and capacitors, as plasticizers to make plastics more flexible, to impregnate cotton and
asbestos, and as additives to some epoxy paints. The extreme stability that led to these uses
also contributed to their widespread dispersion and persistence in the environment.
PCBs are denser than water and tend to accumulate in sediments. One of the worst
incidents of sediment pollution by PCBs occurred during 1950 to 1976 as thousands
of kilograms of PCBs were dumped — legally at the time — into New York’s Hudson
River from electrical equipment manufacturing operations. In an agreement with the
U.S. Environmental Protection Agency and New York state agencies, General Electric
is supposed to remove PCB contamination from Hudson River sediments has been
ordered to dredge and dispose of contaminated Hudson River sediment at a cost as high
as $500 million. As of 2005, the project had suffered a number of delays.
7.16. RADIOACTIVE SUBSTANCES IN WATER
Radioactive isotopes, or radionuclides can get into water from either natural sources
or from the fission of uranium or plutonium in nuclear power reactors or (formerly) above-
ground weapons testing. Radionuclides have unstable nuclei that change spontaneously
to nuclei of atoms of different elements by the emission of ionizing radiation in the form
of alpha particles, beta particles, and gamma rays. These emissions are called ionizing
radiation because they produce reactive ions in materials including flesh. These ions and
other reactive species can damage DNA, impede the body’s ability to make hemoglobin,
and cause a number of biological effects, including severe anemia, mutations, cancer,
and death.
There are 3 major kinds of ionizing radiation that are most commonly given off by
radionuclides. Some of the heavier elements, such as uranium and radium emit alpha
particles, a helium atom nucleus composed of two neutrons and two protons, denoted
by the symbol
2 α
4The penetrating ability of these very heavy particles is low, so that they do not pose
a hazard outside the body. However, if they enter the body, such as by ingestion of
contaminated drinking water, they cause enormous damage to exposed tissue. Beta
radiation is in the form of high-energy electrons designated,
-1β
0or, less commonly, positive electrons, called positrons. These particles are more penetrating
than alpha particles. Gamma rays are not particles, but are electromagnetic radiation
that behave like very short wavelength, high-energy X-rays. The energies of gamma rays
are highly specific and can be measured by sophisticated gamma ray spectrometers as
a means of identifying the kinds and quantities of radionuclides emitting the radiation.
Radionuclides decay with specific half-lives that can range from fractions of a second
to millions of years. After the passage of each half-life, the radioactivity of a specific
radionuclide is half of what it was at the beginning of the half-life. This means that all
radionuclides are eventually converted to nonradioactive forms, although this may take
a very long time.
184 Green Chemistry, 2nd ed