Goethite 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). 

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). 

The specific surface area, determined by adsorption and desorption of nitrogen for titania, goethite and silica samples were 13.5, 42.0, and 261.7 m2/g, respectively. 

Figure 3. Effect of duration cf heating on specific surface area of products prepared from goethite at various temperatures...

The discrepancies in reported a, values are to some degree due to the discrepancies in the specific surface areas reported in different publications for the same material. A few examples of surprisingly low or surprisingly high ap can be found in the literature. A Gq as low as 0.01 (7u in 0.1 M electrolyte, 4 pH units below the PZC, was found for high-surface-area goethite in [569]. An absolute value of do of UO2 exceeding 1 (7m- (on both sides of the PZC) in O.IM electrolyte is reported in [590]. 

Grinding of Goethite (Commercial or Synthetic) for 70 h Properties Contains silica, TEM images, XRD patterns available for particle diameter and BET specific surface area, see Table 3.416 [154]. 

PZC/IEP of maghemite-goethite composite is presented in Table 3.469. Properties Goethite and maghemite, BET specific surface area 135.2 m /g,... 

Compilations of PZC of goethites are presented in [174,715]. A compilation of PZCs and specific surface areas of goethites is presented in [73]. A compilation of surface acidity constants and other model parameters obtained for goethites is presented in [1459]. 

Synthetic Coethites Specific surface areas and other properties of synthetic goethites are reviewed in [1468]. 

Properties Highly disordered goethite, BET specific surface area 306 mVg, particle size 6 1 nm. [569]. 

Oxidation ofthe Product of Reaction Between FeSO and NHfiH Properties Goethite, detailed chemical analysis and electron micrograph available, BET specific surface area 24 rn7g [547,1117],... 

The oxides used in this study were a-Si02 (a-quartz), obtained commercially, and a-FeOOH (goethite), which was prepared in a manner similar to that of Forbes et al. (18). The silica was washed initially in Q.IN nitric acid. Both oxides were washed with double distilled water, dried at 100°C for 24 hr, powdered with a mortar and pestle, and passed through a 200 mesh (75 /xm) sieve. Powdered x-ray diffraction verified the existence of a-quartz and goethite. BET-Ng adsorption indicated specific surface areas of 1.7 m /g for silica and 85 m /g for goethite. Corresponding ZPC values, determined by electrophoresis and turbidity measurements, were 1.7 and 5.5. Dielectrics were taken to be 4.3 for silica and 14.2 for goethite (19). 

Haen Hfo material was determined as 65 m /g (Kofod et al., 1997), which is quite low compared with the specific surface area of amorphous iron hydroxides. These results showed that the Riedel de Haen Hfo material used in the column experiments more closely resembles goethite than freshly precipitated iron hydroxide. 

Column experiments were conducted to simulate the transport of oxoanion aquifers containing iron hydroxides. The Riedel de Haen Hfo material used in these column experiments exhibits a low specific surface area and contains a significant proportion of goethite. PHREEQC2 is equipped to model surface complexation of oxoanions onto iron hydroxides but a consistent data set of surface complexation constants is only available for amorphous hydrous ferric oxides. Tests were conducted to determine whether it is possible to use PHREEQC2 with this data set to model the oxoanion transport in the columns. If the data set of surface complexation constants is also suitable for the Riedel de Haen Hfo material than it should be sufficient to adjust the site density of the iron hydroxide surface in the model. 

Consider as an example of the use of Eq. 1.3 a clay sample containing oethit CTjstals (parallelepipeds) with the dimensions 160 x 26 x 6.5 nmT nce the mass density of goethite is 4.37 x 10 kg crystallographic specific surface area of these crystals is... 

FIG. 1 Relationship between specific surface area and points of zero charge of goethite reported in the literature. (From J. Lutzenkirchen, in preparation.)... 

A phenomenon that has often been explained by artifacts (such as incomplete solid-liquid separation) is the so-called solid-concentration effect. This can, in principle, be verified in several ways (e.g., by measuring the amoxmt of sorbent in the supernatant for goethite one might determine Fe in the supernatant). Honeyman and Santschi [24] labeled their hematite radio-actively to check for possible problems in sohd-hquid separation. They actually found that some hematite was still in the supernatant. Unfortunately, potentially remaining particles are usually the smallest ones, which have the largest specific surface areas and, therefore, relatively large amounts of solute can be sorbed on such particles. 

The main chapters (4-14) are concerned with the preparative methods. For the majority of oxides particularly hematite and goethite more than one preparative method is described. Properties such as crystal morphology and surface area frequently depend on preparative conditions and a selection of methods is presented to enable a range of oxides with specific characteristics to be produced. The production of so-called monodis-perse Fe oxides, i. e. products with a rather narrow particle size distribution, is also included. 

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