Geoscience Reference
In-Depth Information
Catalysis by proton transfer is significant in the subsurface and associated
environment and is common in homogeneous reactions. The strength of the acid or
base is determined by the ionization constant, while its efficiency as a catalyst is
controlled by the reaction rate. This relation, known as the Brönsted catalysis law,
is expressed as
k
a
¼
C
A
K
a
k
b
¼
C
B
K
b
;
ð
14
:
1
Þ
where k
a
and k
b
are the rate constants (catalytic constants) for acid and base
catalytic reactions, k
a
and k
b
are the acid and base ionization constants, and C
A
, C
B
,
a, and b are constants representing characteristics of the reactions, the solvent, and
the temperature, respectively.
Normally a and b are positive with values between 0 and 1. Low values reflect
low sensitivity of the catalytic constant to the strength of the catalyzing acid or
base, while high a and b values indicate an inverse catalytic pathway. In acid
catalysis of organic molecules, the proton located on negatively charged molecules
reduces the negative charge, so that the transfer of electrons is facilitated. It can be
assumed that, under such conditions, a metal ion that generally acts as an acid will
form a metal-organic complex, which reduces the negative charge and enhances
the electron transfer. Unlike a proton, the metal ion can be stabilized by other
ligands. Some metal ions, especially of the transition series, have several stable
oxidation states that enable them to act as catalysts in redox reactions; these ions
can catalyze a wide variety of transformation reactions of organic and inorganic
contaminants (Huang
2000
).
14.2 Surface-Induced Transformation of Organic
Contaminants
The spatial distribution of ions and their charge are affected strongly by the electric
field emanating from charged surfaces. As a result, some organic contaminants in
direct contact with these surfaces can undergo transformation by catalytic processes.
Clay minerals behave like Bronsted acids, donating protons, or as Lewis acids
(
Sect. 6.3
), accepting electron pairs. Catalytic reactions on clay surfaces involve
surface Bronsted and Lewis acidity and the hydrolysis of organic molecules, which
is affected by the type of clay and the clay-saturating cation involved in the
reaction. Dissociation of water molecules coordinated to surface, clay-bound
cations contributes to the formation active protons, which is expressed as a
Bronsted acidity. This process is affected by the clay hydration status, the polar-
izing power of the surface bond, and structural cations on mineral colloids
(Mortland
1970
,
1986
). On the other hand, ions such as Al and Fe, which are
exposed at the edge of mineral clay colloids, induce the formation of Lewis acidity
(McBride
1994
).
