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Chromium as a Contaminant in Water

Chromium (Cr)

Trivalent chromium occurs naturally in the environment and can be found in rocks and soil. It can
also be found in fruits, vegetables and meat. Cr3+
is used to make bricks, metal alloys and chemical
compounds. Hexavalent chromium does not occur naturally in the environment. It is produced by
certain chemical processes and is considered toxic. Metallic elemental chromium or chrome does not
occur naturally in the environment. It is used to make steel and other metal alloys and is produced from
chrome bearing ore.

Trivalent Cr(III): Cr3+
Hexavalent Cr(VI) dichromate Cr2O72-
Hexavalent Cr(VI) chromate CrO42-

Sources of Chromium:
Trivalent chromium is naturally occurring
Hexavalent chromium is produced by certain chemical processes
Potential Health Effects:
Nausea, gastrointestinal distress, stomach ulcers, skin ulcers, allergic reactions
Kidney and liver damage
Reproductive problems
Lung and nasal cancer
Treatment Methods:
-Reverse Osmosis (TFC, CTA)
-Strong Acid Cation Resin
-Reverse Osmosis (TFC, CTA)
-Strong Base Anion Resin
-Weak Base Anion Resin
Organic Complexes
-Reverse Osmosis (TFC, CTA)
-Activated Carbon

It is suggested that Cr3+ is not considered a significant health risk, while the more soluble Cr6+ is
considered toxic. Two of the more common forms of this type of hexavalent chromium are dichromate (Cr2O7-2), found at a pH below 5, and chromate (CrO4
-2 ), found at a pH above 6. Cr6+ thus forms negatively charged anionic complexes. Cr6+ is used in the plating industry, paint production, as well as
an anticorrosive alloy in steel and iron production. Cr6+ was also used as an anti-corrosion agent in boiler water and cooling tower water treatment. It is considered a health risk whether it contacts the
skin, it is inhaled, or it is ingested. It can cause nausea, gastrointestinal distress, stomach ulcers, skin ulcers, allergic reactions, kidney and liver damage, reproductive problems, lung and nasal cancer. It is
believed that hexavalent chromium (Cr6+ ) will be reduced to Cr3+ causing DNA damage in the cell. It is possible, under the right chemical environment for chromium to convert between the different forms.
The EPA has set a maximum contaminant level (MCL) and the MCLG for total chromium of 0.1 ppm (100 ppb) in drinking water. The USEPA is currently evaluating to determine if a new drinking water
standard for hexavalent chromium, Cr6+ is needed. Tap water has an average of 0.4 to 8 ppb total chromium; air has an average of 0.1 ppb (OSHA has set a workplace indoor air limit of 100 ppb based
on a 40 hour workweek). Soil has an average chromium value of 400 ppm.

Trivalent chromium is a cation and can be removed from water with a strong acid cation ionexchange resin in the sodium form. Salt (NaCl) is used as the regenerant, however chromium may not
be completely removed from the resin during the regeneration process. A periodic acid strip will help
remove Cr3+
from the resin, followed by a normal salt regeneration to convert the resin to the sodium
form. A second regeneration may be necessary if the pH of the service cycle is too low. Acid stripping
may be done at a regeneration plant, however, consumers could use a resin cleaner, as with iron, or
add ¼ cup of citric acid per bag of salt once every 3 months. In general, cation resins will have a higher
affinity for chromium than for manganese and iron, therefore iron and manganese will break through
before chromium does. Consult with the resin manufacturer for specific chromium loads on the resin
bed and regeneration recommendations.
Hexavalent chromium can be removed from water by a number of methods. A strong base anion
exchange system will reduce both dichromate and chromate. The resin is regenerated with salt,
however an occasional treatment of a combined salt and caustic solution will be required since a
portion of the chrome will not be stripped from the resin. The service cycle can be extended if the pH of
the raw water is less than 5. A weak base anion resin can also be used if proper pretreatment and
regeneration are used; treatment designers should consult their resin manufacturer for details.
Chromate is an oxidizer and will attack and breakdown ion exchange resin over time. Resin should not
be stored or shutdown when the resin is in the chromate form. Heat may also be generated which can
have a negative impact on the resin.
Organic complexes of chromium can be reduced with a combination of reverse osmosis and
granular activated carbon.
Reverse osmosis and distillation will reduce all types of chromium found in water. Proper
pretreatment for hardness, iron, manganese, and silt or turbidity control is required for successful and
trouble free results.

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