Environmental Engineering Reference
In-Depth Information
11.1.7 The Flow of Sulfur
Phreatophytes that may have part of their root systems in
the water table or capillary fringe may already encounter
ferrous iron in anoxic pore or groundwater, especially at
contaminated sites. This can cause the pore-water concen-
tration of Fe(II) to increase (Jones and Etherington 1970). In
this case, a plant can receive all of the iron it needs without
having to resort to Strategy I or II. If the water-logged
condition is too long, however, Fe(II) concentrations can
reach toxic levels. Although this increases iron availability,
root respiration is limited by a lack of oxygen. In swamps,
this lack of oxygen is overcome, however, by plant develop-
ment of extensive aerenchymal tissues that act as “pipes” to
permit the diffusion of atmospheric oxygen into the plants
that then moves by diffusion into the subsurface, as stated
earlier. The presence of this additional oxygen, however,
tends to, ironically, oxidize the reduced iron, rendering the
originally available Fe(II) unavailable.
The entry of oxygen not only supplies the respiration
requirement but also may be a process that controls ferrous
iron uptake to levels that are not toxic. It also is possible that
excessive ferrous iron is dealt with by differential compart-
mentalization to various plant tissues after uptake, with the
goal of not interfering with the manufacture of chlorophyll.
Comparatively less information is known about the trans-
location of iron from root to shoot after uptake, although iron
does make its way to the xylem by way of the symplastic
pathway and its barrier the Casparian strip. Iron tends to
accumulate in plant leaves rather than in roots. Chloroplasts
contain 80% iron, which is required to synthesize chloro-
phyll and act in proteins in the electron-transport pathway.
Iron is believed to be translocated in plants in the form of
ferric citrate. Tannic acid can inhibit iron absorption through
binding. This may be because Fe(II) if reacted with H
2
O
2
can produce the highly toxic hydroxyl radical (OH). This
reaction, discovered by Fenton in 1876 (Wardsman and
Candeias 1996), is called the Fenton reaction, and is widely
used in the groundwater remediation industry to rapidly
oxidize dissolved-phase organic contaminants.
Finally, Guerinot and Salt (2001) made the observation
that the same process of enhanced iron uptake could be
beneficial not only for phytoremediation but for food crops
as well. The common denominator is that each process is the
result of metal uptake by plants. The redox status of soil is
determined in part by its water content, and this affects plant
health and survival. High concentrations of dissolved iron
generally, as we will see in Chap. 13, are found in ground-
water at sites where reduced organic matter, such as gasoline
or jet fuel, has been released. Conversely, high Fe(II)
concentrations can be found near swamps and forests with
copious amounts of natural organic matter in flood plains. If
present in groundwater, dissolved iron itself can become a
contaminant if levels exceed the National Secondary Drink-
ing Water Standard (NSDWS) MCL for iron.
Like iron, elemental sulfur is an important element for plant
growth (Ernst 2004). Sulfur is necessary for the synthesis of
plant proteins and coenzymes. The sources of most sulfur
used by plants are sulfur-containing rocks and soils, and the
ocean. It is transferred to plants through the atmosphere and
hydrologic cycle. Elemental sulfur cannot directly be used
by plants, however, unless it is first oxidized to sulfate
(SO
4
2
). Plants that contain higher amounts of sulfur as
sulfate tend to also have higher protein contents. In its
reduced form, however, sulfur can be toxic to plants. Ele-
mental sulfur also can be reduced under anoxic conditions to
H
2
S by bacteria such as
Desulfuromonas acetoxidans
.
Sulfur in its dissolved and reduced form as sulfide can
inhibit plant growth. For example, Bradley and Dunn (1989)
reported that the biomass and height of
Spartina alterniflora
were inhibited by hydrogen sulfide concentrations at 1 mM.
The authors clearly point out, however, that this concentra-
tion of dissolved sulfide was one of many possible factors
that can affect such growth in the field. It is important to note
that H
2
S can support chemolithotrophic microbes that oxi-
dize the H
2
S back to elemental S, such as is done by
Beggiatoa
. This S can be oxidized further to SO
4
by
Thiobacillus
.
11.1.8 The Flow of Potassium
Elemental potassium is used by plants to control the intra-
cellular movement of water. Like phosphorus, potassium is
present in large amounts in soil, but typically is present in the
unavailable form, being sorbed onto clay particles in the soil
or aquifer sediments.
11.2
Plants and Natural Chemical
Compounds
The physician Paracelsus (1493-1541) stated that “all
substances are poisons; there is none which is not a poison,
and it is the dosage that differentiates a poison from a
remedy.” That a physician made this statement so long ago
may at first seem surprising but is actually a logical source
for such a comment. Early medicinal practices recognized
that plant-based herbal and chemical approaches to health
restoration were based on providing
small
amounts of the
same compound that had been observed to have lethal
consequences in larger doses. The same goes with other
compounds and their effects on living organisms. Chemicals
can have negative effects on living organisms; it is the dose,
however, that determines the exposure risk.
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