Surfactant Effects on Environmental Behavior of Pesticides - Reviews of Environmental Contamination and Toxicology - page 80

To know not only the penetration of surfactant to cuticle but also its translocation,
radiolabeled surfactants have been utilized (McWhorter 1985). Part of the treated
radioactivity was incorporated into foliage but with insignificant translocation.
Microautoradiograms of transverse sections of tobacco leaf treated with 14 C-labeled
Tween 80 have clearly shown that the surfactant is located only at the upper epider-
mal cells, indicating its penetration from the plant surface through cuticle to
epidermal cells but with no further movement to the mesophyll (Sugimura and
Takeno 1985). As compared with foliar uptake, the behavior of surfactant taken up
by roots has been much less investigated. The 14 C-labeled Soil Penetrant 3685 (lin-
ear alcohol ethoxylates) dissolved in a Hoagland solution was taken up by barley
roots depending on the surfactant concentration (Valoras et al. 1974). Doucette et al.
(2005) utilized crested wheatgrass to study the root uptake and translocation of two
14 C-nonylphenoxy ethoxylates under hydroponic conditions. The larger root
concentration factor ( 14 C concentration in roots divided by that in solution) was
obtained for more hydrophobic surfactant, which was in accordance with the foliar
uptake expressed by K wax/w . The 14 C distribution in leaves, which was unextractable
and so caused by metabolites, was found much less than that in roots by a factor of
less than 0.01, indicating insignificant translocation.
The metabolism of surfactants has been investigated mainly for the foliar appli-
cation of monodisperse 14 C-labeled alcohol and alkylphenoxy ethoxylates.
Successive chromatographic separation of metabolites in barley and rice plants fol-
lowed by GC-MS has revealed that octylphenoxy ethoxylates undergo oxidation in
the alkyl chain and conjugation with glucose at the terminal hydroxyl group to form
polar metabolites, as well as release of the ethoxy units to form nonpolar ones
(Stolzenberg et al. 1982, 1984). The transformation of nonylphenol as one possible
metabolite was examined in wheat cell suspension cultures, and hydroxylation at
the alkyl chain followed by sugar conjugation was the dominant metabolic pathway
(Bokern et al. 1996). Holloway and Silcox (1985) reported the distribution of polar
and nonpolar metabolites from 1-dodecanol octaethoxylate in eight plant species,
each of which was tentatively assigned to the tetraethoxylate derivative and sugar
conjugates, respectively.
Behavior in Soil
Surfactants directly applied to or reaching soil are first partitioned between the
soil matrix and pore water, and some of them are known to modify wetting and water
infiltration of soil. Organosilicone surfactants have been reported to reduce
water infiltration rates of wet soil (Singh et al. 2002a). To understand the effect on
mobility of pesticide in formulation, the adsorption profiles of surfactant should be
investigated. Depending on concentration, surfactant molecules would be adsorbed
to soil surface as monomer, hemimicelle, or admicelle, or dissolved in pore water
as monomer or micelle (Fig. 5) (Edwards et al. 1994). The cation concentration is
known also to modify the structure of adsorbed surfactants, and atomic force
microscopy has recently shown such structural changes of HDTMA + aggregates on