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2.2(10+0)

manganous ion. The values of the equilibrium constants of these complex ions are such that their concentrations diminish the hydroxide ions needed to precipitate the solid hydroxides Fe(OH)2 and Mn(OH)2. For these solids to precipitate out, more hydroxide ions must be added, resulting in a high pH. If these complex ions were destroyed, then the metal ions released would precipitate as the respective hydroxides, preventing the complex equilibria to occur.

To destroy the complex ions requires the use of oxidants. Chlorine, potassium permanganate, and ozone are normally used for this purpose. In the case of iron, the ferrous state is oxidized to the ferric state. This oxidation includes the oxidation of the ferrous complexes. Because Fe(OH)3 precipitates at a wider optimum pH range, the precipitation pH can therefore be reduced to a much lower value, i.e., to even a pH of 5.5.

Let us tackle the situation of precipitating manganese at a lower pH range. Once the complex ions have been destroyed, the reactions left would be those for a precipitation product of a higher oxidation state than 2+. The reactions for the destruction using chlorine, potassium permanganate, and ozone are, respectively, as follows:

Mn2 + + Cl2 + 2H2O ^ MnO2(s) I + 2Cl- + 4H+ (13.23)

3Mn2+ + 2KMnO4 + 2H2O ^ 5MnO2(s) I + 2K+ + 4H+ (13.24) 3Mn2+ + O3 + 3H2O ^ 3MnO2(s) I + 6H+ (13.25)

The previous reactions show that manganese is oxidized from Mn(II) to Mn(IV). This oxidation poses the possibility of Mn(IV) forming a complex with the hydroxides. The review of the literature, however, did not uncover any evidence for this to be so. It did reveal that Mn(IV) forms a complex with fluorine and potassium (Holtzclaw and Robinson, 1988). This complex is K2[MnF6]. The review also did not reveal any solubility product constant for MnO2. It could very well be that it does not have any. If, in fact, MnO2 does not have any solubility product constant, then the previous reactions can be construed to be possible at any pH. Until a study is done to show the complex formation of Mn(IV) and the accompanying solubility constant and to be compatible with the removal of ferrous as Fe(OH)3, we will adopt the practical pH for removal of manganese through its oxidation by the above reactions as 5.5 < optimum pH < 12.5. This range is, however, arbitrary and applies when the complexes have been destroyed. For more accurate results, a pilot plant investigation for a given raw water should be undertaken.

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