Arsenic can be found
in seawater (2-4 ppb), and in rivers (0.5-2 ppb). Half of the arsenic
present is bound to particles. Freshwater and seas algae contain about
1-250 ppm of arsenic, freshwater mycrophytes contain 2-1450 ppm, marine
molluscs contain 1-70 ppm, marine crustaceans 0.5-69 ppm, and fishes
0.2-320 ppm (all values are based on dry mass). In some marine
organisms, such as algae and shrimp, arsenic can be found in organic
compounds.
The legal limit for arsenic in water applied by the World Health
Organization (WHO) is 10 μg/L.
In what way and in what form does arsenic react with water?
Elementary arsenic normally does not react with water
in absence of air. It does not react with dry air, but when it comes in
contact with moist air a layer is formed. The layer has a bronze colour,
and later develops a black surface.
An example of an arsenic compounds that reacts strongly with water is
orpiment. This is an amorphous arsenic compound. Reaction mechanism:
As2S3 + 6 H2O -> 2 H3AsO3
+ 3 H2S
In natural water arsenic participates in oxidation
and reduction reactions, coagulation and adsorption. Adsorption of
arsenic to fine particles in water and precipitation with aluminium or
iron hydroxides causes arsenic to enter sediments. After some time
arsenic may dissolve once again consequential to reduction reactions.
Solubility of arsenic and arsenic compounds
Elementary arsenic is fairly insoluble, whereas
arsenic compounds may readily dissolve. Arsenic is mainly present in
watery solutions as HAsO42-(aq) and H2AsO4-
(aq), and most likely partially as H3AsO4 (aq), AsO43-(aq)
or H2AsO3-(aq).
Examples of solubility of arsenic compounds: arsenic(III)hydride 700
mg/L, arsenic(III)oxide 20 g/L, arsenic acid (H3AsO4.1/2 H2O)
170 g/L, and arsenic(III)sulfide 0.5 mg/L.
Why is arsenic present in water?
Arsenic compounds are abundant in the earth's
crust. Particles are released during mining, and spread throughout the
environment. Arsenic from weathered rocks and soils dissolves in
groundwater. Arsenic concentrations in groundwater are particularly high
in areas with geothermal activity. In aquatic ecosystems inorganic
arsenic derived from rocks such as arsenic trioxide (As2O3),
orpiment (As2S3), arsenopyrite (AsFeS) en realgar
(As4S4) is most prevalent.
Arsenic is applied in different shapes and forms, and can enter water
bodies as such. Large quantities of arsenic that are released from
volcanic activity and from micro organisms are relatively small compared
to the quantities released from for example
fossil fuel combustion.
Metallic arsenic is processed in lead or copper alloys, to increase
hardness. The extremely toxic arsenic gas ASH3 plays an
important role in microchip production. Copper arsenate (Cu3(AsO4)2.4H2O)
is applied as a pesticide in viticulture, but its use is currently
prohibited in many countries. Paxite (CuAs2) is an
insecticide and fungicide.
Other arsenic compounds are applied as a wood preservative, in glass
processing, in chemical industries, or in semiconductor technique
together with gallium
and indium.
Dutch painters applied arsenic as a yellow pigment. In the
First World War
arsenic was applied in chemical weapons. In the
Vietnam War
dimethyl arsenic acid was applied for the destruction of rice cultures.
Although arsenic is applied less and less, it is still present in the
environment in considerable quantities. For example, near abandoned mines
soil quantities of arsenic may still be up to 30 g/kg.
Arsenic was and is applied for medical purposes. In water from safe
sources it probably aids curing asthma, haematological illnesses,
dermatosis and psychosis. In the 19th century watery solutions of
potassium arsenide (Fowler solution) were applied to treat chronic bronchial
asthma and other diseases. At the beginning of the 20th century other
arsenic compounds were applied to treat syphilis. Arsenic may assist in
curing sleeping sickness and leukaemia.
Arsenic compounds may enter the body less specifically through food
intake. This encompasses 90% of the total arsenic intake, mainly from
fish products. Through fish grind in cattle feed arsenic may enter meat,
and through contaminated soils it may enter plant products. In mushrooms
near formed arsenic melting plants concentrations up to 50 mg/kg dry
matter were found.
What are the environmental effects of arsenic
in water?
Arsenic is an essential compounds for many animal
species, because it plays a role in protein synthesis. It is unclear
whether arsenic is a dietary mineral for humans. Arsenic toxicity is
another important characteristic. The boundary concentration of arsenic
is 2-46 ppm for freshwater algae. The
LC50 value
for Daphnia Magna is 7.4 ppm, and for the American oyster it is 7.5 ppm.
These values encompass a time period of 48 hours. The chronic toxicity
values for a time period of three weeks is 0.5 ppm for the large
cladoceran. For rats an LC50 value of 20 mg/kg body mass was
established. This is the value for the carcinogenic arsenic(III)oxide
compound. This compounds also blocks enzymatic processes, increasing its
toxicity. In mice, hamsters and rats the compounds was embryo toxic and
teratogenic. Ferns bioaccumulate large quantities of arsenic.
Naturally, only one stable arsenic isotope exists. Currently 19 other
instable isotopes have been discovered.
What are the health effects of arsenic in water?
Arsenic related illness is usually caused by
consumption of contaminated drinking water. In the old days it was
applied as a poison, because symptoms of arsenic poisoning resemble
cholera symptoms, and therefore the intentional factor was shaded.
Arsenic appears to be essential for some plant and animal species. A
possible safe dose for humans was calculated. If arsenic is a dietary
mineral, this dose would be 15-25 μg. This amount could be absorbed from
food without any trouble. The total amount of arsenic in a human body is
about 0.5-15 mg. Many arsenic compounds are absorbed 60-90%, but they
are also easily excreted. Humans can develop resistance to certain
arsenic concentrations. Shortly after absorption arsenic can be found in
liver, spleen, lungs and digestive tract. Most arsenic is excreted, and
residues may be found in skin, hair, nails, legs and teeth.
Under conditions of prolonged exposure, many organs may be damaged, skin
pigmentation may occur, hair may fall out and nail growth may stop.
Toxicity differs between various arsenic compounds, for example,
monomethyl arsenic acid and inorganic arsenide have a higher toxicity
level than arsenic choline. Acute toxicity is generally higher for
inorganic arsenic compounds than for organic arsenic compounds. Oral
intake of more than 100 mg is lethal. The lethal dose of arsenic
trioxide is 10-180 mg, and for arsenide this is 70-210 mg. The mechanism
of toxicity is binding and blocking sulphur enzymes. Symptoms of acute
arsenic poisoning are nausea, vomiting, diarrhoea, cyanosis, cardiac
arrhythmia, confusion and hallucinations. Symptoms of chronic arsenic
poisoning are less specific. These include depression, numbness,
sleeping disorders and headaches.
Arsenic related health effects are usually not acute, but mostly
encompass cancer, mainly skin cancer. Arsenic may cause low birth weight
and spontaneous abortion.
Arsenic in drinking water is an issue of global importance, therefore
the legal limit was decreased to 10 μg/L. This legal limit is not met in
countries such as Vietnam and Bangladesh, where millions of people
consume drinking water with an arsenic content of over 50 μg/L. This
problem results in long-term chronic health effects, such as skin
disease, skin cancer, and tumours in lungs, bladder, kidneys and liver.
Read more about the
arsenic problem in Bangladesh
on our environmental disasters page
Which water purification technologies can be applied to remove
arsenic from water?
Arsenic removal from water can be carried out in
different ways. Options include ion
exchange, membrane filtration,
and iron and
aluminium
coagulation. Drinking
water mainly contains inorganic arsenic (arsenide or arsenate),
therefore determining the total arsenic concentration suffices.
Distinguishing between different types of arsenic is irrelevant.
Arsenic removal from soils can be achieved by applying ferns that
bioaccumulate large arsenic concentrations.
Literature and the
other elements and their interaction with water
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