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Unit conventions for KINETICS and RATES parameters
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Topic: Unit conventions for KINETICS and RATES parameters (Read 2057 times)
Tom
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Posts: 124
Unit conventions for KINETICS and RATES parameters
«
on:
October 15, 2014, 12:22:23 PM »
I am attempting to understand the necessary conversion of rate parameters for used in the KINETICS and RATES data blocks.
The phreeqc.dat database has an example for pyrite oxidation by oxygen. The specific rate (rk) was published by Williamson and Rimstidt (1994) as:
rk = 10^-8.19 * (mO2^0.51) * (mH+^-0.11)
Where rk has units mol/m2/s and mO2 and mH+ are the molalities of oxygen and hydrogen respectively.
The overall rate equation for mineral dissolution in PHREEQC is:
Rk = rk * (A0/V) * (m/m0)^n
Where Rk is the overall rate and has units mol/m3/s, A0 is the initial surface area (m2), V is the solution volume (m3), m0 is the initial moles of the mineral, m is the remaining moles and n is an exponent.
My question is how to convert the Williamson and Rimstidt (1994) parameters for rk into appropriate units for PHREEQC in terms of 1 litre of solution.
In the phreeqc.dat database, a KINETICS/RATES example is given using the W+R (1994) equation. It converts 10^-8.19 to 10^-10.19 and states that A0/V must be in units of 1/dm.
In the PHREEQC Q+A thread, the conversion from 0^-8.19 to 10^-10.19 is explained as:
"The constant was adapted to comply with units of mol/dm3/s"
(see
http://wwwbrr.cr.usgs.gov/projects/GWC_coupled/phreeqc/mail/msg00195.html
)
-----------
Is anyone able to comment on whether this the correct methodology:
1) Convert constant in rk (in mol/m2/s) to mol/dm3/s:
rk(simplified) = 10^-8.19 (mol/m2/s) = 10^-10.19 mol/dm2/s
Is the latter considered to be for each unit length, i.e dm2/dm in order to account for volume, where the unit length is 1 dm?
and
2) Convert A0/V (in 1/m) to 1/dm:
A0 = 5m2 = 500 dm2
V = 0.001 m3 = 1 dm3
A0/V = 500/dm
Parameter log(A0/V) = 2.7
---------
Hopefully this topic will clarify the issue and ensure consistent input of compatible parameters.
Thanks,
Tom
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Unit conventions for KINETICS and RATES parameters