Removing Iron and Manganese

Two types of precipitation reactions may be used to remove Fe²⁺ and Mn²⁺ from water.

1. The most important of these reactions is oxidation. Using molecular oxygen (O₂) or another oxidant such as potassium permanganate (KMnO₄), Fe(II) is readily oxidized to Fe(III) in solution (Equation 9). If the solution is alkaline (high pH, basic), the Fe(III) forms Fe(OH)₃. As the concentration of Fe(OH)₃ increases, the oxygens start to coordinate between multiple iron ions, and a lattice begins to form. (Recall the definition of a lattice from the discussion above on "The Solvation Process.") At some point in this lattice formation, the Fe(OH)₃ starts to look like Fe₂O₃ (rust) and precipitates. Hence, by adding an oxidant to the water and raising the water's pH at the water-treatment plant, an insoluble precipitate is formed. The insoluble rust can then be removed by sedimentation or filtration.

2. The water-softening agents described in the "Treating Water Hardness" section above can also help to make insoluble precipitates from Fe²⁺and Mn²⁺.

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