FeOOH and feroxyhyte

Synthetic 5-FeOOH has a surface area which ranges from 20-300 m g depending on the thickness of the crystals. In a series of seven synthetic feroxyhytes the surface area increased from 140 to 240 m g (EGME method) as the crystallinity decreased (Garlson and Schwertmann, 1980). 5-EeOOH displays interpartide porosity, i.e. slitshaped micro- or mesopores between the plate like crystals (Jimenez-Mateos et al., 1988 Ishikawa et al., 1992). Both TEM observations and t-plot analysis showed that 0.8 nm micropores formed upon dehydroxylation at 150 °G in vacuo. The surface area rose steeply as the temperature exceeded 100 °G and reached a value close to 150 m g at 200 °C at which temperature, the sample was completely converted to hematite. 

Surface areas of ferrihydrite have been determined by a wide range of methods and are reported as varying between 100 and 700 m g (Table 5.1 see also Jambor Du-trizac, 1998). The BET method is widely used for natural samples this gives areas of between 200-400 m g .  

The small, spherical particles of ferrihydrite often pack together to form aggregates 0.1 pm across. The aggregated structure and interparticle porosity create difficulties in measurement of surface area because the internal area is not fully accessible to all measurement techniques. It is rare for more than two methods of area measurement to have been applied to the same sample. Pyman and Posner (1978) obtained an area of 250 m g using both N2 and water BET measurements. With the EG ME method, however, the same sample had an area of 600 m g . These 

Fitting parameter for Triple-layer model 600-700 Davis and Leckie, 1978 

EGME 221-560 2-6-line with 1.3-5.5%Corg. Carlson and Schwertmann, 1981 

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