# Re: PHREEQC problem

```David Parkhurst (dlpark@xxxxxxxx)
U.S. Geological Survey
Box 25046, MS 413
Denver Federal Center
Denver, CO 80225

Project web page: https://wwwbrr.cr.usgs.gov/projects/GWC_coupled

----- Forwarded by David L Parkhurst/WRD/USGS/DOI on 05/27/03 01:48 PM
-----

David L Parkhurst
To:      xiaomin Mao <xmao@xxxxxxxx>
05/27/03 01:45 PM        cc:
Subject: Re: PHREEQC problem(Document link: David L
Parkhurst)

>About (1), I don't understand why you write: H -2 Cu +1 OH -1. I write in
the kinetic expression:
-Formula  H -2.0 Cu +1
will it be any problem?

I combined your two expressions ( Org_xcu and Org_xh) because they have the
same rate expression. The combined formulas do not appear to be charge
balanced to me.

Your calculated rate is negative I believe, so with -formula H -2 Cu +1,
you would be removing copper and adding H to solution. For Org_xh the rate
is positive, so with -formula O H, you are adding OH to solution. The net
reaction is relative to solution is Cu -1 H +2 OH +1 or Cu -1 H +3 O +1.

What is the reaction that you are trying to model?

SOH + Cu+2 = SOCu+ + H+ ?

You probably need to add the counter ion for such a reaction to make sure
the solution does not get imbalanced. (SURFACE calculations add an inert
charge to account for this effect.)

SOH + Cu+2 + Cl- = SOCuCl + H+

So the -Formula  CuCl -1 H +1 would be appropriate. But maybe this is not
what your are trying to model.

> To (2), I think I have noticed the difference. TOT is used to get the
concentration of Cu in the solution which is required according to the
model. As the water in each cell is only 0.03375 kg, the organic matter is
calculated based on the quantity of soil and ratio of organic matter in
each cell. therefore, it seems not to be the problem.

If you say so. I would still try a simpler rate expression to see whether
the problem is in the rate expression or some other part of the kinetic
definition.

David

David Parkhurst (dlpark@xxxxxxxx)
U.S. Geological Survey
Box 25046, MS 413
Denver Federal Center
Denver, CO 80225

Project web page: https://wwwbrr.cr.usgs.gov/projects/GWC_coupled

```

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