Re: PHREEQC problem

[xiaomin Mao <xmao@xxxxxxxx>]

Sorry I didn't explain the process clearly. I assume the sorption of Cu by
organicmatter site (Org_x) as follow (see Table 6.6, Werner Stumm, 1995, the
book: Aquatic Chemistry):
Cu+2  + Org_xH2  =  Org_xCu  + 2H+
To the formula of Org_xh,  I mean H is 0 (zero), not H O. Maybe as you said,
it's not necessary to mention this kinetic expression as it has the same rate
like Org_xcu.

cheers, xiaomin

David L Parkhurst wrote:

> >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