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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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2.2.1 Removal of arsenic oxoanions from drinking water

To remove arsenite and arsenate anions from water one method is to add particles of iron(III) hydroxide and allow time for the anion exchange to take place. However, where the water is pumped directly from a well for use, a better solution is to use a synthetic ion-exchange medium or resin as in Figure 8 and similar to what you previously saw for the removal of nitrate from water.

Described image
Figure 8 (a) Diagram showing how an ion-exchange medium removes arsenic ions from water. (b) A village water pump in Bangladesh purified using ion-exchange chromatography.
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Figure 8 The diagram in part (a) consists of a tall vertical cylinder, which narrows at the bottom. An arrow labelled 'contaminated water' points vertically down into the top of the cylinder, and an arrow labelled 'purified water' points vertically down from the bottom of the cylinder.

There are eight grey circles arranged in a regular pattern throughout the cylinder. A key identifies the circles as representing 'ion-exchange medium'. The area between the grey circles is coloured pale blue. Near the top of the cylinder there are a number of small yellow pentagons and blue triangles. The key identifies these as representing 'arsenic ions' and 'other contaminants'. There are also a few small green squares, which the key identifies as 'chloride ions'. Most of the yellow pentagons and blue triangles are attached to grey circles near the top of the cylinder. Near the bottom of the cylinder there are no yellow pentagons or blue triangles, but there are many more green squares. Most of the green squares are attached to the grey circles near the bottom of the cylinder.

Part (b) is a photograph which shows a man operating a hand pump attached to the bottom of two vertical cylinders, which are almost as tall as the man. The two vertical cylinders at the top are attached by pipes to the water supply. There is a bucket near the pump, and a child in the foreground of the photograph.

Figure 8 (a) Diagram showing how an ion-exchange medium removes arsenic ions from water. (b) A village water pump in Bangladesh purified...

For arsenic contamination, the ion-exchange medium exchanges chloride ions, Cl−, for arsenate ions, AsO43−. Ion-exchange is almost instantaneous, a big advantage over adsorption processes using iron(III) hydroxide. After a few months the medium becomes saturated with arsenate ions and so is regenerated by eluting with a sodium chloride solution. This process presupposes a safe means of disposing of the eluted arsenate containing solution.

In practice, it has been observed that oxidation is beneficial before the removal of arsenic oxoanions. What is the chemistry behind this observation?

Oxidation favours formation of arsenic acid, H3AsO4, which is a stronger acid than arsenous acid, H3AsO3.

  • At neutral pH, which of the oxoanions, arsenite AsO33− and arsenate AsO43− will bind most strongly to a cation-exchange resin?

  • Arsenate, AsO43−, binds more strongly to a cation-exchange resin. Arsenate will be more fully deprotonated at neutral pH because arsenic acid is a stronger than arsenous acid. Consequently there will be a higher negative charge and greater electrostatic attraction to the cationic resin.