Fractionation of organic matter due to reaction with ferrihydrite: coprecipitation versus adsorption

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 (13)C 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.

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 byX-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.

Scavenging of As from acid mine drainage by schwertmannite and ferrihydrite: a comparison with synthetic analogues

Ochreous precipitates containing 5.5-69.8 g/kg As were isolated from mine drainage in Finland and were composed of schwertmannite, ferrihydrite, and goethite. Schwertmannite formation was favored at pH 3-4, but its structure was degraded at high As levels. A series of coprecipitates were therefore prepared from mixed iron arsenate/sulfate solutions to define the limits of schwertmannite stability. Schwertmannite was replaced as the dominant phase by a poorly crystalline ironIII hydroxy arsenate (FeOHAs) when As/Fe mole ratios exceeded 0.15. The FeOHAs gave an X-ray diffraction pattern similar to that obtained from an "amorphous" ironIII arsenate (As/Fe = 1.0) with broad peaks at 0.30 and 0.16 nm. The FeOHAs possessed a magnetic hyperfinefield of 41.9T at 4.2 K that was intermediate to those of schwertmannite (46.1 T) and the ironIII arsenate (24.8 T). These data indicate a strong disruptive effect of arsenate on magnetic ordering and structure development in schwertmannite. Equilibration of 0.01 M arsenate solutions with freshly prepared schwertmannite and 2-line ferrihydrite at pH 3.0 for up to 60 d gave sorbed As contents of 175 and 210 g/kg, respectively. Arsenate sorption degraded the host schwertmannite and ferrihydrite, perhaps due to the formation of an FeOHAs surface phase.

Properties of Goethites Prepared under Acidic and Basic Conditions in the Presence of Silicate

Goethite in natural environments usually grows in the presence of dissolved silicate. To study silicate-associated goethite with specific properties, goethite was synthesized in an Fe(III) system at RT under acidic (OH/Fe = 2; pH 1.6-1.8) and basic (OH/Fe = 4; pH 12-13) conditions at Si concentrations between 10(-5) and 1 M. The goethites were characterized by transmission (TEM) and scanning (SEM) microscopy, X-ray diffraction (XRD), Mössbauer spectroscopy (MS), and chemical analyses. Despite large differences in size and morphology, all goethite crystals were dominantly bound by 110 and 021 faces. Nanosized crystals (ca. 20 nm) were formed at low pH, where the influence of Si was weak. In the basic system, where Si retarded crystallization, much larger (tens to hundreds of nanometers) crystals were formed whose shape varied from acicular and multidomainic at low Si (10(-5) M) to monodomainic, blocky crystals at high (10(-2) M) Si concentration that had reduced growth along [001] in favor of [100] and [010]. The sizes and shapes of the crystals are discussed in terms of nuclei and growth unit concentrations in the system. Part of the retained Si could be released by phosphate and NaOH (surface-Si), and part was only liberated into HCl congruently with Fe (up to 46 and 17 g kg-1 for the acid and basic goethites, respectively). Neither XRD nor MS were able to prove structural Si-for-Fe substitution, probably because no tetrahedral positions are available to accommodate Si in the goethite structure. It is assumed that this nonsurface Si fraction is located between the crystal domains. Copyright 1999 Academic Press.

From Fe(III) Ions to Ferrihydrite and then to Hematite

The pathways from soluble Fe(III)-aquo ions to various solid polymeric Fe(III) oxides of increasing thermodynamic stability involve intermediate products which are still only partly known. This paper combines results from the literature with own new results from X-ray diffraction, Mössbauer spectroscopy, and electron microscopy. It is maintained that no intermediate phases were positively identified between mono-, di-, and trimers and a range of solid polynuclear phases. This indicates fast polymerisation as the OH/Fe ratio of the system increases. The immediate solid polynuclear phases are poorly crystalline Fe(III)-oxyhydroxy salts and a range of oxyhydroxides, called ferrihydrites in mineralogy, with varying crystallinity and magnetic ordering behavior. The rate of hydrolysis as affected by pH, rate of OH addition and temperature is of paramount importance for the nature of these polymeric phases. In the presence of free or surface-bound water, the transformation of ferrihydrite into the stable end product hematite (alpha-Fe2O3) proceeds by crystallization within the ferrihydrite aggregate and is enhanced as the pH approaches the zero point of charge as well as by Al in the system. This mechanism is different from that in nonaqueous systems. Copyright 1999 Academic Press.

Hydrogen-Aluminum Clays: A Third Buffer Range Appearing in Potentiometric Titration

Wyoming montmorillonite (bentonite, particles 2 to 0.2 micro in diameter) treated with hydrogen-ionsaturated resin shows, on titration in 1N KNO(3) with NaOH by a continuously recording instrument, a third buffer range between pH 5.5 and 7.6 in addition to the first range where exchangeable hydronium is neutralized and the second range where a reaction with exchangeable aluminohexahydronium, Al(OH(2))(6)(3+), occurs. The third range increases considerably when the hydrogen-ion-saturated clay is aged, and is attributed to basic aluminum compounds formed in the presence of negative charges of montmorillonite, comparable to "third range" buffering noted in aged, partially neutralized aluminum chloride solutions.