> If you become irritated by my mailing you please let me know. I don't mind as long as your are thinking and experimenting and not just relying on me to tell you an answer. > Pursuant to your email yesterday I've been trying to simulate goethite to mackinawite inclusion with the sulfate reduction. Including these phases did, as you predicted, initially buffer the pH to a somewhat higher value, however only for small amounts of sulfate reduction and it seemed as though maybe the results were too complex to interpret. So I simplified the system and considered organic matter degradation without sulfate in the presence of goethite. My question here is with regards to this: > Goethite was not reduced... It was reduced, just not very much. > the system underwent methanogenesis. I don't think that this result is possible? From what I've read, iron reduction is much more favourable than methanogenesis having a more negative Gibbs energy (I could cite papers, but I'm sure you're already familiar with these). You need to be careful. The papers usually cite standard free energies, which would apply if all dissolved species were in their standard state (1 molal, ideal activity). Real conditions are far from standard states. > There was some goethite reduction, however, only 1/40 of that required if CH2O used goethite as the terminal electron acceptor. Am I doing something wrong? No, I don't think so. Goethite and hematite are not very soluble, at this pH and even in reducing conditions (calculate the concentration of iron from the log K expression, pH, and pe). For comparison, put in Fe(OH)3a. You will get higher pH, higher iron, but still will eventually make methane without completely consuming the iron oxyhydroxide. David 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 1354 since Jan 22, 1998.