74    ADSORPTION OF HEXAVALENT CHROMIUM ONTO
TAILORED ZEOLITES
Ivonne Santiago, Research Assistant
Vincent P. Worland, Research Assistant
Enrique Cazares-Rivera, Research Assistant
Fernando Cadena, Professor
Department of Civil Engineering
New Mexico State University
Las Cruces, New Mexico 88003
CHROMIUM
Chromium is used in a variety of industrial applications, including electroplating and as a corrosion
inhibitor in cooling towers.1 Up to a few years ago, this metal was the favored corrosion control agent
in cooling towers.2 Cooling water blowdown (waste) was dumped into rivers, pits, lakes and oceans.
During the 1950s chromates were also used as a highway deicing agent in the United States.
Chromium (Cr) occurs in the + 2, + 3 and + 6 oxidation states in water. The divalent slate is rarely
found in field conditions because it becomes rapidly oxidized to + 3 in the presence of atmospheric
oxygen.3
Insoluble Cr(III) occupies much of the Chromium Eh-pH diagram.4 Chromium (III) is less toxic
than Cr(VI),3 however, it can be oxidized to form the carcinogenic Cr(VI) in the presence of oxygen.
The predominance diagrams for Cr(VI) extend over large ranges of Eh and pH.4 Cr(VI) is soluble
and hydrolyses extensively in water, producing only neutral and anionic species as shown in Figure 1.'
These properties insure rapid migration through soils, which are unable to undergo anion exchange
with these hazardous materials.
Hexavalent chromium can be treated by first reducing it to trivalent chromium in the presence of a
reducing agent such as gaseous sulfur dioxide (S02) or a bisulfide solution (NaHS03). In addition,
there are three commercially available methods that use the reducing potential of iron and/or iron
salts to reduce hexavalent chromium.5 Because trivalent chromium is only soluble under acidic condi-
HCrO'
cro;
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Figure 1.    Chemistry' of hexavalent chromium.
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47th Purdue Industrial Waste Conference Proceedings, 1992 Lewis Publishers, Inc., Chelsea, Michigan 481 If
Printed in U.S.A.