Dissolution of pyrite, with and without fixed oxygen fugacity

MOOSE input file

The TimeDependentReactionSolver involves specifying:

• the swaps

• the initial free mole number of Hematite, the pH via the H activity, the bulk mole composition of the aqueous species, and the O2(g) fugacity

• the system is closed at (the default)

• that at the activity constraints are removed from H (so the pH can vary during the simulation because an external agent is no longer fixing it) and O2(g) (meaning the external agent controlling the O2(g) fugacity is no longer adding/removing O2(g) from the aqueous solution)

• that 1mg/second of pyrite is added to the system

[TimeDependentReactionSolver]
  model_definition = definition
  geochemistry_reactor_name = reactor
  swap_out_of_basis = "O2(aq) Fe++"
  swap_into_basis = "O2(g) Hematite"
  charge_balance_species = "Cl-"
  constraint_species = "H2O              Hematite     H+            Ca++             Mg++             Na+              HCO3-            SO4--            Cl-              O2(g)"
  constraint_value = "  1.0              1            -6.5          4                1                2                18               3                5                0.2"
  constraint_meaning = "kg_solvent_water free_mineral log10activity bulk_composition bulk_composition bulk_composition bulk_composition bulk_composition bulk_composition fugacity"
  constraint_unit = "   kg               mg           dimensionless mg               mg               mg               mg               mg               mg               dimensionless"
  remove_fixed_activity_name = "H+ O2(g)"
  remove_fixed_activity_time = '0  0'
  source_species_names = "Pyrite"
  source_species_rates = 8.336E-6 # = 1mg(pyrite)/second, 1mg(pyrite) = 8.34E-6
  ramp_max_ionic_strength_initial = 0 # not needed in this simple problem
  stoichiometric_ionic_str_using_Cl_only = true # for comparison with GWB
  abs_tol = 1E-13
  execute_console_output_on = '' # only CSV output is required
[]

GWB input file

The equivalent Geochemists Workbench input file is

# React script that is equivalent to dissolution_pyrite_1.i
Ca++ = 9.98e-5 mol
Mg++ = 4.114e-5 mol
Na+ = 8.7e-5 mol
HCO3- = 2.95e-4 mol
SO4-- = 3.123e-5 mol
Cl- = 1.4103e-5 mol
balance on Cl-
H+ = 6.5 pH
swap O2(g) for O2(aq)
f O2(g) = 0.2
swap Hematite for Fe++
Hematite = 6.26e-6 free mol
epsilon = 1e-13
react 8.336e-5 mol Pyrite
go

Results

The following graphs demonstrate agreement between the geochemistry module and the GWB software, which may also be compared with Bethke (2007) Figure 14.2 and 14.3.

Figure 1: Precipitated volumes as pyrite is reacted in a system that is closed to oxygen

Figure 2: Solution pH as pyrite is reacted in a system that is closed to oxygen

Figure 3: Species concentrations as pyrite is reacted in a system that is closed to oxygen

MOOSE input file

The TimeDependentReactionSolver is almost identical to Case 1. The differences are:

• that at the activity constraints are removed from H, but not O2(g)

• that 1000mg/second of pyrite is added to the system

[TimeDependentReactionSolver]
  model_definition = definition
  geochemistry_reactor_name = reactor
  swap_out_of_basis = "O2(aq) Fe++"
  swap_into_basis = "O2(g) Hematite"
  charge_balance_species = "Cl-"
  constraint_species = "H2O              Hematite     H+            Ca++             Mg++             Na+              HCO3-            SO4--            Cl-              O2(g)"
  constraint_value = "  1.0              1            -6.5          4                1                2                18               3                5                0.2"
  constraint_meaning = "kg_solvent_water free_mineral log10activity bulk_composition bulk_composition bulk_composition bulk_composition bulk_composition bulk_composition fugacity"
  constraint_unit = "   kg               mg           dimensionless mg               mg               mg               mg               mg               mg               dimensionless"
  remove_fixed_activity_name = "H+"
  remove_fixed_activity_time = '0 '
  source_species_names = "Pyrite"
  source_species_rates = 8.336E-4 # = 0.1g(pyrite)/second over 10 seconds, 1g(pyrite) = 8.34E-3
  ramp_max_ionic_strength_initial = 0 # not needed in this simple problem
  stoichiometric_ionic_str_using_Cl_only = true # for comparison with GWB
  abs_tol = 1E-13
  execute_console_output_on = '' # only CSV output is required
[]

Results

The following graphs demonstrate agreement between the geochemistry module and the GWB software, which may also be compared with Bethke (2007) Figure 14.4.

Figure 4: Precipitated volumes as pyrite is reacted in a system that is open to oxygen

Figure 5: Solution pH as pyrite is reacted in a system that is open to oxygen

The two cases are clearly dramatically different. In Case 1, about 8mg of pyrite dissolves, precipitating hematite, before hematite suddenly dissolves and no further pyrite dissolution occurs. In Case 2, pyrite dissolution and hematite precipitation continues indefinitely in a rather acidic solution.