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Dissolution and precipitation of minerals
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Topic: Dissolution and precipitation of minerals (Read 3589 times)
hmal
Frequent Contributor
Posts: 10
Dissolution and precipitation of minerals
«
on:
December 21, 2017, 11:32:31 PM »
Hi all,
I am new to PhreeqC and to this forum, thus apologies if I ask a question that have been asked before. I am wondering if we could control the dissolution of a mineral using the KINETICS and RATES blocks, while allow the same mineral to precipitate without any kinetic control (for example by using the EQUILIBRIUM PHASE block) when the solution is supersaturated with regard to the mineral. Technically the model works/runs but the result is not what I expect. When I add the same mineral in KINETICS, RATES and EQUILIBRIUM PHASE (with target saturation index of 0.0 and some initial amounts) the saturation index (of the mineral) is forced to 0.0 by PhreeqC thus there is no dissolution of the mineral, only precipitation occurred (and the precipitation was controlled by EQUILIBRIUM PHASE). I think I should write a small script in RATE to allow the mineral to precipitate, however I do not know how to do it as I do not want the precipitation is controlled by kinetics but controlled by supersaturation. Any ideas or suggestion would be highly appreciated!
Many thanks,
hmal
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dlparkhurst
Top Contributor
Posts: 3668
Re: Dissolution and precipitation of minerals
«
Reply #1 on:
December 21, 2017, 11:45:51 PM »
Your conceptual model does not make sense to me. If you include the mineral in EQUILIBRIUM_PHASES with a nonzero moles, it will react to equilibrium (SI=0.0), whether it needs to dissolve or precipitate.
If you want it to react kinetically, then you should use a rate that approaches zero at equilibrium, something like
rate = k * (1 - SR("mineral"))
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hmal
Frequent Contributor
Posts: 10
Re: Dissolution and precipitation of minerals
«
Reply #2 on:
December 22, 2017, 12:33:55 AM »
Thank you David for the prompt reply, which I did not expect at this time of the year. If I do not add some nonzero moles, I got the same results of which only precipitation occurred not dissolution. My expectation was dolomite and siderite will dissolve first then later they will precipitate once the system reach supersaturation. I would like to control the precipitation by thermodynamics not kinetics. Can you please check my model (attached)?
Thank you very much again!
«
Last Edit: December 25, 2017, 10:37:27 PM by hmal
»
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dlparkhurst
Top Contributor
Posts: 3668
Re: Dissolution and precipitation of minerals
«
Reply #3 on:
December 22, 2017, 09:41:56 PM »
You have defined default amounts of 10 moles of siderite and dolomite, so they and kaolinite react instantly to equilibrium.
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hmal
Frequent Contributor
Posts: 10
Re: Dissolution and precipitation of minerals
«
Reply #4 on:
December 23, 2017, 11:22:14 AM »
Hi David,
Thank you very much for your reply. Yes, with the same model, I always have dolomite and siderite precipitate to reach SI = 0.0 (or reach equilibrium) when I enter some initial amounts in EQUILIBRIUM PHASE or or enter a zero amount or leave it as 10 moles as default (for dolomite and siderite). Can you please suggest any solutions that the model will allow dolomite and siderite dissolve first (as initially they are under-saturated) then precipitate when they reach super-saturation? I can add a rate law to allow these minerals precipitate kinetically. However, as mentioned earlier, I really want these minerals precipitate without any kinetic control, somehow similar to the way they would precipitate in EQUILIBRIUM PHASE.
Thank you very much!
Hmal
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dlparkhurst
Top Contributor
Posts: 3668
Re: Dissolution and precipitation of minerals
«
Reply #5 on:
December 23, 2017, 04:01:54 PM »
Enter 0.0 for the amounts of dolomite and siderite.
You can also specify precipitate_only.
«
Last Edit: December 23, 2017, 04:04:38 PM by dlparkhurst
»
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hmal
Frequent Contributor
Posts: 10
Re: Dissolution and precipitation of minerals
«
Reply #6 on:
December 25, 2017, 10:36:39 PM »
Thank you very much, and Merry Christmas and Happy New Year!
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Dissolution and precipitation of minerals