Surface Complexation Calculation

[cmcneece]

Hello,

I am looking for documentation on how the CD-MUSIC model is solved within PHREEQC. I have found this resource which deals with the diffuse layer model: https://wwwbrr.cr.usgs.gov/projects/GWC_coupled/phreeqc.v1/html/phqc_12.htm#HEADING20.

But I can not find a similar document for the CD-MUSIC model.

Thanks,
Colin

[dlparkhurst]

We were a bit lazy and did not provide the same documentation of equation solving for some of the features added in version 3. However, the approach is still a Newton-Raphson solution. For a single site, there are four equations: one site mole balance, and three charge/potential relations, one for each of three planes. The four unknowns are the activity of the master species and three potentials. The equations can be found in the work of Hiemstra, it is a matter of forming the Jacobian for the Newton-Raphson solution.

It has been too long since I dealt with those equations, so you are pretty much on your own unless you want to dig through the PHREEQC source. You can use KNOBS; -debug_model to see the set of linear equations that are solved, plus some information on the CD-MUSIC variables and equations at each iteration, but I am afraid the information is not easy to interpret.