Iron L-edge and K-edge X-ray Absorption Studies of Nontronite Reduction.
Owen Li, Univ of Wisconsin, Environmental Chemistry & Technology, 1525 Observatory Dr., Madison, WI 53706-1299, Kangwon Lee, Univ of Illinois at Urbana-Champaign, Natural Resources & Enviornmental Science, 1102 S. Goodwin, W-321 Turner Hall, Urbana, IL 61801, Joel Kostka, Florida State Univ, Oceanography, Tallahassee, FL 32306, Joseph W. Stucki, Univ of Illinois at Urbana -Champaign, W-321 Turner Hall 1102 South Goodwin Ave, Urbana, IL 61801, and William Bleam, Univ. of Wisconsin, Environmental Chemistry & Technology, 1525 Observatory Dr., Madison, WI 53706-1299.
The reduction and re-oxidation of the iron-rich smectite mineral nontronite causes changes in short-range structure discernible by polarized extended x-ray absorption fine structure (EXAFS) spectroscopy. Reduction causes shifts that indicate internuclear distance changes and a broadening in the overall EXAFS spectrum caused by increased structural disorder. Re-oxidation in air restores the nontronite EXAFS spectrum but not to its original state. Non-reversible structural changes occur mainly during the first reduction cycle; peak width and intensity after a second redox cycle resemble values observed after the first reduction cycle. No differences appear in the polarized EXAFS spectra of nontronite samples reduced by Shewanella oneidensis when compared to samples reduced by sodium dithionite. We also evaluate the iron-migration hypothesis that predicts a significant redistribution of octahedral iron during nontronite reduction. Our polarized EXAFS data and analysis fails to support this hypothesis and proposes a different structural model for ferrous nontronite.Previous studies show iron L-edge XANES is very sensitive to spin state, crystal field strength and symmetry. Our results demonstrate iron reduction changes the intensity but not the position of two peaks dominating x-ray absorption at the L3-edge. The intensity ratio of these two peaks is linearly correlated with the iron oxidation state in the clay, permitting us quantify the Fe(II)/Fe(III) ratio using Fe L3-edge XANES spectroscopy.