354EPA has set an enforceable regulation for selenium, called a maximum contaminant level
(MCL), at 0.05 mg/L or 50 ppb. MCLs are set as close to the health goals as possible,
considering cost, benefits and the ability of public water systems to detect and remove
contaminants using suitable treatment technologies. In this case, the MCL equals the
MCLG, because analytical methods or treatment technology do not pose any limitation.
The Phase II Rule, the regulation for selenium, became effective in 1992. The Safe
Drinking Water Act requires EPA to periodically review the national primary drinking water
regulation for each contaminant and revise the regulation, if appropriate. EPA reviewed
selenium as part of the Six Year Review and determined that the 0.05 mg/L or 50 ppb
MCLG and 0.05 mg/L or 50 ppb MCL for selenium are still protective of human health.
Selenium Water Treatment
Selenium contamination of water systems may result whenever new agricultural runoff
courses through normally dry undeveloped lands. If you have high levels of selenium in
your water the following are recommended selenium water treatment options. Anion
exchange can reduce the amount of selenium in drinking water by 60 - 95%. Reverse
Osmosis Systems are excellent at removing selenium in drinking water.
Selenium shows borderline metalloid or nonmetal behavior. Its most stable form, the grey
trigonal allotrope, is sometimes called 'metallic' selenium. This is because its electrical
conductivity is several orders of magnitude greater than that of the red monoclinic form.
The metallic character of selenium is further shown by the following properties:
Its luster.
Its crystalline structure, which is thought to include weakly 'metallic' interchain
bonding.
Its capacity, when molten, to be drawn into thin threads.
Its reluctance to acquire 'the high positive oxidation numbers characteristic of
nonmetals'.
Its capacity to form cyclic polycations (such as Se2+ 8) when dissolved in oleums
(an attribute it shares with sulfur and tellurium).
The existence of a hydrolyzed cationic salt in the form of trihydroxoselenium (IV)
perchlorate [Se(OH) 3 ]+.ClO– 4.
The Non-metallic Character of Selenium is shown by:
Its brittleness.
Its electronic band structure, which is that of a semiconductor.
The low electrical conductivity (~10−9 to 10−12 Sꞏcm−1) of its highly purified form.
This is comparable to or less than that of bromine (7.95×10–12 Sꞏcm−1), a nonmetal.
Its relatively high electronegativity (2.55 revised Pauling scale).
The retention of its semiconducting properties in liquid form.
Its reaction chemistry, which is mainly that of its nonmetallic anionic forms Se2–,
SeO2−3 and SeO2− 4.
Selenium Explained
Selenium is a chemical element with symbol Se and atomic number 34. It is a nonmetal
with properties that are intermediate between those of its periodic table column-adjacent
chalcogen elements sulfur and tellurium. It rarely occurs in its elemental state in nature,
or as pure ore compounds.