236The 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. Selenium (Greek σελήνη selene meaning "Moon") was discovered in 1817 by Jöns
Jakob Berzelius, who noted the similarity of the new element to the previously-known tellurium
(named for the Earth).
Selenium is found impurely in metal sulfide ores, where it partially replaces the sulfur.
Commercially, selenium is produced as a byproduct in the refining of these ores, most often during
copper production.
Minerals that are pure selenide or selenate compounds are known, but are rare. The chief
commercial uses for selenium today are in glassmaking and in pigments.