Preparation by Transformation of 2-Line Ferrihydrite

Dissolve 40 g Fe(N03)3 9 H2O in 500 mL twice distilled water preheated to 90 °C and precipitate ferrihydrite with 300 mL M KOH also preheated to 90 °C. Add 50 mL 1 M NaHCOs, preheated to 90 °C, to the brown, voluminous precipitate of ferrihydrite and hold the suspension (pH = 8-8.5) in a closed polyethylene flask at 90 °C for 48 hours. 

Method 5 gives around 7 g hematite with a surface area of ca. 20-25 mVg. The crystals are small platelets (Fig. 10-1 e). The sample has a sharp XRD pattern. 

Method 6 produces about 8 g of hematite which consists of grainy-look-ing ellipsoidal (spindle-type) crystals (Fig. 10-5 d) and has a surface area of ca. 80-90 m /g. Again, as seen from the X-ray pattern the sample is reasonably pure and well crystallized (Fig. 10-2) but contains adsorbed oxalate and a trace of goethite (see IR spectrum in Fig. 10-3). 

The mean crystallite dimensions (MCD) along the a and c directions of the six hematites described above, have been determined by Crosa et al. 

As mentioned before, the yield of hematite from forced hydrolysis decreases as the degree of hydrolysis increases, i. e. the pH decreases during the formation process. Bao and Koch (1999) showed that the yield increased from ca. 10 to 60% in 0.1-0.2 M FeCls solutions as the temperature increased from 35 to 140 °C, and, at a given temperature, it was higher in 0.02 M than in 0.2 M solutions, e.g. 90 vs. 60% at 140°C. 

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