Surfactant Effects on Environmental Behavior of Pesticides - Reviews of Environmental Contamination and Toxicology - page 130

Helling (1971) reported increase of R F values for several pesticides in thin-layer
chromatography (TLC) using silty clay loam soil when developed by a 0.01% - 1%
aqueous solution of Tween 20 and 80 surfactants. Similar enhanced movement has
been reported by Foy (1992), but even 10% Tween 80 in a mobile phase was found
not to change the R F values of pesticides including atrazine (13) and trifluralin (15)
in the case of organic soil. The solubilization of pesticide in micelles is considered
to restrict the interactions between pesticide and soil surface, which might be in
accordance with the larger enhancement of mobility by using nonionic surfactants
having the longer ethoxylate unit (Steurbaut 1994). The presence of HDTMA Br in
a mobile phase reduced the movement of organophosphorus pesticides in soil TLC
by solubilization of the pesticides into admicelles and hemimicelles sorbed on soil
via an ion-exchange mechanism (Sharma et al. 1985). When soil treated with
HDTMA Br was used for preparation of soil TLC, a higher effect was observed
(Sánchez-Camazano et al. 1995). In contrast, the usage of SDS either in preparation
of soil TLC or as a mobile phase greatly increased the R F values of pesticides,
which was accounted for by solubilization into the anionic micelles less adsorbed
onto the soil surface because of electrostatic repulsion. Singh and Kumar (2000)
investigated concentration dependency of the surfactant effect in soil- and water-
treated systems. Generally, at concentrations lower than cmc both nonionic and
cationic surfactants reduced the mobility of pesticide by adsorption to sorbed sur-
factants or higher soil dispersion, but at above cmc they increased it by solubiliza-
tion to micelles in a mobile phase.
The effects of adjuvants including surfactants on pesticide mobility in labora-
tory and field column leaching studies are summarized in Table 9. A glass,
acrylic, or stainless steel cylinder with an internal diameter of 5-10 cm and a
length of 10-50 cm is packed with soil and the top of the soil column is treated
with a pesticide or its formulation. The column is eluted by water or 10 mM
CaCl 2 solution and the eluate is periodically analyzed. After elution, the soil
column separated into several sections is extracted and analyzed to examine the
distribution of pesticide and its metabolites. The elution pattern, or breakthrough
curve (BTC), is illustrated in Fig. 9. A tracer species such as chloride ion not
retained in a soil column is used to estimate a pore volume (PV) (Fig. 9a). When
pesticide moves through the column via adsorption/desorption process, the elu-
tion peak is delayed (Fig. 9b). Adjuvants possibly affect both the peak position
and the eluted amount (Fig. 9c,d). Bayer (1967) examined the effects of 23
surfactants at 1% and 10% concentrations on the mobility of four urea herbi-
cides in the soil column eluted by a simulated rainfall. As reported by soil TLC
studies, both nonionic and anionic surfactants mostly increased the downward
mobility of pesticide and some cationic ones markedly lessened the mobility.
The concentration effect of surfactants in a mobile phase on the leaching of pes-
ticide was found to be similar to those in soil TLC. The distribution of lindane
(34) and diuron (41) in the soil column exhibited maxima at shallower depths
when nonionic alcohol ethoxylates were used at lower concentrations, whereas
the higher concentrations resulted in deeper distribution of the pesticides without
clear maxima (Huggenberger et al. 1973).