Geoscience Reference
In-Depth Information
Both inorganic (e.g., metals) and organic substances may be subject to a
hydrolysis reaction in waters. Examples of several hydrolyzable functional groups
are given in Table
13.2
. Water is a weak acid and the acidity of the water mol-
ecules in the hydration shell of a metal ion usually is greater than that of the water.
The acidity of aqueous metal ions is expected to increase with a decrease in the
radius and an increase in the charge of the central ion. In the case of Fe(III), for
example, hydrolysis can extend beyond the uncharged species Fe(OH)
3
(H
2
O)
3(s)
,
to form anions such as ferrate (FeO
4
2-
). All hydrated ions, in principle, can donate
a larger number of protons than that corresponding to their charge and can form
anionic hydroxo-metal complexes (Stumm and Morgan
1996
).
The rate of hydrolysis of an organic contaminant also may be affected by the
pH, due to specific acid-base effects or changes in compound speciation. A change
in the pH can shift the equilibrium in favor of the charged or uncharged species,
which often have different hydrolysis rate constants. Under drastic reaction con-
ditions (i.e., extremely low or high pH, high temperature), many of the major
organic contaminants, such as pesticides, undergo hydrolysis. Functional groups of
organic substances susceptible to hydrolysis include carboxylic acid esters, orga-
nophosphates, amides, anilides, carbamates, triazines, oximes, and nitriles. The
role of hydrolysis in the overall transformation process, however, depends on the
rate of other degradation processes that may occur simultaneously, such as pho-
tolysis, biolysis, or redox reactions. If, in the liquid phase, additional organic or
inorganic chemicals of natural or anthropogenic origin are present, contaminant
hydrolysis could be affected by the presence of other solutes. This is the case of
dissolved metals or humic acids acting on the hydrolysis of organic toxic elements
present in the same water phase. This effect, however, has only minor significance.
Perdue and Wolfe (
1983
) considered the maximum predicted buffer-catalysis
contribution to be B10 % of the uncatalyzed process.
Redox reactions in natural waters involve the transfer of electrons between
chemical species or changes in the oxidation state of species involved in the
reaction. Specifically, oxidation describes the loss of electrons by a molecule,
atom, or ion, while reduction describes the gain of electrons by a molecule, atom,
or ion. Therefore, oxidation is defined as an increase in oxidation number, while
reduction is defined as a decrease in oxidation number. Differences in the oxi-
dation states of natural waters generally exist between surface and ground waters,
between locally inter aggregate stagnant and flowing waters, or in stagnant waters
obstructed by biological or vegetative cover.
Reductants and oxidants are defined as electron donors and proton acceptors
(
Sect. 2.2.2
). Because there are no free electrons, every oxidation is accompanied
by a reduction and vice versa. In aqueous solutions, proton activities are defined by
the pH:
pH ¼
log½H
þ
:
ð
13
:
5
Þ
Similarly, we can define a convenient parameter for the redox intensity:
