Surfactant Effects on Environmental Behavior of Pesticides - Reviews of Environmental Contamination and Toxicology - page 79

C EnvironmentalBehavior
The pesticide formulation is sprayed on crops in most cases, except for direct soil
application using granules, and therefore surfactants in the formulation are consid-
ered to first adhere on foliage and shoots of plants followed by penetration into tis-
sues via the cuticle. Surfactants are considered to partly dissipate via sunlight
photolysis and metabolism on the plants (See Fig. 1). When the formulation reaches
the ground by direct application, or indirectly if not intercepted by leaves or via
washoff from precipitation, the surfactants would be taken up by roots, adsorbed to
soil, or metabolized by soil microorganisms.
Behavior in Plants
Plant foliage is covered by heterogeneous cuticular waxes that change widely
species by species in thickness, composition, and physical structure (Katagi
2004), and the spreading profile of droplets after spraying is highly dependent
on the formulation type (Baker et al. 1983). Because plant cuticles consist of
waxes and the underlying cutin polymer layer, the penetration profiles of sur-
factants have been separately examined. Schönherr et al. (1991) examined the
partition of octylphenoxy ethoxylates having the different number of an ethoxy
unit (N) to the wax-extracted cuticles of citrus leaf and found good correlation
between partition coefficient (K) and N; K = 8634 · exp (−0.389N). The partition
of surfactant to the cuticular wax from an aqueous phase ( K wax/w ) was also exam-
ined for 15 monodisperse alcohol ethoxylates having different lengths of the
alkyl carbon (C) and the ethoxy unit (E) by using barley leaf waxes (Burghardt
et al. 1998). The log K wax/w value of each surfactant was found to be linearly
dependent on C and E as follows: log K wax/w = −2.73 + 0.54C − 0.23E. The maxi-
mum concentration of surfactant in wax at equilibrium (C max ) was only depend-
ent on E; log C max = 2.04 − 0.14E. These results show that the hydrophobicity of
a surfactant is one of the important factors for its penetration to cuticles; this
may be related to the observation that nonionic surfactants, including alcohol
ethoxylates, generally penetrate faster into the waxy leaves of rape and pea than
the less waxy leaves of bean and beet (Holloway and Silcox 1985). In the pene-
tration of five alcohols and an nonylphenoxy ethoxylate, Tween 20 and SDS to
wheat foliage, the molecular weight of the surfactant was not a key factor, but
its physical state on foliage, such as gel, liquid, or solid, seemed to control
uptake (Anderson and Girling 1983). Nielsen et al. (2005) have used five noni-
onic surfactants with molecular weights of 290 to 1350 to investigate their
penetration into fathen ( Chenopodium album ) and wheat leaves. For both species
the total amount taken up for 24hr exponentially decreased with molecular
weight, and the uptake per unit area (nmol/mm 2 ) was found to be proportional to
the applied dose per unit area.