Co-precipitation

[Cami]

Hi everyone 😊 as a beginner, I need help to solve a problem related to co-precipitation (I read the manual and some of the proposed examples, but I still didn’t find the solution since my problem is more complex). I’d like to check the amount of different possible precipitated material obtained at different pH.
This is what I was thinking about:
1)   After inserting my mother solution and adding Ca(OH)2 to rise the pH and saving the intermediates, I run the program for the first time and check the SI>0 of different phases at pH=3; 4 and 8 (consecutive step of pH) to have an idea of the phases that can potentially precipitate.
2)   use the EQUILIBRIUM_PHASES keyword and set the SI at 0 for all the phases that can potentially precipitate and check the amount in the phase assemblage.

My final goal is to find a pH where Gypsum formation is minimized and Ca-P (especially monetite) formation is maximized.

Is this a possible approach or am I totally wrong? Should I insert in every equilbirium_phases block all the phases I'm interested in? I noticed that if I do not set SI=0 for the different phases, P concentration in solution remains the same even if I changed the pH...

I would really appreciate any comments and suggestions!
Have a great day,
Cami

Code: [Select]

SOLUTION 1 Acidic leachate #mother solution
    temp      25
    pH        1.14
    pe        4
    redox     pe
    units     mg/l
    density   1
    Al        990
    Ca        615
    Fe        56.5
    P         3415
    S(6)      19212
       -water    1 # kg
END

Use solution 1

PHASES #I'm interested in, not present in the phreeqc.dat database
Monetite
    CaHPO4 = Ca+2 + HPO4-2
    log_k     -6.81
OCP
    Ca8(HPO4)2(PO4)4 = 8Ca+2 + 2HPO4-2 + 4PO4-3
    log_k     -36.48
TCP
    Ca3(PO4)2 = 3Ca+2 + 2PO4-3
    log_k     -32.63
ACP
    Ca3(PO4)2:H2O = 3Ca+2 + H2O + 2PO4-3
    log_k     -25.46
Brushite
    CaHPO4:2H2O = Ca+2 + 2H2O + HPO4-2
    log_k     -6.6
Ca(OH)2
    Ca(OH)2 = Ca+2 + 2OH-
    log_k     -5.2
pH_fix
    H+ = H+
    log_k     0
EQUILIBRIUM_PHASES 1
Anhydrite  0 0
Gypsum 0 0
    pH_fix    -3 Ca(OH)2
        -force_equality

SELECTED_OUTPUT 1
    -file                 girobase.sel
    -pH                   true
    -totals               P Al Fe Ca S(6)
    -saturation_indices   Anhydrite Brushite Gibbsite Gypsum Monetite TCP ACP Al(OH)3(a) Goethite Hematite Pyrite Vivianite                   
    -equilibrium_phases Anhydrite Brushite Gibbsite Gypsum Monetite TCP ACP Al(OH)3(a) Goethite Hematite Pyrite Vivianite                   
-water
save solution 2
END

USE solution 2
EQUILIBRIUM_PHASES 2
Anhydrite 0 0
Brushite 0 0
Gibbsite 0 0
Gypsum 0 0
Monetite 0 0
TCP 0 0
    pH_fix    -4 Ca(OH)2
        -force_equality
save solution 3
END

Use solution 3
EQUILIBRIUM_PHASES 2
ACP 0 0
Al(OH)3(a) 0 0
Anhydrite 0 0
Brushite 0 0
Gibbsite 0 0
Goethite 0 0
Gypsum 0 0
Hematite 0 0
Monetite 0 0
Pyrite 0 0
TCP 0 0
Vivianite 0 0
    pH_fix    -8 Ca(OH)2
        -force_equality
END

[dlparkhurst]

My first question is about redox. Is it reasonable to precipitate pyrite? If the system is oxygenated, I expect Fe to precipitate as an iron oxyhydroxide.

But even if reducing conditions are plausible, a couple of your minerals are unlikely. In  a rapidly evolving system, you expect more soluble phases to precipitate first: ferrihydrite or goethite rather than hematite, mackinawite rather than pyrite. I guess Fe and Al are relatively small, but depending on your choices, they could affect P.

[Cami]

Thank you very much for your answer and time!
Yes, you are right. Fe will probably precipitate as Fe(OH)3 or at least as FePO4.
So, I should do an initial screening to delete the phases that are unlikely to be present as precipitates (basically remove them from the database).

Sorry for my bad chemistry basis, but I thought that the less soluble phases will precipitate first...so I can compare the Ksp of different phases (e.g. Goethite and hematite) and pick up the one who has the lowest Ksp?

Thank you in advance

[dlparkhurst]

Not sure if you really meant database, just remove phases you do not want to react from EQUILIBRIUM_PHASES.

True, the least soluble phases should ultimately be the stable phase assemblage, but that may take geologic time. When you add Ca(OH)2 rapidly, the idea is that more soluble phases will form initially from high supersaturations, but the phases may then convert to more stable phases over time. The recipes for making Ferrihydrite for surface studies indicate that the phase is only stable for a limited time and then begins to recrystallize. For some phases, like hematite or pyrite, I doubt the transition would occur in any reasonable time.