Characterization of ferrihydrite-soil organic matter coprecipitates by X-ray diffraction and Mössbauer spectroscopy

In soils and sediments ferrihydrite often precipitates from solutions containing dissolved organic matter, which affects its crystallinity. To simulate this process we prepared a series of 2-line ferrihydrite-organic matter coprecipitates using water extractable organic matter (OM) from a forest topsoil. The products were characterized by X-ray diffraction, Mössbauer spectroscopy, N₂-gas adsorption and transmission electron microscopy. With increasing C/Fe ratios of the initial solution the d-spacings of the two major XRD peaks increased, while peak shoulders at 0.22 and 0.16 nm weakened. The asymmetry of the 0.26 nm peak decreased and disappeared at a C/Fe ratio of 0.78. The quadrupole splitting of the Mössbauer spectra at 300 K increased from 0.78 to 0.90 mm s^-1 the mean magnetic hyperfine field at 4.2 K dropped from 49.5 to 46.0 T, and the superparamagnetic collapse of the magnetic hyperfine splitting was shifted toward lower temperatures. These data reflect a strong interference of OM with crystal growth leading to smaller ferrihydrite crystals, increased lattice spacings, and more distorted Fe(O,OH)₆ octahedra. Even small amounts of OM significantly change particle size and structural order of ferrihydrite. Crystallinity and reactivity of natural ferrihydrites will therefore often differ from their synthetic counterparts, formed in the absence of OM.