Chemical Reactions Of The Manganous Ion [MnII

Manganese can be removed as Mn(OH)2 using a suitable OH- source. Upon introduction of the hydroxide source, however, it is not only this solid that is produced. Manganese forms complex ions with the hydroxide. The complex equilibrium reactions are as follows (Snoeyink and Jenkins, 1980):

Mn(OH)2(S) ^ Mn2 + + 2OH- K^oh), = 4.5(10-14) (13.13) Mn(OH)2 ^ Mn2+ + 2OH- KMii(oh)2C = 10-68 (13.14)

The values of the equilibrium constants given above are at 25°C. The complexes are Mn(OH)+, Mn(OH)2, and Mn(OH)-. Also note that the OH- ion is a participant in these reactions. This means that the concentrations of each of these complex ions are determined by the pH of the solution. In the application of the previous equations in an actual treatment of water, conditions must be adjusted to allow maximum precipitation of the solid represented by Mn(OH)2(s). To allow for this maximum precipitation, the concentrations of the complex ions must be held to the minimum. The pH corresponding to this condition is the optimum pH. From the previous reactions, the equivalent mass of Mn2+ is Mn/2 = 27.45.

13.4.1 Determination of the Optimum pH

Let spMn represent the collection of species standing in solution containing the Mn(II) species. Thus,

[ SpMn ] = [ Mn2+] + [ Mn(OH)+] + [ Mn(OH)2 ] + [ Mn(OH)-] (13.16)

All the concentrations in the right-hand side of the above equation will now be expressed in terms of the hydrogen ion concentration. This will result in expressing

[spMn] in terms of the hydrogen ion. Differentiating the resulting equation of [spMn] with respect to [H+] and equating the result to zero will produce the minimum concentration of spMn and, thus, the optimum pH determined.

Using the equations and equilibrium constants of Eqs. (13.12) through (13.15), along with the ion product of water, we proceed as follows:

[M 2+] {Mn2+} Ksp, Mn(OH)2 Ksp, Mn(OH)2{H } Ksp, Mn(OH)27n[H ]

[ Mn(OH)-] = {Mn(OH)-} = {Mn2+}{ OH-}3 = Kp Mn( oh)2Kw

7Mn(OH)3c 7Mn(OH)3cKMn(OH)3c YMn(OH)3cKMn(OH)3c Yh [H+]

YMn, YMnOHc, YMn(OH)2c, and YMn(OH)3c are, respectively, the activity coefficients of Mn(II) and the complexes Mn(OH)+, Mn(OH)0, and Mn (OH)3. K Mn(OH)2 is the solubility product constant of the solid Mn(OH)2(s). KMnOHc, KMn(OH^c, and KMn(OH) c are, respectively, the equilibrium constants of the complexes Mn(OH)+, Mn(OH)0, and Mn(OH)-.

Equations (13.17) through (13.20) may now be substituted into Equation (13.16) to produce

YMnKW YMn( OH) cK MnOHcKw KMn( OH)2c

Ksp, Mn(OH)2Kw

Differentiating with respect to [H+], equating to zero, rearranging, and changing H+ to H„p(, the concentration of the hydrogen ion at optimum conditions, the following expression is produced:

YMnKW YMn(OH) cK MnOH cKw YMn(OH)3cKMn( OH)3cYH

The value of [Hop(] may be solved by trial error.

Example 13.2 Calculate the optimum pH for precipitating Mn(OH)2. Assume the water contains 140 mg/L of dissolved solids.

Solution:

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