Transitions - Sustainable Design: The Science of Sustainability and Green Engineering - page 94

smog-forming reactions incrementally because nitrogen dioxide is very effi-
cient at absorbing sunlight in the ultraviolet portion of its spectrum. This
is why ozone episodes are more common in the summer and in areas with
ample sunlight. Other chemical ingredients (i.e., ozone precursors) in O 3
formation include volatile organic compounds and carbon monoxide. Gov-
ernments around the world regulate the emissions of precursor compounds
to diminish the rate at which O 3 forms. Many compounds contain both ni-
trogen and sulfur along with the typical organic elements (carbon, hydrogen,
and oxygen). The reaction for the combustion of such compounds, in general
form, is

b
2 H 2 O

d
2 N 2 +

C a H b O c N d S e + 4 a + b − 2 c → a CO 2 + + e S (B3.4)

This reaction demonstrates the incremental complexity as additional elements
enter the reaction. In the real world, pure reactions are rare. The environment
is filled with mixtures. Reactions can occur in sequence, in parallel, or both.
For example, a feedstock to a municipal incinerator contains myriad types
of wastes, from garbage to household chemicals to commercial wastes, and
even small (and sometimes large) amounts of industrial wastes that may be
dumped illegally. For example, the nitrogen content of typical cow manure is
about 5 kg per metric ton (about 0.5%). If the fuel used to burn the waste
also contains sulfur along with the organic matter, the five elements will
react according to the stoichiometry of reaction (B3.4). Thus, from a green
engineering perspective, burning municipal waste to generate electricity may
also release harmful compounds.
Certainly, combustion specifically and oxidation generally are very impor-
tant processes that lead to nitrogen and sulfur pollutants. But they are certainly
not the only ones. In fact, we need to explain what oxidation really means. In
the environment, oxidation and reduction occur. An oxidation-reduction (or re-
dox ) reaction is the simultaneous loss of an electron (oxidation) by one substance
joined by an electron gain (reduction) by another in the same reaction. In oxi-
dation, an element or compound loses (i.e., donates) electrons. Oxidation also
occurs when oxygen atoms are gained or when hydrogen atoms are lost. Con-
versely, in reduction, an element or compound gains (i.e., captures) electrons.
Reduction also occurs when oxygen atoms are lost or when hydrogen atoms
are gained. The nature of redox reactions means that each oxidation-reduction
reaction is a pair of two simultaneously occurring half-reactions . The formation
of sulfur dioxide and nitric oxide by acidifying molecular sulfur is a redox
reaction:

NO 3 (aq) S(s) + → SO 2 (g) + NO(g) (B3.5)