[secondary mineral formation]

Dear Brian,

 

thank you very much for your detailed explanation, it really helped.

 

I am still having problems swapping aqueous vs. mineral species, i am sure this is complete basic stuff and has been discussed many times. Sorry for this naive, newbie problem.

 

I have updated the example above, one file having a sliding Eh path, one example adding H2 to decrease Eh, using the thermo.dat as database as you recommended. Since I dont know how much lactate / NOM was in fact used, i increased H2 conc. until calculation gave ~-350 mV, which is what we measured.

 

When i now start swapping aqueous vs. mineral species, some phases work, others result in either "inital solution too supersaturated" or simply "residuals too large". For instance, if I use "H2 addition", I can swap Fe->Goethite, but not As->Scorodite; Mn-> Birnessite.

 

In particular I do not really understand the probelm with "intial solution too supersaturated". To me it seems intuitive that the solution needs to be supersaturated (for most elements) since despite some reductive dissolution, the soil will never "dissolve" entirely.

 

I would be glad, if you could explain where my misunderstanding is.

 

Many thanks

Best regards,

Markus

 

H2 addition.rea

sliding Eh.rea

[secondary mineral formation]

Hi Brian,

 

thanks for your help. Sorry, but I am still a bit puzzled. I would swap then for most probable minerals. You write "You should not enter solid phase concentrations as if they were dissolved, though". I dont get what you mean with this. If I have 5300 ppm of Fe in solid in the reactor, I should not enter this as "goehtite 5300 free mg/kg as Fe"? How should I enter it instead?

 

To the second point, it is now more clear to me what epsilon is representing. Is there a numerical maximum for epsilon that still represents a reasonable accurate solution (and would be acceptable for a publication)?

 

In one of the experiments, the Eh lowered because of natural organic matter present, in the other experiment we added lactate. I will try using titration instead. Still, when using the sliding Eh, do you know why H2O in fluid increased from 55 mol to >10^6 mol at reducing Eh ? When I plot "elemental composition of system (fluid + rock)" vs. Eh, both hydrogen and oxygen are ok. Also, system parameters -> "Mass H2O" and "Mass solution" vs. Eh are ok. Maybe there is some general problem with the model/calculations causing the program to have problems in the first place. Or maybe just a plotting bug (no prob, I would anyhow not plot this) ?

 

Thanks a lot,

Best regards,

[secondary mineral formation]

sorry, forgot, Pd was a type-o in the model, the soil contained 140 ppm Pb.

Best

Markus

[secondary mineral formation]

Hi Brian,

 

thank you very much for your quick response. To the description of the system, it is correct that the medium was fed as dissolved salts, the soil elemental composition is based on XRF bulk values. We only know minerology for the As phase (xafs), not for any other phase (Fe, Mn). The soil was aerobic, probably it would make sense to swap Fe/Mn diss. to Fe/Mn solid phases characteristic for such soils. The intial soil Na, K, Cl, Mg, Ca, PO4 etc. I realize now, I should have included as well. However, again I do not as which phase they are present, should I add the concentration to the dissolved content or make an assumption on probable solid phases?

 

Regarding the configuration -> iteration setting, I can see how this works for the calculation. Still, I do not really understand what the convergence criterion "epsilon" stands for and what are typical values that make sense. I need to set it to 10^-8 instead of 10^-11 to have all steps calculated, is that ok? I would very much appreciate some help here.

 

I realized somethind strange when I started playing with the plots. H2O in fluid increased considerately from ~ -100 mV onwards. Could you comment on this please?

 

Thanks again for your help,

Best regards

Markus

[secondary mineral formation]

Hello,

 

I am working on the following problem. We have been looking at As mobilization from soil when reducing conditions are induced. We see a decrease in As concentration in liquid when reducing conditions (i.e. Mn, Fe and sulfate reducing) persist for an extended time and suspect secondary mineral formation of either As containing minerals; or precipitation of further minerals (Fe, else) that can sorb / co-precipitate As. This I would like to model now.

I am intending to use react, feed it with both soil bulk element concentrations and medium composition and slide Eh from oxidizing (400 mV) to reducing (-400 mV) conditions (simulating subsequent Mn, Fe, SR conditions). Alternatively, I could use dissolved element concentrations instead of soil bulk concentrations.

 

 

The soil elemental composition is [mg / kg] (only few selected elements); we have used 100 g in 1 L volume

Mn 1200

Fe 53000

Zn 400

As 2010 as Scorodite

Pb 140

 

The medium was composed of different salts that add up to (all in mol / L)

 

Na 0.0319

K 0.0054

Cl 0.0361

Mg 0.0020

Ca 0.0010

PO4 0.0015

CO3 0.0071

SO4 0.0021

 

The pH was controlled by acid base dosing at 8.2.

 

Attached is the react file. When I press run, the program gives the following error:

 

Step 83, Xi = .7758 (15 iterations)

Charge balance: Cl- molality adjusted from .03237 to .03192

Checking basis

N-R didn't converge after 400 its., maximum residual = 2.84e-11, Xi = 0.7776

Cutting step size to find solution

Checking basis

N-R didn't converge after 999 its., maximum residual = 2.84e-11, Xi = 0.7758

-- Didn't wake up, abandoning path

 

Seems like some of the paths are calculated, but then the software stops. I can plot some mineral phases, but only until ~-200 mV. Do you have an advice on what to change ?

 

 

Thanks a lot,

Best regards,

Markus

 

soil As experiment.rea