Selenium in Water

Selenium as a Water Contaminant

Selenium (Se)
selenate (SeO42-)
biselenite (HSeO3-)
selenite (SeO32-)

Selenium is a metal found in natural deposits as ores containing other elements. The greatest use of
selenium compounds is in electronic and photocopier components, but they are widely used in other
products as well. Selenium releases to the environment have been primarily from copper smelting
industries. The largest releases from 1987 to 1993 occurred in Utah. The largest direct releases to
water occurred also at that time period in Indiana.
The main source of selenium intake for humans is through plant foods in which the plants
concentrate the selenium found in soil or taken from groundwater used for irrigation. Selenium (Se)
contamination of potable water supplies at concentrations above the current MCL of 0.05 milligram per
liter (mg/L) is rare. In a 1982 study AWWA concluded that only 44 water supplies needed to treat for
this contaminant. At typical groundwater pH values (7.0 to 9.5) only anionic forms of selenious (Se IV) or selenic (Se
VI) acid are found.

Se (IV) Selenious Acid Dissociation Equilibria:
-H2SeO3 = H+ +HSeO3
-HSeO3 = H+ + HSeO2
Se (VI) Selenic Acid Dissociation Equilibria:

  • H2SeO4 = H+ + HSeO4

– HSeO4 = H+ + HSeO2

Under oxidizing conditions, Se (VI) will predominate at pH values less than 8.15 and the divalent
selenate (SeO42-)anion will be found demonstrating chemical behavior similar to the sulfate ion. The
predominant species under reducing conditions will be the Se (IV) species. At pH less than 8.15 the
monovalent biselenite anion (HSeO3-) will dominate and at pH values greater than 8.15, the divalent
selenite (SeO32-) will form.

Sources of Contaminant
Natural deposits
Releases from copper smelting

Potential Health Effects
Hair and fingernail changes
Damage to the peripheral nervous system
Fatigue and irritability
In trace quantities, selenium appears to be essential for nutrition of human beings while larger
concentrations produce definite toxic symptoms. Although retained in the liver and kidney in small
amounts, selenium salts, for the most part, are excreted. Signs of selenium toxicity occur at selenium
ingestion levels of 0.7 –7.0 mg/day while 200 μg/day (0.2 mg/day) is nutritionally adequate
The USEPA has set the Maximum Contaminant Level (MCL) and the Maximum Contaminant Level
Goal (MCLG) in drinking water for selenium at 0.05 mg/L.
EPA has found selenium to potentially cause the following health effects when people are exposed
to it at levels above the MCL for relatively short periods of time: hair and fingernail changes; damage to
the peripheral nervous system; fatigue and irritability.
Long-term, selenium has the potential to cause the following effects from a lifetime exposure at
levels above the MCL: hair and fingernail loss; damage to kidney and liver tissue, and problems with
the nervous and circulatory systems.

Treatment Methods (Residential):
Activated alumina adsorption (85-95% reduction)
Strong base anion type I Cl- exchange (60-95% reduction)
Distillation (>98% reduction)
Reverse osmosis (>90% reduction)

Treatment Methods (Municipal):
Activated alumina
Coagulation/filtration
Lime softening

Several techniques may be used to reduce the level of selenium from drinking water: anion
exchange, activated alumina (AA), reverse osmosis (RO), and distillation.
Anion Exchange can reduce selenium by 90-95%, where the selenate ion is strongly preferred.
Although Se (IV) is more difficult to oxidize than As (III) is to As (V), this can readily be accomplished
with free chlorine. The optimal rate of oxidation is found to be between pH 6.5 and 8.0 where Se (IV)
can be converted to Se (VI) within 5 minutes at a free chlorine concentration of 2 mg/L. At pH 9.0, only
15% of the Se (IV) has been found to be converted with 2 mg/L of free chlorine. Pure oxygen was found
to be ineffective at oxidizing Se (IV) to Se (VI). The run lengths that can be achieved when using
oxidation prior to a strong base anion exchange resin system in chloride form are approximately 275
bed volumes (BV’s). While this may not be considered an outstanding capacity, it should be noted that
anion exchange resin’s high affinity for the selenate ion will prevent chromatographic dumping and runs
could be safely terminated at sulfate breakthrough.
Other techniques used for selenium removal include distillation (> 98% reduction), reverse osmosis
(RO) (> 90% reduction), and activated alumina (AA) (85-95% reduction). Several point-of-use (POU)
RO and POU distillation products have been tested and certified by ANSI-accredited third party
organizations for selenium reduction.
Between pH 5 – 6, AA is very effective in removing Se (IV). But when the predominant species is Se
(VI), AA is poor due to competition from the sulfate ion. In general, AA can be used from pH 3.0 – 8.0
for biselenite, Se (IV), removal. The media in such systems can be removed and recycled using the
standard acid-base regeneration procedure normally employed when AA is used for fluoride removal.
At the present time, it appears that any of these methods can be made practical, feasible, and
economical for selenium reduction when considering point-of-entry (POE) or POU devices.

Author

@AngieY

Leave a comment

Your email address will not be published. Required fields are marked *