Define the specific adsorbed arsenic amount in surface complexation

[tongg]

Hi Dr. Parkhurst

Hope you are doing well!

I am simulating the desorption process of arsenic, we have the specific arsenic concentration in hematite, but it seems in PHREEQC we can't set the initial adsorbed amount directly. Besides, I am also somewhat confused about using "USE SURFACE 1" in the script. In my conceptual model, the initial aquifer contains hematite with 7 ppm arsenic adsorbed on it. Then the reducing fluid recharges the aquifer to dissolve hematite and change pH of aquifer. This process should influence the sorption of arsenic and arsenic concentration in aqueous. When I set "USE SURFACE 1" in reducing fluid, the result shows that the complexations are relatively stable, after hematite fully dissolves, arsenic concentration is pretty low, lower than the arsenic concentration in the recharge fluid itself. Here below is my code. I appreciate your help!

Best,
Tong

Code: [Select]

Arsenic concentration evolution - advective transport, cation exchange, surface complexation and fluid-rock interactions
#
# Use phreeqc.dat
# Hematite as iron oxide mineral in this model
#
SURFACE_MASTER_SPECIES
    Hema_s        Hema_sOH
    Hema_w        Hema_wOH
SURFACE_SPECIES
#strong bonding site, Hema_sOH
Hema_sOH = Hema_sOH
    log_k     0
H+ + Hema_sOH = Hema_sOH2+
    log_k     7.29
Hema_sOH = Hema_sO- + H+
    log_k     -8.93
AsO4-3 + 3H+ + Hema_sOH = Hema_sH2AsO4 + H2O
    log_k     29.31
AsO4-3 + 2H+ + Hema_sOH = Hema_sHAsO4- + H2O
    log_k     23.51
AsO4-3 + Hema_sOH = Hema_sOHAsO4-3
    log_k     10.58
# weak bonding site, Hema_wOH
Hema_wOH = Hema_wOH
    log_k     0
H+ + Hema_wOH = Hema_wOH2+
    log_k     7.29
Hema_wOH = Hema_wO- + H+
    log_k     -8.93
AsO4-3 + 3H+ + Hema_wOH = Hema_wH2AsO4 + H2O
    log_k     29.31
AsO4-3 + 2H+ + Hema_wOH = Hema_wHAsO4- + H2O
    log_k     23.51
AsO4-3 + Hema_wOH = Hema_wOHAsO4-3
    log_k     10.58
SOLUTION_MASTER_SPECIES
    As            H3AsO4          -1     74.9216         74.9216
SOLUTION_SPECIES
        H3AsO4 = H3AsO4
                log_k           0.0
        H3AsO4 = AsO4-3 + 3H+
                log_k   -20.7
        H+ + AsO4-3 = HAsO4-2
                log_k   11.50
        2H+ + AsO4-3 = H2AsO4-
                log_k           18.46

SOLUTION 1 Initial water
    temp      15
    pH        6.6
    pe        0.54 #change pe
    redox     pe
    units     mmol/l
    density   1
    Alkalinity 1.52 meq/l
    As       1 ug/L
    Ca        0.12
    Cl        0.51
    Fe        0.02
    K         0.08
    Mg        0.44
    Na        0.92
    S(6)      0.64
    Si        0.32
    -water    1 # kg
END

USE solution 1
EQUILIBRIUM_PHASES 1
    Calcite   0 3
    Hematite  0 0.44
    Illite    0 0.87
    K-Feldspar 0 2.87
    Kaolinite 0 0.75
    Quartz    0 138.15
SAVE solution 1
END

EXCHANGE 1
    CaX2    0.2
    -equilibrate with solution 1
    -pitzer_exchange_gammas true

SURFACE 1
    -equilibrate with solution 1
    Hema_wOH Hematite        equilibrium_phase 0.044  533
    Hema_sOH Hematite        equilibrium_phase 0.0044
END

USE solution 1
USE SURFACE 1
EQUILIBRIUM_PHASES 0 #Use solution 1 to equi with reducing gas
    CO2(g)    1.28 100 #original value is 1.28
    CH4(g)    -0.84 10 #original value is -0.84
SAVE solution 0

END
ADVECTION
    -cells 1
    -shifts 600
    -print_frequency 600
    -warnings true
USER_GRAPH 1 Arsecnic test
    -headings               PV As(ppb) Ca(M) Fe(M) Na(M) strong weak
    -axis_titles            "Pore volumes or shift number" "Log(Concentration, in ppb or molal)" ""
    -chart_title            "Arsenic Evolution of the Navajo Sandstone Aquifer"
    -axis_scale x_axis      0 600 auto auto
    -axis_scale y_axis      auto 100 auto auto log
    -initial_solutions      false
    -connect_simulations    true
    -plot_concentration_vs  x
  -start
10 GRAPH_X STEP_NO
20 GRAPH_Y TOT("As") * 74.92e6, TOT("Ca"), TOT("Fe"), TOT("Na")
  -end
    -active                 true
USER_GRAPH 2 Arsenic test2
    -headings               PV Hema_sOHAsO4-3 Hema_wOHAsO4-3 Hema_wH2AsO4 Hema_wHASO4- pH
    -axis_titles            "Pore volumes or shift number" "Concentration, in molal" "pH"
    -chart_title            "Arsenic Evolution of the Navajo Sandstone Aquifer"
    -axis_scale x_axis      0 500 auto auto
    -initial_solutions      false
    -connect_simulations    true
    -plot_concentration_vs  x
  -start
10 GRAPH_X STEP_NO
20 GRAPH_Y MOL("Hema_sOHAsO4-3"),MOL("Hema_wOHAsO4-3"),MOL("Hema_wH2AsO4"), MOL("Hema_wHAsO4-")
30 GRAPH_SY -LA("H+")
  -end
    -active                 true
USER_GRAPH 3 mineral change
    -headings               PV Calcite Hematite
    -axis_titles            "Pore volumes or shift number" "molal" "pH"
    -chart_title            "Arsenic Evolution of the Navajo Sandstone Aquifer"
    -axis_scale x_axis      0 500 auto auto
    -axis_scale y_axis      1e-06 auto auto auto
    -initial_solutions      false
    -connect_simulations    true
    -plot_concentration_vs  x
  -start
10 GRAPH_X STEP_NO
20 GRAPH_Y EQUI("Calcite"),EQUI("Hematite")
30 GRAPH_SY -LA("H+")
  -end
    -active                 true
END

[dlparkhurst]

Let's simplify a little.

Code: [Select]

SOLUTION_MASTER_SPECIES
    As            H3AsO4          -1     74.9216         74.9216
SOLUTION_SPECIES
        H3AsO4 = H3AsO4
                log_k           0.0
        H3AsO4 = AsO4-3 + 3H+
                log_k   -20.7
        H+ + AsO4-3 = HAsO4-2
                log_k   11.50
        2H+ + AsO4-3 = H2AsO4-
                log_k           18.46
Arsenic concentration evolution - advective transport, cation exchange, surface complexation and fluid-rock interactions
#
# Use phreeqc.dat
# Hematite as iron oxide mineral in this model
#
SURFACE_MASTER_SPECIES
    Hema_s        Hema_sOH
    Hema_w        Hema_wOH
SURFACE_SPECIES
#strong bonding site, Hema_sOH
Hema_sOH = Hema_sOH
    log_k     0
H+ + Hema_sOH = Hema_sOH2+
    log_k     7.29
Hema_sOH = Hema_sO- + H+
    log_k     -8.93
AsO4-3 + 3H+ + Hema_sOH = Hema_sH2AsO4 + H2O
    log_k     29.31
AsO4-3 + 2H+ + Hema_sOH = Hema_sHAsO4- + H2O
    log_k     23.51
AsO4-3 + Hema_sOH = Hema_sOHAsO4-3
    log_k     10.58
# weak bonding site, Hema_wOH
Hema_wOH = Hema_wOH
    log_k     0
H+ + Hema_wOH = Hema_wOH2+
    log_k     7.29
Hema_wOH = Hema_wO- + H+
    log_k     -8.93
AsO4-3 + 3H+ + Hema_wOH = Hema_wH2AsO4 + H2O
    log_k     29.31
AsO4-3 + 2H+ + Hema_wOH = Hema_wHAsO4- + H2O
    log_k     23.51
AsO4-3 + Hema_wOH = Hema_wOHAsO4-3
    log_k     10.58
END
SOLUTION 1 Initial water
    temp      15
    pH        6.6
    pe        0.54 #change pe
    redox     pe
    units     mmol/l
    density   1
    Alkalinity 1.52 meq/l
    As       1 ug/L
    Ca        0.12
    Cl        0.51
    Fe        0.02
    K         0.08
    Mg        0.44
    Na        0.92
    S(6)      0.64
    Si        0.32
    -water    1 # kg
END
USE solution 1
EQUILIBRIUM_PHASES 1
    Calcite   0 3
    Hematite  0 0.44
    Illite    0 0.87
    K-Feldspar 0 2.87
    Kaolinite 0 0.75
    Quartz    0 138.15
SAVE solution 1
END
SURFACE 1
    -equilibrate with solution 1
    Hema_wOH Hematite        equilibrium_phase 0.044  533
    Hema_sOH Hematite        equilibrium_phase 0.0044
END

Here you define the surface reactions and aqueous arsenic speciation. Then SOLUTION 1 is defined, which reacts with EQUILIBRIUM_PHASES 1. Now solution 1 has a pH over 10. Is that what you want? An oxyanion like arsenic will not sorb well at high pH.

My next question is what do you mean by 7 ppm? 7 mg/kg of water? or 7 mg/kg of solid?

The SURFACE definition will determine the amount of arsenic sorbed on the surface that is consistent with the concentrations of SOLUTION 1. SOLUTION 1 does not change; the surface composition adjusts to equilibrium with the water. If you want a different amount of arsenic initially sorbed, (1) you can adjust SOLUTION 1 to have a different pH or As concentration, (2) you can adjust the number of surface sites, or (3) you can adjust the equilibrium constants for the arsenic sorption reactions.

Logically, if you know the solution composition, and you know the amount of arsenic sorbed, and you know the number of surface sites, then I think you would adjust the equilibrium constants to get the measured sorption at the composition of the solution. Usually the number of sites and equilibrium constants are highly correlated, so you probably need to fix one or the other.

[tongg]

Thank you, Dr. Parkhurst. That makes sense!