Environmental Engineering Reference
In-Depth Information
As with other cycles, the phosphorus cycle has been altered as a result of fertilization with
phosphorus that has been mined from concentrated phosphate rock deposits. This has created
two major concerns:
1. Phosphorus enrichment in freshwater bodies with the subsequent excessive growth of
plants, algae, and other organisms and the disruption of ecosystems.
2. The depletion of concentrated phosphate rock deposits, which is not sustainable in the
long run (see Chapter 3).
The sulfur cycle
Sulfur is widely distributed in the lithosphere and the hydrosphere primarily in the form of
sulfate (SO
4
=
), and combined to metals in the form of sulfide (S
=
) in soils and rocks. In a
preindustrial era, mobilization of sulfur occurred primarily as a result of weathering of rocks
and soils, volcanic eruptions, production of volatile sulfur by biota, and the dispersion by the
atmosphere of sea spray. The weathering process, which is connected to the hydrologic cycle,
is the responsible for mobilizing sulfur in the form of sulfates to rivers that eventually transport
the sulfur to the oceans (Turner et al., 1991).
In oceans, reducing bacteria transforms sulfates into insoluble sulfides, which precipitate to
the sediments. Sulfide compounds, along with sulfur-containing organic matter, are then taken
deeper into the crust by subduction and eventually returned to the atmosphere by volcanism in
the form of sulfur dioxide (SO
2
) or by uplifting and weathering. Another route that mobilizes
sulfur from oceans to the atmosphere is through the formation of dimethyl sulfide (DMS), a
volatile compound formed by phytoplankton by attaching to methyl groups to a sulfur atom
(Andreae et al., 1985). Most of the DMS remains in the ocean, but some escapes to the atmosphere.
Sulfate is an essential nutrient for plants that is taken from the soil. When organisms die,
decomposition and other processes release sulfur from organic molecules and return sulfate
into the ground. In soils, as well as in the sediments of freshwater ecosystems, sulfates can be
reduced into sulfides in a reversible reaction. Sulfides can be converted into SO
2
and transferred
to the atmosphere.
Human activities have impacted the sulfur cycle by adding extra sulfur to the system as a
result of burning sulfur-containing fossil fuels and mining concentrated sulfur deposits. The
result is the emission of sulfur dioxide that is first transformed into sulfur trioxide (SO
3
) and
after the addition of a water molecule, it turned into sulfuric acid (H
2
SO
4
) and sulfate ions:
SO
+
½ O
→
SO
[2.12]
2
2
3
SO
+
H O
→
H SO
[2.13]
3
2
2
4
Sulfuric acid and sulfate ions adsorbed into particles are returned to the land in the form of
wet or dry depositions, collectively designated as acid rain. Acid rain affects soils, ecosystems,
forests, human health, and man-made structures.
IMPORTANCE OF EARTH'S ECOSYSTEMS
Natural ecosystems
An ecosystem consists of the combination of a biological community, also known as the biotic
factor (e.g., plants, animals, fungi, or bacteria), and an abiotic factor, such as a mountain, a
lake, a pond, a river, a wetland, or a desert, and is considered a unit. Both the biotic and abiotic
