Geoscience Reference
In-Depth Information
Table 13.1 Concentration of selected organic and inorganic species in interstitial water, com-
pared to river water and seawater (Wolfe et al.
1990
)
Species
World average (mol/L)
Interstitial water
River
Seawater
2.00 9 10
-4
Ammonia
9.09 9 10
-3
9.78 9 10
-4
2.33 9 10
-3
Carbonate
2.00 9 10
-4
1.00 9 10
-4
5.00 9 10
-6
Fulvic acid
1.80 9 10
-5
2.84 9 10
-6
Phosphate
3.96 9 10
-7
Sulfide
2.18 9 10
-4
1.03 9 10
-4
Silicate
4.10 9 10
-4
Borate
13.1.2 Reactions in Natural Waters
Hydrolysis is a bond-making-bond-breaking process, which may be described by
the equation
RX
þ
H
2
O
!
ROH
þ
H
þ
þ
X
ð
13
:
3
Þ
where R denotes an organic moiety. The general expression for the observed
global rate constant K
obs
is given by
K
obs
¼ K
H
½H
þ
þ
K
OH
½OH
þ
K
w
þ
X
i
ð
K
HA
½HA
þ
K
A
½A
Þ
i
;
ð
13
:
4
Þ
where K
H
and K
OH
are specific acid-base catalyzed second-order rate constants; K
w
is the neutral hydrolysis rate constant; K
HA
and K
A
are, in general, acid-base
catalyzed rate constants; and the summation is over all components i. In Eq. (
13.4
),
[H
+
] and [OH
-
] are the hydrogen and hydroxyl ion concentrations (activities),
respectively, while [HA] and [A
-
] are, respectively, the concentrations (activities)
of acid and bases in the reaction mixture. The term K
obs
can include contributions
from acid-or base-catalyzed hydrolysis, nucleophilic attack by water, or catalysis
by buffers in the reaction medium.
Abiotic hydrolysis of pollutants in subsurface waters is pH dependent. The
predominant pathways are acid-catalyzed, base-mediated, and neutral (pH-inde-
pendent) hydrolysis. The acid-catalyzed hydrolysis reaction rate is dependent on
proton concentration increases with a decrease in pH. This behavior occurs
because the proton is not consumed in the reaction.
Increases in pH as a direct proportional augmentation of the hydroxyl ion
activity leads to a base-mediated hydrolysis process. In this case, the hydroxyl
behaves as a nucleophile and is consumed in the reaction. Neutral and alkaline
hydrolysis are the most frequent reactions over the common environmental pH
ranges. The relation between first-order hydrolysis rate constants and the pH often
is presented as a pH rate profile (Wolfe et al.
1990
).