Click here to donate to keep PhreeqcUsers open
Welcome,
Guest
. Please
login
or
register
.
Did you miss your
activation email
?
1 Hour
1 Day
1 Week
1 Month
Forever
Login with username, password and session length
Forum Home
Login
Register
PhreeqcUsers Discussion Forum
»
Beginners
»
PHREEQC basics
»
Solubility of Fe(OH)3(a) in presence of Goethite
« previous
next »
Print
Pages: [
1
]
Go Down
Author
Topic: Solubility of Fe(OH)3(a) in presence of Goethite (Read 600 times)
arif
Contributor
Posts: 2
Solubility of Fe(OH)3(a) in presence of Goethite
«
on:
June 02, 2023, 06:05:52 PM »
Hi Users,
I am developing a surface complexation model with Fe(OH)3(a) as sorbing mineral. The input script is below:
SOLUTION 1
temp 25
pH 7
pe 4
redox pe
units mg/l
density 1
Cu 10
-water 1 # kg
EQUILIBRIUM_PHASES 1
Al(OH)3(a) 0 0.1
Fe(OH)3(a) 0 0.01
Fix_pH -4 HNO3 10
Goethite 0 0.001
SURFACE 1
Hfo_sOH Fe(OH)3(a) equilibrium_phase 0.0005 600
Hfo_wOH Fe(OH)3(a) equilibrium_phase 0.02
PHASES
Fix_pH
H+=H+
log_k 0
END
and the out put is
Phase SI log IAP log K(T, P) Initial Final Delta
Al(OH)3(a) -0.90 9.90 10.80 1.000e-01 0 -1.000e-01
Fe(OH)3(a) -5.89 -1.00 4.89 1.000e-02 0 -1.000e-02
Fix_pH -4.00 -4.00 0.00
HNO3 is reactant 1.000e+01 9.703e+00 -2.965e-01
Goethite 0.00 -1.00 -1.00 1.000e-03 1.100e-02 1.000e-02
Hfo_s
0.000e+00 moles [0.0005 mol/(mol Fe(OH)3(a))]
Mole Log
Species Moles Fraction Molality Molality
Hfo_sOH2+ 2.715e-31 0.000 2.700e-31 -30.569
Hfo_sOCu+ 4.034e-32 0.000 4.012e-32 -31.397
Hfo_sOH 7.589e-33 0.000 7.547e-33 -32.122
Hfo_sO- 4.859e-36 0.000 4.833e-36 -35.316
Hfo_sOFe+ 8.514e-48 0.000 8.467e-48 -47.072
Hfo_w
0.000e+00 moles [0.02 mol/(mol Fe(OH)3(a))]
Mole Log
Species Moles Fraction Molality Molality
Hfo_wOH2+ 6.272e-28 0.000 6.238e-28 -27.205
Hfo_wOH 1.753e-29 0.000 1.744e-29 -28.759
Hfo_wOCu+ 4.780e-31 0.000 4.754e-31 -30.323
Hfo_wO- 1.123e-32 0.000 1.116e-32 -31.952
Hfo_wOFe+ 1.836e-46 0.000 1.826e-46 -45.739
Hfo_wOFeOH 2.675e-49 0.000 2.660e-49 -48.575
In the presence of goethite the Fe(OH)3(a) gets dissolved 100%, irrespective of mol of goethite (even at 0 moles). Because of that the surface area is zero. But when I remove goethite from the equilibirum phase the solubility of Fe(OH)3(a) reduces. I wonder whether anyone has observed the same output. Any lead would be highly appreciated.
Thanks in advance.
Arif
Logged
dlparkhurst
Top Contributor
Posts: 3668
Re: Solubility of Fe(OH)3(a) in presence of Goethite
«
Reply #1 on:
June 02, 2023, 08:57:47 PM »
Yes, your observation is an expected result. The IAP (ion activity product) definitions are almost the same between goethite and Fe(OH)3(a); they only differ by the amount of water in the expressions. Given that the activity of water is near 1.0 for dilute solutions, you can consider that IAPs are equal.
The K for Fe(OH)3 is larger than goethite, so saturation ratio for Fe(OH)3 is smaller than for Goethite (SR = IAP / K). So when goethite is in equilibrium (SR = 1), Fe(OH)3 must be less than 1, indicating it should dissolve. Under any reasonable conditions, all of the Fe(OH)3 will dissolve and precipitate as goethite.
This is a long way of saying that Goethite is more stable than Fe(OH)3(a), and PHREEQC is calculating the stable phase assemblage among the minerals included in EQUILIBRIUM_PHASES.
Logged
arif
Contributor
Posts: 2
Re: Solubility of Fe(OH)3(a) in presence of Goethite
«
Reply #2 on:
June 03, 2023, 10:38:14 PM »
Thanks for the explanation. Much appreciated.
Best regards,
Arif
Logged
Print
Pages: [
1
]
Go Up
« previous
next »
PhreeqcUsers Discussion Forum
»
Beginners
»
PHREEQC basics
»
Solubility of Fe(OH)3(a) in presence of Goethite