Attempting to quantify secondary reactions in Fo80 olivine weathering

[JackPTRoden]

Hello,

I am very new to PHREEQC and I have decided to use it as part of my dissertation for uni. Specifically, I want to try to work out how much CO2 is consumed per mole of Fo80 olivine.

I am trying to approach it in the same way as J. Griffioen (2017) (http://dx.doi.org/10.1016/j.scitotenv.2016.09.008) did with different seawater composition and variation of temperature. I figured the best way to do this would be to try to emulate Griffioen's results so that I know I am doing it properly before entering my own inputs.

However, I have struggled quite a lot to get anything close to the results shown in Griffioen's paper and I figure this is down to my ineptitude at the program. In order to include Fo80 in the llnl.dat database, I entered the following (based off of thermodynamic data produced by Stefánnson (2001) (https://doi.org/10.1016/S0009-2541(00)00263-1), which Griffioen adapted and supplied a logk value for).

Code: [Select]

Fo80
        Mg1.6Fe0.4SiO4 +4.0000 H+  =  + 1.0000 SiO2 + 2.0000 H2O + 1.6000 Mg++ + 0.4000 Fe++
        log_k           25.9126
-delta_H 0       # Not possible to calculate enthalpy of reaction Fo80
# Enthalpy of formation: -483.48 kcal/mol
#       -Range:  0-300

As for my actual input, I am also fairly confident that I have made mistakes. N.b., the following are the relevant reactions and I intend to work out efficiency by means of looking at each constituent's flux in equilibrium.

Mg1·6Fe0.4SiO4 + 3.2 CO2 + 0.1 O2 + 4.2 H2O → 1.6 Mg2+ + 0.4 Fe(OH)3+H4SiO4 + 3.2 HCO3−
Mg1·6Fe0.4SiO4 + 1.6 CO2 + 0.1 O2 + 1.6 Ca2+ + 2.6 H2O → 1.6 Mg2+ + 0.4 Fe(OH)3+H4SiO4 + 1.6 CaCO3
Mg1·6Fe0.4SiO4 + 1.6 CO2 + 0.1 O2 + 2.6 H2O → 1.6 MgCO3 + 0.4 Fe(OH)3+H4SiO4
3 Mg1·6Fe0.4SiO4 + 5.6 CO2 + 0.3 O2 + 8.1 H2O →½Mg4Si6O15(OH)2·6H2O + 2.8 Mg2+ + 1.2 Fe(OH)3 + 5.6 HCO3−

This is my input:

Code: [Select]

SOLUTION 1 Seawatergriffioen
    temp      25
    pH        8.2
    pe        8.5
    redox     pe
    units     mg/l
    density   1
    Alkalinity 142 as HCO3
    Ca        400
    Cl(-1)    19000
    K         380
    Mg        1350
    Na        10500
    S(6)      2700
    Si        6.4
    -water    10 # kg

EQUILIBRIUM_PHASES 1
    Aragonite 0 0
    Calcite   0.7 0
    Fo80      0 0.1
    Magnesite 1.03 0
    Sepiolite 3.46 0
    SiO2(am)  -1.32 0


GAS_PHASE 2
    -fixed_volume
    -equilibrium with solution 1
    -pressure 1
    -volume 250
    -temperature 25
    O2(g)     0.1995262
GAS_PHASE 1
    -fixed_volume
    -equilibrium with solution 1
    -pressure 1
    -volume 250
    -temperature 25
    CO2(g)    0.000398

I am constantly mixing things up and changing variables but nothing seems to be improving for me. I am sure this is a woefully incorrect input file, so any and all advice would be massively appreciated, as my due date is very quickly approaching and I want to make sure I can get this done in time.

Thank you all!

[dlparkhurst]

I'm not entirely sure what you have in mind. Maybe this will get you started.

The calculation incrementally dissolves Fo80, maintaining equilibrium with atmospheric O2 and CO2 and allowing  Goethite, Calcite, Magnesite, Sepiolite, and SiO2(am) to precipitate if they become saturated. The solution composition is essentially invariant with this combination of minerals, as is the saturation index of Fo80; the mass added by dissolution is almost identically precipitated in secondary minerals. The plot shows the change in moles of minerals per mole of Fo80 dissolved.

Code: [Select]

PHASES
Fo80
        Mg1.6Fe0.4SiO4 +4.0000 H+  =  + 1.0000 SiO2 + 2.0000 H2O + 1.6000 Mg++ + 0.4000 Fe++
        log_k           25.9126
-delta_H 0       # Not possible to calculate enthalpy of reaction Fo80
# Enthalpy of formation: -483.48 kcal/mol
#       -Range:  0-300
END
SOLUTION 1 Seawatergriffioen
    temp      25
    pH        8.2
    pe        8.5
    redox     pe
    units     mg/l
    density   1
    Alkalinity 142 as HCO3
    Ca        400
    Cl(-1)    19000
    K         380
    Mg        1350
    Na        10500
    S(6)      2700
    Si        6.4
    -water    10 # kg
END
EQUILIBRIUM_PHASES 1
    O2(g)     -0.7 10
    CO2(g)    -3.4 10
    Goethite  0 0
    Calcite   0 0
    Magnesite 0 0
    Sepiolite 0 0
    SiO2(am)  0 0
END
INCREMENTAL_REACTIONS
USE solution 1
USE EQUILIBRIUM_PHASES 1
REACTION
Fo80 1
1 in 10
USER_GRAPH 1
    -headings               rxn O2(g) CO2(g) Goethite Calcite Magnesite Sepiolite SiO2
    -axis_titles            "Fo80 dissolved, moles" "Moles per mole Fo80" ""
    -initial_solutions      false
    -connect_simulations    true
    -plot_concentration_vs  x
  -start
10 GRAPH_X 1/10 * STEP_NO
20 GRAPH_Y EQUI_DELTA("O2(g)")/RXN, EQUI_DELTA("CO2(g)")/RXN
30 GRAPH_Y EQUI_DELTA("Goethite")/RXN, EQUI_DELTA("Calcite")/RXN
40 GRAPH_Y EQUI_DELTA("Magnesite")/RXN, EQUI_DELTA("Sepiolite")/RXN
50 GRAPH_Y EQUI_DELTA("SiO2(am)")/RXN
  -end
    -active                 true
END



[JackPTRoden]

I forgot to respond to this sooner - thank you very much for the suggestion, it was a lot of help. Apologies for not being clearer on what I was wanting to do, but your advice definitely pointed me in the right direction and have now been able to do what I was intending on doing.