Agriculture Reference
In-Depth Information
A reduced form of sulfur that is highly toxic and an important pollutant is
hydrogen sulfide (H
2
S). Certain microbes, especially bacteria, reduce nitrogen
and sulfur, using them as energy sources through the acceptance of electrons.
For example, sulfur-reducing bacteria can produce hydrogen sulfide (H
2
S) by
chemically changing oxidized forms of sulfur, especially sulfates (SO
4
). To
do so, the bacteria must have access to the sulfur; that is, it must be in the
water, which can be surface water, groundwater, or the water in soil and
sediment. These sulfur reducers are often
anaerobes
, bacteria that live in water
where concentrations of molecular oxygen (O
2
) are deficient. The bacteria
remove the O
2
molecule from the sulfate, leaving only the sulfur, which
in turn combines with hydrogen (H) to form gaseous H
2
S. In groundwater,
sediment, and soil water, H
2
S is formed from the anaerobic or nearly anaerobic
decomposition of deposits of organic matter (e.g. plant residues). Thus, redox
principles can be used to treat H
2
S contamination; that is, the compound
can be oxidized using a number of different oxidants (see Table B3.1). Strong
oxidizers such as molecular oxygen and hydrogen peroxide oxidize the reduced
forms of sulfur, nitrogen, or any reduced compound most effectively.
Table B3.1
Theoretical Amounts of Various Agents Required to Oxidize 1 mg L
−
1
of Sulfide Ion
Amount (mg L
−
1
) Needed to
Oxidize 1 mg L
−
1
of S
2
−
Theoretical
Oxidizing Agent
(based on practical observations)
Stoichiometry (mg L
−
1
)
Chlorine (Cl
2
)
2.0-3.0
2.2
Chlorine dioxide (ClO
2
)
7.2-10.8
4.2
Hydrogen peroxide
(H
2
O
2
)
1.0-1.5
1.1
Potassium permanganate
(KMnO
4
)
4.0-6.0
3.3
Oxygen (O
2
)
2.8-3.6
0.5
Ozone (O
3
)
2.2-3.6
1.5
Source
: Water Quality Association, Ozone Task Force Report, “Ozone for POU, POE and small water
system applications,” WQA, Lisle, IL, 1999.
Ionization is also important in environmental reactions. This is due to the
configuration of electrons in an atom. The arrangement of the electrons in
the atom's outermost shell (i.e., valence) determines the ultimate chemical
behavior of the atom. The outer electrons become involved in transfer to and
sharing with shells in other atoms (i.e., forming new compounds and ions). An
atom will gain or lose valence electrons to form a stable ion that has the same
number of electrons as the noble gas nearest the atom's atomic number. For
example, the nitrogen cycle (see Figure B3.1) includes three principal forms
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