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Arsenic: An Abundant Natural Poison
(Released March 2009)

  by Andreas Saldivar & Vicki Soto  


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Arsenic has the chemical symbol As, the atomic number 33, and an atomic weight of 74.92. It is present throughout the Earth's crust at varying concentrations with an average concentration of 1.7mg/kg. (Robinson and Ayotte 2006) The twentieth most abundant element in the Earth's crust, it is found in sedimentary, igneous, and metamorphic rocks. In sedimentary iron ores the average concentration is extremely high at 400 mg/kg. (ISSI Consulting Group, et al. 2000)

overlapping triangles
Ancient Alchemical Arsenic Symbol
Arsenic occurs in 5 different valence states:

Arsine -3

Elemental Arsenic 0

Arsonium Metals +1

Arsenites +3

Arsenates +5

Elemental arsenic is rarely found in nature, whereas arsenite and arsenate are the two most common states found.

Arsenic can occur both inorganically and organically. If the compound contains carbon (symbol C) then it is considered organic. Organic arsenic can be found in nature in water, natural gas, and shale oil. It is found in the human body due to liver enzyme activity. (ISSI Consulting Group, et al. 2000) Some examples of organic arsenic are:

Monomethylarsonic acid


Dimethylarsinic acid

  (CH3) 2AsO(OH)

Trimethylarsine oxide


Inorganic arsenic is found in almost all rocks and many minerals. The most common minerals are:



Found in high temperature veins associated with tin and tungsten ores



See below



Realgar and Orpiment often appear together in lead and sliver deposits

glittering blue rock
Weathering of rocks results in arsenic being present in almost all soils and sediments. The concentration of arsenic in sediments is largely dependent on source rock. Sediments derived from volcanic rocks generally have higher arsenic concentrations. The arsenic concentration in soil normally varies from 0.1 to 40 mg/kg. Extremely high concentrations of up to 8000 mg/kg can occur in soils associated with sulfuric ores. (ISSI Consulting Group, et al. 2000)

Anthropogenic arsenic compounds used in agriculture, industry, and wood preservation are other sources of arsenic in soil and sediment. Arsenic in rock, soil, and sediment eventually makes it into ground and surface water where it can be found in both the arsenite state (AS+3) and the arsenate state (AS+5). Arsenate is found in oxidizing conditions while arsenite is found in sufficiently reducing conditions. (ISSI Consulting Group, et al. 2000)

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