gridFeAsS(cd-music).ppi
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# predominance diagram for As in the presence of goethite which adsorbs As according to the CD-MUSIC adsorption model
SPECIATION
calculationType grid
calculationMethod 1
mainSpecies As
xmin 2.0 # range of pH
xmax 12.0
ymin -90.0 # range of log fO2(g) to control redox
ymax 0.0
resolution 101 # controls the resolution (big resolution means small step size)
PLOT
plotTitle "Fe-As-S-H<sub>2</sub>O (CD-MUSIC)"
xtitle pH
ytitle "log <i>f </i>O<sub>2</sub>(g)"
extratext extratextFeAsS.dat
CHEMISTRY
# simulation 1
include 'ht1.inc' # standard predominance-calculating file, also sets -high_precision true
include 'cdmusic_hiemstra.dat' # CD-MUSIC database, resets convergence criterion, therefore must FOLLOW ht1.inc
SOLUTION 1 # initial solution calculation
Temp 20
pH 1.8
units mol/kgw
Fe(2) 3.5e-1 # total concns
As 1e-2
S(6) 1e-2 # sulphide minerals can form but not adsorb
Na 1e-1
Cl 8e-1 charge
END
# simulation 2
USE solution 1
EQUILIBRIUM_PHASES 1
Fix_H+ -<x_axis> NaOH
-force_equality true
O2(g) <y_axis> 0.1
As_native 0 0 # most likely minerals given the database
Orpiment 0 0 # hematite, magnetite removed to make goethite stable
Realgar 0 0
As2S3(am) 0 0
Pyrite 0 0
Arsenolite 0 0
Claudetite 0 0
Mackinawite 0 0
FeS(ppt) 0 0
Sulfur 0 0
Fe(OH)3(a) 0 0
Greigite 0 0
Scorodite 0 0
Mirabilite 0 0
Melanterite 0 0
Thenardite 0 0
As2O5(cr) 0 0
Jarosite-Na 0 0
JarositeH 0 0
Goethite 0 0 # common Fe oxide under oxidising conditions
# Magnetite 0 0 # stable below log fO2(g) = -60 but not in cd-music database (yet) so ignore!!
SURFACE 1
Goe_uniOH1.5 Goethite 0.049886874 8707.79 # 3.5 sites/nm2, 98 m2/g, MWt = 88.855 g/mol
Goe_triOH0.5 Goethite 0.039041901 8707.79 # 2.7 sites/nm2
-cd_music
-cap 0.85 0.75 # C1 C2 (in F/m2)
-equil 1
END
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