Fe(OH)3 precipitation

[cevans3098]

All,

I have a quick question regarding precipitation of Fe(OH)3.  If I create a solution of Fe(3) and Cl- and vary the pH by adjusting the Cl- concentration, PHREEQC returns a value for Fe(OH)3.  I am trying to understand what this value means.  Since the pKa for this reaction is positive

Fe(OH)2+ + H2O --> Fe(OH)3 + H+

I would not expect to see a steady state result.  How should I interpret the results from PHREEQC for Fe(OH)3?

Thanks,

Craig

[dlparkhurst]

Are you referring to a saturation index for say Fe(OH)3(a), where (a) refers to amorphous, or the aqueous species Fe(OH)3(aq)?

The saturation index will change as a function of pH related to the reaction

Fe(OH)3(a) + 3H+ = Fe+3 + 3H2O.

At very low pH, the concentration and activity of Fe+3(aq) will be somewhat constant, so the saturation index would vary as the pH varied. At higher pH, Fe+3 will hydrolyze to Fe(OH)+2, Fe(OH)2+, and others, which will affect the distribution of Fe(III) species. The following simulation, which simply titrates an FeCl3 solution with base, shows that the SI reaches a maximum at pH 10, after which it decreases. (The O(0) keeps the iron in the +3 state.)

USER_GRAPH 1
    -headings               pH SI(Fe(OH)3a) Fe+3 FeOH+2 Fe(OH)2+ Fe(OH)3 Fe(OH)4-
    -axis_titles            "pH" "SI(Fe(OH)3a)" "Molality"
    -initial_solutions      false
    -connect_simulations    true
    -plot_concentration_vs  x
  -start
10 graph_x -LA("H+")
20 graph_y SI("Fe(OH)3(a)")
30 graph_sy MOL("Fe+3"), MOL("FeOH+2"), MOL("Fe(OH)2+"), MOL("Fe(OH)3"), MOL("Fe(OH)4-")
  -end
    -active                 true
END
SOLUTION 1
pH  3 charge
Fe  1
Cl  3
O(0) 6
END
INCREMENTAL_REACTIONS
USE solution 1
REACTION 1
NaOH 1
100*0.0001 9*0.01 9*0.1 9*1
END

[cevans3098]

David,

Thank you for your response.  I have a follow-up:

Suppose I have a solution:

SOLUTION 1
       temp      50
       pH        7 charge
       pe        4
       units     mol/kgw
       density   1
       Cl        0.1
       Fe(2)     0.001
       Fe(3)     0.001
       K         0.003
       water    1 # kg

I am trying to determine what amount of precipitate I will have at different pH levels.  Using your titration example I add KOH to the solution to increase the pH.  I am trying to understand the level of precipitation in the solution and what is precipitating. 

Would you propose I just look at the Saturation indexes and see when Fe(OH)3, Goethite and Hematite are greater than zero? are Fe(OH)3, Goethite and Hematite the only potential insoluables? is there a way to quantify the amount of each species that precipitates? Not sure how Hematite would form in my solution

Warm regards,

Craig

[dlparkhurst]

I would not expect hematite to form either, and goethite will be less soluble than Fe(OH)3(a). If you add

EQUILIBRIUM_PHASES
Goethite 0 0

after the REACTION block and before the END, then the solution will precipitate goethite if it becomes supersaturated. The Basic function EQUI("Goethite") will give the amount of Goethite that has precipitated.

If you include Fe(OH)3(a) in the EQUILIBRIUM_PHASES data block (assuming the phreeqc.dat or wateq4f.dat database), then the more stable of the two phases will form, which will be goethite. If you use Fe(OH)3(a) instead of Goethite, you will find the amount of Fe(OH)3(a) that will form.

[cevans3098]

David,

That worked perfectly.  Thank you again.

Craig