Anthropogenic Sources of Arsenic to the Environment

Anthropogenic environmental sources of arsenic stem from

• the continuing use of its compounds as pesticides;

• its unintended release during the mining and smelting of gold, lead, copper, and nickel, in whose ores it commonly occurs (the leachate from abandoned gold mines of previous decades and centuries can still be a significant source of arsenic pollution in water systems);

• the production of iron and steel;

• the combustion of coal, of which it is a contaminant; and

• arsenic-contaminated water brought to ground level by wells.

The arsenic present in raw coal can become a serious pollutant, especially around areas where the fossil fuel is burned. The total pollution from arsenic can be substantial where the coal is burned in small, unventilated stoves rather than in large power plants. In these cases, which occur in some developing countries, the arsenic not only becomes an indoor air pollutant but also contaminates the food and water stored indoors. A particularly acute example occurs in the Chinese province of Guizhou, where arsenic levels in the coal are extraordinarily high, exceeding 1% (i.e., 10,000 ppm) in some cases. Many of the residents of Guizhou suffer arsenic-related health problems, since they use this coal for domestic cooking and heating. By contrast, the level of arsenic in U.S. coal averages about 22 ppm, and most coal worldwide has arsenic levels of less than 5 ppm.

Arsenic compounds found widespread use as pesticides before the modern era of organic chemicals. Although its use in these applications has decreased, arsenic contamination from pesticides remains an environmental problem in some areas of the world. The common arsenic-based pesticides include the insecticide lead arsenate, Pb3(As04)2, and the herbicide calcium arsenate, Ca3(As04)2, both of which contain As(V) as As043~~. The herbicides sodium arsenite, Na3As03, and Paris Green, Cu3(As03)2, both contain As(III) as As033^. An organic compound containing As(V) is routinely used in chicken feed to stimulate growth and prevent disease; some scientists have worried about the contamination of soil and water by arsenic leached from chicken litter. Some methylated derivatives of arsenic acids are still used as herbicides, even in developed countries. The sodium salt of the arsenate ion in which one —OH has been replaced by a methyl group, producing the methanearsonate ion, 0=As(0H) (CH3)0"~, is a herbicide widely used on golf courses and cotton fields in the United States. Such As(V) compounds act as weedkillers because they enter into plant metabolism in place of phosphate ion. The environmental consequences of using another heavy metal, tin, in a pesticide are discussed in Box 15-1.

BOX 15-1

Continue reading here: Organotin Compounds

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