sulfate reduction

[pia said]

Hi I`m modelling Fe remotion in a close system, I want to represent FeS2 formation. I do not how to add the sulfate redution in my code, i was thinking of using solution_species and the formula CH4 + SO4-2 = HCO3− + HS− + H2O, but i dont know the logK and delta_h (where can i found it?). Also I tried with EQUILIBRIUM_PHASES pyrite 0 0 but it didn't work because there isn't S(2) (that's why i need to incorporate the sulfate reduction).

Beforehand thank you very much.

This is my code:

Code: [Select]

SOLUTION 1
    temp      25
    pH        2
    redox     pe
    units     ppm
    density   1
    As(5)     4
    B         30
    Fe(2)     50
    S(6)      1000 charge
    -water    1 # kg
END

REACTION 1
    Calcite 1
    0.015 moles in 10 steps #verificar concentracion disuelta de calcita

EQUILIBRIUM_PHASES 1
    Calcite   0 0
    #Pyrite    0 0
    #H2S(g) 0 0
END

[John Mahoney]

You need to add a reductant  to your system.   Normally a reductant such as CH2O is added as a reactant.  Simple C will also work; some folks use acetate.   Then include possible reduced phases in the Equilibrium_Phases block, which you have already done.

 Get hold of Appelo and Postma and look at the example 9.6 in that book, actually you should look at the whole chapter.  Or if you cannot get hold of that book, you can also get a version of the input file for PhreePlot (PP) at the PhreePlot website  under Custom Plots,  development of redox zones, by clicking on that figure.   All the PHREEQC input is below the word CHEMISTRY in the PP file.   It might need a couple of changes if you are going to run in PHREEQC.    The PhreePlot model is a little different as it also includes arsenic, you will need to add a selected_output file name if you use  the PP version.  But it is a good example and it runs through the various reductions of oygen consumption, then nitrate reduction ,  manganese, arsenic, iron, sulfur and finally methanogenesis. 

 

[pia said]

Thanks for your answer, I saw the example but i don't know the amount of C that I have to add for simulatting the effects of microorganism

[John Mahoney]

Well the example adds small amounts or reductant in increments and it reduces the more oxidized species first.  So for a first start you need enough to get you to sulfate reduction sulfide mineral formation and that will depend upon the amounts of easily oxidized species in your solution or even in the system if you have ferrihydrite for example as a phase. But maybe not enough to get to methanogenesis unless that is also observed.   In the model, the C or CH2O is merely the food for the microorganisms.  If uyou are alos looking into microorganism growth you will need include kinetics in the model.