Geoscience Reference
In-Depth Information
OH
ð
dP
=
dt
Þ
¼ k OC
½
½
ð
13
:
1
Þ
where P is the product, k denotes a second-order rate constant, [OC] is the organic
contaminant concentration and [OH
-
] is the hydroxide ion concentration (see also
Sect. 13.1.2
). Subsurface waters contain organic and inorganic species
(Table
13.1
), which, as a function of their character, affect hydrolysis reactions in
different ways. The presence of carbonate, fulvic acid, and silicate favor acid-base
reactions in aerobic waters. In anaerobic interstitial waters, these reactions are
induced by the presence of ammonia, carbonate, fulvic acid, phosphate, and sul-
fides. Both dissolved and suspended organic material can enhance or impede
hydrolysis of selected organic contaminants. For example, dissolved fulvic acids
accelerate atrazine hydrolysis, while humic acids retard alkaline hydrolysis of the
hydrophobic n-octyl ester of 2, 4-D herbicide (Perdue and Wolfe
1983
).
Metal ions in aerobic, natural waters, such as Cu
2+
,Fe
3+
,Mn
2+
,Mg
2+
, and
Ca
2+
, may catalyze hydrolysis of organic contaminants. Blanchet and St. George
(
1982
), for example, showed that interaction of organophosphate esters with Cu
2+
and Mn
2+
led to the hydrolysis of pesticides. However, similar studies with Mg
2+
and Ca
2+
did not induce any transformation process.
The main environmental factors that control transformation processes are
temperature and redox status. In the subsurface, water temperature may range from
0 C to about 50 C, as a function of climatic conditions and water depth. Gen-
erally speaking, contaminant transformations increase with increases in tempera-
ture. Wolfe et al. (
1990
) examined temperature dependence for pesticide
transformation in water, for reactions with activation energy as low as 10 kcal/
mol, in a temperature range of 0-50 C. The results corresponded to a 12-fold
difference in the half-life. For reactions with an activation energy of 30 kcal/mol, a
similar temperature increase corresponded to a *2,500-fold difference in the half-
life. The Arrhenius equation can be used to describe the temperature effect on the
rate of contaminant transformation, k:
k ¼ Ae
Ea
=
RT
;
ð
13
:
2
Þ
where Ea is the activation energy for the reaction, R is the gas constant, T is the
absolute temperature, and A is a constant.
The redox potential in subsurface water varies with alterations from aerobic to
anaerobic conditions. In and around anaerobic environments, conditions for
reduction exist and contaminants are transformed accordingly. Under aerobic
conditions, O
2
is the predominant oxidation agent (mainly through biological
processes), because the transformation of contaminants is mainly through oxida-
tive pathways. Aerobic and anaerobic states may occur both in surface waters and
in deeper subsurface water.
