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What chemical compounds might be present in drinking water?
What chemical compounds might be present in drinking water?

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2.2 Arsenic pollution in the environment

Inorganic arsenic is naturally present at high levels in the groundwater of a number of countries, including Bangladesh, China, India, Mexico, and the USA. Note when articles discuss arsenic levels in natural waters they are talking about the oxoanions arsenite, AsO33−, and arsenate, AsO43−.

  • What are the oxidation numbers of arsenic in arsenite and arsenate?

  • Arsenate(III) and arsenate(V) or +3 and +5 respectively, which are bioavailable forms of arsenic.

The World Health Organization (2015) suggests a maximum safe level for drinking of arsenic of only 10 µg l−1.

  • Which vital element do you think arsenic might replace in the body?

  • Phosphorus displays similar chemistry as it is above arsenic in Group 15 and is indeed replaced by arsenic in organs and tissues within the body. For example, phosphorus can be substituted by arsenic in adenosine triphosphate (ATP) which plays a fundamental role in metabolism.

In natural waters one of the main sources of arsenic oxoanions is believed to be the weathering of pyrite, FeS2, minerals which contain a trace of arsenic. Weathering also oxidises some of the pyrite to form iron(III) hydroxide, Fe(OH)3, and sulfate. Remarkably, this iron(III) hydroxide then has a capacity to exchange its surface hydroxide anions and so is able to adsorb arsenite and arsenate anions from natural water effectively restricting their mobility in the environment. However, this adsorption on iron(III) hydroxide is pH-dependent, and so the arsenate ions can be re-mobilised into the environment by a pH change.

  • Which other anion in water do you predict iron(III) hydroxide might adsorb?

  • Phosphate, PO43−, is similar to arsenate as in both anions the central elements are in Group 15. So phosphate can be similarly absorbed.