Geoscience Reference
In-Depth Information
Fig. 8.3 Ammonia-N
volatilization as a function of
time from sewage sludge
applied to soils at 0, 0.01, and
1.5 MPa, and air-dry initial
moisture levels (Donovan and
Logan
1983
)
8.2.1 Inorganic Contaminants
Ammonia volatilization illustrates the behavior of inorganic chemicals in the
subsurface under aerobic or anaerobic conditions. It is recognized that ammonia
volatilization is affected by the time and depth of release, pH, temperature, and
moisture content as well as by the cation exchange capacity (CEC).
The relative concentrations of NH
3
and NH
4
+
in an aqueous solution are pH
dependent, in accordance with the following reaction equilibrium:
NH
4
þ
NH
3
ð
aq
Þ
þ
H
þ
ð
8
:
1
Þ
½NH
3
ð
aq
Þ
½H
þ
=
½NH
4
þ
¼K ¼ 10
9
:
5
ð
8
:
2
Þ
¼
9
:
5
þ
pH
;
log ½NH
3
ð
aq
Þ
=
½NH
4
þ
ð
8
:
3
Þ
where NH
3(aq)
is NH
3
in solution. According to these equations, the concentrations
of NH
3(aq)
at pH 5, 7, and 9 are 0.0036, 0.36, and 36 %, respectively, of the total
ammonia-N in solution.
Loss of NH
3
from calcareous subsurface media is considerably greater than
from noncalcareous media and involves production of (NH
4
)
2
CO
3
or NH
4
HCO
3
.
The fate of (NH
4
)
2
SO
4
added to calcareous material is expressed by the reaction
between (NH
4
)
2
SO
4
and CaCO
3
, and as a result, volatile ammonia is produced
according to the equation
CaCO
3
þð
NH
4
Þ
2
SO
4
2NH
3
þ
CO
2
þ
H
2
O
þ
CaSO
4
ð
solid
Þ
:
ð
8
:
4
Þ
A similar pattern of ammonia formation and volatilization occurs when sewage
effluent is disposed on the land surface or in sludge-enriched soils. Donovan and
Logan (
1983
) showed that ammonia loss increases with increasing pH and tem-
perature and is affected by the type of sludge added to the land surface. Loss from
air-dry soil is much lower than from a soil at moisture tension \1.5 MPa, the
kinetics of volatilization with time being depicted in Fig.
8.3
. Donovan and Logan
