Agriculture Reference
In-Depth Information
liquid phase is limiting; if it is greater than about 500, transport in the gas phase
is limiting.
However, this simple picture only applies to gases that do not undergo reactions
in the boundary layers. For gases that do react, for example through hydration
and acid-base reactions, the net flux depends on the simultaneous movement
of all the solutes involved, and the flux will not be the simple function of con-
centration expressed in Equation (3.25). An example is CO
2
, which reacts with
water to form carbonic acid and carbonate species-H
2
CO
3
,
HCO
3
−
and CO
3
2
−
.
The situation is complicated because the exchange of H
+
ions in the carbonate
equilibria results in a pH gradient across the still layer, and it is therefore nec-
essary to account for the movement of H
+
ions across the still layer as well as
the movement of carbonate species. The situation is further complicated in the
case of CO
2
by the kinetics of hydration and dehydration, which may be slow in
comparison with transport.
3.4.1 CO
2
TRANSFER ACROSS THE AIR-WATER INTERFACE
Under equilibrium conditions, the bulk of the dissolved CO
2
is present as HCO
3
−
or CO
3
2
−
or both if the pH is greater than about 6. Therefore, where a gradi-
ent of CO
2
concentration exists across a solution, the net flux of CO
2
will be
greatly increased if there is rapid equilibration between the dissolved CO
2
and
carbonate species. Consequently, most plants and animals have evolved enzyme
systems to catalyse the hydration-dehydration equilibria and the enzyme respon-
sible—carbonic anhydrase—is present in most plant and animal cells. It is
likely that this enzyme will often be present extracellularly in natural waters.
This is because many aquatic plants use HCO
3
−
for photosynthesis under low
CO
2
conditions by catalysing the conversion of HCO
3
−
to CO
2
outside the
plasma membranes of leaf cells. The mechanism involves catalysis by extra-
cellular carbonic anhydrase in conjunction with H
+
extrusion across the plasma
membrane (Graham
et al
., 1984; Tsuzuki and Miyachi, 1989). Since at least some
forms of the enzyme are soluble, appreciable concentrations should arise in the
water under intense algal growth, though the stability of the enzyme under high
light and O
2
conditions is unknown. The presence of carbonic anhydrase or
similar enzymes catalysing CO
2
hydration has been demonstrated in seawater
with corresponding differences in rates of CO
2
exchange (Berger and Libby,
1969).
The following calculations show the range of effects from infinitely slow hydra-
tion-dehydration to infinitely fast. Emerson (1975) and Kirk and Rachhpal-Singh
(1992) and have made calculations allowing for the kinetics of the uncatalysed
hydration-dehydration reactions, giving intermediate results.
We have for the flux of CO
2
across the still air layer an equation of the type
D
G
δz
G
(C
G
−
C
L0
/H )
F
G
=−
(
3
.
33
)
