Mackinawite precipitation as a function of pH

[Sifat Azad Papry]

Hi,
I am trying to plot the amount (moles) of Mackinawite precipitates as a function of pH.

The code I used (shown as below) gives me the plot with only one point only and does not show any variation with pH. I would highly appreciate if anyone could help me figure out the problem in my codes and help me generate a graph that shows the moles of Mackinawite formed as a function of pH.

#SOLUTION 1
    temp      25
    pH        9
    pe        4
    redox     pe
    units     mole/L
    density   1
    Fe        5.37e-04
    S(-2)     9.07e-05
    Na        1
    Cl        1
    -water    1 # kg

PHASES
Fix_H+
H+ = H+
log_k 0.0

EQUILIBRIUM_PHASES 1
Mackinawite 0 0


USER_GRAPH 1 Mackinawite precipitation
-headings pH Mackinawite
-chart_title "HS- Sorption onto Montmorillonite (I = 1 M)"
-axis_titles "pH" "MOLES/GRAM BENTONITE"
-axis_scale x_axis 9 12 1 0.25
-axis_scale y_axis 1e-11 1e-3 1 1 log
-start
10 GRAPH_X -LA("H+")
20 GRAPH_Y EQUI("Mackinawite")
END

 

[dlparkhurst]

Not sure what you have in mind. Here is a simulation that equilibrates with mackinawite as acid is added to achieve a series of pH values. Positive values represent precipitation, negative, dissolution.

Code: [Select]

PHASES
Fix_H+
H+ = H+
log_k 0.0
END
SOLUTION 1
    temp      25
    pH        9
    pe        4
    redox     pe
    units     mole/L
    density   1
    Fe        5.37e-04
    S(-2)     9.07e-05
    Na        1
    Cl        1
    -water    1 # kg

END
USER_GRAPH 1 Mackinawite precipitation
-headings pH Mackinawite
-chart_title "HS- Sorption onto Montmorillonite (I = 1 M)"
-axis_titles "pH" "MOLES/GRAM BENTONITE"
#-axis_scale x_axis 9 12 1 0.25
#-axis_scale y_axis 1e-11 1e-3 1 1 log
-start
10 GRAPH_X -LA("H+")
20 GRAPH_Y EQUI_DELTA("Mackinawite")
END
USE solution 1
EQUILIBRIUM_PHASES 1
Mackinawite 0 10
END
USE solution 1
EQUILIBRIUM_PHASES 1
Mackinawite 0 10
Fix_H+ -10 HCl 10
END
USE solution 1
EQUILIBRIUM_PHASES 1
Mackinawite 0 10
Fix_H+ -8 HCl 10
END
USE solution 1
EQUILIBRIUM_PHASES 1
Mackinawite 0 10
Fix_H+ -6 HCl 10
END
USE solution 1
EQUILIBRIUM_PHASES 1
Mackinawite 0 10
Fix_H+ -5.25 HCl 10
END



[Sifat Azad Papry]

Hi Dr. Parkhurst,
Thanks a lot for your reply. I highly appreciate the help. You got it right. I wanted to simulate the precipitation of Mackinawite due to reaction of Fe and bisulfide (HS-)at various pH conditions. Applying your code, the graph now shows the moles of Mackinawite precipitated at various pH levels. 
However, the trend observed does not fully comply with my experiment results. To my knowledge, Mackinawite formation is negligible below pH 6.5 (the model output also reflects that). The experiments and literature suggests that Mackinawite formation  is maximum around pH 9 and then start decreasing with increasing pH, and reaches equilibrium at around pH 10.5 and no more precipitation after pH 10.5. My point is, between pH 9-11 there should be a downward trend of the amount of mackinawite precipitation. But the model result is showing an upward/ increasing trend upto pH 11. I would highly appreciate if you could insight me to the the reason of this discrepancy and how it can be fixed? Is there any other issue (e.g, ionic strength or redox condition involved). Please advise.

I have slighly changed the code (as shown below):

PHASES
Fix_H+
H+ = H+
log_k 0.0
END
SOLUTION 1
    temp      25
    pH        9
    pe        4
    redox     pe
    units     mole/L
    density   1
    Fe        5.37e-04
    S(-2)     9.07e-05
    Na        1
    Cl        1
    -water    1 # kg

END

USER_GRAPH 1 Mackinawite precipitation
-headings pH Mackinawite
-chart_title "Mackinawite Precipitaion(I = 1 M)"
-axis_titles "pH" "MOLES/GRAM BENTONITE"
#-axis_scale x_axis 9 12 1 0.25
#-axis_scale y_axis 1e-11 1e-3 1 1 log
-start
10 GRAPH_X -LA("H+")
20 GRAPH_Y EQUI_DELTA("Mackinawite")
END


USE solution 1
EQUILIBRIUM_PHASES 1
Mackinawite 0 0
Fix_H+ -12 NaOH 10
END

USE solution 1
EQUILIBRIUM_PHASES 1
Mackinawite 0 0
Fix_H+ -11 NaOH 10
END

USE solution 1
EQUILIBRIUM_PHASES 1
Mackinawite 0 0
Fix_H+ -10.5 NaOH 10
END

USE solution 1
EQUILIBRIUM_PHASES 1
Mackinawite 0 0
Fix_H+ -10 NaOH 10
END
USE solution 1
EQUILIBRIUM_PHASES 1
Mackinawite 0 0
Fix_H+ -8 HCl 10
END
USE solution 1
EQUILIBRIUM_PHASES 1
Mackinawite 0 0
Fix_H+ -6 HCl 10
END
USE solution 1
EQUILIBRIUM_PHASES 1
Mackinawite 0 0
Fix_H+ -5.25 HCl 10
END

[dlparkhurst]

Your initial solution conditions distribute Fe between Fe(2) and Fe(3). If you specify that the dissolved iron  is ferrous, you will get a very different result. Precipitation from the initial solution lowers the pH to 5.5, and it is necessary to add base to achieve the pH series.

Using phreeqc.dat, the addition of base causes essentially all of the sulfide to precipitate at pH 7 and above. If the log K for Mackinawite is set 2 units larger, you get a maximum precipitation between 9 and 11, similar to what you described. You may be precipitating a fresh solid that is more soluble than the phase that is represented in the database. You should check the literature for the range of log Ks for Mackinawite, or perhaps you can fit your own log K.

I'm not sure how well the HS-/S-2 log K is known. You could check databases and literature for uncertainty in the aqueous sulfide log Ks.

There is also the possibility of transitioning from Fe(2) to Fe(3) at high pH. You will have to decide if this is possible. If it is, then you may need to consider the precipitation of an iron oxyhydroxide, such as goethite. Addition of goethite as a potential phase generates a maximum in precipitation over the range that you suggest when using the database mackinawite log K.

Code: [Select]

PHASES
Fix_H+
H+ = H+
log_k 0.0

Mackinawite
FeS + H+ = Fe+2 + HS-
-log_k -4.648
      #-log_k -2.648
-Vm 20.45
END
SOLUTION 1
    temp      25
    pH        9
    pe        4
    redox     pe
    units     mole/L
    density   1
    Fe(2)     5.37e-04
    S(-2)     9.07e-05
    Na        1
    Cl        1
    -water    1 # kg

END
USER_GRAPH 1 Mackinawite precipitation
-headings pH Mackinawite
-chart_title "HS- Sorption onto Montmorillonite (I = 1 M)"
-axis_titles "pH" "MOLES/GRAM BENTONITE"
#-axis_scale x_axis 9 12 1 0.25
#-axis_scale y_axis 1e-11 1e-3 1 1 log
-start
10 GRAPH_X -LA("H+")
20 GRAPH_Y EQUI_DELTA("Mackinawite")
END
USE solution 1
EQUILIBRIUM_PHASES 1
Mackinawite 0 10
#Goethite 0 0
END
USE solution 1
EQUILIBRIUM_PHASES 1
Mackinawite 0 10
#Goethite 0 0
Fix_H+ -7 NaOH 10
END
USE solution 1
EQUILIBRIUM_PHASES 1
Mackinawite 0 10
#Goethite 0 0
Fix_H+ -8 NaOH 10
END
USE solution 1
EQUILIBRIUM_PHASES 1
Mackinawite 0 10
#Goethite 0 0
Fix_H+ -9 NaOH 10
END
USE solution 1
EQUILIBRIUM_PHASES 1
Mackinawite 0 10
#Goethite 0 0
Fix_H+ -10 NaOH 10
END
USE solution 1
EQUILIBRIUM_PHASES 1
Mackinawite 0 10
#Goethite 0 0
Fix_H+ -11 NaOH 10
END
USE solution 1
EQUILIBRIUM_PHASES 1
Mackinawite 0 10
#Goethite 0 0
Fix_H+ -12 NaOH 10
END


[Sifat Azad Papry]

Dear Dr. Parkhurst,
Thanks a lot again for your response with the thoughtful suggestions. You got it right that I want to simulate freshly precipitated Mackinawite rather than previously presented in the solution.

Actually, my project is to investigate the sorption of bisulfide (HS-) onto bentonite clay. To my knowledge from experimentation and literature, the principal sorption mechanism may include 2 types of processes: (i) the reaction of  HS- with the Fe bearing minerals in bentonite and subsequent precipitation of Mackinawite (governing process) (ii) formation of inner and outer sphere surface complexation with the OH groups of bentonite.

The code we are working now is to simulate the first part (i.e., reaction of HS- with Fe in bentonite) in which I considered the total amount of Fe in 1g of bentonite ( as both Fe2+ and Fe3+ may react with HS- and form Mackinawite). Please let me know if you have any alternative suggestions on simulating this part of the model.

As per your suggestions, I will try to find the most appropriate logk values that fits my experimental data. Also, I will look for the log k values in literature.

Thanks again for your time. I am a beginner level Phreeqc user but the discussion form is helping me  a lot.

[Sifat Azad Papry]

Hi David,
Further to the mackiwite precipitaion questions
Earlier I considered only Ferrous ion (Fe2+) reaction with HS- but I need to add also the ferric ion reaction with HS- . I know the reaction with Fe3+ occurs in 2 steps as follows. But could not find the log k values for these reactions. Any insights to this will be helpful.

Fe+3 + HS-→Fe+2 +S + H+
FeS + H+ = Fe+2 + HS-

I would highly appreciate If you could help me with some references of the logK values.

Also, if you could help me how I can model the effect of ionic strengths (i.e., 0.01 M, 0.1 M and 1 M background NaCl) on the mackinawite precipitation that would be great too. My experiment results show a decreasing trend with increasing ionic strength.

The code used for mackinawite precipitation with pH is pasted below. The graphs need to be generated for the ionic strength conditions mentioned above.

PHASES
Fix_H+
H+ = H+
log_k 0.0

Mackinawite
   FeS + H+ = Fe+2 + HS-
   -log_k   -4.648
      #-log_k   -2.648
   -Vm 20.45
END
SOLUTION 1
    temp      25
    pH        9
    pe        4
    redox     pe
    units     mole/L
    density   1
    Fe(2)     5.37e-05
    S(-2)     6.07e-05
    Na        0.1
    Cl        0.1
    -water    1 # kg

END
USER_GRAPH 1 Mackinawite precipitation
-headings pH Mackinawite
-chart_title "HS- Sorption onto Montmorillonite (I = 0.1 M)"
-axis_titles "pH" "MOLES/GRAM BENTONITE"
#-axis_scale x_axis 5 12 1 0.25
#-axis_scale y_axis 1e-11 1e-3 1 1 log
-start
10 GRAPH_X -LA("H+")
20 GRAPH_Y EQUI_DELTA("Mackinawite")
END
USE solution 1
EQUILIBRIUM_PHASES 1
Mackinawite 0 0
Goethite 0 0
Fix_H+ -9 NaOH 10
END
USE solution 1
EQUILIBRIUM_PHASES 1
Mackinawite 0 0
Goethite 0 0
Fix_H+ -9.5 NaOH 10
END
USE solution 1
EQUILIBRIUM_PHASES 1
Mackinawite 0 0
Goethite 0 0
Fix_H+ -10 NaOH 10
END
USE solution 1
EQUILIBRIUM_PHASES 1
Mackinawite 0 0
Goethite 0 0
Fix_H+ -10.5 NaOH 10
END
USE solution 1
EQUILIBRIUM_PHASES 1
Mackinawite 0 0
Goethite 0 0
Fix_H+ -11 NaOH 10
END


Thanks a lot for your time and help.

[dlparkhurst]

Sorry, I don't have any special knowledge for the source of the log Ks. You will have to do some literature searching. Wateq4f gives the following reference Berner, R. A., 1967, Comparative dissolution characteristics of carbonate minerals in the presence and absence of aqueous magnesium ion: American Journal of Science, v. 265, p. 45-70.

Your script gives the results for 0.1 ionic strength. I leave it to you to produce the other ionic strengths.