> 1) for oxidized Mn minerals containing Mn4+, the major PHREEQC databases have written the Ksp reactions using Mn3+. That is perfectly rational as Mn4+ is not listed as a species in databases. But why is Mn4+ not included? I recognize that equilibrium data for the Mn3+/Mn4+ couple would be hard to come by and perhaps not terribly meaningful if they were....but I would appreciate your perspective. From a practical point of view, does this mean that in many cases the reported Mn3+ concentrations and speciation are in fact equivalent to Mn4+ concentrations (or species????hmmm...) any references to how this is handled in PHREEQC documentation would be appreciated My understanding is that aqueous Mn+4 is virtually non existent. Under natural conditions, Mn+2 is the only significant manganese species. Permanganate is metastable and tends to decompose to Mn+2 relatively quickly. The question seems to imply that because Mn+4 exists in solids, it must exist in solution, but there is no reason to require this. Sulfur in pyrite is effectively valence -1, but there is no -1 aqueous species. For manganese, it simply means that there must be a redox reaction in the dissolution of +4 minerals, generally reducing manganese and oxidizing something else, water to oxygen if necessary. > 2) Both ferric and manganic phases have their IAP's calculated in PHREEQC according to some convention besides that indicated by the mineral reaction given in PHREEQC.DAT and elsewhere. They also report Kt that differs from the database and is following the same alternate reaction. I am confident the SI's that are reported are fine but what basis is used for calculating the IAP's and modifiying the Ksp's from the database values for minerals like (say) goethite or Fe(OH)3 (a)? This is a "feature" of the program. I think I just got lazy at some point. The different K's result from "basis switching", which means the program rewrites the equations for the minerals to the predominant valence state at intermediate calculations in the numerical method. Thus if ferric ion is 10 times greater than ferrous ion, the equations are rewritten to ferric, resulting in a different log K. I should have gone back to the original log K's in the printout, but didn't. David Parkhurst (dlpark@xxxxxxxx) U.S. Geological Survey Box 25046, MS 413 Denver Federal Center Denver, CO 80225
Please note that some U.S. Geological Survey (USGS) information accessed through this page may be preliminary in nature and presented prior to final review and approval by the Director of the USGS. This information is provided with the understanding that it is not guaranteed to be correct or complete and conclusions drawn from such information are the sole responsibility of the user.
Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply endorsement by the U.S. Government.
The URL of this page is:
Last modified: $Date: 2005-09-13 21:04:21 -0600 (Tue, 13 Sep 2005) $
Visitor number 1782 since Jan 22, 1998.