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Arsenic (As) and water

Arsenic and water: reaction mechanisms, environmental impact and health effects

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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