Environmental Engineering Reference
In-Depth Information
concept, an
acid
is a substance that has a tendency to lose a proton (H
+
), and conversely, a
base
is a substance that has a tendency to accept a proton. With this acid-base scheme, an
acid
is a
proton donor.
It is a
protogenic
substance. Similarly, a
base
is a
proton acceptor
, i.e., it is a
protophilic
substance. Water is both a
protophilic
and a
protogenic
solvent, i.e., it is
amphiprotic
in nature. It can act either as an acid or as a base. It can undergo self-ionization, resulting
in the production of the conjugate base OH
−
and conjugate acid H
3
O
+
. The self-ionization
of water is called
autoprotolysis.
Neutralization
is the reverse of autoprotolysis. Substances
that have the capability to both donate and accept protons such as water and alcohols are
called
amphiprotic
substances.
Chemical reactions in the porewater include (a) acid-base reactions and hydrolysis,
(b) oxidation-reduction (redox) reactions, (c) speciation and complexations. Acid-base
reactions and equilibrium in the porewater have important consequences on the partition-
ing and transport of contaminants in the soil. Acid-base reactions are
protolytic
reactions
resulting from a process called
protolysis
, i.e., proton transfer between a proton donor (acid)
and a proton acceptor (base).
To assess the bonding and partitioning relationships between heavy metals and soil sol-
ids, it is useful to use the Lewis (1923) concept of acids and bases. This concept deines an
acid as a substance that is capable of accepting a pair of electrons for bonding, and a base
as a substance that is capable of donating a pair of electrons. This means that
Lewis acids
are electron acceptors, and
Lewis bases
are electron donors. All metal ions
M
nx
are Lewis
acids. The Lewis acid-base concept permits us to treat metal-ligand bonding as acid-base
reactions. Hydrated metal cations can act as acids or proton donors, with separate
pk
val-
ues for each. The dissociation constant
k
is a measure of the dissociation of a compound.
This constant
k
is generally expressed in terms of the negative logarithm (to the base 10) of
the dissociation constant, i.e.,
pk =
−log(
k
). The smaller the
pk
value, the higher is the degree
of ionic dissociation, and the more soluble is the substance. A comparison of the various
pk
values between compounds will tell us which compound would be more or less soluble in
comparison to a target compound.
Oxidation-reduction reactions involve the transfer of electrons between the reactants
and the activity of the electron e
−
in the chemical system plays a signiicant role. There is
a link between redox reactions and acid-base reactions since the transfer of electrons in
a redox reaction is accompanied by proton transfer. Redox reactions involving inorganic
solutes result in a decrease or increase in the oxidation state of an atom. Organic chemi-
cal contaminants, meanwhile, show the effects of redox reactions through the gain or loss
of electrons in the chemical. Biotic redox reactions are of greater signiicance than abiotic
redox reactions. These reactions are signiicant factors in the processes that result in the
transformation, persistence, and fate of organic chemical compounds in soils.
The stability of inorganic solutes in the porewater is a function of such factors as pH, the
presence of ligands, temperature, concentration of the inorganic solutes, and the
Eh
or
pE
of the porewater.
Eh
is the redox potential and
pE
is a mathematical term that represents
the negative logarithm of the electron activity e
−
. The redox potential
Eh
is a measure of
electron activity in the porewater, and is described by the following relationship:
=+
a
a
RT
nF
0
iox
,
Eh
E
ln
,
(9.2)
i red
,
where
E
0
is the standard reference potential,
n
is the number of electrons,
R
is the gas con-
stant,
T
is the absolute temperature,
F
is the Faraday constant,
a
i
is the activity of the
i
th
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