OCCURRENCE AND USES OF SELENIUM
Selenium, a non-metallic element, is a metalloid, Atomic Number 34, Atomic Weight 78.96, in Group VIA of the Periodic Table of the Elements, has six stable isotopes, and is analogous to sulfur in many of its chemical combinations.
Selenium occurs in some soils as basic ferric selenite, calcium selenate, in its elemental form, and in organic selenium compounds derived from decayed plant tissue.
The greatest use of selenium compounds is in electronic and photocopier components, but these compounds also find uses in glass, pigments, rubber, metal alloys, textiles, petroleum, medical therapeutic agents and photographic emulsions.
Soil concentrations of selenium vary widely. For example, New Zealand’s typical human daily selenium intake approximates 56 micrograms per day. By contrast, local soils in South Dakota are high in selenium, and inhabitants may take in as much as 7,000 micrograms per day.
Animals grazing in high seleniferous areas have been reported to incur renal, hepatic and heart damage. Selenium reacts in vivo with other elements against heavy metals toxicity from mercury, cadmium, silver, thallium and molybdenum.
Selenium concentrations in drinking water typically are quite low. The U.S. National Interim Primary Drinking Water Regulation compliance monitoring showed only 150 groundwater and six surface water systems with selenium in excess of ten (10) micrograms per liter. When selenium is found in groundwater, it is as a result of natural, not artificial [man-made] contamination. The United States Environmental Protection Agency (USEPA) reports selenium releases to water and to land for the years 1987 to 1993 as 13, 556 pounds and 1,010,686 pounds, respectively.
At typical groundwater pH levels (7.0 →9.5), only the anionic forms of selenious (SeIV) and selenic (SeVI) acids are found.
Selenious acid dissociates according to:
H2SeO3 = H+ + HSeO3 – and HSeO3 = H+ + SeO3 2-
While selenic acid dissociates as:
H2SeO4 = H+ + HSeO4 – and HSeO4 – = H+ + SeO4 2-
Under oxidizing conditions Se(VI) will predominate, and divalent selenate (SeO4 2-), an anion with chemical behavior similar to that of sulfate, will be found. Under reducing conditions Se(IV) will predominate, and at pH values below 8.15 the monovalent biselenite anion (HSeO3-) will dominate.
TOXICITY AND HEALTH EFFECTS
It is widely believed that selenium is highly toxic to man, although, according to MeKee and Wolf, proof of human injury by selenium is scanty, and definite symptoms of selenium poisoning have not been identified.
Selenium can form a highly toxic hydrogen selenide, a gas that is a close relative, and equally as lethal, as hydrogen sulfide.
Selenium is an essential micronutrient at low levels. USEPA has found selenium to potentially cause changes in hair and fingernails, damage to the peripheral nervous system, fatigue and irritability, when exposed to levels above the MCL for relatively short periods of time.
Long-term, life time exposure to selenium has the potential to cause hair and fingernail loss, damage to kidney and liver tissue, and damage to the nervous and circulatory systems.
The World Health Organization (WHO) and the European Drinking Water Standards (EDWS) prescribe a mandatory limit of 0.05 milligrams per liter. Because of inadequate evidence, the USEPA has not classified selenium as a carcinogen, however, in 1992, USEPA promulgated an MCL and an MCLG of 0.05 mg/L.
REMOVAL OF SELENIUM
Several unit operations for the removal of selenium water are available. Since the choice of removal process largely depends upon the species of selenium present, accurate laboratory data acquisition, including speciation, is essential. Processes include:
- Reverse Osmosis
- Ion Exchange
- Activated Alumina
- Coagulation / Filtration
- Lime Softening