(C 4 H 9 )
(C 4 H 9 )
(C 4 H 9 ) Sn
(C 4 H 9 )
Tetra-n-butyltinHeavy Metal Pollutants and Green Chemistry
From the discussion above, it is seen that several heavy metals are among the more
troublesome water pollutants, with a number of others in more isolated cases. Obviously,
it is important to prevent such elements from getting into water. Here the practice of
green chemistry plays an important role. One approach is to strictly forbid the release of
heavy metals into water. This has worked reasonably well but, as with all command and
control measures, it is subject to human oversight, accident, and even deliberate releases
made to try to avoid disposal costs.
A much better approach, where possible, is to use the principles of green chemistry
to avoid any possibility of pollutant release. For example, a command and control
approach to preventing the release of cadmium in electroplating operations would be
to strictly control any releases. But, a green chemistry approach is to come up with
safer substitutes for cadmium in metal treatment so that there is never any cadmium
around to be released. One of the major objectives of green chemistry is to avoid the
use and dispersal of heavy metals. Similar approaches can be tried with any other water
pollutant.
7.12. Inorganic Water Pollutants
Cyanide is deadly as volatile hydrogen cyanide, HCN, or as cyanide ion, CN-;
as little as 60 mg of cyanide can be fatal to a human. Cyanide is produced from coke
ovens and is widely used in the metals industry for metal extraction from ores and for
metal cleaning and electroplating. Cyanide is sometimes released to water, especially
from metal extraction operations. One such incident occurred in 1995 when cyanide-
containing water mixed with red clay from mine tailings was released from a gold mining
operation in the South American country of Guyana. A breached dam allowed release of
approximately 2.7 billion liters of cyanide-contaminated wastes. The cyanide present in
the water at a level of approximately 25 parts per million killed all the fish in the small
Omai Creek leading from the site of release to the Essequibo River, where the dilution
from the river flow reduced levels of cyanide to below fatal concentrations. In 1992,
cyanide and heavy metals spilling from the Summitville mine in southern Colorado
killed all life in a 17-mile section of the Alamosa River. The state of Colorado agreed to
settle for damages totaling $30 million in late 2000.
Although no human fatalities resulted from these incidents, the scope of the spill
and the extreme toxicity of cyanide point to the dangers of using large quantities of a
reagent so toxic as cyanide. Regulations forbidding such a release were not helpful in
Chap. 7. Water, The Ultimate Green Solvent 175