Arsenic (Environmental & Earth Science)
Arsenic is an element (atomic number 33) classed as a semi-metal or metalloid. This means it has some properties of metals, and some properties of non-metals. Arsenic occurs in two distinct solid forms. One is a brittle, gray metal, while the other is a yellow, non-metallic form, rarely seen outside the laboratory. Arsenic and its compounds often have a garlic-like odor when crushed or when scratched with a hard object.
Elemental arsenic had very few uses, until recent decades, when large quantities of arsenic began to be used in solar voltaic panels (in harnessing solar power) and in lithium ion batteries (the main energy source for electric vehicles. Nearly all the applications are as salts or oxides of arsenic. Arsenic compounds can be very toxic, and their uses are sometimes controlled by environmental regulations. Since these panels and batteries are not routinely recycled, large quantities of arsenic have begun to enter earth's soil and groundwater, as evidenced by increasing arsenic in groundwater. In the year 2010 a lifeform capable of metabolizing arsenic was found in Mono Lake, California. This discovery has implications for the possibility of finding extremophile organisms on other planets than Earth, which environments may have substantially different chemical composition from the Earth.
Name
| Prior Element: Germanium Next Element: Selenium |
| |
| Physical Properties | ||
|---|---|---|
| Color | gray | |
| Phase at Room Temp. | solid | |
| Density (g/cm3) | 5.778 | |
| Hardness (Mohs) | 3.5 | |
|
Melting Point (K) |
886 | |
|
Boiling Point (K) |
--- | |
| Heat of Fusion (kJ/mol) | 27.7 | |
| Heat of Vaporization (kJ/mol) | --- | |
| Heat of Atomization (kJ/mol) | 302 | |
| Thermal Conductivity (J/m sec K) | 50.2 | |
| Electrical Conductivity (1/mohm cm) | 30.03 | |
| Source | Arsenopyrite enargite | |
| Atomic Properties | ||
| Electron Configuration | [Ar]3d104s24p3 | |
|
Number of Isotopes |
1 | |
| Electron Affinity (kJ/mol) | 78 | |
| First Ionization Energy (kJ/mol) | 946.5 | |
| Second Ionization Energy (kJ/mol) | 1797.8 | |
| Third Ionization Energy (kJ/mol) | 2735.4 | |
| Electronegativity | 2.18 | |
| Polarizability (Å3) | 4.3 | |
| Atomic Weight | 74.92 | |
| Atomic Volume (cm3/mol) | 13 | |
| Ionic Radius2- (pm) | --- | |
| Ionic Radius1- (pm) | --- | |
| Atomic Radius (pm) | 120 | |
| Ionic Radius1+ (pm) | --- | |
| Ionic Radius2+ (pm) | --- | |
| Ionic Radius3+ (pm) | 72 | |
| Common Oxidation Numbers | -3, +3, +5 | |
| Other Oxid. Numbers | +2 | |
| Abundance | ||
| In Earth's Crust (mg/kg) | 1.8 | |
| In Earth's Ocean (mg/L) | 3.7-3 | |
| In Human Body (%) | 0.00001% | |
| Regulatory / Health | ||
| CAS Number | 7740-38-2 | |
| OSHA Permissible Exposure Limit (PEL) | TWA: 0.5 mg/m3 | |
| OSHA PEL Vacated 1989 | No limits | |
|
NIOSH Recommended Exposure Limit (REL) |
No limits | |
|
Sources: |
||
The name arsenic comes from the Greek word arsenikon, which means orpiment. Orpiment is a bright yellow mineral composed of arsenic sulfide (As2S3), and is the most highly-visible common arsenic mineral. Historians say that arsenic was discovered in 1250 AD by Albertus Magnus, a German monk who spent his life studying and classifying natural materials. It is believed that he heated soap and orpiment together and isolated elemental arsenic.
Sources
Arsenic metal very rarely occurs in its pure form in nature. The most common arsenic mineral is arsenopyrite, a compound of iron, arsenic, and sulfur. Several other, less-common minerals contain arsenic, including orpiment, realgar, and enargite, which are arsenic sulfides. Most arsenic is obtained not from an ore mineral of arsenic, but as a by-product in the treatment of gold, silver, copper, and other metal ores. In fact, environmental laws require that arsenic be removed from ores, so that it does not enter the environment in effluent gases, fluids, or solids.
Significant quantities of arsenic are associated with the copper-gold deposits in Chile, the Philippines, and many other countries. However, many countries produce by-product arsenic from smelting of metal ores. China is by far the largest producer, with Chile, Mexico, and Peru also important, and lesser production from about a dozen other countries with metal smelters.
The United States imports all the metallic arsenic and arsenic compounds that it consumes. Very little is recycled, except in waste from factories that make arsenic compounds.
Uses
As of 2017, USA industrial consumption of arsenic was 6920 tons per annum. Only about 5% of arsenic consumption is of the metallic element. Most of this is used to alloy (mix) arsenic with lead, copper, or other metals for specific uses. As a metalloid, arsenic is a semiconductor, like silicon. This means it conducts some electricity like a metal, but not all the electricity a true conductor like copper would conduct. Manufacture of solar panels is a major use of arsenic; most governmental agencies do not have controls on disposal of solar panels to safely isolate and dispose of the arsenic. The International Renewable Energy Agency estimated that there were "about 250,000 metric tons of solar panel waste in the world at the end of 2016 and that the figure could reach 78 million metric tons by 2050" (Institute of Energy Research. 2018); a major amount of this toxic waste is arsenic. Furthernore, about 1/10% of arsenic is consumed in the manufacture of gallium arsenide semiconductors for use in other miscellaneous electronics. Some arsenic is also used in glass-making.
The majority of U.S. consumption is in the form of chromated copper arsenate (CCA), a chemical used as a wood preservative for telephone poles, fence posts, pilings, and foundation timbers. The CCA significantly reduces rot and eliminates wood destruction by termites, ants and other insects. However, the use of CCA is being phased out in the U.S.; however, due to major growth in USA solar panel use, about thirteen percent annual growth rate of arsenic use has been seen in the USA since 2013, and a major increase in the arsenic market is expected as a result.
Formerly the most important use of arsenic compounds, was as an insecticide sprayed in fields and orchards. This use has entirely disappeared in most countries, due to the poisonous nature of arsenic compounds. Arsenic contamination is a problem in some well-water and may be associated with mine drainage.
Arsenic is not recovered from any waste materials or in any recycling program.; in particular, there is no national standard in the USA for disposal or recycling of arsenic in solar panels; also, there are few states with disposal standards, so that considerable amounts of arsenic in solar panels wind up in landfills or leach into soil and groundwater. Some is recovered from runoff at wood treatment facilities.
Interestingly, a trace amount of arsenic is necessary for good health and growth of animals, including humans. 0.00001% is needed for growth and for a healthy nervous system.
Toxicity
Arsenic and many compounds containing arsenic are highly toxic to many animals, including humans, especially potent poisons. Toxicity levels in the USA have been set at 10 micrograms per liter in drinking water, indicating the level where carcinogenic incidence is unsafe. For example, this is important since there are numerous water supplies proximate to mines are contaminated by arsenic; many other groundwater basins are present where landfills that accept solar panels are present.
Common adverse health effects of arsenic ingestion above ten micrograms per liter are cancer of the lungs, kidney, bladder and skin as well as skin discoloration and heart arrhythmia. A bout eght percent of Americans use water supplies in excess of this toxic standard. According to the National Academy of Science, it is projected that about two million Americans will experience a lifetime cancer resulting from chronic exposure to arsenic in drinking water. Arsenic disrupts Adenosine (ATP) production through several mechanisms. Within the the citric acid cycle, arsenic inhibits lipoic acid which is a cofactor for pyruvate dehydrogenase; by competing with phosphate, the arsenic uncouples oxidative phosphorylation, thereby inhibiting energy-linked reduction of Nicotinamide adenine dinucleotide, mitochondrial respiration, and ATP synthesis. Hydrogen peroxide production is also increased, which may form reactive oxygen species and oxidative stress. These metabolic interferences can lead to death from multi-system organ failure, likely from necrotic cell death. Post mortem necropsy reveals brick red coloured mucosa, due to severe hemorrhage.
Although arsenic causes toxicity, it can also play a protective role in certain very small concentrations. Worldwide exposure to arsenic including outcome of a high number of excess deaths appears to be an increasing trend in the last decade, correlated with an explosion of production of solar panels, which now number more than 1.5 million in the USA alone. One recent study showed eight percent of US households have unsafe levels of arsenic in drinking water. (THIS ARTICLE DOES NOT ENCOURAGE USE OR EXPERIMENTATION WITH ARSENIC SUBSTANCES, DUE TO HIGH HEALTH RISKS)
Substitutes and Alternative Sources
A variety of alternative wood preservatives are available to replace CCA, as is plastic wood lumber.
References
- Sabina C. Grund, Kunibert Hanusch, Hans Uwe Wolf. 2005. Arsenic and Arsenic Compounds, Ullmann's Encyclopedia of Industrial Chemistry, Weinheim: Wiley-VCH\
- ,Otis A. Glazebrook IV (January 18, 2009) Not so Green Solar Energy. American Thinker
- Nicholas C. Norman. 1998. Chemistry of Arsenic, Antimony and Bismuth. Springer. ISBN 9780751403893.
- Institute of Energy Research (Sept. 12, 2018) The Mounting Solar Panel Waste Problem Egon Biberg,Nils Wiberg, and Arnold Frederick Holleman. 2001. Inorganic Chemistry. Academic Press. ISBN 9780123526519.
- C.Michael Hogan. 2010. Heavy metal Encyclopedia of Earth. Topic ed. Emily Monosson. Ed.-in-Chief Cutler J.Cleveland. Washington DC.
- Ryan Felton and Lisa Gill of Consumer Reports and Lewis Kendall for the Guardian. (Mar 31, 2021) Nine-month investigation by the Guardian and Consumer Reports found alarming levels of forever chemicals, arsenic and lead in samples taken across the US. The Guardian and Consumer Reports\ Common Minerals and Their Uses, Mineral Information Institute.
- More than 170 Mineral Photographs, Mineral Information Institute.
| Disclaimer: This article contains information that was originally published by the Mineral Information Institute. Topic editors and authors for the Encyclopedia of Earth have edited its content and added new information. The use of information from the Mineral Information Institute should not be construed as support for or endorsement by that organization for any new information added by EoE personnel, or for any editing of the original content. |
