Re: Goethite reduction

["David L Parkhurst" <dlpark@xxxxxxxx>]

> 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