Y

10-11

2.3(10-5)

2.51(10-5)

4.81(10-5)

10-12

2.3(10-8)

2.51(10-7)

2.74(10-7)

10-12 2.74 (10-7) Therefore, pH = -log101.08( 10-12) = 11.97 Ans

Example 13.3 From the optimum pH's of 11.97, calculate the concentrations spMn in mg/L. Assume the water contains 140 mg/L of dissolved solids.

Solution:

Ksp, Mn(OH)2Yh[H ] Ksp, Mn(OH)2Yh[H ] Ksp, Mn(OH)2 [ spMn] = rj +~ j; ~ +'

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

Ksp, Mn(OH)2Kw

Ksp, Mn(OH)2 — 4.5( 10 ) 7e — 7Mn(oh)c — 7Mn(OH)3c — 0'94 7Mn —

Therefore, r p ] — (4.5)(10-14)(0.94)2[10-n.91f + (4.5)(10-14X0.94)[10-1:L91] PMn (0>11X10-14)2 (0>94)(10-34X10-14)

13.4.2 Practical Optimum pH Range for the Removal of Manganese

From Example 13.2, at 25°C and at a solids concentration of 140 mg/L, the optimum pH for the removal of manganese is 11.97. The corresponding concentration for spMn is 0.0179 mg/L. As in the case for the removal of ion, there is a practical pH range at which units used for the removal of manganese can be operated and effect good results. Tables 13.6 and 13.7 show the respective concentrations of spMn at other conditions of pH and total solids. The values for [spMn] were obtained using Equation (13.21).

Again, as in the case of the removal of iron, the tables show that total solids (or equivalently, the activities of the ions) do not have a significant effect on the optimum pH value for the removal of manganese as Mn(OH)2. The optimum pH remains at around 12.0. For the practical optimum pH, we adopt the following:

manganese removal as Mn(OH)2: 11.5 < optimum pH < 12.5, which is the same as that for ferrous removed as Fe(OH)2.

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