Complex Composition Rate Law for Mineral Dissolution

[Jeff Catalano]

I have read through the available documentation for React and GWB more broadly and am stumped on a topic. I want to create a complex composite rate law for Olivine (Forsterite) dissolution following Wogelius and Walthers (1991). They have a three term rate equations:

Rate = 9.07e-12*activity(H+)^0.54 + 5.25e-15 + 2.33e-17*activity(H+)^-0.31

The overall rate is in units mol cm-2 s-1, so to write this complex composite rate law surface and (1-Q/K) terms are needed. None of the documentation I have read explains clearly how to do such a rate law, and you cannot add more than one kinetic mineral term in React. I suspect I just need guidance on how to format this on the command line, so insight for someone more knowledgeable than me would be appreciated.

[Brian Farrell]

Hi Jeff, 

For this kinetic mineral reaction, you'll need to specify a custom rate law instead of using the GWB's Built-in rate law. I think the simplest option, the rate law equation, would work for your purposes, but you can set more complicated scripts or functions if you'd like. The process is described in section 5.1, Setting the rate law form directly, in the GWB Reaction Modeling Guide. After you've added the kinetic Forsterite, change the rate law pulldown from "built in" to "equation", then type in the equation to be evaluated. You can access the Internal parameters in Table 5.1 and the Helper functions in Table 5.2. The latter will be needed to include the H+ ion activity in your rate law.

Regards,

Brian Farrell
Aqueous Solutions LLC