A10.2 Removal of Heavy Metals

## Absorption

Polluted water can be filtered through activated carbon. The metal ions are adsorbed onto the surface of the carbon and so are removed from the solution.
In recent years, other organic materials like chitosan and agar have been used to remove heavy metals from water and waste water.

## Did you know? 💡

Chitosan is a polysaccharide derived from the exoskeleton of crustaceans and insects. Agar is a polymer present in some types of seaweed.

## Precipitation

Heavy metals are typically precipitated from polluted water as hydroxides, sulphides and sometimes sulphates or carbonates.
The water is then either filtered or allowed to settle and in this way the heavy metal solids are removed from the solution.

## Chelation

Special chemical agents (chelating agents) are used which bind strongly to the heavy metal ions and effectively “capture them”. This method is used by doctors to treat heavy metal poisoning.

## Precipitation of Heavy Metals as Hydroxides

Precipitation as the metal hydroxide is the most common. The precipitation follows the reaction:
$\textsf M{^\textsf{n+}}\textsf{(aq)} \textsf{+ nOH}{^{-}\textsf{(aq)}}$
$\textsf{M(OH)}_\textsf{n}\textsf{ (s)}$
Many heavy metals are amphoteric which means that their solubility reaches a minimum at a different pH for each metal.
The addition of soluble hydroxide ions by adding alkaline substance materials is used to raise the pH.
$\textsf{NaOH}$
or
$\textsf{Ca(OH)}_2$
(often known as lime) solutions are both used.
Calcium hydroxide is less expensive than
$\textsf{NaOH}$
and also has the advantage of acting as a coagulant, making the metal hydroxide particles clump together and settle more easily.
$\textsf{NaOH}$
does not have this effect.