Dissolution of Ferrihydrite following reaction with organic carbon

[lswerhone]

Hello,

I am trying to simulate the redox change and SI of ferrihydrite in tailings under differing organic carbon addition rates. A secondary goal is to model the release of arsenic that is sorbed to the tailings ferrihydrite surfaces at the different organic carbon addition rates. I have written the following code:

Code: [Select]

TITLE Tailings Porewater Redox Calculations with Organic C additions
#using minteq.v4 database
#tailings porewater chemistry is at the time of discharge
#model assumes 0.3 porosity (1 L of porewater per 4.66 kg tailings)

SOLUTION 1
    temp      7
    pH        9.6
    pe        6.5
    redox     pe
    units     mg/l
    density   1
    Al        1.73
    Alkalinity 44 as Ca0.5(CO3)0.5
    As        0.09
    Ba        0.151
    Ca        564
    Cl        79.6
    Fe        0.117
    K         59.7
    Mg        13.8
    Mo        9.3
    N(-3)     25 as N
    N(5)      18.6 as N
    Na        146
    Ni        0.174
    Pb        0.052
    S(6)      1994 charge
    Se        0.048
    U         0.088
    Zn        0.01
    -water    1 # kg

SURFACE 1
    -equilibrate with solution 1
    Hfo_sOH Ferrihydrite    equilibrium_phase 0.005  600
    Hfo_wOH Ferrihydrite    equilibrium_phase 0.02

PHASES
    Fix_H+
    H+ = H+
    log_k 0.0

#Ferrihydrite moles calculated assuming all Fe in tailings (avg. 17,100 mg/kg) is Fe(OH)3, dry density 2000 g/cc, 2.33L tails solids
#fixed pH at 9.6 assuming excess lime in tailings acts as buffer
EQUILIBRIUM_PHASES 1
    Ferrihydrite 0 1.43
    Fix_H+    -9.6 Ca(OH)2   10

USE solution 1
USE surface 1

SAVE solution 1
SAVE surface 1

SELECTED_OUTPUT 1
    -file                 selected_output_1.sel
    -reset                false
    -simulation           true
    -pH                   true
    -pe                   true
    -reaction             true
    -ionic_strength       true
    -charge_balance       true
    -percent_error        true
    -saturation_indices   Ferrihydrite

USER_PUNCH
    -headings Aqueous_As_ugL Sorbed_As_ug Aqueous_OrgC_mgL Aqueous_Fe_mgL Aqueous_Fe(3+)_mgL Aqueous_Fe(2+)_mgL Aqueous_N(5+)_mgL Aqueous_N(3-)_mgL Aqueous_N(tot)_mgL
10 PUNCH TOTMOLE("As")*74.92*1000*1000,\
         SURF("As","Hfo")*74.92*1000*1000,\
         TOTMOLE("C")*12.011*1000,\
         TOTMOLE("Fe")*55.847*1000,\
         TOTMOLE("Fe(3)")*55.847*1000,\
         TOTMOLE("Fe(2)")*55.847*1000,\
         TOTMOLE("N(5)")*14.0067*1000,\
         TOTMOLE("N(-3)")*14.0067*1000,\
         TOTMOLE("N")*14.0067*1000
END

USE solution 1
USE surface 1

REACTION 1 CH2O addition to bring organic C into porewater at various concentrations
    CH2O       1
    0.083  0.83  1.67  2.09  2.5  2.58  2.66 
    2.75  2.83  2.92  3.33  4.16  5  5.84  6.66 
    7.49  8.33 millimoles

END

My problem is that while the model seems to calculate the redox change and resulting SI for ferrihydrite effectively, the ferrihydrite I've defined as an equilibrium phase and surface doesn't appear to be dissolving and adding Fe3+ to the system (total Fe is constant under all reactions). I expected that Fe would be released as the ferrihydrite dissolves and "buffer" the redox change with Fe3+ as the electron acceptor. Right now the point at which redox and ferrihydrite SI drop off seems to be entirely dependent on the initial NO3- concentration

Is there anything obviously wrong with my code that would result in ferrihydrite not dissolving and releasing Fe to solution? Am I misunderstanding how the redox is being calculated within the program?

Thank you,
Lawrence

[dlparkhurst]

The calculation involving REACTION does not include EQUILIBRIUM_PHASES. Perhaps you intended to add "USE equilibrium_phases 1".

Code: [Select]

END
USE solution 1
USE surface 1

REACTION 1 CH2O addition to bring organic C into porewater at various concentrations
    CH2O       1
    0.083  0.83  1.67  2.09  2.5  2.58  2.66
    2.75  2.83  2.92  3.33  4.16  5  5.84  6.66
    7.49  8.33 millimoles

END

[lswerhone]

Bonehead oversight on my part, that fixed it. Appreciate you taking a look!

Lawrence