Calculating precipitation

[KirstyH]

Hi all PHREEQC users!

I am using PHREEQC to estimate how much calcite might precipitate from my initial solution composition.

I was just wondering how PHREEQC calculates the amount of calcite which should precipitate given the saturation index? From the below input I get +9.468E-3 moles of calcite, but when I manually calculate it (using the Ksp value to work out the molar solubility or equilibrium concentrations (sqrt(Ksp)) and then subtracting this from the initial Ca concentration) I get a slightly different value (1.013E-2). Perhaps I am manually calculating it wrong?


EQUILIBRIUM_PHASES 1
    Calcite   0 0

SOLUTION 1
    temp      10
    pH        9.019
    pe        4
    redox     pe
    units     mmol/l
    density   1
    Alkalinity 43.66
    Ca        10.15625
    Mg   9.122
    Na        5.624
    K         1.328
    S(6)      0.4
    N(5)      0.31
    Cl        0.717
    -water    1 # kg

I am so sorry if this is a stupid question!

Many thanks,
Kirsty

[dlparkhurst]

You are on the right track, it is just that PHREEQC is doing a more complex calculation. Depending on the database, PHREEQC includes activity coefficients to calculate activity for the ion-activity product part of saturation index. For ion-association databases, the activity coefficients are functions of ionic strength, so all other ions affect the calculation. Ion-association databases also include ion pairs and complexes, which reduce the concentration of the free ions relative to total concentrations.

[KirstyH]

Thank you for this explanation Dr Parkhurst, very helpful!

[Jeisonb]

Hello, I have a question, is there a function that allows me to determine the amount of precipitates in the solution, the saturation indices (SI) of the phases can allow me to identify which substances are dissolved or precipitated but can I find the total difference of the precipitates formed in the neutralization of an acid water with calcite ? Me solution is :
TITLE  Neutralización de agua acida
SOLUTION 1  Agua ácida
    temp         25
    pH           3.62   
    units        mg/L           
    density     1.01
    pe           12
    AL      36.9
    Ca      9.6
    Cl      4.0
    Cu      6.4
    Fe      74.4
    Mg      8.4
    Mn      2.2    
    K      3.4
    Na      2.6
    S      52.5
    Zn      1.2
    O(0)        8.36
   -water    1       # kg
 reaction  1
  CaCO3   0.1
 
 END

[dlparkhurst]

Here are a couple calculations that address your question. The first simply calculates the amount of calcite that dissolves to reach equilibrium. The result is in the output file.

The second adds calcite incrementally, but precipitates excess calcite addition once equilibrium is reached.

Code: [Select]

TITLE  Neutralización de agua acida
SOLUTION 1  Agua ácida
    temp         25
    pH           3.62   
    units        mg/L           
    density     1.01
    pe           12
    AL      36.9
    Ca      9.6
    Cl      4.0
    Cu      6.4
    Fe      74.4
    Mg      8.4
    Mn      2.2   
    K      3.4
    Na      2.6
    S      52.5
    Zn      1.2
    O(0)        8.36
   -water    1       # kg
END
# Calcite equilibrium
USE solution 1
EQUILIBRIUM_PHASES 1
Calcite 0 10
USER_PRINT
10 PRINT "Calcite dissolved to equilibrium: ", -EQUI_DELTA("Calcite")
END
# incremental addition of calcite, precipitate at equilibrium
USE solution 1
REACTION 1
    CaCO3      1
    0.01 moles  in 100
USER_GRAPH 1
    -axis_titles            "CaCO3 added, moles" "pH" ""
    -initial_solutions      false
    -connect_simulations    true
    -plot_concentration_vs  x
  -start
10 GRAPH_X RXN
20 GRAPH_Y -LA("H+")
  -end
    -active                 true
EQUILIBRIUM_PHASES
Calcite 0 0
END


[dlparkhurst]

You should also consider adding Al, Mn, and Fe oxyhydroxide minerals to EQUILIBRIUM_PHASES. Several minerals that are likely to precipitate should be added, like gibbsite, pyrolusite, and goethite.

[Jeisonb]

Perfect, I will try to start from scratch using the template but not with the acidic water but with pure water and using situations of adding calcite, then quicklime (CaO) and Hydrated Lime Ca(OH)2 this in order to corroborate with the theory the results of solubility in water, after checking the results I do enter the acidic water template and I will take into account the phases that present positive saturation indexes (SI) in order to take those phases that precipitate as the "sludge" of the acidic water formed.
Thank you very much, it is very helpful, I will simulate and I will comment the results. Thank you again.

[Jeisonb]

in the output file it shows possible minerals and Iron Oxyhydroxide formed like Goethite and Hematite but when adding more than one phase of the same element to Phase Equilibrium, the output file only recognizes the formation of Iron for Goethite, my question is yes by the element (Fe, Al, Mn) you can only see the precipitate of one phase, or I could see the amount of Hematite and Goethite formed ?

DATABASE c:\phreeqc\database\ThermoddemV1.10_15Dec2020.DAT
TITLE  Neutralización de agua acida
SOLUTION 1  Agua ácida
    temp         25
    pH           3.62   
    units        mg/L           
    density     1.01
    pe           12
   
    Fe      74.4
    S(6)    52.5
    O(0)    8.36
   
   -water    1       # kg
END
# Calcite equilibrium
USE solution 1
EQUILIBRIUM_PHASES  1
Calcite 0 10

USER_PRINT
10 PRINT "Calcite precipato to equilibrium: ", EQUI_DELTA("Calcite")
20 PRINT "Goethite  precipato to equilibrium: ", EQUI_DELTA("Goethite ")
30 PRINT "Hematite precipato to equilibrium: ", EQUI_DELTA("Hematite")
END
# incremental addition of calcite, precipitate at equilibrium
USE solution 1
REACTION 1
    CaCO3 1     
    0.1 moles in 100
USER_GRAPH 1
    -axis_titles            "CaCO3 added, moles" "pH" ""
    -initial_solutions      false
    -connect_simulations    true
    -plot_concentration_vs  x
  -start
10 GRAPH_X RXN
20 GRAPH_Y -LA("H+")
  -end
    -active                 true
EQUILIBRIUM_PHASES
Calcite        0 0
Goethite       0 0
Hematite           0 0
END

Guidance would be helpful

[dlparkhurst]

When you have multiple phases in EQUILIBRIUM_PHASES, PHREEQC will calculate the most stable phase assemblage. I don't know the details of the log Ks in your database, but hematite should be more stable than goethite. The reaction from goethite to hematite requires only water (which has roughly constant activity of 1), so the most stable phase can be determined from the log Ks alone. Thermodynamically, the log Ks indicate that any goethite present in the system should react to form hematite.

Now, reality (kinetics) and thermodynamics are not always the same, otherwise, we would have a nitric acid atmosphere. In this case, goethite is much more likely to form at earth surface conditions. So, I would exclude hematite from the calculation.