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SUMMARY:EESS talk on "Catalysis of Fe(III)-Mineral Transformations by Fe(I
 I)"
DTSTART:20240220T121500
DTEND:20240220T131500
DTSTAMP:20260503T014333Z
UID:88ff8a93c841afb67423b4158f0a5a35cbd8bab7cc98df093cb7d6a7
CATEGORIES:Conferences - Seminars
DESCRIPTION:Dr. Kevin Rosso\, Pacific Northwest National Laboratory\, USA\
 nAbstract:\nFe(III)-(oxyhydr)oxides are critical secondary minerals in soi
 ls and sediments that control iron bioavailability and the cycling of coup
 led elements. The nanomineral ferrihydrite is a poorly crystalline and met
 astable initial form\, typically resulting from aqueous Fe(II) oxidation d
 uring meteoric recharge of otherwise stagnant suboxic pore water. Because 
 it is metastable\, ferrihydrite will spontaneously transform into more cry
 stalline and persistent bulk phases such as goethite or hematite. However\
 , in the absence of a redox catalyst such as Fe(II)\, this transformation 
 tends to be kinetically hindered\, consistent with both 1) the extremely l
 ow aqueous solubility of Fe(III)\, which restricts the dissolution/repreci
 pitation pathway\, and 2) the extremely low solid-state diffusivity of lat
 tice iron expected at room temperature\, which restricts the topotactic tr
 ansformation pathway. Particle-based recrystallization to goethite has bee
 n proposed\, but this cannot transform ferrihydrite without recruiting one
  or both of these two slow pathways.  Furthermore\, in the presence of mi
 llimolar aqueous Fe(II)\, the goethite product and its physical characteri
 stics are essentially unchanged\, but the kinetics are\, intriguingly\, ab
 out 1000x faster.\nI will present a comprehensive experimental/computation
 al investigation into this transformation at suboxic conditions and circum
 neutral pH\, resolved at the iron site occupancy level. In the absence of 
 added Fe(II)\, two-line ferrihydrite aged over two years was periodically 
 characterized using in situ mXRD with detailed line shape analysis\, elect
 ron microscopy\, and synchrotron Fe L-edge and O K-edge XAS/XMCD spectrosc
 opy quantitatively analyzed using MRCI ab initio calculations. The results
  are compared to DFT calculations of the relative thermodynamic stabilitie
 s of several possible ferrihydrite structure models as a function of hydra
 tion state\, and goethite\, along with activation energies for solid-state
  diffusion of iron along hypothetical topotactic channels. Tetrahedral Fe(
 III) is clearly resolved in 2 d ferrihydrite\, diminishing concomitantly w
 ith increasing octahedral Fe(III) and decreasing net magnetic moment until
  the first appearance of goethite at ~ 100 d. Calculated iron diffusion ac
 tivation energies are as low as 1 eV per unit cell. In the presence of Fe(
 II)\, the accelerated transformation kinetics appear to involve facile mas
 s transfer of a labile Fe(III) pool created on ferrihydrite surfaces durin
 g oxidative adsorption of Fe(II).  The findings are being assembled into 
 transformation kinetics models that will help resolve the relative importa
 nce of dissolution/reprecipitation vs. solid-state pathways\, to lay out t
 he first atomically-resolved mechanism of this important Fe(III)-(oxyhydr)
 oxide mineral transformation.\n\nShort Biography:\nDr. Rosso is best known
  for his pioneering research on electron transfer reactions between aqueou
 s ions\, mineral surfaces\, and bacterial enzymes. Beginning with topics s
 uch as metal sulfide oxidation\, bacterial reduction of metal oxides\, con
 taminant interactions with clay minerals\, and mechanisms of crystal growt
 h and dissolution\, his research expanded into geologic carbon sequestrati
 on\, stress corrosion cracking in alloys\, performance optimization of lit
 hium battery materials\, and the design of semiconductor materials for sol
 ar photocatalysis. Dr. Rosso is well recognized as being at the center of 
 the field of molecular geochemistry\, a field he helped create with the in
 ception of advanced tools such as scanning probe microscopy\, quantum mech
 anical molecular simulations\, and massively parallel supercomputers.
LOCATION:GC B1 10 https://plan.epfl.ch/?room==GC%20B1%2010 https://epfl.zo
 om.us/j/69011077410
STATUS:CONFIRMED
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