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Re: pH to high

> I have another problem that I hope you can help me with:  I have mixed a
groundwater with 129 ppm SO4-2 (specified as S) with Fe_metal and observed
that the water reaches saturation with respect to several Fe oxide and
sulfide phases and as well as As2S3.  As expected, pe drops like a rock.
Magenite has the greatest degree of saturation of the Fe phases.

> Allowing magnetite, pyrite, and orpiment to "exist" shows that only
magnetite ppts, leaving pH high but perhaps unrealistically high (12+) so
that HS- is trivial relative to other S species and the sulfides do not

> Do I need to define reactions for the other S species and the orpiment,
pyrite, etc.?

If I understand you correctly, no, you do not need to define additional
reactions. The current definitions allow for equilibrium among all of the
aqueous species. Reactions defining the minerals can be written in terms of
any of the aqueous species.

> Also, can you comment on reaction precipitation sequence?

I can't comment. Equilibruim with iron metal would put you outside the
stability field for water, so you need to be careful how you define your
system. I think the program is probably doing what you are asking it to do
if it converges to a solution. You just need to think about what is


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

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