Geoscience Reference
In-Depth Information
earth's crust. Arsenic occurs in trace quantities in all rock, soil, water, and air (WHO
2001
). Under reducing conditions, arsenite (As
III
) is the dominant form, while
arsenate (As
V
) generally is the stable form in oxygenated environments. Arsenic salts
exhibit a wide range of solubilities, depending on pH and the ionic environment.
The average subsurface abundance of arsenic is 5-10 mg/kg (Han et al.
2003
),
and it is present in more than 200 mineral species. Approximately 60 % of natural
arsenic minerals are arsenates, 20 % sulfides and sulfosalts, and the rest are ar-
senides, aresnites, oxides, alloys, and polymorphs of elemental arsenic. Inorganic
arsenic of geological origin is found in groundwater used as drinking water in
several parts of the world, especially in the Bengal Basin. Organic arsenic com-
pounds are found mainly in marine organisms.
Mining, smelting of nonferrous metals, and burning of fossil fuels are the major
industrial processes that contribute to anthropogenic arsenic contamination of air,
water, and soil. Use of chromated copper arsenate (CCA) for wood preservation is
still in widespread use in many countries and was used heavily during the latter
half of the twentieth century as a structural and outdoor building component,
where there was a risk of rot or insect infestation in untreated timber. Lead
arsenate has been used, well into the twentieth century, as a pesticide on fruit trees,
and copper arsenate has even been recorded in the nineteenth century as a coloring
agent in sweets.
Arsines released from microbial sources in soils or sediments undergo oxidation
in the air, reconverting the arsenic to nonvolatile forms, which settle back to the
ground. Dissolved forms of arsenic in the water column include arsenate, arsenite,
methylarsonic acid (MMA), and dimethylarsinic acid (DMA). Some arsenic spe-
cies have an affinity for clay mineral surfaces and organic matter, and this can
affect their environmental behavior. There is a potential for arsenic release when
there is fluctuation in Eh, pH, soluble arsenic concentration, and sediment organic
content. Many arsenic compounds tend to adsorb to soils, and leaching usually
results in transportation over only short distances in soil. Three major modes of
arsenic biotransformation have been found to occur in the environment: redox
transformation between arsenite and arsenate, reduction and methylation of
arsenic, and biosynthesis of organo-arsenic compounds. Arsenic levels in
groundwater average about 1-2 lg/L except in areas with volcanic rock and
sulfide mineral deposits, where arsenic levels can reach up to 3 mg/L. Naturally
elevated levels of arsenic in soils may be associated with geological substrata such
as sulfide ores. Anthropogenically contaminated soils can have concentrations of
arsenic up to several grams per 100 mL. Nonoccupational human exposure to
arsenic in the environment is primarily through the ingestion of food and water.
Contaminated soils, such as mine tailings, also are a potential source of arsenic
exposure. The daily intake of total arsenic from food and beverages generally is
between 20 and 300 lg/day.
Soluble inorganic arsenic is acutely toxic, and ingestion of large doses leads to
gastrointestinal symptoms, disturbances of cardiovascular and nervous system
functions, and eventually death. In survivors, bone marrow depression, hemolysis,
hepatomegaly, melanosis, polyneuropathy, and encephalopathy may be observed.
