Chemical Reactions Of The Ferric

The ferric salts used as coagulant in water and wastewater treatment are FeCl3 and Fe2(SO4)3. They have essentially the same chemical reactions in that both form the Fe(OH)3(s) solid. When these coagulants are dissolved in water, they dissociate according to the following equations:

As in any coagulation process, these ions must be rapidly dispersed throughout the tank in order to effect the complete coagulation process. The solid precipitate Fe(OH)3(s) and complexes are then formed. The reactions, together with the respective equilibrium constants at 25°C, are as follows (Snoeyink and Jenkins, 1980):

Fe(OH)3(s) ^ Fe3+ + 3OH- Kp,Fe(OH)3 = 10-38 (12.33)

Fe (OH )3(s) ^ FeOH2+ + 2OH- K FeOHc = 10-2616 (12.34)

Fe(OH)3(s) ^ Fe(OH) ++ OH- Kft(OH)2c = 10-1674 (12.35)

2Fe(OH>3(s) ^ Fe2(OH)4+ + 4OH- K^ohV = 10-508 (12.37)

The complexes are FeOH , Fe(OH)+, Fe(OH)-, and Fe2(OH)4+. 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 above equations in an actual coagulation treatment of water as in all applications of coagulants, conditions must be adjusted to allow maximum precipitation of the solid which in the present case is represented by Fe(OH)3(s). To allow for this maximum precipitation, the concentrations of the complex ions must be held to a minimum.

12.7.1 Determination of the Optimum pH

For effective removal of the colloids, as much of the ferric ions should be converted to the solid Fe(OH)3(s). Also, as much of the concentrations of the complex ions should neutralize the primary charges of the colloids to effect their destabilization. Overall, this means that once the solids have been formed and the complex ions have neutralized the colloid charges, the concentrations of the complex ions standing in solution should be at the minimum, which corresponds to the optimum pH for the coagulation process.

Let spFeIII represent all the species that contain the Fe(III) ion standing in solution. Thus, the concentration of all the species containing the ion is

[ spFeIII ] = [ Fe3+] + [ FeOH2+ ] + [ Fe(OH)+] + [ Fe(OH)-] + 2 [ Fe2(OH)2+ ] (12.38)

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 the expressing of [spFeIII] in terms of the hydrogen ion. Differentiating the resulting equation of

[spFeIII] with respect to [H+] and equating the result to zero will produce the minimum concentration of spFeIII and, thus, the optimum pH determined. Using the equilibrium reactions, Eqs. (12.33) through (12.37), along with the ion product of water, we now proceed as follows:

3+ { Fe3+ } Ksp,Fe(OH)3 Ksp,Fe(OH)3 {H } Ksp,FeCOH),7n[H ]

7FeIII 7FeIIl{OH- }3 7FeInKW Yf^K

FeOHc FeOHc {H+}2 FeOHc 7ih[H+]2 [ FeOH ] - —-- - -— - -— - -—

7Fe°Hc 7FeoHc {OH } YFeOHcKw YFeOHcKw

rr- T\ —I {Fe(OH)4} KFe(OH)4c{OH } KFe(OH)4cKw KFe(OH)4cKw

7Fe(°h)4c 7Fe(°h)4c 7Fe(OH)4c {H+} 7Fe(OH)4c7H{H+}

YFem, Yh, fteOHc, YFe(OH)2c, YFe(OH)4c, Y=e2(OH)2c are, respectively, the ac+ivity coeffi-cients of the ferric and the hydrogen ions and the complexes FeOH +, Fe(OH)+, Fe(OH)-, and Fe2(OH)2. Ksp, Fe(OH)3 is the solubility product constant of the solid Fe(OH)3(s) and Kw is the ion product of water. KFeoHc, Kpe(OH)2c Kpe(OH)4c, and KFe (OH)2c are, respectively, the equilibrium constants of the complexes FeOH2+, Fe(OH)2+, Fe(OH)-, and Fe2(OH)2+.

Equations (12.39) through (12.43) may now be substituted into Equation (12.38) to produce

Ksp,Fe(OH)37H[H ] K FeOHc YH[H+]2 K Fe(OH)2c7H[H+ [SpFeIII] - -3--1 2--+ '


Differentiating with respect to [H+], equating to zero, rearranging, and changing H+ to H+pt, the concentration of the hydrogen ion at optimum conditions,

I"8 K Fe2(OH)2c7H


I KFe(OH)2cTH 7Fe(OH)2cKw

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