Arsenic in drinking-water
Background document for development of WHO Guidelines for Drinking-water Quality
Arsenic exists in oxidation states of -3, 0, 3 and 5. It is widely distributed throughout the Earth’s crust, most often as arsenic sulfide or as metal arsenates and arsenides. In water, it is most likely to be present as arsenate, with an oxidation state of 5, if the water is oxygenated. However, under reducing conditions (<200 mV), it is more likely to be present as arsenite, with an oxidation state of 3 (IPCS, 2001).
|Compound||CAS No.||Molecular formula|
|Dimethylarsinic acid (DMA)||75-60-5||(CH3)2AsO(OH)|
|Monomethylarsonic acid (MMA)||124-58-3||(CH3)AsO(OH)2|
2 Physicochemical properties (IARC, 1980; Lide, 1992–1993)
|Compound||Melting point (°C)||Boiling point (°C)||Density (g/cm3)||Water solubility (g/litre)|
|As||613||–||5.727 at 14 °C||insoluble|
|As2O3||312.3||465||3.738||37 at 20 °C|
|As2O5||315 (decomposes)||–||4.32||1500 at 16 °C|
|As2S3||300||707||3.43||5 × 10-4 at 18 °C|
|(CH3)2AsO(OH)||200||–||–||829 at 22 °C|
|PbHAsO4||720 (decomposes)||–||5.79||very slightly soluble|
|KH2AsO4||288||–||2.867||190 at 6 °C|
3 Major uses
Arsenicals are used commercially and industrially as alloying agents in the manufacture of transistors, lasers and semiconductors, as well as in the processing of glass, pigments, textiles, paper, metal adhesives, wood preservatives and ammunition. They are also used in the hide tanning process and, to a limited extent, as pesticides, feed additives and pharmaceuticals.
4 Environmental fate
Arsenic is introduced into water through the dissolution of rocks, minerals and ores, from industrial effluents, including mining wastes, and via atmospheric deposition (IPCS, 1981; Nadakavukaren et al., 1984; Hindmarsh & McCurdy, 1986). In well oxygenated surface waters, arsenic(V) is generally the most common arsenic species present (Irgolic, 1982; Cui & Liu, 1988); under reducing conditions, such as those often found in deep lake sediments or groundwater, the predominant form is arsenic(III) (Lemmo et al., 1983; Welch et al., 1988). An increase in pH may increase the concentration of dissolved arsenic in water (Slooff et al., 1990).