Ferrihydrite specific surface area

Addition of sufficient base to give a > 3 to a ferric solution immediately leads to precipitation of a poorly ordered, amorphous, red-brown ferric hydroxide precipitate. This synthetic precipitate resembles the mineral ferrihydrite, and also shows some similarity to the iron oxyhydroxide core of ferritin (see Chapter 6). Ferrihydrite can be considered as the least stable but most reactive form of iron(III), the group name for amorphous phases with large specific surface areas (>340 m2 /g). We will discuss the transformation of ferrihydrite into other more-crystalline products such as goethite and haematite shortly, but we begin with some remarks concerning the biological distribution and structure of ferrihydrite (Jambor and Dutrizac, 1998). 

The BET method requires that the sample be dried and outgassed to remove adsorbed water. This procedure may, if the outgassing temperature is too high, lead to a phase change at the surface of the oxide hydroxide and hence, an alteration in the specific surface area of the sample. Clausen and Eabricius (2000) recommend that ferrihydrite be outgassed at room temperature, at which temperature, a stable BET surface area is obtained after 19 hr of outgassing. 

Dos Santos Alfonso and Stumm (1992) suggested that the rate of reductive dissolution by H2S of the common oxides is a function of the formation rate of the two surface complexes =FeS and =FeSH. The rate (10 mol m min ) followed the order lepidocrocite (20) > magnetite (14) > goethite (5.2) > hematite (1.1), and except for magnetite, it was linearly related to free energy, AG, of the reduction reactions of these oxides (see eq. 9.24). A factor of 75 was found for the reductive dissolution by H2S and Fe sulphide formation between ferrihydrite and goethite which could only be explained to a small extent by the difference in specific surface area (Pyzik Sommer, 1981). 

Properties 60% lepidocrocite, 40% ferrihydrite, XRD results available, BET specific surface area 222 m /g [1611]. 

Properties Two-line ferrihydrite [539,1510], BET specific surface area 327 m7g [1510], aggregates 235 jiun in diameter [539]. 

Properties Two-line ferrihydrite [157,1629], structure confirmed by XRD, three-point BET specific surface area 202 m-/g [1630]. 

Rapid Hydrolysis of FeCI, at pH 7-8 by KOH Properties One-point BET specific surface area 269 mVg, two-line ferrihydrite structure confirmed by XRD [173],... 

Modified recipe. Properties Two-line ferrihydrite [741,1633,1634], BET specific surface area 245 in7g. X-ray micrographs at different levels of preparation available [1633]. 

Properties 36% ferrihydrite (FejOj 2.2H2O), XRD pattern available, BET specific surface area 213 mVg, TEM image available [1611]. 

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