Reaction Calculations

1
Reaction Calculations
or

EQUILIBRATION REACTOR
2
EQUILIBRIUM REACTIONS

• SURFACE
• EXCHANGE
• SOLID_SOLUTIONS
• EQUILIBRIUM_PHASES
• GAS_PHASE
• MIX

3
NON-EQUILIBRIUM REACTIONS

• REACTION
• REACTION_TEMPERATURE
• KINETICS

4
CONCEPTUAL MODEL

• Define initial solution (or mixture of initial
  solutions).
• 2. Define irreversible REACTION
• 3. Define EQUILIBRIUM_PHASES
• Moles of each phase
• SI or gas partial pressure

5
Reactions

• Implicit redox reactions

6
MIX One or more SOLUTIONS

• Type solution number
• Type mixing fraction

7
R.1. Exercise Implicit Redox Reactionsmg/L

1. Define rainwater as SOLUTION 1 with log partial
   pressure of O2 -0.7 and CO2 -3.5.
2. Define END.
3. Define MIX by using solution 1 and mixing
   fraction 1.
4. Define END.
5. Run.

8
R.2. Questions

• 1. Explain the differences between the initial
  solution composition and the reaction (mixed)
  solution composition, particularly pH, pe, N(5),
  N(0), and N(-3).

9
Reactions

• Sequential reactions

10
SAVE and USE

• Save results of calculations
• Use previously defined SOLUTIONS,
  EQUILIBRIUM_PHASES, REACTIONs, etc
• Use previously SAVEd SOLUTIONS,
  EQUILBRIUM_PHASES, etc

11
SAVE results from a Reaction Calculation
Index numbers are used to keep track Index
numbers do not need to be sequential

• Note SOLUTION defines an initial solution
  calculation, which is automatically saved.

12
USE Previously defined or SAVEd

• USE includes KINETICS, MIX, REACTION, and
  REACTION_TEMPERATURE
• Can USE previously SAVEd EQUILIBRIUM_PHASES,
  EXCHANGE, SOLID_SOLUTION, SOLUTION, or SURFACE

13
Initial Solution and Reaction Calculations
14
REACTION Reactants and stoichiometry

• Choose phase or type formula
• Define relative stoichiometry

15
REACTION Reaction amounts

• Steps are a number of equal increments
• or
• Steps are a specified list
• Specify units

16
Initial Solution and Reaction CalculationsThe
inscrutable END

• Initial Solution/Speciation calculationevery
  SOLUTION
• Reaction calculationSOLUTION reactant before
  END
• Reactant may be any reactant keyword data block
• USE can define solution or reactant
• ENDDo speciation and reaction
• ENDdefines a Simulation

17
Speciation and Reaction CalculationsThe
inscrutable END

• Simulation 1
• Speciation calculation solution 1
• Speciation calculation solution 2
• Reaction calculation
• SOLUTION 1
• EQUILIBRIUM_PHASES 2
• REACTION 3
• Simulation 2
• Speciation calculation solution 3
• Reaction calculation
• SOLUTION 2
• EQUILIBRIUM_PHASES 1
• REACTION 3
• Simulation 3
• No speciation calculation
• No reaction calculationNo Reactant defined

18
R.3. Exercise Sequential Reactions

1. Make an unsaturated zone water. Build on previous
   exercise with rainwater. Add SAVE solution 2
   after MIX step and before END. After the END, add
   USE solution 2 and equilibrate (EQUILIBRIUM_PHASES
   ) with CO2, log partial pressure 2, and calcite
   (SI 0), save solution as solution 3.
2. Use solution 3, add 1 mmol CO2 (REACTION),
   equilibrate with calcite (EQUILIBRIUM_PHASES),
   save as solution 4.

19
R.4. Questions

1. What is the log pCO2 of the rainwater, rainwater,
   in redox equilibrium (the mixture), the mixture
   equilibrated with CO2 and calcite, and after
   reaction with CO2?
2. How many millimoles of calcite and CO2 reacted to
   make solution 3?
3. How many millimoles of calcite reacted to make
   solution 4 from solution 3?

20
R.5. Extra Credit Exercise

1. Use MIX and REACTION to evaporate rainwater 20
   fold (at constant pCO2) before reaction with CO2
   and calcite. Hint You must remove water and
   water has 55.5 mol per kg.

21
Reactions

• Dedolomitization

22
Dedolomitization

• Anhydrite dissolution
• Calcite precipitation
• Dolomite dissolution

23
R.6. Exercise

1. Make a ground water with log pCO2 -2,
   equilibrium with calcite and dolomite.
2. React 50 mmol of anhydrite (CaSO4) in increments
   of 10 mmol. Maintain equilibrium with calcite and
   dolomite, allow anhydrite to precipitate if it
   becomes saturated.

24
R.7. Questions

• What trends do you expect in water composition
  with anhydrite-driven dedolomitization?
• Why is the following reaction misleading?
• CaSO4 CaMg(CO3)2 2CaCO3 Mg2 SO4-2
• 3. How does the water composition change from
  step 4 to step 5?

25
R.8. QuestionsPlummer and Sprinkle, 2001mg/L,
S(-2) as H2S

• 1. Are these the trends of dedolomitization?
• 2. Anything else?

26
EQUILIBRIUM_PHASEDissolve Only

• Force mineral not to precipitate

27
R.9. Extra Credit Exercise

1. Equilibrate seawater (major ions only) with
   calcite and dolomite.
2. Make a ground water with log pCO2 -2,
   equilibrium with calcite and dolomite.
3. Mix ground water with seawater in fractions of
   .25, 0.5, .75. Maintain equilibrium with calcite
   allow dolomite only to dissolve.

28
R.10. Questions

1. What reaction is calculated for seawater
   equilibration with calcite and dolomite?
2. What reactions are calculated for a carbonate
   reactions for the ground-water/seawater mixtures?

29
Reactions

• Organic decomposition

30
Organic Decomposition

• Sequential removal of electron acceptors, usually
  in the sequence
• O2
• NO3-
• MnO2
• Fe(OH)3
• SO4-2
• HCO3-

31
Redox Environments

• OxicDissolved O2 reduction
• CH2O O2 CO2 H2O
• Post-oxicNO3-, MnO2, Fe(OH)3 reduction
• CH2O 4Fe(OH)3 7CO2 4Fe2 8HCO3- 3H2O
• SulfidicSO4-2 reduction
• 2CH2O SO4-2 2HCO3- H2S
• MethanicCH4
• CH2O CO2 CH4

32
Redox Environments
33
R.11. Exercise

• Dilute seawater by 50 percent.
• React diluted seawater with 50 mmol of CH2O in
  steps of 0.1, 0.2, 10, 20, 30, 40, and 50 mmol.
  Equilibrate with .1 moles of Fe(OH)3(a) and 0.0
  mol of mackinawite.
• Print moles of reaction (-rxn) and total
  concentrations of O(0), C(4), C(-4), Fe(2),
  Fe(3), S(6), S(-2) (-totals) to the
  selected-output file (SELECTED_OUTPUT).

34
R.12. Questions

• What sequence of electron acceptors is used?
• Where is Fe(3) important?
• Use Excel to plot the concentrations in the
  selected-output file (rxn is the x variable, set
  first line rxn from 99 to 0, omit
  d_mackinawite).
• 4. Why is C(4) not a straight line?
• 5. Why does Fe(2) increase after 30 mmol of CH2O
  is reacted.
• Should a gas bubble form?
• What trends are observed for sulfate reduction?

35
R.13. QuestionsPotomac Estuary Sediments

1. How is this similar to the sulfate reduction
   simulation results?
2. How does this differ from the simulation results?

36
R.14. Exercise

• React pure water with 10 mmol of CH2O, maintain
  equilibrium with barite.

37
R.15. Questions

1. What is the barium concentration in mg/L?
2. Are there reactions other than sulfate reduction?
3. What other reactions could affect barium?

38
Sulfate ReductionNorman Landfill

• Barium concentration appears insignificant unless
  barium is lost to cation exchange or mineral
  precipitation.

39
R.16. Extra Credit Exercise

• At pH 7.0, determine the pe indicated when
  concentrations are equal for each redox state of
  the following redox couples As(5)/As(3),
  C(4)/C(-4), Fe(3)/Fe(2), N(5)/N(0), N(5)/N(-3),
  N(0)/N(-3), O2(/H2O), S(6)/S(-2), U(6)/U(4).
• Hint Use only SOLUTION and enter 1 umol/kgw of
  each valence state.
• No entry is allowed in SOLUTION for H2O.

40
R.17. Questions

1. Iron is soluble as reduced ferrous iron, uranium
   is soluble as oxidized U(6). As organic carbon
   reacts, which would you observe first (a)
   increase in iron, (b) decrease in uranium?
2. Thermodynamically, which nitrogen species to you
   expect to see in an oxygenated ground water? In a
   methanic water?

41
Cape Cod Sewage Plume

• Iron reduction
• No sulfate reduction

Blue Plains, Washington D.C. Potomac River
Sediment Pore Water

• Iron reduction
• Minor sulfate reduction
• Intense methanogenesis

42
Reactions

• Sulfide oxidation

43
Sulfide Oxidation

• Pyrite/Marcasite are most important reactants
• Need Pyrite, Oxygen, Water, and bugs
• Oxidation of pyrite and formation of ferric
  hydroxide complexes and minerals generates acidic
  conditions

44
R.18. Exercise

1. React the pure water with 10 mmol of pyrite,
   maintaining equilibrium with atmosphreric oxygen.
2. React the pure water with 10 mmol of mackinawite,
   maintaining equilibrium with atmosphreric oxygen.
3. React the pure water with 10 mmol of sphalerite,
   maintaining equilibrium with atmosphreric oxygen.

45
R.19. Questions

1. Write qualitative reactions that explain the pH
   of the 3 solutions.
2. What pH buffer starts to operate at pHs below 3?
3. Run the input file with wateq4f.dat database.
   What minerals may precipitate during pyrite
   oxidation?

46
R.20. Extra Credit Exercise

1. React the pure water with 20 mmol of pyrite,
   maintaining equilibrium with atmospheric oxygen
   and goethite.
2. Acid mine drainage is usually treated with
   limestone. Use the results of exercise 1 and
   equilibrate with O2, Fe(OH)3(a), and calcite.

47
R.21. Questions

1. Write a net reaction for the PHREEQC results for
   the low-pH simulation.
2. Looking at the results of the calcite-equilibrated
   simulation, what additional reactions should be
   considered?

48
Picher Oklahoma Abandoned Pb/Zn Minemg/L

• Mines are suboxic
• Carbonates are present
• Iron oxidizes in stream

49
Reactions

• Aluminosilicate reactions

50
Aluminosilicate Reactions

• Disseminated calcite important in silicate
  terranes

• Bowen (Goldich) reaction series

Ca-Feldspar Plagioclase Na-Feldspar K-Feldspar Mus
covite Quartz

• Olivine
• Pyroxene (augite)
• Amphibole (hornblende)
• Biotite mica

51
Aluminosilicates

• Primary minerals react to form gibbsite,
  kaolinite, smectite, zeolites, SiO2
• Thermodynamic data is not reliable
• Compositional uncertainties
• Range of stabilities
• Difficulty of measurement
• Kinetics are slow

52
Add Reactant to Phase Boundary

• KAlSiO8 is added (or removed) until gibbsite
  equilibrium is reached
• Amount is amount of KAlSiO8, not gibbsite

53
R.22. Exercise (phreeqc.dat)

1. Dissolve just enough K-feldspar (KAlSi3O8) to
   come to equilibrium with gibbsite.
2. Dissolve just enough K-feldspar to come to
   equilibrium with kaolinite.
3. Dissolve just enough K-feldspar to come to
   equilibrium with K-mica.

54
R.23. Questions

1. Given the thermodynamic data, which mineral
   should precipitate first?
2. Does quartz need to be included in this
   calculation?

55
Sierra Nevada Springsmmol/kgw

• Increase in Ca, Alkalinity, pH
• Increase in SiO2
• Slight increase in Mg, K, Cl, SO4

56
Summary

• MIX
• EQUILIBRIUM_PHASES
• REACTION

57
Summary

• Carbonate minerals and CO2
• Dedolomitization
• Organic decomposition
• Sulfate reduction
• Other electron acceptors O2, NO3, FeOOH, CH2O
• Sulfide oxidation
• Aluminosilicate reactions