Surfactant Effects on Environmental Behavior of Pesticides - Reviews of Environmental Contamination and Toxicology - page 128

fresh soil, partly by the entrapment of pesticide in soil matrix (Katagi 2006).
Triton X-100 has been shown to desorb trifluralin (15) from freshly treated and
aged soils very similarly, with the extent proportional to its concentration (Mata-
Sandoval et al. 2000). Therefore, Triton X-100 is considered to access the interior
of soil particles and release tightly adsorbed pesticide. A similar ability for
releasing pesticides from the aged soils was reported for several nonionic sur-
factants, and their extent gradually decreased with a period of aging (Ahmad
et al. 2004; Rodriguez-Cruz et al. 2006).
Clay mineral is one of the main components in soil, and surfactant can be either
adsorbed onto its surface or intercalated into its interlayers. The adsorption of atra-
zine (13) to sodium-saturated smectite was influenced by nonionic Brij-35 sur-
factant (Chappell et al. 2005). The K F values decreased with Brij 35 concentration
up to 2.1 g/L but increased afterward. Based on the X-ray diffraction data, Brij-35
was considered to first compete with (13) for intercalation to the clay but at the
higher concentration the admicelles on the clay surface would solubilize (13). The
cationic surfactant DDTMA Br has been shown to replace paraquat (1,1-dimethyl-
4,4 -bipyridinium) intercalated into laponite clay, as evidenced by batch equilibra-

tion method and X-ray diffraction measurements (Esumi et al. 1998). The character
of clay sorbing surfactant molecules into its interlayer has been applied extensively
to prepare organoclay formulations of hydrophilic pesticides to improve their
mobility in soil. 2,4-D (26) is an ionizable herbicide that is scarcely adsorbed to
montmorillonite clay, but treatment of the clay with HDTMA Br at a concentration
equivalent to cation-exchange capacity (CEC) greatly enhances its adsorption
(Hermosín et al. 2006). The desorption process showed insignificant hysteresis,
indicating weak hydrophobic interactions between (26) and HDTMA Br interca-
lated into the clay.
Enhanced adsorption of organophosphorus pesticides has been examined using
montmorillonite modified by quaternary ammonium surfactants. Rodríguez-
Gonzalo et al. (1993) reported increase of K F values for parathion (3) and its oxon
(4) by 1.0 and 0.3, respectively, in a logarithm unit when the natural montmorillon-
ite was treated with 84% CEC equivalent TDTMA Br. The larger affinity of (3) to
the organoclay than (4) corresponds to the more hydrophobicity of (3), which is in
accordance with the adsorption mechanism of partition to the intercalated sur-
factants. The adsorption isotherm of azinphos-methyl (39) was found to depend on
the extent of a countercation exchange in montmorillonite by HDTMA Br and
DDTMA Br (Sánchez-Camazano and Sánchez-Martin 1994). In the case of
HDTMA Br, the 50% exchanged clay showed a linear isotherm, but 99% and 25%
exchange resulted in type-L and type-S isotherms, respectively. The log K F values
of (39) increased with the alkyl chain length of the surfactants, while insignificant
change as compared with the untreated clay was observed for the more water-
soluble dichlorvos (36). Although the water solubility is similarly high, hexazinone
(59) was found to be more adsorbed to montmorillonite treated with HDTMA Br,
presumably because of incorporation into the unoccupied interlayer space (Celis
et al. 2002).

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