Chemical Reactions Of The Ferrous And The Ferric Ions

The chemical reactions of the ferrous and the ferric ions were already discussed in a previous chapter. From the topic in the preceding section, iron is more efficiently removed as ferric hydroxide. The natural iron is in the form of Fe(II), so this ferrous must therefore oxidize to the ferric form in order to precipitate as the ferric hydroxide, if, in fact, the iron is to be removed in the ferric form. In Chapter 12, this was done using the dissolved oxygen that is relatively abundant in natural waters. It must be noted, however, that based on the Ksp values, iron can also be removed in the ferrous form as Fe(OH)2. For convenience, the reactions are reproduced next. Ferrous:

Fe(OH)2(S) + OH- ^ Fe (OH)- K Fe(OH)3c = 10-51 (13.3)

Fe(OH)2 + 4-O2 + 2-H2O ^ Fe(OH)3 I (conversion from ferrous to ferric) (13.4)

Ferric:

Fe(OH)3(S) ^ FeOH2 + + 2OH- KP6ohc = 10-2616 (13.6)

Fe(OH)3(S) ^ Fe(OH)+ + OH- KFe(oh)2c = 10-1674 (13.7)

Fe(OH)3(s) + OH- ^ Fe( OH)- K Pe(OH)4c = 10-5 (13.8)

2Fe(OH)3(s) ^ Fe2(OH)2+ + 4OH- KFe2(OH)2c = 10-50 8 (13.9)

Remember that the values of the solubility product constants, Ksp,Fe(OH)2 and Ksp,Fe(OH) , and all the other equilibrium constants for the complex ions apply only at 25°C. 3

The presence of the complex ions increase the solubility of the iron species and therefore increase the concentration of these species in solution. For the ferrous and the ferric species, they are spFeII and spFeIII and were derived in Chapter 12, respectively, as

= ^-sp.FetOH^yH^ ] + K-FeOHc7H[H+] + kfe(oh)3cKw (1310)

/FellKW 7FeOHcKw YFe(OH)3c Yh[H+]

[ spFeiii ] = [ Fe3+] + [ FeOH2+] + [ Fe( OH)+] + [ Fe( OH)-] + 2 [ Fe2( OH) 2+]

Ksp, Fe(OH)37n[H ] KFeOHc7lH[H+]2 KFe(OH)2c7H[H ] KFe(OH)4cKw

YFelllKW YFeOHcKW YFe(OH)2cKw YFe(OH)4c [H+]

7Fe2( OH)2cKw

Example 13.1 From the respective optimum pH's of 11.95 and 8.2 for spFeII and spFeIII, calculate the concentrations [spFeII] and [spFeIII], respectively. Assume the water contains 140 mg/L of dissolved solids.

Solution:

r _ Ksp,Fe(OH)27H[H ] KFeoHc7h [H+] KFe(OH)3cKw [spFeII] _--;2--1 "-j---+

ksp,Fe(oh)2 _ 10-145 / _ 2.5( 10^)TDS y _ 10 1+1-14 / _ 2.5(10-5)( 140) _ 3.5(10-3)

Therefore, r ] _ 1 0-145 ( 0.94 ) 2 r 1 0-1 1 9 ] 2 (10-9.4) ( 0.94 ) r 1 0-1 1 .95 ] (1 0-5. 1 ) (1 0- 1 4 )

(0.77)( 10-14)2 (0.94)( 10 14) (0.94)(0.94)[10 1195 ]

_ 4.57( 10-11) + 4.47(10-8) + 8.01( 10-8) _ 1.0( 10-7) gmol/L Ans _ 0.0056 mg/L

[ ] Ksp,Fe(OH)37H[H ] KFeOHcYlH[H+]2 KFe(OH)2cYH[H ] KFe(OH)4cKw [spFeIII] _ -773--1 772--1 Z-1?--+ '

7FeIII Kw 7FeOHcK w

7Fe2(OH)2cK w

Ksp,Fe(OH)3 _ 10 7FeIII _ 10 1 + U4[ 3 5(10 )] _ 0.56 KFeOHc _ 10

YFeOHc _ 10-1 + u4[a/,3.5(10 3)1 _ 0.77 KFe(OH)2c _ 10"16 74

0.5 (4)2[V 3.5(10-3)] 1° 1 + 1.14[V 3.5( 10-3)] = °36

Therefore,

_ (10-38) ( 0.94 ) 3 [ 1 0-8'2] 3 + ( 1 0-26-1 ( ( 0.94 ) 2 [ 1 0-82] 2 + ( 1 0-16'74 ) ( 0.94 ) [ 1 0-8,2] (0.56)(10-14)3 (0.77)(10-14)2 (0.94)(10-14)

+ (10-5)(10-14) + 2(10-5a8X0)94)4[10-8'2]4 (O^m-8'2] (0.36)(10-14)4

_ 2i09(10-63) + 22.44)3(10"43)) + l.Oi^C 10-.225)) + L0( 10-19) + 3.92( 10-84) 5)6(10-43) 7.7(10-29) 9.4(10-)5) 5.93(10-9) 3.6( 10-57)

= 3.73( 10-21) + 3.16( 10-15) + 1.15 (10-11) + 1.69 (10-11) + 1.08 (10-27)

Practical Optimum pH Range for the Removal of Ferrous and Ferric

As shown in Chapter 12, at 25°C and at a solids concentration of 140 mg/L, the optimum pH's correspond to 11.95 and 8.2 (or around 12 and 8), respectively, for ferrous and ferric. The respective concentrations for spFeII and spFeII at these conditions as obtained in the previous example are [spFeII] = 0.0056 mg/L and [spFeIII] = 0.0000016 mg/L. A pH range exists, however, at which units used for the removal of the elements can be operated and effect good results. This range is called the practical optimum pH range.

Tables 13.2 through 13.5 show the respective concentrations of spFeII and spFeIII at other conditions of pH and total solids. The values for [spFeII] were obtained using Equation (13.10) and the values for [spFeIII] were obtained using Equation (13.11). Note that these equations require the values of the activity coefficients of the ions. The activity coefficients are needed by the equations and, since activity coefficients are functions of the dissolved solids, dissolved solids are used as parameters in the tables, in addition to pH.

The previous tables indicate that the total solids (or equivalently, the activities of the ions) do not have a significant effect on the optimum pH values, which for spFeII remain at about 12.0 and for spFeIII remain at about 8.0. For practical purposes, however, the practical optimum pH for spFeII ranges from 11 to 13 and for spFeIII, it ranges from 5.0 to 13.0. Note that for the range of pH for spFeII, it is assumed the element is to be removed as Fe(OH)2. If it is to be removed as Fe(OH)3, the pH of the solution during its oxidation by dissolved oxygen or any oxidizer need not be adjusted since the practical optimum pH for the precipitation of ferric hydroxide varies over a wide range from 5.0 to 13.0 and already includes the range for the ferrous removal.

K Fe(OH)4c

7Fe2(OH)2c

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