case of the lamella-forming surfactant, the
adsorption maximum followed by its decrease with an increasing concentration of
surfactant was observed. In the case of adsorption of Triton X-100 on peat soil, a
skewed Gaussian-type sorption isotherm was reported, which may be accounted for
by the solubilization of soil organic matter by surfactant (Lee et al. 2000). When
octylphenoxy and dodecyl ethoxylates were adsorbed on montmorillonite clay,
greater adsorption at plateau for the surfactant having a longer ethoxy chain was
detected (Lee et al. 2006).
A better correlation of the distribution ratio with the clay fraction in sediment
than the organic matter content was observed for adsorption of monodisperse pen-
tadecyl nonaethoxylate (Cano and Dorn 1996). These results show the interaction
of a polyethoxy chain with the mineral surface possibly occurs via hydrogen bond-
ing (Krogh et al. 2003). The Freundlich adsorption coefficients of nonionic sur-
factants are larger than for the anionic ones (Sánchez-Camazano et al. 2003; Urano
et al. 1984). A similar trend was reported for adsorption of Triton X-100 and SDS
to 18 soils, and the mechanism of adsorption was examined from the aspect of soil
characteristics (Rodriguez-Cruz et al. 2005). The K F values of Triton X-100 were
positively correlated with a clay content via interaction with montmorillonite and
illite, whereas the largest positive correlation with those of SDS was content of soil
organic matter. Therefore, the hydrophobic interaction seems to predominate in the
adsorption of SDS and either ion-dipole interaction or hydrogen bonding controls
that of Triton X-100. A sediment adsorption study of 10 14 C-labeled LAS sur-
factants showed that the K d values increased by a factor of 2.8 for each additional
methylene group in the linear alkyl chain, which was in accordance with the hydro-
phobic mechanism (Hand and Williams 1987). Theoretical assessment using a
particle interaction model explained the hydrophobic interactions of several anionic
surfactants with soil and sediment, but its contribution is diminished as cation-
exchange capacity (CEC) of solid increases (Di Toro et al. 1990).
Greater adsorption of cationic HDTMA Br than SDS was considered to origi-
nate from the strong electrostatic interaction of HDTMA + with the negatively
charged sediment surface (Jones-Hughes and Turner 2005). Cationic surfactants
tend to be adsorbed strongly onto soil or sediment via an ion-exchange mechanism
(Hand et al. 1990). The extent of a surfactant adsorption increased with the length
of an alkyl chain in the order of ODTMA + > HDTMA + > DDTMA + (Boyd et al.