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Topic: Rate Expressions (Read 467 times)
rfembilejr
Top Contributor
Posts: 66
Rate Expressions
«
on:
July 11, 2017, 10:34:31 AM »
While tinkering on my script for transport modeling, it just occurred to me that there are different ways to write rate equations in phreeqc depending on completeness of experimental data. If i am using rate information from Palandri and Kharaka (2004), is it actually correct to use a rate expression in the form similar to that of pyrite for all other minerals present in my model (below)?
Palandri and Kharaka (2004) provided log k values and reaction orders with respect to H+ or Fe3+ ie. for acid mechanism.
Pyrite
-start
rem PARM(1) = log10(A/V, 1/dm)
rem PARM(2) = exp for (M/M0)
rem PARM(3) = exp for O2
rem PARM(4) = exp for H+
10 if (M <= 0) then goto 200
20 if (SI("Pyrite") >= 0) then goto 200
30 lograte = -10.19 + PARM(1) + PARM(2)*LOG10(M/M0)
40 lograte = lograte + PARM(3)*LM("O2") + PARM(4)*LM("H+")
50 moles = (10^lograte) * TIME
60 if (moles > M) then moles = M
200 SAVE moles
-end
Or what is the best way of expressing the rate info from Palandri and Kharaka (2004) for phreeqc.
I also wanted to clarify the exponent for (M/M0). Is it always fixed to n = 2/3 for uniformly dissolving spheres/cubes? I noticed that changing this would lead me to interesting results. What if the mineral, for example, pyrrhotite is partly to fully enclosed within olivine, n will probably be different. Perhaps this is one reason why I am not seeing Ni coming out into solution as what I would have wanted. I tried changing it to 1.34 but from the form M/M0, n cannot exceed 1. But for something that is being liberated, M may actually increase than M0 making n>1?
Thanks,
RE
«
Last Edit: July 11, 2017, 11:40:23 AM by rfembilejr
»
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dlparkhurst
Top Contributor
Posts: 1241
Re: Rate Expressions
«
Reply #1 on:
July 11, 2017, 02:31:53 PM »
Selecting a rate expression has a lot of flexibility. You are on firmer ground if you use published expressions and physically reasonable constants, but you are free to use other expressions.
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rfembilejr
Top Contributor
Posts: 66
Re: Rate Expressions
«
Reply #2 on:
July 11, 2017, 03:04:00 PM »
Copy. Thanks.
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