Surfactant Effects on Environmental Behavior of Pesticides - Reviews of Environmental Contamination and Toxicology - page 119

implied by the photoreaction with H 2 O, and neither dechlorination nor N -demethylation
was affected by molecular oxygen. Therefore, monuron is most likely to be solubilized
in the Stern layer of micelles. Detection of the corresponding biphenyl dimers together
with its reduction by addition of quenchers showed that photodegradation was initiated
by homolytic cleavage of the C-Cl bond (Tanaka et al. 1982; Tanaka 1989).
Photoinduced isomerization and decarboxylation in micelles were examined for
some pyrethroids. UV irradiation of cypermethrin ( ( RS )- a -cyano-3-phenoxybenzyl
(1 RS )- cis - trans -3-(2,2-dichlorovinyl)-2,2-dimethylcyclo-propanecarboxylate) in
SDS micelles specifically gave the corresponding acid, benzaldehyde, and benzoic
acid, which showed the effective hydrogen abstraction from the surfactant by the
radicals formed via homolytic cleavage of the O—CH(CN) bond (Ruzo 1983). In
the case of cyhalothrin, the 2-chloro-2-trifluoromethylvinyl analogue of cyper-
methrin, the main reactions in SDS and DDTMA Br micelles were photoinduced
isomerization and decarboxylation from the greater stability of the generated radi-
cals (Ruzo et al. 1987).
Photoreduction of nitrodiphenyl ether herbicides has been examined in micelles.
By exposure to UV-A light, the several nitrodiphenyl ethers in Triton X-100 micelles
were found to be reduced by the cosolubilized b -carotene (Orr and Hogan 1985). The
reactive species was a nitroxide radical, as demonstrated by spin trapping technique
using electron spin resonance (ESR) (Piekarski et al. 1990). The broadening of the
asymmetrical ESR signal in the higher field together with a redshift of absorption
spectrum indicated that the nitrodiphenyl ether is dissolved near the core of SDS and
HDTMA Br micelles. Photodegradation of sulfonylurea herbicides having a high
water solubility is also accelerated by nonionic surfactants. Both Tween 80 and Triton
X-100 markedly enhanced the sunlight photodegradation of chlorimuron (22) and
metsulfuron (23) with a larger extent by the latter surfactant, possibly because of sen-
sitization of an aryl moiety of the surfactant (Thomas and Harrison 1990). Similar
photosensitization has been reported for rimsulfuron (29) in formulation (Scrano
et al. 1999). The effect of various adjuvants on the photostability of cyclohexene
oxime herbicides has been well examined from the aspect of preparing a stable for-
mulation (Falb et al. 1990; Hazen and Krebs 1992; McInnes et al. 1992). Factors
determining photostability of pesticides in formulation cannot be easily identified
because of confidentiality of the adjuvants, and either enhancement or retardation of
photodegradation in formulation was reported (Mekkaoui et al. 2000; Santoro et al.
2000; Wamhoff and Schneider 1997).

VI Effects on Mobility and Biodegradation in Soil

A Adsorption and Desorption

There are many possible processes involved in the adsorption and desorption of a
chemical to soil in the presence of surfactant (see Fig. 5). In the pore water of soil,
surfactant can be present as a monomer or micelle depending on its concentration.