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Questions about alkalinity

Dear David

In our department (Chemical Engineering, Royal Institute of Technology), we
have used PHREEQC in several projects with excellent results. However, we
have some questions about the way as PHREEQC calculate alkalinity.

We are currently assessing the generation of acid mine drainage using the
PHREEQC model. We have run into problems regarding the contribution to
acidity from aluminium species in the model. As far as we understand, the
alkalinity of a specie is defined from its formation reaction from master
species and the defined alkalinities for those master species in the data

For a trivalent atomic ion (e.g. M=Al or Fe(III)), the formation of
hydrolysis complexes would be written

M3+ + xH2O = M(OH)x+(3-x) + xH+

and the associated alkalinity of the M(OH)x+(3-x) would be alk(M(OH)x+(3-x))
= alk(M3+) + x.

However, on the one hand, for the Fe(III) master specie, the alkalinity is
defined as - 2 (alk(Fe(3+) = -2) in the PHREEQC-, MINTEQ-, and the
WATEQ4F-databases, such that the Fe3+ ion contributes to acidity as
expected. On the other hand, for the aluminium master specie, the alkalinity
is defined as zero (alk(Al3+) = 0), which seems to imply that the Al3+ ion
(the aqueous or "free" Al3+ ion) in fact does not contribute to acidity.

This puzzled us, as the aluminium acidity (Al_acid) commonly is calculated
from Al_acid = 3[Al3+] + 2[Al(OH)+2] + [Al(OH)2+] (see e.g. Drever (1997)
"The geochemistry of natural waters", 3ed, p. 291). Does PHREEQC handle
acidity from Al-species in a special way, that we have not understood?

We would very much appreciate if you could help us sort out why the acidity
of Fe3+ and Al3+ are handled so differently in the PHREEQC code and how we
should run the model to account also for the aluminium acidity.

I will appreciate very much, if you can help us in this problem
Best regards

Luis Moreno

Luis Moreno
Associate Professor
Chemical Engineering and Technology /Chemical Engineering
Royal Institute of Technology
S 10044 Stockholm, Sweden
Phone: +46 - (8) 790 6412
Fax: +46 - (8) 10 52 28
Mobile: +46 - 070 593 32 13

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