The Toxicity of Chromium
R.  S.  INGOLS
Research Professor
Georgia Institute of Technology
Atlanta, Georgia
In a study of toxicity there are several problems that one would
like to answer. First, how much of the material is required to show
a toxic level, or in other words, how much material will be required
to bring about a sensible, i.e., major, reduction in the respiration or
metabolism of the organism affected ? Second, in order that some means
may be found to overcome this toxicity, what is the mechanism of the
reaction? And third, what can be understood concerning the chemical
reactions and structure of the organism itself from the amount and
chemical nature of the toxic agent used under varying conditions?
This report brings out how the state of oxidation of chromium
(trivalent or hexavalent) affects the organisms and the relationship of
the state of oxidation to the environment of the organisms. The variations in the environment include both aerobic and anerobic conditions,
as well as several organic matter concentrations. Then, too, the report
indicates what practical application these factors have in sewage treatment and in the disposal of sewage or industrial effluents containing
some chromium. The report also includes a comment concerning the
structure and metabolism of the microorganisms.
A study of the literature reporting the effect of chromium upon
various sewage-treatment processes, rivers or analytical results indicates
confusion about the amount of chromium necessary to give toxicity.
One anonymous article (13) indicates that one part per million of
hexavalent chromium can not be tolerated in a digestion tank, whereas
Barnes and Braidech (3) indicate that 10,000 p.p.m. of chromate give
only a 34 percent reduction in the amount of gas generated. This
tremendous discrepancy points out a problem; how can two people get
such different results? Is the discrepancy the result of the method
for determining the amount of chromium that is present, or of the
method for evaluating the amount that is added? One can also raise
the question of what happens to hexavalent chromium after it has
been added to the digestion tank. Does the one report mean that one
part per million of chromium remained as chromate after a portion
had been reduced, so that the toxicity includes the excess chromate ion
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