Environmental Engineering Reference
In-Depth Information
Large concentrations of humic materials found in natural water can
complex with iron so successfully that they make iron unavailable to or-
ganisms. Thus, some dystrophic systems are
unproductive not only because of high con-
centrations of humic materials that absorb
light but also because they make iron less
available. At intermediate concentrations,
these chelators make iron more available by
interfering with precipitation, but they are
detrimental at high concentrations.
Ferric iron can also precipitate phos-
phate, as mentioned previously. These floc-
culant precipitates will settle to the sedi-
ments in an oligotrophic lake. If the
hypolimnion is anoxic, the FePO
4
precipi-
tate will dissociate into phosphate and fer-
rous iron. This process is a key interaction
between elemental cycles and has a major
impact on algal production.
Iron pyrite forms when ferrous iron re-
acts with sulfide. This precipitate has lim-
ited solubility and forms black deposits in
anoxic sediments. Thus, in wetlands or lakes
with O
2
in the water column immediately
above the sediments, the first few centime-
ters of sediment are light colored, but as the
deeper anoxic portions are reached the dark
pyrites color the sediments dark brown or
black. This reaction and its indirect media-
tion of the phosphorus cycle have been
known to alter management of nutrient pol-
lution (Sidebar 13.3).
Ferrous iron has potential energy in the
presence of O
2
and can serve as an energy
source for microbes able to oxidize it before
spontaneous conversion occurs. This reaction
is confined to areas with O
2
that are close to
an anoxic habitat that provides ample iron in
the reduced (ferrous) form. This oxidation is
one route for dissociation of iron pyrite (FeS)
in oxic environments.
Iron-oxidizing bacteria can cause prob-
lems in groundwater wells in anoxic habi-
tats high in iron. In these situations, O
2
dis-
solved in the well water moves down the
well. The aquifer immediately outside the
bottom of the well has a zone in which O
2
and Fe
2
are found together. The iron oxi-
dizers can utilize this potential energy source
to incorporate CO
2
and grow. Bacterial
Sidebar 13.3.
Sulfur and Iron Dynamics Increase
Wetland Phosphorus and Associated
Growth of Noxious Plants
Large areas of The Netherlands are composed
of peaty lowlands. These wetland areas are fed
with river water to compensate for groundwater
withdrawals for agriculture (Smolders and
Roelofs, 1993). The inflow of river water was hy-
pothesized to cause deterioration in water qual-
ity (increases in turbidity, filamentous algae, and
duckweed) and a shift away from characteristic
emergent wetland plants. Attempts were made
to halt this deterioration by stripping incoming
river water of phosphate, but the water quality
did not improve.
Further investigation revealed that the sulfate
and iron contents in the river water were alter-
ing the biogeochemical cycling. The river water
entering the wetlands was stripped of phos-
phate, high in sulfate, and low in iron. The sul-
fate diffused into the anoxic sediments where it
was reduced by microbes to sulfide. Increased
sulfide concentrations in the anoxic sediments
led to greater precipitation of ferrous iron as iron
pyrite into the sediments. Because the iron was
removed from the system, it could no longer pre-
cipitate with phosphate in the oxic waters of the
wetland. The lowered amount of phosphate pre-
cipitation allowed higher concentrations of phos-
phorus to be maintained in the surface waters
and led to stimulation of algal growth. Concur-
rent increases in the sulfide levels in the pore
waters combined with low iron concentrations
were hypothesized to harm the native macro-
phytes via sulfide toxicity and growth limitation
by low iron.
This European case study is an excellent
example of the importance of understanding
nutrient cycling and how interactions among
the cycles influence primary producers. The
management of the system appeared to be sim-
ple, but complex interactions among nutrient
cycles conspired against the feasibility of a
“dilution solution to pollution.”
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