Re: PHREEQ-C question

> I have a question for you about PHREEQ-C: you mention in the manual that
the
usage of mole fraction for the activity of surface species instead of
molarity (which sounds reasonable to me) does affect multidentate
complexes - I'd be interested in how this affects bi- or tridentate
complexes.

If you consider mass-action equations

(1) Surf + Me+ = SurfMe+, K = [SurfMe+]/[Surf][Me+]

and

(2) 2Surf + Me+2 = Surf2Me+2, K = [Surf2Me+2]/[Surf]^2[Me+2]

If mole fraction is used [SurfMe+] = (SurfMe+)/(Surf, total) and [Surf]
= (Surf)/(Surf,total), where [] indicates activity and () molality. In
equation (1) the (Surf, total), the total number of sites, cancel relative
to molality.

In equation 2, [SurfMe+2] = 2(Surf2Me+2)/(Surf,total) and [Surf]
= (Surf)/(Surf, total). In this case a mass-action equation formed from
molality and one formed with mole fraction will differ by a factor of
2(Surf, total). I think using molality gives K's that are dependent on the
amount of surface sites and use of mole fraction is better.

>In addition, I was wondering if it is possible to include surface
precipitation as well in PHREEQ-C (accounting for the phenomenon that near
the surface concentrations may be high enough for precipitation whereas
overall solution concentrations are too low for precipitation to occur).

Basically, no, I did not implement Dzombak and Morel's surface
precipitation model.
There are solid solution capabilities and general kinetics capabilities. It
might be possible to use them to try to recreate the effects of surface
precipitation, but it would not be straightforward.

> I simulated As(V) sorption onto a quartz sand using the diffuse double
layer
model, and I had some problems at "higher" As(V) concentrations - at 10^-7
and 10^-8 moles/l As(V) the model works fine, but it underpredicts sorption
at 10^-6 moles/l and I was wondering if that could be due to bidentate
complexes involved (which was observed in other studies) or surface
precipitation with dissolved Fe.

I'm not sure. Surface complexation is not my specialty. It sounds like you
have your ideas and they are probably better than mine.

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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