Aggregation iron oxyhydroxides

Figure 2. Cartoon illustrating features of the structure of ferrihydrite proposed by Drits et al. (1993). Note the severd octahedra-wide spaces. Although not discussed by Drits et al., these might arise if the solid forms by aggregation of smaller iron oxyhydroxide subunits.

Banfield JF, Welch SA, Zhang H, Ebert TT, Perm RL (2000) Aggregation-based crystal growth and microstructnre development in natural iron oxyhydroxide biomineralization products. Science 289 751-754... 

Further development of these methods will allow the examination of structural effects of systems, such as iron oxyhydroxides or natural organics, on filtration behaviour. In this thesis it was shown that there are structural effects, but their quantification for these systems (that is, oxyhydroxide floes and organic-calcium aggregates) was not possible. 

The three examples of the effects of pedochemical weathering on the surface structures in soil clays just described illustrate the complexity of the reactive solid materials in natural soils. To these examples can be added many others, including the formation of iron oxyhydroxide or calcium carbonate coatings on the external surfaces (as opposed to interlayer surfaces) of phyllosilicates, the development of thick envelopes of colloidal organic matter on aggregates of metal oxides and aluminosilicates, and the... 

Hydrous Oxides. This term is generally taken to include the oxides, hydroxides, and oxyhydroxides of aluminium, iron and manganese, which form in soil when these elements are released from primary minerals by weathering. They exist mainly as small particles in the claysized fraction of a soil (<2 pm), and also as coatings on other soil minerals or as components of larger aggregates. 

Perera er at." added iron to an aluminosilicate based geopolymer using either ferric nitrate solution or freshly precipitated ferric hydroxide. It was determined that the iron was present in octahedral sites, either as isolated ions in the geopolymer matrix, or as unreacted oxyhydroxide aggregates. In this study, typical aluminosilicate chemistries were used (i.e. metakaolin added to sodium silicate solution) and the iron was added as a minor compoitfrtt (vt%). Under these conditions, the highly reactive aluminosilicate geopolymers set fairly quickly and may have not allowed sufficient time for iron to participate in the reaction process. 

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