Reaction of Iron with bisulfide

[Sifat Azad Papry]

Hi,
I am trying to model the reaction of Fe with HS- which occurs in two steps:
(i) ferric iron is reduced to ferrous iron and bisulfide(HS-) is oxydized to elemental sulfur (redox reaction)
(ii) HS- again reacts with the ferrous iron and forms FeS (mackinawite).

In the code,I have kept the SI and initial moles of both elemental Sulfur and mackinawite zero to determine the newly formed Sulfur and FeS. however, the model output only shows the formation of mackinawite but no Sulfur. I will highly appreciate if you could look into the codes and help me fix the issue to see the formation of Sulfur.

SOLUTION_SPECIES
Fe+2 = Fe+3 + e-
   -log_k   -13.02
   -delta_h 9.680   kcal
   -gamma   9.0   0


PHASES
Fix_H+
H+ = H+
log_k 0.0


Mackinawite
   FeS + H+ = Fe+2 + HS-
   -log_k   -3.54
      #-log_k   -2.648
   #-Vm 20.45
Sulfur
   S + H+ + 2e- = HS-
   log_k       -2.1449
   delta_h   -16.3   kJ
END



SOLUTION 1
    temp      25
    pH        9
    pe        4
    redox     pe
    units     mole/L
    density   1
    Fe        3.4e-05
    S(-2)     7.8e-05
    Na        0.01
    Cl        0.01
    -water    1 # kg
End

USER_GRAPH 1 HS- sorption onto bentonite (I= 1 M)
-headings pH Sorption_Capacity(model) Sulfur FeS(Mackinawite) Mont_sS-(Inner_sphere) Mont_sOH2HS(Outer_sphere)
-chart_title "HS- Sorption onto bentonite (I = 1 M)"
-axis_titles "pH" "MOLES/GRAM BENTONITE"
-axis_scale x_axis 8 12 1 0.25
-axis_scale y_axis 1e-8 1e-3 1 1 log
#-plot_tsv_file IS_1.tsv
-start
10 sor= EQUI_DELTA("Sulfur")+EQUI_DELTA("Mackinawite")+ mol("Mont_sS-") + mol("Mont_sOH2HS")
20 GRAPH_X -LA("H+")
30 GRAPH_Y sor, EQUI_DELTA("Sulfur"), EQUI_DELTA("Mackinawite"), mol("Mont_sS-") , mol("Mont_sOH2HS")
END
USE solution 1
#USE Surface 1
EQUILIBRIUM_PHASES 1
Mackinawite 0 0
#Goethite 0 0
Sulfur 0 0
Fix_H+ -9 NaOH 10
END
USE solution 1
#USE Surface 1
EQUILIBRIUM_PHASES 1
Mackinawite 0 0
Sulfur 0 0
#Goethite 0 0
Fix_H+ -9.5 NaOH 10
END
USE solution 1
#USE Surface 1
EQUILIBRIUM_PHASES 1
Mackinawite 0 0
#Goethite 0 0
Sulfur 0 0
Fix_H+ -10 NaOH 10
END
USE solution 1
#USE Surface 1
EQUILIBRIUM_PHASES 1
Mackinawite 0 0
Sulfur 0 0
#Goethite 0 0
Fix_H+ -10.5 NaOH 10
END
USE solution 1
#USE Surface 1
EQUILIBRIUM_PHASES 1
Mackinawite 0 0
Sulfur 0 0
#Goethite 0 0
Fix_H+ -11 NaOH 10

END

 

[dlparkhurst]

Sorry, using phreeqc.dat database, the formation of sulfur is not thermodynamically favored under your conditions.

Here is a calculation that generates equilibrium with sulfur. pH is

Code: [Select]

SOLUTION 1
    temp      25
    pH        9
    pe        4
    redox     pe
    units     mole/L
    density   1
    Fe        3.4e-05
    S(-2)     7.8e-05
    Na        0.01
    Cl        0.01
    -water    1 # kg
End
USE solution 1
EQUILIBRIUM_PHASES
Sulfur 0 10
END