Jump to content
Geochemist's Workbench Support Forum

zenolevy

Members
  • Posts

    3
  • Joined

  • Last visited

Posts posted by zenolevy

  1. Hi Brian,

     

    Thank you- this has been hugely helpful. I will look into how the rate laws I used were derived.

     

    I have one final problem that is holding me up on this project. I wanted to incorporate DO and have Fe(OH)3 precipitate instead of Fe(OH)2. I added DO and Fe+++ (I made Fe+++ starting concentrations negligible) to the basis, and suppressed minerals I thought to be unlikely to form under these conditions. However, my model fails to converge when DO is included.

     

    When I run the model Fe(OH)3 (ppd) is supersaturated and is swapped for Fe(OH)4-, and the model fails to converge soon after.

     

    This should (hopefully) be the last query on this thread, and I thank you for your help thus far.

     

    Zeno

    siderite_firstorder_customratelaws_DO.x1t

  2. Hi Brian,

     

    Thank you, this was very helpful.

     

    I want to confirm that the custom rate law equation in GWB calculates a dissolution rate in the units of mol/sec, is that correct?

     

    I think the problem with my siderite rate law was that I was using the "mass" helper function, instead of the "moles" function. I rewrote my rate law in this form:

     

    (9.77e-12) * (moles("Siderite") * mw("Siderite")) * (1 - QoverK)

     

    and it seems to be working...

     

    However, my calcite rate law does not seem to be working

     

    (6.75e-10) * (QoverK - 1)

     

    I am still getting a negative and extremely small precipitation rate. When I check "System Parameters" in xtplot I get low decimals fractions for Q/K, Calcite; but when I check "mineral saturation" for calcite I have Q/K's in the 1000's, which makes more sense since I am running very high Ca and alkalinity waters through the model domain, and siderite is dissolving.

     

    Any idea why this might be? I am attaching my script.

     

    Thanks,

    Zeno

    siderite_firstorder_customratelaws.x1t

  3. Hello,

     

    I am trying to simulate high-pH leachate (~12) flowing through a reactive porous medium composed of siderite (starting out with 20% by volume). I have rate laws that are in the form of mol/sec:

     

    siderite dissolution:

     

    (rate_con + 5.55e-22 * activity("H+")) * (mass("siderite")) * 115.86) * (1 - Q/K)

     

    with a rate_con of 9.77e-12 mol/g/sec

     

    (note - the equation I have specifies the middle term as mass of siderite in grams - so I manually altered it to multiply the mass("siderite") term by the gram formula weight since this command returns mass in terms of moles)

     

    and

     

    calcite dissolution:

     

    rate_con * (Q/K - 1)

     

    with a rate_con of 6.75e-10 mol/sec

     

    I am trying to send the high Ca and pH leachate through a 30 m permeable reactive barrier and achieve pH control by dissolution of siderite, precip of calcite, and precip of iron hydroxides, which I will assume as equilibrium controlled because this is a very fast reaction.

     

    My main problems are my rate constants are not in the form specified by GWB (mol/cm2 sec), and I don't have surface areas in my equations. However, GWB requires inputs for both these variables. I am getting a dissolution rate of 0 for siderite and precipitation rate of -6e-11 for calcite when I run the model, which is not surprising because the form I am using does not appear to be compatible with GWB.

     

    I am attaching my x1t script (GWB v. 9).

     

    Heeeellllpppp!!!!!

     

    Zeno

     

     

     

    siderite_firstorder.x1t

×
×
  • Create New...