The nature of an iron oxide—organic iron association in a peaty environment

Abstract

Fe oxides deposited in ditches draining a bog in the Harz Mountains, Northern Germany, consist essentially of a two-XRD line ferrihydrite that has a magnetic hyperfine field of only 46·5 T at 4·2 K. Pyrophosphate extraction led to an incomplete separation of this from organically-bound Fe, which was depleted in the residue and particularly enriched in the dialysate of the extract. Decomposition of humics with H2O2 led to a further depletion of organic Fe, but also increased the number of XRD lines of the ferrihydrite to six, and raised the hyperfine field by ∼1 T. These modifications are probably the result of an improved interparticle coherence.

Iron solid-phase differentiation along a redox gradient in basaltic soils

“…Here small differences in the LHT-Q components for the surface and subsurface 4200 mm yr À1 site soils are consistent with an assignment of organic-Fe III complexes in the surface horizons and isomorphic substitution of Fe III in silicates in the subsurface soils. The carbon-rich (480 g kg À1 ) surface soil has a CS value of 0.53 mm s À1 , which is close to LHT CS values of 0.54 mm s À1 for organically-complexed Fe in peat soils reported by Schwertmann and Murad (1988). Whereas, the kaolinite/halloysite-rich subsurface soil has a CS of 0.48 mm s À1 , which is similar to that of 0.47-0.49 mm s À1 reported for kaolin-group minerals by Fysh et al (1983).…”

“…Several recent studies have highlighted the effects of various organic components, organic matter and organic coatings on the magnetic properties of ferrihydrite (Schwertmann and Murad, 1988;Schwertmann et al, 2005;Eusterhues et al, 2008;Mikutta et al, 2008;Berquo et al, 2009;Michel et al, 2010). These studies illustrate that different carbon sources impact the Fh structure more substantially than others (Mikutta et al, 2008) decreasing Bhf between 1 and 3.5 T (Schwertmann et al, 2005;Eusterhues et al, 2008) and decreasing ordering temperature (T N ) up to 18 K. Even when changes to the Fh structure were minimal, Mikutta et al (2008) found dramatic changes in the point of zero charge, surface area and mineral porosity for organic-Fh co-precipitates.…”

Comparison of Pedogenic and Sedimentary Greigite by X-ray Diffraction and Mössbauer Spectroscopy

“…The approximate ratio of width/length has been inferred from the saturation magnetization values, which are given in Figure 1 of Ricci and Kirschvink, 1992. s for macroscopic measurements to 10 -8 s for Mtssbauer spectroscopy (dashed curve in Figure 3), both greigites still lie within the SP region. This behavior is similar to that of a ferrihydrite--organic matter association, in which Schwertmann and Murad (1988) observed an enhancement of magnetic coupling across the ferrihydrite crystallites after removal of the organic matter.…”

Sorptive stabilization of organic matter in soils by hydrous iron oxides

“…Dissolution of OC from organo-Fe(III) complexes by dithionite extraction is less clear but likely. Dithionite extraction almost fully dissolved precipitated, organically complexed Fe from a peat soil (Schwertmann and Murad, 1988). While dithionite effectively disperses Fe-cemented microaggregates (Pinheiro-Dick and Schwertmann, 1996), insoluble organic matter (e.g., plant detritus) in these aggregates will not appear in the dithionite-extractable phase.…”