Читать книгу Encyclopedia of Renewable Energy - James Speight G., James G. Speight - Страница 106

Arsenic is a notoriously poisonous metalloid with many allotropic forms, including a yellow (molecular non-metallic) and several black and gray forms (metalloids). Three metalloid forms of arsenic, each with a different crystal structure, are found free in nature. However, it is more commonly found as arsenide and in arsenate compounds, several hundreds of which are known. Arsenic and its compounds are used as pesticides, herbicides, insecticides, and in various alloys.

Selenium a nonmetal, chemically related to sulfur and tellurium, which rarely occurs in its elemental state in nature. Isolated selenium occurs in several different forms, the most stable of which is a dense purplish-gray semimetal (semiconductor) form that is structurally a trigonal polymer chain. It conducts electricity and is used in photocells. Selenium is found in economic quantities in sulfide ores such as pyrite FeS₂), partially replacing the sulfur in the ore matrix. Minerals that are selenide or selenate compounds are also known, but all are rare.

Selenium salts are toxic in large amounts, but trace amounts of the element are necessary for cellular function in most, if not all, animals, forming the active center of the enzymes glutathione peroxidase and thioredoxin reductase (which indirectly reduce certain oxidized molecules in animals and some plants) and three known deiodinase enzymes (which convert one thyroid hormone to another).

Arsenic and selenium occur in biomass to the extent of several parts per million and, on combustion of the coal, a varying quantity of these elements are released or retained in the ash, depending largely on the conditions under which the combustion takes place and on the nature of the ash.

Arsenic and selenium are determined by mixing a weighed sample with the Eschka mixture followed by ignition 750°C (1,380°F). The mixture is dissolved in hydrochloric acid, and the gaseous hydride of each element is generated from the appropriate oxidation state and determined by atomic absorption spectrophotometry. The method permits measurement of the total arsenic and selenium content of coal for the purpose of evaluating these elements where they can be of concern, for example, in coal combustion. When coal samples are prepared for analysis in accordance with standard procedure, the arsenic and selenium are quantitatively retained and are representative of the total amounts in the coal

See also: Periodic Table of the Elements.