Reaction Path Modeling

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Reaction Path Modeling

• Chpt. 8 Zhu Anderson

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Reaction Path Models

• Used to model open systems
• Variable composition relative time scale
  (reaction progress variable ?)
• Types
• Titration or mixing (most common) (B Fig. 2.3)
• Polythermal (B Fig. 2.2)
• Buffering or sliding fugacity (B Fig. 2.4)
• Flow-through tracks evolution of fluid
  composition as it flows through rock. Reaction
  products left behind, isolated from further
  reaction (like fractional crystallization).
• Flush tracks chemical evolution of system
  through which fluid migrates unreacted fluid
  displaces reacted fluid (B Fig. 2.7, ZA Fig.
  2.5).
• Kinetic reaction path model

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Reaction Path Models

• Describe irreversible reactions or processes
  using a series of partial equilibrium states.
• Thought of as a very slow titration.
• Results do not always correspond exactly to
  reality because we are using thermodynamics to
  describe a series of disequilibrium states. But
  still provide insights into the processes
  involved.

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Alkalinity titration (ZA 8.2)

• React script, saved Sun Feb 23 2003 by
  Dabieshan
• title "Titrate Bear Creek sample MW-36 with
  HCl"
• data "E\Program Files\Gwb\Gtdata\thermo.com.v8.
  r6.dat" verify
• temperature 15
• 1 kg free H2O
• total mg/l Ca 158
• total mg/l Fe .01
• total mg/l Mg 21
• total mg/l Mn .11
• total mg/l K 17
• balance on Na
• total mg/l Na 61
• total mg/l SO4-- 425
• total mg/l Cl- 25
• total mg/l Al .01
• total mg/l SiO2(aq) 5.6
• pH 7.4
• total molality HCO3- .00305
• react .004 mol of HCl(aq)

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Compare w/ Fig. 8.2 calculated using Phreeqc
(script in Table 8.1)
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Acidity of Acid Mine Water (ZA 8.3)

• React script
• title "Titrate Bear Creek sample TS-3 with
  Calcite"
• data C\Program Files\Gwb\Gtdata\thermo.com.v8.
  r6.dat" verify
• temperature 16
• 1 kg free H2O
• total mg/l Ca 310
• total mg/l Fe 1950
• total mg/l Mg 1000
• total mg/l Mn 66.3
• total mg/l K 60
• balance on Na
• total mg/l Na 89
• total mg/l SO4-- 16500
• total mg/l Cl- 550
• total mg/l Al 1020
• total mg/l SiO2(aq) 40.5
• pH 3.8
• total mg/l HCO3- 5
• react .25 mol of Calcite

• For acidic water the carbonate acidity is a small
  part of total acidity.
• Titrate solution with calcite to measure
  theoperational acidity, the amount of CaCO3
  required to saturate the solution.

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Compare with Fig. 4 Zhu Anderson.
8
All minerals suppressed except gypsum, gibbsite
Fe(OH)3. Compare with Fig. 8.4 Zhu Anderson.
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Weathering of K-feldspar

• If water/rock ratio high, all K-feldspar
  converted to kaolinite, composition of weathered
  rock determined by its environment, i.e., the
  composition of water.
• If water/rock ratio low, water dissolves soluble
  primary minerals (K-spar) precipitates
  insoluble secondary minerals (kaolinite),
  concentrations of dissolved K and H4SiO4 ?
• 2 KAlSi3O8 9 H2O 2 H ? Al2Si2O5(OH)4 2 K
  4 H4SiO4

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• log Keq 4 log H4SiO4 2 log K 2 log
  H
• log Keq 4 log H4SiO4 2 log K/H
• log K/H ½ log Keq 2 log H4SiO4
• plot y K/H and x log H4SiO4, get
  straight line with m -2 and b ½ log Keq .

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React Script K-feldspar weathering

• data "e\program files\gwb\gtdata\thermo.dat"
  verify
• work_dir E\Users\Ayers\GEO320\GWB
• temperature 25
• 1 kg free H2O
• total mg/kg Na 5
• total mg/kg K 1
• total mg/kg Ca 15
• total mg/kg Mg 3
• total ug/kg Al 1
• total mg/kg SiO2(aq) 3
• total mg/kg Cl- 30
• total mg/kg SO4-- 8
• total mg/kg HCO3- 50
• pH 5
• react 2 mmol of K-feldspar
• suppress Clinoptil-K Mordenite-K

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Extracting the Overall Reaction

• When mmoles of secondary minerals produced are
  plotted as a function of mmoles of primary
  mineral consumed, the slopes of the lines give
  the reaction coefficient for each species and
  mineral in the overall reaction (only if both
  axes use linear scales and consistent units).
• Three reaction segments 1) precipitation of
  kaolinite, 2) transformation of kaolinite to
  muscovite, 3) further formation of muscovite once
  kaolinite exhausted.

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Segment 1 Segment 2 Segment 3
CO2(aq) -1 0 -.67
HCO3- 1 0 .67
K 1 0 .67
H2O -1.5 1 -.67
Quartz 2 2 2
Kaolinite .5 -1
Muscovite 1 .33

1. KAlSi3O8 3/2 H2O CO2(aq) ? 2 SiO2 ½
   Al2Si2O5(OH)4 HCO3- K
2. KAlSi3O8 Al2Si2O5(OH)4 ? 2 SiO2
   KAl3Si3O10(OH)2 H2O
3. KAlSi3O8 2/3 CO2(aq) 2/3 H2O ? 2 SiO2
   1/3 KAl3Si3O10(OH)2 2/3 HCO3- 2/3 K

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K-feldspar weathering Ex6 in Phreeqci

• Example illustrates how to locate
• points representing intersection of reaction
  trace with phase boundaries.
• trace reaction path between phase boundaries.
• use kinetic equations to calculate amount of time
  required for each segment of reaction path.
• Other features
• can use Phases keyword block to add new phases
  or modify existing phases in thermodynamic
  database (allows calculation at temperatures
  significantly different from 25C).
• cant create activity diagram in Phreeqci use
  same database (LLNL) to create diagram in GWB
  Act2 and import trace from reformatted Phreeqci
  output file.

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K-feldspar weathering script in Phreeqci

• SOLUTION_SPREAD
• -redox O(-2)/O(0)
• -units mg/kgw
• Na K Ca Mg Al Si Cl
  S(6) pH pe C(4)
• ug/kgw charge
  CO2(g) -2 O2(g) -0.7
• 5 0.0001 1 3 1 0.0001
  30 8 5 10 50
• INCREMENTAL_REACTIONS True
• EQUILIBRIUM_PHASES 1
• Gibbsite 0 0
• Kaolinite 0 0
• Muscovite 0 0
• Pyrophyllite 0 0
• SiO2(am) 0 0
• K-Feldspar 0 0
• REACTION 1
• K-feldspar 1
• 0.2 millimoles in 100 steps
• SELECTED_OUTPUT
• -file Kfeldspar.out

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Figure 6.--Phase diagram for the dissolution of
K-feldspar (microcline) in pure water at 25oC
showing stable phase-boundary intersections
(example 6A) and reaction paths across stability
fields (example 6B). Diagram was constructed
using thermodynamic data for gibbsite, kaolinite,
K-mica (muscovite), and microcline from Robie and
others (1978).
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Hydrothermal Fluids Fluorite Deposits

• React script, saved Wed Mar 31 2004 by ayersj
• data "c\program files\gwb\gtdata\thermo.dat"
  verify
• temperature initial 175, final 125
• 1 kg free H2O
• total mol SiO2(aq) .00157131343
• total mol Al .271987822
• total mol F- .767745894
• total mol SO4-- .00144353827
• total mol H .236805901
• total mol Ca 1.09283049
• total mol Na 3.71859882
• total mol K .524567232
• total mol Mg .0233140992
• total mol Cu .0116570496
• total mol HCO3- .233140992
• balance on Cl-
• total mol Cl- 6.53610768
• TDS 247500
• density 1.14

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AlF3 ½ CaCl AlF2 ½ CaF2 ½ Cl-
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Are results reliable?

• Fluorite is the only phase that precipitates in
  significant amounts, consistent with observation
  that deposit is 90 fluorite.
• Decrease in concentration (stability) of neutral
  complex AlF3 is consistent with known behavior of
  the dielectric constant of water (charged species
  favored at low T).
• However,
• get very different results using Phreeqci (see
  script C\Documents and Settings\Administrator\My
  Documents\GMEAP\Phrqci\Fluorite_deposition.pqi).
• Ionic strength much higher than recommended range
  of B-dot equation used to calculate activities in
  GWB.
• Thermodynamic data required to accurately
  calculate values of equilibrium constants of
  critical reactions at these elevated temperatures
  are not present in the GWB databases (see output
  from program Rxn using LLNL database on next
  slide).

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Rxn input

• Rxn script, saved Thu Apr 01 2004 by ayersj
• data "C\Program Files\Gwb\Gtdata\thermo.com.v
  8.r6.dat" verify
• react Fluorite
• swap CaCl for Ca
• swap AlF3(aq) for F-
• swap AlF for Al
• activity CaCl ?
• activity AlF3(aq) ?
• activity Cl- ?
• activity AlF ?
• long

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Rxn output

• Fluorite Cl- AlF CaCl AlF3(aq)
• Log K's
• 0 C ??? 150 C
  ???
• 25 C -1.0326 200 C
  ???
• 60 C ??? 250 C
  ???
• 100 C ??? 300 C
  ???
• Polynomial fit
• log K -1.033 0 T 0 T2 0
  T3 0 T4
• Equilibrium equation
• log K log aCaCl log aAlF3(aq) -
  log aCl- - log aAlF

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Rxn script

• Rxn script, saved Thu Apr 01 2004 by ayersj
• data "C\Program Files\Gwb\Gtdata\thermo.com.v
  8.r6.dat" verify
• react Fluorite
• swap Anhydrite for Ca
• swap Muscovite for Al
• swap Quartz for SiO2(aq)
• swap Alunite for SO4--
• swap Kaolinite for H
• swap AlF3(aq) for F-
• activity AlF3(aq) ?
• activity K ?
• activity H2O ?
• long

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Rxn output

• Fluorite .8333 Quartz .5 Alunite .3333
  Kaolinite
• Anhydrite .6667 AlF3(aq) .5
  Muscovite 1.667 H2O
• Log K's
• 0 C ??? 150 C
  ???
• 25 C -2.6268 200 C
  ???
• 60 C ??? 250 C
  ???
• 100 C ??? 300 C
  ???
• Polynomial fit
• log K -2.627 0 T 0 T2 0
  T3 0 T4
• Equilibrium equation
• log K .6667 log aAlF3(aq) 1.667
  log aH2O

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Seawater Evaporation

• React script, saved Tue Feb 25 2003 by
  Dabieshan
• data "e\program files\gwb\gtdata\thermo_hmw.dat
  " verify
• temperature 25
• decouple ALL
• swap CO2(g) for H
• 1 kg free H2O
• fugacity CO2(g) .000316227766
• total mg/kg Na 10760
• total mg/kg Mg 1290
• total mg/kg Ca 411
• total mg/kg K 399
• balance on Cl-
• total mg/kg Cl- 19350
• total mg/kg SO4-- 2710
• total mg/kg HCO3- 142
• TDS 35000
• react -996 gram of H2O
• flow-through
• delxi .001 linear

Flow-through model described on pg. 118 of GWB
Users Guide. Equivalent to fractional
crystallization solids separated from liquid,
traces chemical evolution of liquid.
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Calcite precipitation in hot water heater

• React script, saved Wed Mar 03 2004 by ayersj
• data "c\program files\gwb\gtdata\thermo.dat"
  verify
• temperature initial 25, final 50
• swap Calcite for Ca
• swap CO2(g) for HCO3-
• 1 kg free H2O
• free gram Calcite 1
• balance on H
• fugacity CO2(g) .000316227766
• fix fugacity of CO2(g)

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Ca 2HCO3- CaCO3 H2CO3
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Heat calcite-saturated water w/ 0.1 mmolal HCl

• First run previous script at T 25C, then
  pickup fluid
• React script, saved Wed Mar 03 2004 by ayersj
• data "c\program files\gwb\gtdata\thermo.dat"
  verify
• temperature initial 25, final 50
• swap CO2(g) for HCO3-
• 1 kg free H2O
• total mol Ca .00044154404
• fugacity CO2(g) .000316227766
• balance on H
• total mmolal H .1
• total mmolal Cl- .1

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• Effect of 0.1 mmolal HCl inhibitor

• Saturation Indices