Akaganeite tunnels

Fig. 2.6 Structure of akaganeite. a) Arrangement of octahedral double chains in tunnels with chloride ions in the centre ofthe tunnels, b) Ball-and-stick model with unit cell outlined.

Aicagan ite. can incorporate up to 0.06 mol mol Cu and much smaller amounts of Cr, Mn, Co, Ni and Zn in the structure (Inouye et al., 1974 Holm, 1985 Buch-wald Clarke, 1989). Incorporation of Al, Cr and Ga has also been reported (Lorenz Kempe, 1987). Cornell (1992) produced akaganeite from acid Si-containing Fe " solutions and found by congruent dissolution that up to 0.04 mol mol Si could be incorporated. The Si species were probably located in the tunnels (0.5 nm ) of the akaganeite structure. 

An example involving the akaganeite hematite transformation is shown in Figure 14.8 where the average pore diameter increased from 1.1 nm at 150 °C to 3.7 nm at 350 °C and then to > 15 nm at 500 °C. The t-plot method using H2O as an adsorbate has also been used to investigate the location of H2O in the tunnel structure of akaganeite (Naono et al., 1993). 

The crystals of akaganeite are not microporous. Micropores observed by TEM are considered to be due to irradiation in the electron beam (Galbrait et al., 1979 Naono et al., 1982). Open ended, cylindrical, interparticular micropores have been reported these arose as a result of alignment of the rod-like crystals into parallel arrays (Paterson and Tait, 1977). Akaganeite does possess a potential structural microporosity arising from the presence of 0.21-0.24 nm across tunnels in the structure. At room... 

Two poorly ordered compounds both having the akaganeite structme but with sulfate or nitrate instead of chloride in the tunnel have been found recently, The sulfate form oecurs frequently in nature as an oxidation product of pyrite and has, therefore, been reeognized as a mineral with the name schwertmannite (Bigham et al. 1994). Chemically it can be considered a Fe oxyhydroxy sulfate with the ideal formula Feg0s(0Ff)6S04. The corresponding nitrate form can be synthesized by forced hydrolysis of an acidic Fe (N03)3 solution at 80°C and is a precursor of ferrihydrite (Schwertmann et al. 1996). 

Akaganeite may also be precipitated from Fe fluoride solutions (Bemal et al. 1959 Naono et al. 1993), but not from bromide or iodide solutions the latter ions are too large to fit into the tunnels in the structure. 

This material contains 2.3 mol% Si. Si is released congruently with Fe upon dissolution and is, therefore, considered to be homogenously distributed within the akaganeite crystals, probably in the tunnels (Cornell, 1992). 

Further candidates for the tunnel position in the schwertmannite structure is selenate. It seems that the oxyanions of hexavalent elements (S, Se, Cr) can be accomodated in the akaganeite structure whereas tliose of pentavalent ones (P, As) may only adsorb on the surface (Waychunas et al. 1995). 

Schwertmannite, is a common nanoparticle-product of neutralization of sulfuric acid-rich solutions (Bigham et al. 1994). The original structural analysis indicated that sulfate was contained within tunnels similar to those found in akaganeite (FeOOH). However, recent work by Waychunas et al. (2001) suggests that this is a defective, nanoporous phase and that sulfate occupies inner and outer sphere positions on the surface, and probably on the internal surfaces of defect regions within the structure. 

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