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 ).
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