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In soil and water, ferrihydrite frequently forms in the presence of dissolved organic matter. This disturbs crystal growth and gives rise to coprecipitation of ferrihydrite and organic matter. To compare the chemical fractionation of organic matter during coprecipitation with the fractionation involved in adsorption onto pristine ferrihydrite surfaces we prepared ferrihydrite-organic matter associations by adsorption and coprecipitation using (i) a forest-floor extract or (ii) a sulfonated lignin. The reaction products were studied by 13C CPMAS NMR, FTIR, and analysis of hydrolyzable neutral polysaccharides. Relative to the original forest-floor extract, the ferrihydrite-associated organic matter was enriched in polysaccharides, especially when adsorption took place. Moreover, mannose and glucose were bound preferentially to ferrihydrite, while fucose, arabinose, xylose, and galactose accumulated in the supernatant. This fractionation of sugar monomers was more pronounced during coprecipitation and led to an enhanced ratio of (galactose + mannose)/(arabinose + xylose). Experiments with lignin revealed that the ferrihydrite-associated material was enriched in its aromatic components but had a lower ratio of phenolic C to aromatic C than the original lignin. A compositional difference between the adsorbed and coprecipitated lignin is obvious from a higher contribution of methoxy C in the coprecipitated material. Coprecipitated organic matter may thus differ in amount and composition from adsorbed organic matter.

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.

Ferrihydrite (Fh) - even if present at low concentrations - may control the available surface area and therefore, the behaviour of nutrients and pollutants in soils. Its pre- cipitation often takes place in the presence of dissolved organic matter. This involves processes like adsorption, but also coprecipitation, flocculation/coagulation and poi- soning of crystal growth. To simulate this process we prepared a series of organic matter-Fh coprecipitates using water extractable organic matter (OM) from a forest topsoil. Products were investigated by N 2 -adsorption, TEM, XRD and Mößbauer spectroscopy. Precipitating Fh from solutions with increasing OM/Fe ratios yield products with up to 170 mg g − 1 organic C. While the OM-free sample consists of Fh particles a few nm in size (TEM) and specific surface areas of 386 m 2 g − 1 , Fh- OM coprecipitates are significantly smaller and display a surface area of only 5-23 m 2 g − 1 . With increasing amounts of associated OM the d-spacings of the major XRD peaks increase linearly from 0.258 to 0.269 nm and from 0.149 to 0.151 nm, while peak shoulders at 0.22 and 0.16 nm weaken. The asymmetry of the 0.26 nm peak de- creases and disappears at carbon contents > 120 mg kg − 1 C. The quadrupole split of the Mößbauer spectra at 300 K increases from 0.78 to 0.90 mm s − 1 and the magnetic hyperfine field at 4.2 K drops from 49.5 to 46.0 T. These data reflect the interference of OM with crystal growth leading to smaller Fh crystals, increased lattice spacings and a lower crystallinity. Specific surface area values suggest an almost complete OM- coverage on all OM-Fh coprecipitates. By comparison to sorption experiments we conclude that coprecipitation produces OM loadings similar to adsorption and does not encapsulate large amounts of OM.

Humic–iron oxide associations are believed to exist in various surface environments, such as soils and surface waters, and may add substantially to the stability of organic matter under oxidizing surface conditions. However, a nondestructive, solid-state characterization of such associations is still lacking. In this paper synthetic coprecipitates between humic material (dissolved organic matter; DOM) obtained from a Podzol and synthetic ferrihydrite are examined using X-ray diffraction (XRD) patterns and Fe-specific Mössbauer spectra at temperatures between 4.2 K and room temperature. Lepidocrocite formed in the absence of DOM. However, DOM induced the formation of a four (XRD)-line ferrihydrite that contained 96 mg C/kg. In contrast to a pure four-line ferrihydrite, which was completely magnetically ordered at 4.2 K, the synthesized DOM–ferrihydrite was not fully ordered at 4.2 K and had a magnetic hyperfine field 1 to 2 T lower than the pure ferrihydrite. Such an effect was not observed when DOM was only surface-adsorbed. We conclude that organic components of the DOM coprecipitated with the ferrihydrite. Their interaction with the Fe atoms of the oxide prevents complete spin freezing at 4.2 K. Solid-state 13C nuclear magnetic resonance (NMR) spectra suggested that O-alkyl C of the DOM was mainly responsible for the interaction with the Fe in the oxide.