Environmental Engineering Reference
In-Depth Information
The structure of surfactant sorbed to clay has been reported to change the
adsorption mechanism of pesticide to organoclay (Singh et al. 2004). Mont-
morillonite treated with phenyltrimethylammonium chloride (PTMA Cl) exhib-
ited the S-type isotherm in the adsorption of fenamiphos (ethyl 4-methylthio- m -tolyl
isopropylphosphoramidate), while linear adsorption was obtained for that with
HDTMA Br. The adsorption of the pesticide onto the clay surface modified with
PTMA Cl via side-by-side association is the controlling mechanism for the
former organoclay, but partition to the hydrophobic phase of HDTMA Br is the
controlling mechanism for the latter.
Interactions between pesticide and surfactant molecules have been investigated
in more detail for sulfonylurea herbicides. By separating the micelles and mono-
mers of octadecyltrimethylammonium bromide (ODTMA Br) with a dialysis bag,
Mishael et al. (2002, 2003) investigated the interactions of each species with
montmorillonite by X-ray diffraction and freeze-fracture electron microscopy. At
the lower concentrations, the monomers are adsorbed as bilayers lying parallel to
the clay interlayer, and its structure changed to pseudo-trimolecular layers as the
surfactant concentration increased, together with adsorption of the micelles onto
the clay surface. The interactions of sulfometuron (11) and sulfosulfuron (12) with
each surfactant species resulted in enhanced adsorption to the organoclay. The UV
absorption spectrum of sulfentrazone (56) in aqueous suspension of the same orga-
noclay has shown that (56) is present in a dissociated form and, therefore, the tight
association of (56) with ODTMA Br is likely to originate from both electrostatic
and hydrophobic interactions (Polubesova et al. 2003). The interactions of alachlor
(53) and the intercalated surfactant molecules have been studied using the infrared
difference spectrum (El-Nahhal et al. 1998). Enhancement of adsorption to
montmorillonite was minimal for the treatment of HDTMA Br, but the incorpora-
tion of benzyltrialkylammonium ion markedly increased the adsorption of (53)
because of the p - p interactions of the phenyl rings and hydrogen bonding via
the anilidic moiety. Similar interactions were reported for metolachlor (55)
(Singh 2006).
B Leaching
Adsorption and desoprtion are the determining processes for the distribution of
pesticide in soil and sediment systems, and the corresponding parameters such as a
K F value are estimated in an equilibrium state. However, the distribution of pesti-
cide in the real environment is highly dependent on either diurnal water movement
at a soil surface affected by precipitation or its continuous movement in a river
water-sediment system (Katagi 2004, 2006). To investigate the mobility of pesti-
cide in soil, a soil thin layer coated on a glass plate has been conveniently utilized,
but a soil column leaching study is more favorably applied to examine the possibility
of groundwater contamination by pesticide.
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