Ferrihydrite water content

A 6-line ferrihydrite whose water content was reduced from 146 to 26 g kg by heating for 3000 hr at 123 °C, while the oxalate soluble proportion decreased from 100% to 12% and the unit cell volume from 0.3091 to 0.3079 nm ... 

Two-line ferrihydrite, for use as a catalyst, can also be synthesized by thermal decomposition of iron penta carbonyl, Fe(CO)5, in a stream of moist air at 500°C (Kosowski, 1993 Zhao et al.l993). It is a free flowing, reddish brown powder with a much lower bulk density than freeze-dried ferrihydrite prepared as above, probably because of much weaker aggregation. Small changes in the reaction conditions (water content of the air, duration of heating) may induce hematite formation. An analogous recipe involved slow thennal decomposition of trinuclear aceto-hy-droxy Fe "-nitrate for 20-40 hr in air in an attempt to simulate the red pigment on the Martian surface (Morris et al. 1991). 

Typically, mammalian ferritins can store up to 4500 atoms of iron in a water-soluble, nontoxic, bioavailable form as a hydrated ferric oxide mineral core with variable amounts of phosphate. The iron cores of mammalian ferritins are ferrihydrite-like (5Fe203 -9H20) with varying degrees of crystallinity, whereas those from bacterioferritins are amorphous due to their high phosphate content. The Fe/phosphate ratio in bacterioferritins can range from 1 1 to 1 2, while the corresponding ratio in mammalian ferritins is approximately 1 0.1. 

A number of observations help to understand the mechanism of hematite formation from ferrihydrite in aqueous systems i. e. under conditions essentially different from those for solid-state transformation by dry heating (see 14.2.6). Air-dry storage of ferrihydrite containing 100-150g H20/kg of water (found by weight loss) at room temperature for 20.4 years in closed vessels led to partial transformation to fairly well crystalline hematite with a little goethite (Schwertmann et al., 1999). In contrast, no hematite was formed from ferrihydrite if the content of adsorbed water was substantially reduced (Stanjek and Weidler, 1992 Weidler, 1997) as seen from the following examples ... 

Ferrous sulfate (feosol, others) is the hydrated salt, FeSO 7Hff), which contains 20% iron. Ferrous fumarate (feostat, others) contains 33% iron and is moderately soluble in water, stable, and almost tasteless. Ferrous gluconate (fergon, others), which contains 12% iron, also is used in the therapy of iron-deficiency anemia. Polysaccharide—iron complex (niferex, others), a compound of ferrihydrite and carbohydrate, has comparable absorption. The effective dose of these preparations is based on iron content. 

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