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.
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.
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).
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.
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.
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.
Read more about the arsenic problem in Bangladesh on our environmental disasters page
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.