Agriculture Reference
In-Depth Information
The designations in parentheses give the physical phase of each reactant and
product: “s” for solid, “aq” for aqueous, and “g” for gas. The oxidation half-
reactions for this reaction are
S
→
SO
2
(B3.6)
4H
+
+
4e
−
S
+
2H
2
O
→
SO
2
+
(B3.7)
The reduction half-reactions for this reaction are
NO
3
→
NO
(B3.8)
NO
3
4H
+
+
3e
−
→
+
NO
+
2H
2
O
(B3.9)
Therefore, the balanced oxidation-reduction reactions are
4NO
3
+
3S
+
16H
+
+
6H
2
O
→
3SO
2
+
16H
+
+
4NO
+
8H
2
O
(B3.10)
4NO
3
4H
+
→
+
3S
+
3SO
2
+
4NO
+
2H
2
(B3.11)
Oxidation-reduction reactions are not only responsible for pollution but
are also very beneficial. Redox reactions are part of essential metabolic and
respiratory processes. Redox is commonly used to treat wastes (e.g., to amelio-
rate toxic substances and to detoxify wastes) by taking advantage of electron-
donating and electron-accepting microbes or by abiotic chemical redox re-
actions. For example, in drinking water treatment, a chemical oxidizing or
reducing agent is added to the water under controlled pH. This reaction raises
the valence of one reactant and lowers the valence of the other. Thus, re-
dox removes compounds that are “oxidizable,” such as ammonia, cyanides,
and certain metals, including selenium, manganese, and iron. It also removes
other “reducible” metals, such as mercury (Hg), chromium (Cr), lead (Pb),
silver (Ag), cadmium (Cd), zinc (Zn), copper (Cu), and nickel (Ni). Oxidizing
cyanide (CN
−
) and reducing Cr
6
+
to Cr
3
+
are examples in which the toxicity
of inorganic contaminants can be greatly reduced by redox.
*
*
Redox reactions are controlled in closed reactors with rapid-mix agitators.
Oxidation-reduction probes are used to monitor reaction rates and product formation. The
reactions are exothermic and can be very violent when the heat of reaction is released, so
care must be taken to use only dilute concentrations, along with careful monitoring of batch
processes.
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