Environmental Engineering Reference
In-Depth Information
The concentration of surfactant would be mostly below its cmc in the aquatic envi-
ronment, and the effect of micelles on hydrolysis of a pesticide is generally of
minor importance, except with a limited area of water after a runoff event or a sew-
age treatment plant. There are many excellent reviews of the effects of micelles on
hydrolysis of many kinds of organic chemicals (Bunton and Savelli 1986; Bunton
et al. 1991; Cordes and Gitler 1972; Fendler and Fendler 1975; Ta¸cio˘ lu 1996).
A Mechanism
Abiotic hydrolysis of pesticides in water mostly obeys first-order kinetics, and the
observed hydrolysis rate (k
obs
) can be expressed by summing the reaction rates of
specific acid-catalyzed, neutral, and specific base-catalyzed hydrolysis (Katagi
2002). As hydrolysis is a bimolecular reaction between the pesticide molecule and
reactants such as H
2
O, H
+
, and OH
−
, both the solubilization site of pesticide in the
micelles and the electrostatic field generated by the ionic head groups on the micel-
lar surface are considered to significantly affect the reaction kinetics. The concen-
tration and medium effects are known for reactions in aqueous micellar systems
(Ta¸cio˘lu 1996). The former effect originates from either solubilization of a noni-
onic reactant in micelles or electrostatic attraction of an ionic reactant to the micel-
lar surface with an opposite charge. In practicice, many pesticides are considered to
be located in the Stern layer; the extent of the reactant accessing this region would
determine if the micellar reaction is catalyzed or inhibited. The latter effect consists
of many factors. The cage effect forces reactants in the neighboring space and
enhances a reaction probability. Preorientation effect means the specific orientation
of a solubilizate in micelles and may control regio- and stereoselectivity, together
with increased microviscosity in micelles. Furthermore, a transition state having a
partially ionic character may be stabilized in micelles.
To describe reactions in the micellar system, many kinds of kinetic approaches,
including a Hill model for an enzymatic reaction (Piszkiewicz 1977), have been exam-
ined. In general, by assuming the two-phase model in Fig. 7, the observed pseudo-first-
order rate constant (k
obs
) can be expressed below (Fendler and Fendler 1975):
k
obs
= {k
w
+ k
m
.
K
s
([S] − cmc)} / {1+
K
s
[S] − cmc)}
The rate constants k
w
and k
m
are the first-order constants in aqueous and micellar
phases, respectively. [S] is a total concentration of surfactant, and
K
s
is a micelle-substrate
partition constant. For the reactions in charged micelles whose surface is taken as
a selective ion exchanger, the pseudo-phase ion-exchange (PPIE) model has been
successfully applied by assuming that a fraction of the surface (
b
) occupied by the
counter ions is constant (Bunton and Savelli 1986; Bunton et al. 1991). Ion-
exchange constant (
K
N
X
) and
b
are defined below:
K
N
X
= [N
w
][X
m
] / [N
m
][X
w
]
b
= ([N
m
] + [X
m
]) / ([S] − cmc)
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