2. Where does environmental arsenic come from?
- 2.1 What are the natural sources of environmental arsenic?
- 2.2 What are the man-made sources of environmental arsenic?
- 2.3 How is arsenic transported and distributed in the environment?
2.1 What are the natural sources of environmental arsenic?
The Earth’s crust is an abundant natural source of arsenic. It is present in more than 200 different minerals, the most common of which is called arsenopyrite.
About one-third of the arsenic in the Earth’s atmosphere is of natural origin. Volcanic action is the most important natural source. The next most important source is arsenic-containing vapor that is generated from solid or liquid forms of arsenic salts at low temperatures.
Inorganic arsenic of geological origin is found in groundwater used as drinking water in several parts of the world, for example Bangladesh, India and Taiwan.
Organic arsenic compounds, that is, those containing carbon, are mainly found in sea-living organisms, although some of these compounds have also been found in species living on land.
As seen in questions 5 to 8, inorganic arsenic poses more problems than organic arsenic. More...
2.2 What are the man-made sources of environmental arsenic?
Elemental arsenic is produced commercially from arsenic trioxide. Arsenic trioxide is a by-product of metal smelting operations. About 70% of the world production of arsenic is used in timber treatment, 22% in agricultural chemicals, and the remainder in glass, pharmaceuticals and metallic alloys.
Mining, metal smelting and burning of fossil fuels are the major industrial processes that contribute to arsenic contamination of air, water and soil. The use of arsenic-containing pesticides in the past has left large areas of agricultural land contaminated. The use of arsenic in the preservation of timber has also led to contamination of the environment. More...
2.3 How is arsenic transported and distributed in the environment?
The transport and distribution of arsenic in the environment is complex, due to the many chemical forms in which it may be present and because there is continuous cycling of different forms of arsenic through air, soil and water.
Much of the arsenic in the atmosphere comes from high-temperature processes such as coal-fired power plants, burning vegetation and volcanic activity. The arsenic is released into the atmosphere primarily as arsenic trioxide where it adheres readily onto the surface of particles. These particles are dispersed by the wind and eventually fall back to the earth due to their weight or during rain.
Natural, low-temperature biological reactions involving microbes also release arsenic into the atmosphere. Microbes acting on arsenic in soils and sediments generate arsine gas or other volatile arsenic compounds. Arsine reacts with oxygen in the air and is converted back to non-volatile forms of arsenic, which settle back to the ground.
Arsenic dissolved in water can be present in several different forms. In well-oxygenated water and sediments, nearly all arsenic is present in the stable form of arsenate. Some arsenite and arsenate forms are less stable and are interchangeable, depending on the chemical and biological conditions. Some chemical forms of arsenic adhere strongly to clay and organic matter and this can affect how they behave in the environment. There is potential for arsenic to be released from water and sediments, again depending on the chemical and biological conditions.
Lastly, weathered rock and soil containing arsenic may be transported by wind or water erosion. Since many arsenic compounds tend to adhere strongly to soils, water percolating down does not usually move arsenic through more than a short distance in soil. More...