Unit conventions for KINETICS and RATES parameters

[Tom]

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

[Charlie]

Is this the case that PHREEQC always has to have A0/V for rates equations?

[dlparkhurst]

In RATES definitions, PHREEQC really doesn't care how you arrive at the SAVE command. At SAVE, PHREEQC expects a quantity in moles that is the number of moles that react over the time period given by TIME.

If the reaction is, say, a kinetic decomposition of a dissolved pesticide to a dissolved degradation product, the rate would not have any relation to a surface area.

If the reaction is dissolution of a mineral, then usually, the rate would depend on the surface area available to react. More surface area, faster reaction.

Precipitation of a mineral may also depend on surface area or nucleation sites, but if precipitating in an initially mineral-free system, you have to account for precipitation in the absence of surface area or nucleation sites, at least initially.