in orchards and on crops. Among the deadly compounds used for this purpose were lead
arsenate, Pb 3 (AsO 4 ) 2 ; sodium arsenite, Na 3 AsO 3 ; and Paris Green, Cu 3 (AsO 3 ) 2.
In modern times, the most tragic occurrence of water pollution by arsenic has taken
place in Bangladesh, an impoverished nation on the Indian subcontinent. Around 1980 a
program sponsored by the United Nations to drill wells in this country brought abundant,
pathogen-free drinking water to many areas of Bangladesh. However, about 20 years
later, symptoms of arsenic poisoning appeared among the people using this water source,
leading to debilitating illnesses and death. These otherwise wholesome water sources
were contaminated with dangerous levels of arsenic.
Organically Bound Metal Water Pollutants
In a number of instances, simple hydrated metal ions are not soluble enough in water
to cause pollution problems. However, organically bound metals may be considerably
more mobile and in some cases more toxic. Binding of metals as metal chelates was
discussed in Section 7.9. Another form of metal binding occurs when metals are bonded
directly to carbon in hydrocarbon groups such as the methyl group (-CH 3 ) to produce
organometallic compounds.
A nasty surprise related to the formation of organometallic compounds in water
was revealed in 1970 when it was found that fish in some areas, such as in Lake Saint
Clair located between Michigan and Ontario, Canada, had dangerously high levels of
mercury. It was known that the electrically driven chloralkali method of producing
sodium hydroxide and elemental chlorine — both important industrial chemicals —
used mercury electrodes in electrolyzing sodium chloride solutions, and that each unit
in this process was releasing up to 14 kilograms of mercury per day. However, it was
known that the inorganic forms of mercury released formed very insoluble precipitates
in water and were thought to be safely buried with lake and river sediments. Subsequent
investigation showed that anoxic bacteria growing in the oxygen-free sediments were
attaching methyl groups, -CH 3 , to mercury:
HgCl 2
Anaerobic
bacteria^ CH^3 HgCl + Cl- (7.11.1)
The monomethylmercury ion in this compound, CH 3 Hg+, is soluble and mobile in water
and the dimethylmercury, (CH 3 ) 2 Hg, also produced is volatile as well. Because of these
physical characteristics and their solubility in lipid (fat) tissue, these methyl mercury
species were released from the sediments and concentrating in fish tissue.
In recent years, the organometallic compounds of most concern in water have been
the organotin compounds. Up to 40,000 metric tons per year of organotin compounds,
such as tetra-n-butyltin (below) have been produced each year as industrial biocides to
prevent biological growths on surfaces. Boat and ship hulls were painted with organotin-
containing paints to prevent the growth of “aufwuchs,” organisms that attach themselves
to such surfaces and greatly increase the friction, hence the fuel costs, of propelling these
vessels through water. Because of water pollution concerns, in 1998 the International
Maritime Organization agreed to ban organotin antifouling paints on all ships by 2003.
174 Green Chemistry, 2nd ed