Environmental Engineering Reference
In-Depth Information
making the study of chemicals, and the field of chemistry,
more quantitative. Indeed, the chemicals during combustion
were undergoing a combination with oxygen from the air.
Combustion, alas, was not a
release
of something to the
air but an
absorption
of oxygen from the air. This led
Lavoisier to develop the idea that respiration in animals
and plants was the interaction of atmospheric oxygen with
organic matter, and that both water and CO
2
were released.
He also made the connection that the process of the burning
candle was analogous to breathing by humans and animals, a
process that was driven by the blood. Indeed, our lives are
“burning” as much as is the wax of a candle. As such, he
provided the data necessary to extinguish the phlogiston
theory of combustion. Unfortunately, Lavoisier's own life
was extinguished in 1794 when he was executed by guillo-
tine upon being suspected of profiteering from association
with a tax-collecting agency.
The interface between plants and contaminated groundwa-
ter brings to light a few concepts with regard to the oxygen
cycle (Fig.
11.2
). As we will see in the next few sections,
minerals and nutrients rarely are cycled by themselves.
Rather, they are associated with oxygen and, therefore,
become soluble in water. Examples include nitrate and iron
oxyhydroxides. In addition, in pristine systems, the unsatu-
rated zone provides a source of atmospheric oxygen to plant
roots. Under steady-state conditions, the input of oxygen is
balanced by uptake during aerobic respiration. Even as trees
grow larger and the demand for oxygen increases, the plant
will survive as long as oxygen can reach the roots. If the
oxygen source is removed, however, by soil compaction or
if the water table rises and permanently floods the air-filled
pore spaces, oxygen consumption demands will exceed oxy-
gen supply and respiration will cease and roots will die.
Some plants, however, can handle oxygen limitations by
shunting oxygen to the roots. Plants still need to respire under
these anoxic conditions, and those that can transport oxygen
by diffusion from the atmosphere to the roots have a selective
advantage in environments where water is not limiting but
oxygen is. The cortex and aerenchymal tissues that have
interconnected pore spaces permit the diffusion of oxygen to
the rhizosphere to assist with the rate of respiration necessary
for cell growth. Again, as long as the delivery of oxygen is at a
rate equal to respiration, the plant will remain alive. These
anoxic conditions and the effect on plant survival must be
considered when planting a site where groundwater is known
to be anoxic, as is generally the case when petroleum
hydrocarbons have been spilled or released to the subsurface.
These rocks also contain almost 90% of the near-surface
store of oxygen in a form no longer available for respiration.
Carbon is contained in previously synthesized organic mat-
ter that was buried over time, such as fossil fuels (around
8
10
6
g/m
2
). From a biological standpoint, therefore, the
atmosphere and hydrosphere are the major sources, 4
10
3
10
3
g/m
2
, respectively, and sinks for CO
2
.In
each of these two compartments, carbon can take many
forms, such as calcium carbonate, CaCO
3
, in limestone,
crude oil, CO
2
in the atmosphere, or as bicarbonate in neutral
pH water. Unlike the hydrogen and oxygen in water, carbon
primarily is present as a gas.
Plants need inorganic carbon to synthesize organic
molecules, but this is only part of the story. These organic
molecules are needed to capture the solar energy for later
conversion into a source of energy to support plant metabo-
lism and growth. As stated previously, plants take in simple
CO
2
with H
2
O for use in synthesizing all the complex
organic compounds necessary for plant survival and repro-
duction. These reactions consist of a series of successive
reductions designed to generate the compound adenosine
triphosphate (ATP) a more useable and transferable source
of energy. When plants or the consumers of plants die and
are not rapidly buried, organic matter is mineralized to
CO
2
by decomposition, and what is not decomposed is
buried and the carbon thereby stored, especially under
anoxic conditions. This cycle between carbon production
and consumption or sequestration by the oceans is roughly
in balance, because the rate of cycling is fast, as is evident in
the relatively low concentration of CO
2
in air (only 0.032%
or 320 ppm).
The reaction of photosynthesis, as part of the carbon
cycle, was introduced briefly in Chap. 3. In this chapter the
complexities of photosynthesis will be explored, as will the
process of plant respiration. These details are important for
those interested in implementing phytoremediation at sites
characterized by contaminated groundwater, because photo-
synthesis supports plant growth, water use, and plant-detox-
ification reactions. Factors that affect photosynthesis and
water use, therefore, affect the efficiency and effectiveness
of a phytoremediation planting. This is similar to the imple-
mentation of an air-sparge system at some site with
contaminated groundwater, in which air is added to the
groundwater to remove and trap volatile contaminants.
Such a system would never be permitted to be implemented
unless data had been collected to support the soil's ability
to transfer gases to and from the contaminated zone.
Photosynthesis requires light energy that is then trans-
formed into chemical energy. Light required as the input of
energy to split water is called the light reaction
and 270
11.1.3 The Flow of Carbon
H
2
O
þ
ADP
þ
P
i
þ
NADP
Most carbon is stored, rather than flows, in the earth where
up to 6.6
!
O
2
þ
H
þ
ATP
þ
NADPH
(11.3)
10
7
g/m
2
of carbon is in rocks such as limestone.
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