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 base. 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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